Biological Treatment and Secondary sedimentation

Chapter 12: Biological Treatment

This chapter describes biological treatment processes including design, construction and operational considerations. Suspended growth systems using the activated sludge process with its variations, lagoons, and fixed-film systems are described in this chapter. A summary of the design loadings for conventional biological processes is provided in Appendix V, which should be used in conjunction with the details in this chapter.

12.1 Process Selection

The activated sludge process (ASP) and its variations (including the sequencing batch reactor process) is the most common secondary treatment process used in Ontario. Other processes including fixed-film treatment systems are also capable of meeting secondary effluent quality (15 mg/L CBOD₅, 15 mg/L total suspended solids).

A list of the common biological processes including suspended growth, fixed film, and hybrid (combined suspended and fixed-film) systems that are well known and proven technologies for use in North America is provided below:

• Suspended Growth Processes:
  • Conventional Activated Sludge (CAS) process;
    • Plug Flow;
    • Complete Mix;
    • Contact Stabilization;
    • Extended Aeration;
    • Step-Feed ASP; and
    • High-Rate ASP.
  • Membrane Bioreactor (MBR);
  • Sequencing Batch Reactor (SBR); and
  • Lagoon (Facultative and/or Aerated).
• Fixed Film Processes:
  • Rotating Biological Contactor (RBC); and
  • Trickling Filter (TF).
• Hybrid Processes:
  • Integrated Fixed-film Activated Sludge (IFAS);
  • Trickling Filter/Solids Contact (TF/SC);
  • Rotating Biological Contactor/Solids Contactor (RBC/SC); and
  • Biological Aerated Filter (BAF).

12.2 Activated Sludge Process

12.2.1 General

The activated sludge technology and its several variations rely on aeration tanks for biological treatment and a means of sludge retention within the process. The ASP may be utilized to accomplish varied degrees of removal of total suspended solids and reduction of carbonaceous and/or nitrogenous oxygen demand. The major ASP types include: plug-flow, complete-mix, high-rate, contact stabilization, extended aeration and step-feed systems. Choice of the most appropriate process type or mode of operation should depend on the effluent quality criteria, consistency of treatment required, characteristics of sewage, proposed plant size, anticipated degree of operation with maintenance requirements, operating and capital costs. All designs should provide for flexibility in operation and, if feasible, should allow for operation in various modes.

The activated sludge process requires close operational attention and control, including routine monitoring and laboratory analyses. These requirements, which are relatively independent of plant size, should be considered when proposing this type of treatment.

The activated sludge process requires considerable amounts of energy to satisfy aeration demands. The energy demand is often 40 to 60 percent of the total energy usage for the overall sewage treatment plant (STP). Capability of energy usage reductions while still maintaining viability of the treatment process, both under normal and emergency conditions, should be included in the ASP design.

Protection against low temperatures and excessive heat loss should be provided to ensure continuity of operation and performance. Insulation of the tanks by earthen banks should be considered.

12.2.2 Pretreatment

Effective removal or exclusion of grit, debris, excessive oil or grease and screening of solids should be accomplished prior to the activated sludge process. Description of the facilities that may need to be provided is included in Chapter 10 - Preliminary Treatment.

Where primary treatment is used, provision should be made for discharging raw sewage (after screening and grit removal) directly to the aeration tanks to facilitate plant start-up and operation during the initial stages of the plant’s design life and to provide operational flexibility.

12.2.3 Selectors

Selectors can be used to enhance the selection of desired organisms in the ASP, to reduce the growth of filamentous organisms and to enhance the settling of the mixed liquor suspended solids. Selectors can be aerobic, anoxic or anaerobic. All selectors should be designed to provide a substrate concentration gradient, a high initial Food-to-Microorganism (F/M) ratio and adequate time for the absorption of soluble organic material by the microorganisms. The F/M ratios indicated in sections 12.2.3.1 through 12.2.3.3 are calculated as follows:

F = Influent BOD₅ (g/m³) · Influent flow rate (m³/d)

M = MLVSS in the tank(s) (g/m³) · Volume of tank(s) (m³)

Where:

MLVSS = Mixed Liquor Volatile Suspended Solids concentration in the tank(s) (g/m³)

F = BOD₅ mass loading rate to first selector compartment. The same F is used in the F/M calculations for the other compartments as well.

M = Total mass of MLVSS in the compartments considered. Mass for the first compartment is MLVSS concentration multiplied by the first compartment volume. For the second compartment, the MLVSS concentration is multiplied by the combined volume of the first and second reactors and for the third compartment, MLVSS concentration is multiplied by the volume of all three compartments.

12.2.3.1 Aerobic Selectors

Three-compartments should be used. The F/M ratio of the first compartment is critical. The following F/M gradient, which would result in two equally sized compartments followed by a third compartment with twice the volume of the first compartment, is recommended:

• 1^st compartment 24 d^-1
• 2^nd compartment 12 d^-1
• 3^rd compartment 6 d^-1

Aerobic selectors should maintain 1 to 2 mg/L of dissolved oxygen (DO). Provision should be made to satisfy oxygen uptake rates (OUR) of at least 65 to 80 mg of oxygen/g of MLVSS/hr [65 to 80 lb/1000 lb of MLVSS/hr].

12.2.3.2 Anoxic Selectors

Anoxic selectors are used in nitrifying activated sludge systems. A portion of the nitrified mixed liquor is recycled to the anoxic zone for denitrification. Sufficient nitrate-N concentration needs to be maintained in the recycled stream to the anoxic zone to remove the soluble BOD₅ and maintain anoxic conditions. A single-stage selector design with an F/M of 0.5 to 1 d-1 will generally be effective for filamentous control.

More efficient selection may be achieved with a three-compartment anoxic configuration. The following F/M gradient is recommended:

• 1^st compartment 12 d^-1
• 2^nd compartment 6 d^-1
• 3^rd compartment 3 d^-1

The anoxic zone or components should be mixed with mechanical mixers or by very low aeration rates. If low aeration is used for mixing, the DO should be kept below 0.5 mg/L.

12.2.3.3 Anaerobic Selectors

The anaerobic selector retention time should be 0.75 to 2.0 hours. The zone may be divided into three compartments with similar F/M ratios as the anoxic zone. Dissolved oxygen and nitrate cannot be present for the selector to act as an anaerobic selector. Mechanical mixers need to be used to maintain the solids in suspension in anaerobic selectors.

12.2.4 Aeration

12.2.4.1 General

The designer of aeration tanks and associated equipment should consider the following:

• Expected oxygen demands, including variations, exerted by sewage flows from upstream treatment units;
• Hydraulic loading rates, including variability;
• Treatment requirements, including reduction of CBOD₅, and nitrification if necessary;
• Temperature, pressure, relative rate of oxygen transfer (Alpha factor) and relative oxygen saturation values (Beta factor) for the sewage; and
• Other factors including type of recycle streams, aeration equipment and surfactants in the sewage.

The design parameters for the aeration systems associated with various activated sludge treatment processes are given in Table 12-1. These design parameters are applicable to both complete-mixed and plug-flow systems.

12.2.4.2 Aeration System Alternatives

Both mechanical and diffused aeration systems should be considered including standby equipment needs.

The designer should evaluate the aeration system alternatives considering the following factors:

• Oxygen transfer efficiencies;
• Power requirements;
• Diffuser clogging problems;
• Mixing capabilities;
• Air pretreatment requirements;
• Aerator tip speed of mechanical aerators;
• Icing problems;
• Misting problems;
• Cooling effects on aeration tank contents;
• Estimated installed capital cost;
• Expected maintenance costs; and
• Estimated operating costs.

12.2.4.3 Oxygen Transfer Efficiencies and Rates

The typical process oxygen transfer efficiencies and rates for water at 0 mg/L DO, 20 °C and 101 kPa (1 atm) atmospheric pressure, commonly used for aeration devices are given below:

• Coarse bubble diffusers - 4 to 6 percent [based on average tank depth of 4.5 m (15 ft)];
• Fine bubble diffusers - 6 to 15 percent [based on average tank depth of
• 4.5 m (15 ft)];
• Low speed mechanical aerators (70 rpm or less) - 1.5 to 2.7 kgO₂/kWh [2.5 to 4.4 lb/(hp·hr)];
• Submerged turbines - 1.0 to 1.5 kgO₂/kWh [1.6 to 2.5 lb/(hp·hr)];
• High-speed mechanical aerators - 1.2 to 1.5 kgO₂/kWh [2.0 to 2.5 lb/(hp·hr)]; and
• Brush rotors - 1.5 to 2.1 kgO₂/kWh [2.5 to 3.5 lb/(hp·hr)].

Higher oxygen transfer efficiencies and rates than stated above may be considered if the designer or equipment supplier can document or show through pilot-scale or full-scale testing that higher rates can be achieved.

There are also other aeration methods such as pure oxygen addition systems, jet aerators, tubular aerators which may be considered.

Table 12-1 - Aeration System Design Parameters ^{11, 13}

Treatment Process	Organic Loading Rate12 (kg BOD5/(m3·d))	F / Mv1 (d-1)	Minimum Retention Time (Based on Q Avg.)	Return Sludge Rate2 (% Q Avg.)	Oxygen Demand in Typical Municipal Sewage at Standard Conditions 9	Solids Retention Time (SRT) (Days)	MLSS Concentration (mg/L)
Conventional A.S.3 Without Nitrification	0.31 - 0.72	0.2 - 0.5	6 h	25 - 100	1.0 kg O2/kg BOD5	4 - 6	1000 - 3000
Conventional A.S. With Nitrification	0.31 - 0.72	0.05 - 0.25	6 h	50 - 200	1.0 kg O2/kg BOD5 + 4.6 kg O2/kg TKN	>4 at 20°C >10 at 5°C	3000 - 5000
Extended Aeration (Provides Nitrification)	0.17 - 0.24	0.05 - 0.15	15(10) h	50 - 200	1.5 kg O2/kg BOD5 + 4.6 kg O2/kg TKN	>15	3000 - 5000
High-Rate Without Nitrification 4	0.72 - 0.96	0.4 - 1.0	4 h	50 - 200	1.0 kg O2/kg BOD5	4 - 6	1000 - 3000
Contact Stabilization Without Nitrification4	0.31 - 0.725	0.2 - 0.55	0.33 (6) h 4 (7) h	50 - 150	1.0 kg O2/kg BOD5	4 - 10	1000 - 3000
Aerated Facultative Lagoons8	0.031 - 0.048	-	4 - 5 d	-	1.0 kg O2/kg BOD5	-	N/A

12.2.4.4 Aeration Tank Capacities and Loadings

The size of the aeration tank should be determined by pilot plant studies, or calculations based on solids retention time, food-to-microorganism ratio and mixed liquor suspended solids levels. Other factors, such as size of treatment plant, diurnal load variations, degree of treatment required and data from similar full-scale STP should be considered. In the case of the nitrification process, temperature, alkalinity, pH and DO concentration are important factors that the designer needs to consider.

Calculations should be carried out to justify the basis for design of aeration tank capacity. Calculations using values differing substantially from those in Table 12-1 should reference actual full-scale operational plants. Mixed liquor suspended solids (MLSS) levels greater than 5000 mg/L may be considered if pilot or other operational data shows that the aeration and clarification system are capable of supporting such high solids concentrations.

The aeration tanks are normally designed on average daily BOD₅ loading at the design average daily flow (Section 8.5.11 - Design Basis for Various Plant Components). When nitrification is required, the designer should evaluate the Total Kjeldahl Nitrogen (TKN) loading variations to prevent the effects of effluent ammonia bleed through during peak TKN loadings by establishing appropriate safety factors. A ratio of 7.14 mg/L alkalinity destroyed per mg/L of total ammonia nitrogen (TAN) should be used and a minimum residual alkalinity of 50 mg/L as calcium carbonate should be available. When process design calculations are not carried out, the aeration tank capacities and loadings for the processes shown in Table 12-1 should be used. The values apply to plants receiving daily load ratios of peak hourly BOD₅ to average daily BOD₅ ranging from about 2:1 to 4:1.

12.2.4.5 Arrangement of Aeration Tanks

Aeration basin depth affects the aeration efficiency and mixing capabilities of diffused aeration devices and mechanical aerators. The dimensions of each independent aeration tank or return sludge re-aeration tank should be such as to maintain effective mixing and utilization of air. Liquid depths should not be less than 3 m (10 ft) or more than 9 m (30 ft) except in special design cases such as horizontally mixed aeration tanks. An aeration basin depth of between 3.5 and 4.6 m (11.5 to 15.0 ft) is recommended. Complete mixed tanks have length-to-width (L/W) ratios of 1:1 to 3:1. Plug flow tanks have much larger L/W ratios of generally greater than 4:1, with baffling to simulate plug flow. Plug flow tanks provide the capability to perform step-feed, biological nutrient removal (with separate aerated and non-aerated zones) and improved nitrification kinetics. Arrangement of the aeration tanks in terms of the entire plant layout is discussed in more detail in Section 8.1.3 - General Plant Layout and Section 8.1.4 - Provisions for Future Expansion.

For very small tanks or tanks with special configuration, the shape of the tank, the location of the influent and sludge return and the installation of aeration equipment should provide for positive control to prevent short-circuiting through the tank.

Total aeration tank volume should be divided among two or more units, each capable of independent operation.

Inlets and outlets for each aeration tank unit should be suitably equipped with valves, gates, stop plates, weirs, or other devices to permit controlling the flow to any unit and to maintain reasonably constant liquid level. The effluent weir for a horizontally mixed aeration tank system should be easily adjustable by mechanical means and should be sized based on the design peak instantaneous flow plus the maximum return sludge flow. The hydraulic properties of the system should permit the design peak instantaneous flow to be carried through with any single aeration tank unit out of service. Overflow devices are preferred to avoid trapping foam and scum.

Channels and pipes carrying liquids with solids in suspension should be designed to maintain self-cleansing velocities or should be agitated to keep such solids in suspension at all rates of flow within the design limits. Adequate provisions should be made to drain segments of channels, which are not being used due to alternate flow patterns.

All aeration tanks should have a freeboard of not less than 460 mm (18 in). However, if a mechanical surface aerator is used, the freeboard should be not less than 0.9 m (3 ft) to protect against windblown spray freezing on walkways.

12.2.4.6 Mixing Requirements

The aeration system which is selected should not only satisfy the oxygen requirements of the mixed liquor, but should also provide sufficient mixing to ensure that the mixed liquor remains in suspension. The designer should be aware that it is important to avoid both insufficient and excessive mixing. One exception to this is with aerated facultative lagoons where mixing is only provided to the extent necessary to ensure uniform DO levels in the upper layers of the aeration cell. The power transferred to the mixed liquor and diffused air flow rates to achieve uniform DO and MLSS concentrations are shown in Table 12-2.

12.2.4.7 Oxygenation Capacity

Consideration should be given to reducing power requirements of aeration systems by varying oxygenation capacity to match oxygen demands within the system. Such a system would utilize automatic DO probes in each aeration basin to measure dissolved oxygen levels. Consideration should be given to the operations and maintenance requirements to maintain these systems. An output signal could then be used to change the number of aerators in operation, aerator speed, immersion of surface aerator impellers, or air flow rate to submerged turbine and diffused aeration systems to maintain the required minimum DO levels.

Table 12-2 Aeration Mixing Requirements¹

Aeration System	For Uniform DO Levels	For Uniform MLSS Levels
Mechanical	1.6 to 2.5 W/m3 [(0.06 to 0.09 hp/(103ft3)]	16 to 25 W/m3 [0.61 to 0.95 hp/(103ft3)]
Coarse Bubble Diffusers2	-	0.33 L/(m3.s) (0.02 cfm/ft3)
Fine Bubble Diffusers3	-	0.61 L/(m2.s) (0.12 cfm/ft2)

12.2.4.8 Aeration Equipment

Oxygen requirements depend on maximum diurnal organic loading, degree of treatment and level of total suspended solids concentration to be maintained in the aeration tank mixed liquor. Aeration equipment should be capable of maintaining a minimum of 2 mg/L of dissolved oxygen in the mixed liquor at all times and provide thorough mixing of the mixed liquor.

In the absence of experimentally determined values (recommended method), the design oxygen requirements for all activated sludge processes should be 1.0 kg O₂/kg design average daily BOD₅ (1.0 lb O₂/lb design average daily BOD₅) applied to the aeration tanks, with the exception of the extended aeration process, for which the value should be 1.5 kg O₂/kg design average daily BOD₅ to include endogenous respiration requirements.

Where nitrification is required or will occur, such as within the extended aeration process, the oxygen requirement for oxidizing ammonia should be added to the above requirement for BOD₅ removal and endogenous respiration needs. The nitrogenous oxygen demand (NOD) should be taken as 4.6 times the TKN content of the influent.

In addition, the oxygen demands due to recycle flows (e.g., anaerobic digester supernatant, heat treatment supernatant, dewatering centrate or filtrate, elutriates) need to be considered due to the high concentrations of BOD₅ and TKN associated with such flows. Similarly, contaminant loads associated with any septage or landfill leachate additions to the STP for co-treatment need to be considered.

Careful consideration should be given to maximizing oxygen transfer per unit of power input. In some site-specific situations (e.g. wide variations in industrial loadings) the aeration system should be designed to match the diurnal organic load variation while economizing on power input.

The design requirements of an aeration system should accomplish the following:

• Maintain a minimum of 2.0 mg/L of DO in the mixed liquor at all times throughout the tank or basin;
• Maintain all biological solids in suspension (Table 12-2);
• Meet maximum oxygen demand and maintain process performance with the largest unit out of service; and
• Provide for varying the amount of oxygen transferred in proportion to the load demand on the STP.

12.2.4.9 Diffused Air Systems

Having determined the oxygen requirements as discussed earlier in this section, air requirements for a diffused air system should be calculated by incorporating such factors as:

• Tank depth;
• Alpha factor of sewage;
• Beta factor of sewage;
• Certified aeration device transfer efficiency;
• Minimum aeration tank DO concentration;
• Critical sewage temperature; and
• Altitude of plant.

In the absence of experimentally determined alpha and beta factors, sewage transfer efficiency should be assumed to be not greater than 50 percent of clean water efficiency (i.e., accounting for aeration parameters Alpha times Beta) for plants treating primarily (90 percent or greater) domestic sewage. Treatment plants where sewage contains higher percentages of industrial wastes should use a correspondingly lower percentage of clean water efficiency and should have calculations performed to justify such a percentage. The design transfer efficiency should be included in the specifications.

The designer should also consider the following:

• Additional air supply should be provided for aerated channels, air driven pumps and aerobic digesters above the air requirements for secondary treatment. Separate aeration supply for aerobic digester is preferred for control purposes;
• The specified capacity of blowers or air compressors, particularly centrifugal blowers, should take into account that the air intake temperature may reach 45 °C (115 °F) or higher and the pressure may be less than normal. The specified capacity of the motor drive should also take into account that the intake air may be -30 °C (-20 °F) or less and may require over sizing of the motor or a means of reducing the rate of air delivery to prevent overheating or damage to the motor;
• The blowers should be provided in multiple units, so arranged and in such capacities as to meet the maximum air demand with the single largest unit out of service. The design should also provide for varying the volume of air delivered in proportion to the load demand of the plant. Aeration equipment should be easily adjustable in increments and should maintain solids in suspension within these limits;
• Diffuser systems should be capable of handling the air output from all blowers installed (including standby). The air diffusion piping and diffuser system should be capable of delivering normal air requirements with minimal friction losses;
• Air piping systems should be designed such that total head loss from blower outlet (or silencer outlet, where used) to the diffuser inlet does not exceed 3.4 kPa (0.5 psi) at average operating conditions;
• The spacing of diffusers should be in accordance with the oxygen uptake through the length of the channel or tank and should be designed to facilitate adjustment of the spacing without major revisions to air header piping;
• Individual assembly units of diffusers should be equipped with control valves, preferably with indicator markings, for throttling or complete shutoff. Diffusers in any single assembly should have uniform pressure loss; and
• Air filters should be provided in numbers, arrangements and capacities to furnish at all times an air supply sufficiently free from dust to prevent damage to blowers and clogging of the diffuser system.

12.2.4.10 Fine Bubble Diffuser Systems

With increased emphasis being placed on energy conservation in STP design, fine bubble diffuser systems are generally considered for new facilities and retrofits. Such systems have oxygenation efficiencies under process conditions of approximately 12 per cent with conventional tank depth (4.6 m) and greater efficiencies with increased depths.

Due to their increased oxygenation efficiencies, the air flows satisfying oxygen demand may not provide adequate mixing. The designer should ensure that proper mixing will occur. The fine bubble systems may under certain circumstances foul with slime. The slime formation is caused high F/M ratios, high soluble BOD₅ levels, low DO concentrations and also by low mixing levels.

The designer should also consider the following:

• Pilot testing should be carried out to determine if sliming will occur. This is particularly important where industrial waste contribution is significant or where soluble BOD₅ concentrations are expected to be high due to other causes;
• Manufacturer’s recommended maximum and minimum air flow rates should be complied with;
• The ability to vary air flow rates should be possible to take full advantage of the efficiency of the diffusers and to minimize fouling; when satisfying peak oxygen demands, diffusers should be operating at close to their maximum air flow rating; air valves should be provided for each grid of the aeration system; automatic variation of air flow rate is desirable, but as a minimum a DO probe should be located in the area of the aeration tank near the raw sewage inlet and set to alarm if DO falls to 1 mg/L or less;
• To facilitate dome diffuser cleaning, equipment should be provided to allow for rapid tank draining, diffuser removal and diffuser cleaning; cleaning of ceramic domes may be carried out by hosing and scrubbing, steaming or acid cleaning, or combinations of these methods; a clean water source should be available to refill the tanks following cleaning;
• Air cleaning should be provided; replaceable air filters, using coarse pre-filters and fine final units may be the simplest and least expensive; electrostatic precipitators or bag houses may also be used; with retrofit plants, old piping may have to be replaced since the flaking of rust from cast iron lines may clog the diffusers from inside; equipment should be provided to remove liquid accumulations from inside the headers following power failure or repair shut downs;
• Spare parts should be provided including diffusers, gaskets, bolts and air supply piping;
• Since dome or disk diffusers are better vertical mixers than horizontal mixers, tanks should be built as deep as possible to minimize the horizontal travel required; oxygen transfer efficiency, however, may taper off at depths greater than 6.1 m due to oxygen depletion in the air bubbles; and
• Although diffusers work best under conditions of uniform loading, some degree of plug flow appears to be desirable for good sludge settleability; L/W ratios of approximately 8:1 are recommended; the aeration system should be divided into approximately 4 grids with the number of dome diffusers per grid being gradually reduced to match oxygen supply to demand; full-floor coverage should be provided in the first 3/4 of the aeration tank; in the last 1/4 of the aeration tank, the dome diffusers should be positioned along the centre line of the tank to induce a double spiral roll mixing effect; to avoid over-design of the oxygen supply, 50 percent blanks should be provided in at least the first half of the aeration system for possible addition of more diffusers in the future, if necessary; step feeding to at least mid-tank length should also be allowed for in design in case it is needed to reduce sliming problems.

Manufacturers' mixing power recommendations should be considered and compared to values in Table 12-2.

12.2.4.11 Mechanical Aeration Systems

The mechanism and drive unit should be designed for the expected conditions in the aeration tank in terms of the power performance. Certified testing should be provided to verify mechanical aerator performance. Design transfer efficiencies should be included in the specifications.

The design requirements of a mechanical aeration system should provide that motors, gear housing, bearings and grease fittings be easily accessible and protected from inundation and spray as necessary for proper functioning of the unit.

Where extended cold weather conditions occur, the aerator mechanism and associated structure should be protected from freezing due to splashing.

12.2.4.12 Return Sludge Equipment

The minimum permissible return sludge rate from the final sedimentation tank of the ASP is a function of the concentration of suspended solids in the mixed liquor entering it, the sludge volume index (SVI) of these solids and the length of time these solids are retained in the sedimentation tank. See Chapter 13 Secondary Sedimentation.

Since undue retention of solids in the final sedimentation tanks may be deleterious to both the aeration and sedimentation phases of the activated sludge process, the rate of sludge return expressed as a percentage of the design average daily flow of sewage should generally be variable between the limits set forth in Table 12-1.

The RAS rate should be varied by means of variable speed motors, drives, or timers (small plants) to pump RAS at the recommended rates (Table 12-1). All designs should provide for flexibility in operation in various process modes, if feasible.

If motor driven return sludge pumps are used, the maximum return sludge capacity should be obtained with the largest pump out of service. A positive head should be provided on pump suctions. Pumps should have at least 80 mm (3 in) suction and discharge openings.

If air lifts are used for returning sludge from each sedimentation tank hopper, no standby unit may be needed if the design of the air lifts facilitate their rapid and easy cleaning and other suitable standby measures are provided (i.e., blower capacity). Air lifts should be at least 80 mm (3 in) in diameter. Due to air within the RAS pipe, direct flow measurement within the pipe is difficult.

Discharge piping should be at least 100 mm (4 in) in diameter and should be designed to maintain a velocity of not less than 0.6 m/s (2 ft/s) when return sludge facilities are operating at normal return sludge rates. Suitable devices for observing, sampling and controlling RAS flow from each sedimentation tank hopper should be provided.

Waste sludge control facilities should be provided so that the excess activated sludge may be wasted from the RAS lines or directly from the aeration tank. Wasting from the return sludge is more common and provides a more concentrated sludge; however wasting from the mixed liquor provides a simpler control. The waste pumps and pipelines should be sized based on the expected maximum sludge production rates and minimum sludge concentrations. Although continuous wasting is preferred, for non-continuous wasting the capacity of pumps and pipelines should be designed to handle the wasting rates expected.

Means for observing, measuring (i.e., flow rate and total), sampling and controlling waste activated sludge (WAS) flow should be provided. Waste sludge may be discharged to the primary sedimentation tank, sludge digestion tank, sludge thickening or dewatering processes, storage tank or any practical combination of these units.

12.2.5 Flow Monitoring

Flow monitoring devices should be installed in all activated sludge plants for raw sewage or primary effluent, return sludge, waste sludge and air to each aeration tank. For plants designed for design average daily sewage flows of 4000 m³/d (1 mUSgd) or more, these devices should totalize and record, as well as indicate flows. Where the design provides for all return sludge to be mixed with the raw sewage (or primary effluent) at one location, then the influent flow rate to each aeration tank should be measured.

12.3 Sewage Treatment Lagoons

12.3.1 General

This section provides design guidelines for sewage treatment lagoons (also referred to as waste stabilization ponds) capable of achieving equivalent to secondary treatment (annual average concentrations of 25 mg/L CBOD₅ and 30 mg/L TSS) or better. The sewage treatment lagoons are classified based on either the bioactivity type (facultative and/or aerated lagoons) or mode of operation (seasonal discharge or continuous discharge). Aerated lagoons are further classified based on their design and purpose as either aerated facultative lagoons, completely mixed aerated lagoons and post-aeration polishing cells. Most of the lagoons in Ontario are facultative lagoons with seasonal discharge. Combinations of various types of lagoons are also used based on site-specific needs.

Lagoons utilized for equalization, infiltration, evaporation and sludge storage are not discussed in this section.

Lagoon systems are often capable of providing secondary equivalent sewage treatment at a lower cost than mechanical STP when land costs are considered. This is generally the case with rural small municipalities, where sufficient low cost land is available in the vicinity of the service area and where low permeability soils are available for lagoon cell construction.

Seasonal discharge lagoons have advantages over continuous discharge lagoons and mechanical sewage treatment plants where receiving streams experience insufficient flows during at least part of the year to provide adequate dilution for continuous effluent discharges or where downstream recreational water uses make summer effluent discharges undesirable.

It is generally accepted practice in Ontario to design sewage treatment lagoons based upon average daily sewage flow rates and BOD₅ loading and making no special allowance for net precipitation entering the cells.

12.3.1.1 Facultative Lagoons

At the feasibility or pre-design planning stage for facultative lagoons, the designer should consider the following:

• Possible nuisances such as odours, algae and vectors (e.g. mosquitoes). For the recommended land use surrounding lagoons refer to Section 4.5 - Separation Distances between Sewage Works and Sensitive Land Use;
• Whether the lagoon can be continuously discharged or needs to operate on a seasonal discharge basis;
• The minimum time and calendar dates for discharge of the lagoon cell contents;
• Industrial wastewater component and its effects on lagoon treatment. In some cases, it may be necessary to pretreat industrial wastewater;
• Whether phosphorus removal will be necessary and if required, to what level;
• Whether effluent ammonia and/or hydrogen sulphide concentrations will need to be reduced; and
• What discharge rates will be permitted from seasonal discharge lagoons and what provision may be required for controlling effluent discharge rates in proportion to the receiving stream flow rates.

Facultative lagoons that need to be discharged prior to or soon after the ice cover leaves the lagoon in the spring, may have hydrogen sulphide levels in the effluent high enough to cause fish toxicity in the receiving stream. In such cases, the designer should consider oxidation of hydrogen sulphide in the effluent in a post-aeration cell (Section 12.3.1.7 - Post-Aeration Polishing Cell).

Ammonia may be stripped from the lagoon contents during the summer months of high algal growth and pH. However, higher ammonia levels in spring especially under ice cover, cannot be effectively treated using a postaeration cell. In such cases, the designer should consider the use of intermittent sand filters (Section 12.3.6 - Intermittent Sand Filters) or other ammonia removal technology.

12.3.1.2 Facultative Lagoons with Supplemental Aeration

In some cases supplemental aeration may be the most economical means of upgrading or expanding a facultative lagoon. The additional aeration will supplement the insufficient level of oxygen provided by photosynthetic activity and natural surface reaeration of the upper layer. The required level of supplemental aeration should be established under the site-specific conditions.

12.3.1.3 Seasonal Discharge

Facultative lagoons, when operated on a seasonal discharge basis with phosphorus removal by batch dosing with alum or iron salts, are able to achieve an effluent quality (CBOD₅ of 15 mg/L, TSS of 20 m/L and TP of 0.5 to 1.0 mg/L) comparable to conventional activated sludge plants with phosphorus removal. To achieve such quality, the lagoon cells should be ice free at the time of planned discharge so that batch dosing can be used for phosphorus removal. Continuous addition of alum to the raw sewage entering lagoon cells has not proven to be as effective as batch dosing; however, effluent TP of 1 mg/L can be achieved.

The ability to introduce raw sewage to all lagoon cells is desirable, but as a minimum there should be a capability to divide raw sewage flows among enough cells to reduce the design average BOD₅ loading to 22 kg/(ha·d) (20 pounds per acre per day) or less, at the mean operating depth in the primary cells.

The required hydraulic detention time should be determined using the volume between 0.6 m (2 ft) (recommended minimum operating depth) and the maximum operating depth of the entire lagoon system and the design average daily flow. The hydraulic detention time should not be less than:

• The hydraulic detention time as set by the area needed to meet the design BOD₅ loading;
• The largest number of consecutive days of a year when discharge is not allowed; and
• The number of days the lagoon is under ice cover.

12.3.1.4 Continuous Discharge

For continuous discharge facultative lagoons, the design average BOD₅ loading distribution should be similar to that of a seasonal discharge lagoon (Section 12.3.1.3 - Seasonal Discharge).

Design variables such as lagoon depth, multiple units, detention time, supplemental aeration and additional treatment units should be considered with respect to effluent quality requirements for CBOD₅, TSS, E. coli, ammonia, hydrogen sulphide, DO and pH. The major factor in the design is the duration of the cold weather period where the lagoon contents are at temperatures of less than 5 °C (41 °F).

During the summer/fall months, the presence of algae may considerably increase the effluent TSS and CBOD₅ concentrations. If the effluent quality criteria are provided as average monthly concentrations, the designer needs to consider the appropriate facilities for algae removal like microscreening (Section 15.3 - Microscreening) or intermittent sand filters (Section 12.3.6 – Intermittent Sand Filters).

12.3.1.5 Aerated Lagoons

Aerated lagoons can be classified into two categories depending on the degree of aeration achieving partial or complete mixing. These types of lagoons are generally used in conjunction with continuous discharge operations, but may also be a part of a seasonal discharge lagoon system.

12.3.1.6 Aerated Facultative Lagoons

Aerated facultative lagoons are designed and operated to ensure that enough oxygen is transferred to satisfy the applied BOD₅ loading and maintain an adequate dissolved oxygen level. The lagoon contents should be mixed sufficiently to maintain uniform DO levels throughout the aerobic layer. No attempt is made to supply enough mixing to maintain a uniform suspended solids concentration. Mixing is kept low enough to permit solids settling. Solids settling to the lagoon bottom will undergo anaerobic decomposition and the products of this decomposition are released and treated in the upper aerobic layers.

The most common application for the use of aerated facultative lagoons is as pretreatment of raw sewage prior to discharge into subsequent lagoons. With 4 to 5 days of retention time, typical effluent quality from an aerated facultative lagoon treating domestic sewage will generally be a CBOD₅ concentration of 60 mg/L, TSS of 100 mg/L and TP of 6 mg/L. With a total retention time of 30 days in the lagoon system, effluent quality (annual average concentrations) equivalent to that produced by conventional activated sludge treatment may be achieved.

Aerated facultative lagoon systems (i.e., aerated facultative lagoons plus subsequent lagoons) designed to treat domestic sewage should consist of two or more aerated cells. It is recommended that the first two cells should be of equal size. If more than two cells are proposed, any cell should not provide more than 50 percent of the total required volume.

12.3.1.7 Post-Aeration Polishing Cell

A post-aeration polishing cell may be considered in cases where the period of discharge from a facultative lagoon falls at the time when the lagoon may have significant ice cover or soon after ice melt, resulting in high hydrogen sulphide levels.

The following design criteria should be incorporated in the design of a postaeration cell to permit biochemical oxidation of hydrogen sulphide (H2S) and to minimize stripping of the gas:

• The cell should be 3 to 4 m deep (10 to 13 ft); have a L/W ratio of 4:1 and provide at least 12 hours of retention time;
• The influent should be fed to the bottom of the cell and dispersed at a minimum of three locations in the first two-thirds of the cell; and
• The aeration system should consist of a fine bubble diffuser to ensure high oxygen transfer and minimize mixing. Oxygen should be supplied to provide both 1.2 kg O₂/kg CBOD₅ (1.2 lb O₂/lb CBOD₅) and 1.0 kg O₂/kg H₂S (1.0 lb O₂/lb H₂S).

12.3.1.8 Completely Mixed Aerated Lagoons

Completely-mixed aerated lagoons are another type of aerated lagoon system, wherein complete mixing is achieved within the lagoon cells. The aeration systems are designed in a similar way to those of activated sludge processes, except that earthen berm construction is used for the aeration basin.

The design cell depth should be 3 to 4.6 m (10 to 15 ft). This depth limitation may be adjusted depending on the aeration equipment, waste strength and climatic conditions.

The completely-mixed aerated lagoon should be followed by a sedimentation basin.

12.3.2 Aeration Equipment

Various types of aeration systems may be used, including bridge mounted mechanical surface aerators, floating mechanical surface aerators and diffused aeration using submerged diffusers or aeration tubing. Where extreme winter temperatures are experienced, submerged aeration systems are recommended. If mechanical surface aerators are used, they should be of the low speed bridge mounted type to avoid icing damage. Erosion protection will generally be required below mechanical aerators to prevent bottom scour. For a completely mixed cell, power requirement of the aeration equipment to maintain solids in suspension would control the power input to the system and meet the oxygen demand.

Aeration requirements will generally depend on the BOD₅ loading, degree of treatment required, temperature and the concentration of suspended solids to be maintained in the cell. The final sizing of the aeration equipment should be based on guaranteed performance by the equipment manufacturer with verification of mixing and oxygen dispersion capabilities of the proposed aerators.

The designer should ensure the operational reliability of the aeration system by providing the following:

• The blowers serving diffused air systems should be provided in multiple units, so arranged and in such capacities as to meet the maximum air demand with the largest unit out of service;
• The air diffusion system for each aeration cell should be designed such that the largest section of diffusers can be isolated without losing more than 50 percent of the oxygen transfer capability within each cell;
• The floating or fixed mechanical aerators should be provided in sufficient numbers to enable the design oxygen demand of a particular cell to be satisfied with the largest capacity aerator in that cell out of service;
• At least two mechanical aerators should be installed in each primary cell for a mechanical aeration-based system; and
• A backup aerator should be provided. The backup aerator may be a complete uninstalled unit or a motor. In the latter case, a prop assembly (drive train) should be provided so that the installed aerator or parts can be easily removed and replaced.

Suitable protection from the elements should be provided for electrical controls, aerators and piping.

12.3.3 Design Considerations

The minimum number of cells should be two for small installations. Larger installations should have a minimum of three cells designed to facilitate both series and parallel operations.

The maximum sewage depth in facultative lagoons should be 1.8 m (6 ft) in primary cells. Greater cell depths can be used if preceded by supplemental aeration or mixing. The bottom 0.3 m (1 ft) of cell liquid depth (i.e., retained sediment) should be retained at the completion of lagoon drawdown..

The shape of all lagoon cells should be such that there are no narrow or elongated portions. Rectangular lagoons (length not exceeding three times the width) are considered most desirable; long dimension(s) should not align with prevailing wind direction. The maximum size of each lagoon cell should be 8 ha (20 acres), but 4 ha (10 acres) is preferred.

The hydraulic capacity for seasonal discharge lagoons should be sized to permit all cells to be discharged in the minimum time specified in the design but not less than a minimum rate of 150 mm (6 in) of lagoon water depth per day at the available head.

The hydraulic capacity for continuous discharge structures and piping should allow for a minimum of 250 percent of the design maximum day flow of the system or be at least equal to the expected future peak raw sewage pumping rate.

Effluent from each cell should be drawn from 0.3 m (1 ft) above the cell bottom. Outlets should lead to effluent chamber(s) which permit level regulation. All cells should be provided with an emergency overflow system to overflow when the liquid contents reach within 0.6 m (2 ft) of the top of the berms.

Cross connection piping between adjacent cells should be interconnected to permit flow between cells. Where cells are at or near the same elevation, the pipes should be valved. Where cell elevations differ significantly, the cross connection pipe should have a chamber with a weir to control flow from the higher cell. The valve or chamber should be provided with suitable locking devices and be located off the traveled portion of the top of the berm.

Short-circuiting in continuous discharge lagoons should be minimized, especially to avoid the need for effluent disinfection. Two or more cells should be provided and designed to allow series (as well as parallel) operation. Effluent piping should be as far removed as possible from inlet or cross connection piping. Wind induced currents should be considered when lagoon orientation is being selected. Installation of baffles or curtains can be considered to reduce short-circuiting.

12.3.4 Lagoon Construction

A soil consultant’s report should be prepared to address the following:

• The suitability of the native soils for the proposed construction and the need for a liner;
• The maximum groundwater elevation and depth to bedrock;
• The soil strata which will be suitable for use (i.e., for forming the cell bottom and berm cores), soils to be removed and solids suitable for topdressing and estimates of their permeabilities; and
• The estimated initial clear water leakage rate which should be experienced from the cell structures.

When a lagoon is being considered for a site where leakage is expected and where there are nearby groundwater uses or surface water bodies which are likely to be adversely affected, the above factors need to be evaluated by a hydrogeologist, as part of a hydrogeological assessment in accordance with ministry Guideline B-7, Incorporation of the Reasonable Use Concepts into Ground Water Management Activities. A system of wells or lysimeters may be needed around the perimeter of the lagoon site to facilitate groundwater monitoring where needed.

Soil used in constructing the lagoon bottom (not including the seal) and dike cores should be relatively incompressible and tight and compacted at or up to 4 percent above the optimum water content as required based on the soils report.

Under certain soil circumstances, liners may be required in order to minimize excessive leakage. Where clay liners are used, precautions should be taken to avoid erosion and desiccation cracking prior to placing the system in operation.

Berms should have a minimum top width of 3.0 m (10 ft) to allow for access by liquid alum trucks and maintenance vehicles. Minimum freeboard above maximum lagoon operating level should be 0.9 m (3 ft). Berm slopes should not exceed 4:1 (horizontal:vertical) inside and 3:1 outside unless greater slopes are recommended by a soil consultant. Adequate provision should be made to divert stormwater runoff around the lagoons and protect lagoon embankments from erosion.

Influent lines may be located along the bottom of the lagoon with the top of the pipe just below the average upper elevation of the lagoon seal or liner. However, the full seal depth needs to be maintained below the bottom of the pipe and throughout the transition area from the bottom of the pipe to the lagoon bottom.In situations where pipes penetrate the lagoon seal, provisions to prevent seepage (such as anti-seep collars) need to be made. The lagoon site should be fenced and provided with a locked access gate of sufficient width to accommodate mowing equipment.

12.3.5 Control Structures and Interconnecting Piping

A manhole or vented cleanout wye should be installed prior to entrance of the influent line into the primary cell and should be located as close to the dike as topography permits. Its invert should be at least 150 mm (6 in) above the maximum operating level of the lagoon and provide sufficient hydraulic head without surcharging the manhole.

Flow distribution structures should be designed to effectively split hydraulic loads equally between the primary cells.

All primary cells should have individual influent lines which terminate approximately at the midpoint of the width and at approximately two-thirds of the length away from the outlet structure so as to minimize short-circuiting.

The influent line should discharge horizontally into a shallow, saucer-shaped depression. The end of the influent discharge line should rest on a suitable concrete apron large enough to prevent the terminal velocity at the end of the apron from causing soil erosion. A minimum size apron of 0.6 m (2 ft) square should be provided.

The designer should consider the use of multi-purpose control structures to facilitate normal operational functions such as drawdown and flow distribution, flow and depth measurement, sampling, pumps for recirculation, chemical additions and mixing and minimization of the number of construction sites within the dikes.

As a minimum, control structures should be:

• Accessible for maintenance and adjustment of controls;
• Adequately ventilated for safety and to minimize corrosion;
• Locked to discourage vandalism;
• Equipped with controls to permit sewage level and flow rate control and complete shutoff;
• Constructed of non-corrodible materials (metal-on-metal contact in controls should be of similar alloys to discourage electrochemical reactions); and
• Located to minimize short-circuiting within the cell and avoid freezing and ice damage.

Recommended devices to regulate sewage level are valves, slide tubes or dual slide gates. Stop logs should not be used. Regulators should be designed so that they can be preset to prevent the lagoon surface elevation from dropping below the desired operational level.

12.3.6 Intermittent Sand Filters

Use of the intermittent sand filter (ISF) process is a viable method for polishing lagoon effluents. The process involves application of lagoon effluent on a periodic or intermittent basis onto the surface of a sand filter bed. As the lagoon effluent passes through the sand, suspended and soluble matter are removed through a combination of physical straining and biochemical transformations. A mature ISF is a complex ecosystem with the majority of the biochemical activity concentrated near the surface of the filter. This allows ammonia to be nitrified and a portion of the BOD₅ to be removed. A properly designed and operated ISF system provides a very high removal of BOD₅ and TSS and can produce a completely nitrified effluent with high dissolved oxygen. In Ontario, intermittent sand filters have been demonstrated to be functional only during warmer non-freezing periods.

Filter surface is typically flooded once or twice per day with lagoon effluent. The influent system should be capable of applying the tota1 dai1y hydraulic load in less than 6 hours to ensure maximum head development and maximum bed reaeration after drainage. The length of the filter run is controlled by the size of the sand, the hydraulic loading rate and the total suspended solids concentration in the lagoon effluent. Night time applications to the bed have been shown to significantly extend filter runs by inhibiting the growth of algae in effluent applied onto the filters and filter beds.

Typical ISF filter run lengths may range from 30 days with lagoon effluent TSS concentrations of greater than 50 mg/L to one year with low lagoon effluent solids. To allow flexibility for cleaning, all systems should have at least two filter beds (three are preferred), each designed to receive the total flow.

The depth of the sand in the filter bed should be 0.9 m (3 ft) initially to allow removal of the top 2 - 5 cm (1 - 2 in) layer during each cleaning cycle and replacement of that sand about once per year. The fi1ter should not be operated with less than 0.6 m (2 ft) of sand on the bed. The effective particle size of the sand should be 0.15 to 0.30 mm, with a uniformity coefficient of less than 5.

The sand layer should be underlain by a graded gravel layer to prevent intrusion of sand into the underdrain piping. A grave1 bed should be 0.3 m (1 ft) deep and contain 10 cm (4 in) of 6 mm (0.25 in) pea gravel on top, 10 cm (4 in) of 20 mm (0.8 in) gravel in the middle and 10 cm (4 in) of 30 mm (1.25 in) gravel at the bottom.

The underdrain piping should have maximum spacing of 1.5 m (5 ft) with minimum lateral pipe size of 15 cm (8 in) in diameter and connected to an outlet manifold. This manifold should be designed to allow complete drainage of the underdrain network so that air can circulate through the drain system into the filter bed. The base of the filter bed should be lined with clay or membrane liners.

A dosing basin with a siphon or electrically actuated valves and timer controls should be used to apply the lagoon effluent to the filter bed at a hydraulic loading rate of 500 L/(m²·d) (12 USgpd/ft²) at one or more equal dosings per day. The influent zone should be provided with a gravel splash pad using 50 mm (2 in) gravel.

When site topography permits, gravity flow or automatic dosing siphons should be used for application of lagoon effluent. The use of pumps is necessary when fi1ter beds are operated in series to lift the effluent to the second stage filter unit. The containing walls for the filter unit are earthen embankments, but concrete or other materials can be used for smaller systems where space is limited. Washing and reuse of the sand is feasible when local sources of low cost sand are not available.

In Ontario, the major application of intermittent sand filters has been for ammonia removal and polishing of lagoon effluents. The major limitations associated with intermittent sand filters are the large land area requirements for construction of the system, the need to periodically remove or replace the upper layers of sand on the bed and to either clean or dispose of the removed sand.

12.4 Other Biological Systems

12.4.1 General

Alternative biological systems include a wide range of suspended growth, fixed film and hybrid processes. Some of the processes described in this section are not common in Ontario or in Canada, however these may find common usage in the future. Some of these processes may include proprietary equipment or processes that will require coordination with the manufacturer or supplier of the technologies. Care should be taken to obtain sufficient pilot or full-scale process performance results consistent with the design conditions.

12.4.2 Sequencing Batch Reactors

The fill-and-draw mode of the activated sludge process commonly termed the Sequencing Batch Reactor (SBR) may be used in a similar fashion to the activated sludge process. Continuity and reliability of treatment equal to that of the continuous-flow-through modes of the activated sludge process should be provided. The SBR process uses control strategies that permit optimization of the system. Manufacturer input should be included in the design and sizing of these units. Provision for emergency maintenance (e.g. spare parts) to minimize downtime should be considered.

12.4.2.1 Design Considerations

The designer should consider the following:

• More than two tanks should be provided. Influent baffling using a baffle wall and adequate physical separation of the influent from the decanter is recommended for any basin which may operate with a continuous feed during the settle and decant phases. The baffling should direct the influent wastewater below the sludge blanket. Average horizontal velocities through each baffle wall opening should not exceed 0.3 m/s (1 ft/s);
• All SBR tanks should have a minimum freeboard of not less than 600 mm (24 in);
• The decantable volume and decanter capacity of the SBR system with the largest basin out of service should be sized to pass at least 75 percent of the design peak daily flow without changing cycle times. A decantable volume providing at least 4 hours retention time with the largest basin out of service based on 100 percent of the design peak daily flow is recommended;
• System reliability with any single tank unit out of service and the instantaneous delivery of flow should be evaluated in the design of decanter weirs and approach velocities. The treated effluent from each reactor should be free of scum and have a total suspended solids concentration of no greater than 30 mg/L at any time. Scum removal should be provided. An adequate zone of separation between the sludge blanket and the decanter(s) should be maintained throughout the decant phase;
• Decanters should draw treated effluent from below the water surface and exclude scum or have a means to exclude scum and floatables;
• Protection against ice build-up on the decanter(s) and freezing of the discharge piping and decant valve(s) should be provided;
• Treatment facilities with fixed decanters, or any other system where the low-water depth cannot be adjusted quickly by the operator, should be designed to end the decant phase at a higher water level than other types;
• The water depth of any basin where simultaneous fill and decant may occur should be limited to not less than 3.7 m (12 ft) at the end of the decant phase. The minimum water depth can be reduced to 3 m (10 ft) for SBR with non-continuous feed;
• Adequate means to accommodate basin dewatering should be provided. All sludge transfer and wasting pumps should be accessible for maintenance without dewatering the tank;
• The capability to transfer sludge between SBR tanks should be provided. If the decant pumps are used for sludge transfer, all solids in the decant piping need to be flushed and recycled back to the SBR;
• The blowers should be provided in multiple units, so arranged and in such capacities as to meet the maximum air demand in the aerated portions of the fill/react and react phases of the cycle with the single largest unit out of service;
• Oxygen transfer rates from the aerators based on average water depth between the low-water level and the maximum water level should be considered to provide a DO residual of 2.0 mg/L during aeration. Credits for oxygen recovery through denitrification should only be considered for those systems designed to denitrify;
• Independent aeration mixing should be provided for all systems where biological phosphorus removal or denitrification is required. The mixing equipment should be sized to thoroughly mix the entire basin from a settled condition within 5 minutes without aeration;
• Downstream processes need to be sized to handle peak discharge rates that will occur during decant phase unless equalization is provided for decant flow;
• All 24-hour effluent quality composite samples for compliance reporting or monitoring plant operations should be flow-paced and include samples collected at the beginning and end of each decant phase; and
• Programmable logic controllers (PLC) should be provided. Multiple PLCs should be provided as necessary to ensure rapid process recovery or minimize the deterioration of effluent quality from the failure of a single controller. An uninterruptible power supply with electrical surge protection should be provided for each PLC to retain program memory (i.e., process control program, last-known set points and measured process/equipment status) through a power loss. A hard-wired backup for manual override should be provided in addition to automatic process control. Both automatic and manual controls should allow independent operation of each tank. In addition, a failsafe control should be provided which cannot be adjusted by the operator allowing at least 20 minutes of settling between the react and decant phases.

12.4.2.2 Unit Sizing

Activated sludge process design considerations in Section 12.2 - Activated Sludge Process) should be reviewed. The aeration tank volumetric loading should not exceed 0.24 kg BOD₅/(m³·d) (15 lb BOD₅/d/1000 ft³). Design F/M ratios should be within the range of 0.05 to 0.1 d -1. The reactor MLVSS and MLSS concentrations and aeration tank volumetric loading rate should be calculated at the low-water level.

12.4.3 Membrane Bioreactors

The Membrane Bioreactor (MBR) process consists of a suspended growth biological reactor (activated sludge system variation) integrated with a microfiltration or ultrafiltration membrane system. The key to the technology is the membrane separator which allows elevated levels of biomass in the reactor to degrade or remove the pollutants from the waste stream. These systems typically operate in the microfiltration or ultrafiltration range which results in removal of particles having a nominal size larger than 0.1 μm and 0.01 μm, respectively.

The benefits of these processes are consistent high effluent quality, reduced footprint and increased expansion capabilities within the same tankage and ease of operation. Tertiary quality effluent (Table 8-1) is the normal output of a membrane bioreactor. Virtually no solids are lost via the permeate stream and the unintentional wasting of solids is reduced. As a result, the sludge age can be very accurately determined. Nitrification for ammonia removal is easily achieved by optimizing reactor and sludge age to specific sewage characteristics and effluent requirements.

If required, denitrification can be achieved with MBR processes that operate at MLSS concentrations of 10,000 mg/L and higher. The mixed liquor rapidly becomes anoxic in the absence of a continuous stream of air. Furthermore, the high level of biomass ensures that at all times there are enough microorganisms in the anoxic zone to efficiently convert the nitrates into nitrogen gas.

12.4.3.1 Design Considerations

MBRs can be configured in a number of different ways. The two main configurations differ by those in which the membranes are submersed directly in the bioreactor and those which contain external membrane process tanks. When membrane modules are submersed into the bioreactor, they are in direct contact with the mixed liquor. A vacuum is created within hollow fiber or flatplate membranes by the suction of a permeate pump. The treated effluent passes through the membrane, enters the hollow fibers or a permeate collection zone and is pumped out by the permeate pump. An air flow may be introduced into the bottom of the membrane module to create turbulence which scours and cleans the membrane surface to maintain a satisfactory permeate flux. The permeate (treated effluent) is then collected for reuse or discharge.

Externally-coupled membrane processes operate in a similar manner, however, the membranes are contained in a separate tank through which the mixed liquor from the bioreactor requiring filtration constantly flows. Recycled mixed liquor flow can provide cross-flow velocity required for membrane scouring and flux control. Air is often added for both treatment and membrane scouring purposes.

The main difference between the two MBR configurations lies in the membrane cleaning processes where membranes submersed within the aeration tanks should be removed or isolated for cleaning while externallycoupled membranes are cleaned by evacuating the membrane tanks and providing for equalization during the cleaning procedures within the main aeration tank.

Adequate pretreatment of raw sewage by either microscreening or fine screening may be required upstream of MBR processes in order to prevent operational difficulties (i.e., buildup of trash, fat, hair, lint, and other fibrous materials in the membrane modules and/or integrated aeration devices).

12.4.3.2 Unit Sizing

The biological components of the MBR process can be designed similar to the activated sludge process (Section 12.2 - Activated Sludge Process). Owing to the typical elevated MLSS concentrations, the MBR process has characteristic long solids retention times (SRT).

The type and design of the membranes is dependent on the orientation of the membrane and the manufacturer. The membrane manufacturer should be consulted for particulars to their units and to provide verified design parameters for their units. Verified MBR design parameters should be based on pilot- and full-scale systems with an adequate period of operation. Special care should be given to peak flow operations and consideration for units being out-of-service for cleaning.

12.4.4 Biological Aerated Filters

The Biological Aerated Filter (BAF) process comprises submerged, granular media filters which treat sewage by biologically treating carbonaceous and nitrogenous matter using biomass growth fixed to the media and by physically capturing suspended solids within the media. No downstream secondary clarification is required.

BAFs are aerated to degrade carbonaceous biodegradable matter and convert ammonia-nitrogen to nitrates via nitrification. Non-aerated filters in the presence of supplemental organic matter can convert nitrates into nitrogen gas through denitrification.

BAFs are designed either as co-current backwash or countercurrent backwash systems. The co-current backwash design has a nozzle deck supporting a granular media that has a specific gravity (SG) greater than 1.0. Pretreated sewage is introduced under the nozzle deck and flows up through slightly expanded media bed and effluent leaves the filter from above the media. Process air is introduced just above the nozzle deck (the bed is not aerated for denitrification). During backwash, wash water and air scour are introduced below the nozzle deck and flow up through the bed. Wash water is pumped to the STP headworks or directly to solids handling.

The countercurrent backwash BAF operates under the same general principles, except that the granular media has a SG less than 1.0 (e.g. polystyrene bead media). Therefore, the media float and are retained from above by a screen. During backwash, wash water flows by gravity through the media. Process air is introduced below the media; therefore, scour air moves countercurrent to the wash water flow.

12.4.4.1 Design Considerations

The performance of BAFs in terms of allowable loading rates and effluent quality depends on influent sewage quality and temperature. In general, higher organic or suspended solids influent loadings result in higher effluent concentrations. Adequate water velocity is necessary to provide scouring of the media and biomass and for an even flow distribution across the media bed. Inadequate water velocity can result in premature bed plugging; this is especially the case for denitrification reactors in which the effects of air scouring are not present.

Factors that positively affect nitrification include:

• Warm sewage temperature;
• Adequate aeration and good air distribution; and
• Low BOD₅ and suspended solids loading.

Most manufacturers have estimated that solids production from the BAF process is comparable to that of a conventional activated sludge process. Effluent contaminant concentrations from a single BAF cell increases for approximately 30 minutes following a backwash event and therefore a minimum of four cells should be included in any design to dampen these spikes.

The nozzle deck features nozzles that prevent media loss and assist in evenly distributing flow across the bed. The reported media loss from the BAF system is less than 2 percent per year. The nozzle openings are slightly smaller than the media and require that influent be pretreated with a fine screen to prevent plugging. Headloss across the media bed can be more than 2 m (6.6 ft) prior to backwash. In existing installations, the filters are constructed above grade. The combination of the tall structure [6 m (20 ft)] and headloss across the bed requires influent pumping to the BAF in most situations. In addition, the co-current designs require pumping of wash water which is a significant, but intermittent, energy demand.

Process air is required in BAF cells that are removing carbonaceous organic matter (CBOD₅) and ammonia. The process aeration system consists of coarse- to medium-bubble diffusers on a stainless steel piping grid. The diffusers should be simple and reliable as possible because of the difficulty in accessing the aeration grid. Energy for process air can represent more than 80 percent of the energy demand of a BAF system.

12.4.4.2 Unit Sizing

The granular media bed for both BAF designs is typically 3 to 4 m (9 to 12 ft) deep with media size of 3 to 6 mm (0.012 to 0.024 in) in diameter. The specific surface area of media ranges from 500 to 2000 m²/m³ (150 to 610 ft²/ft³). Contact time in the media is typically 0.5 to 1.0 hour. The media bed is backwashed every 24 to 48 hours for 20 to 40 minutes using a wash water volume about three times the media volume. Backwash water from a single event is collected in a storage tank and returned to the head of the STP or directly to solids processing over a 1- to 2-hour period. Backwash water typically contains 400 to 1200 mg/L of suspended solids. The backwash water recycle flow can represent up to 20 percent of the influent sewage flow.

BAFs can operate in different process configurations, depending on the facilities, effluent goals and sewage characteristics. The process can follow primary sedimentation (with or without chemical addition) or an activated sludge system. Adequate pretreatment is required to ensure that the BAF media and nozzles do not become plugged. Enhanced primary treatment (with chemical addition and/or polymer) can assist the BAF process in providing combined organic removal and nitrification in a single-pass orientation. Following primary sedimentation, BAF cells can be operated for BOD₅ removal at loadings of between 2.5 to 5.0 kg BOD₅/(m³·d) [0.16 to 0.31 lb/(ft³·d)] or, under lower loading rates (less than 1.5 kg BOD₅/(m³·d) [0.09 lb/(ft³·d)]) for both carbonaceous BOD₅ and ammonia-nitrogen removal. A cell can operate in a nitrification mode following an activated sludge system or another BAF cell removing carbonaceous matter.

A denitrification biological filter process can follow either an activated sludge or BAF system that is nitrifying. Denitrification usually requires methanol addition and sewage flow velocities should be greater than 10 m/h (32.8 ft/h).

12.4.5 Trickling Filters

A trickling filter (TF) is a fixed film process that is suitable to biologically treat municipal sewage, although consideration needs to be given to the impact of temperature loss that occurs through the trickling filter during winter periods. Trickling filters should be preceded by effective primary sedimentation tanks equipped with scum and grease removal devices or other suitable pretreatment facilities. Solids separation is an important part of the TF process; accordingly, downstream secondary clarification is required. (Chapter 13 – Secondary Sedimentation)

Trickling filters should be designed to provide for reduction in carbonaceous and/or nitrogenous oxygen demand in accordance with established sitespecific effluent quality requirements or to properly condition the sewage for subsequent treatment processes. Multi-stage TFs should be considered if required to meet more stringent effluent quality criteria.

12.4.5.1 Design Considerations

The influent sewage may be distributed over the filter by rotary distributors or other suitable devices, to ensure uniform distribution over the surface area. For rotary distributors, reverse reaction nozzles, hydraulic brakes or motordriven distributor arms should be provided to not exceed the maximum speed recommended by the manufacturer and to attain the desired media flushing rate.

For reaction-type distributors, a minimum head of 610 mm (24 in) is required between the low water level in the siphon chamber and centre of the arms. Similar allowance in design should be provided for added pumping head requirements where pumping to the reaction-type distributor is used. A minimum clearance of 300 mm (12 in) between media and distribution arms should be provided.

Influent sewage may be applied to TFs by siphons, pumps or by gravity discharge from preceding treatment units. Influent to the trickling filter should be continuous and therefore the piping system should be designed for recirculation as required to achieve the design efficiency. The recirculation rate should be variable and subject to plant operator control at the range of 0.5:1 up to 4:1 (ratio of recirculation rate versus design average daily flow). A minimum of two recirculation pumps should be provided.

Forced ventilation should be provided for covered trickling filters to ensure adequate oxygen for process requirements. The design of the ventilation facilities should provide for operator control of air flow depending on the outside seasonal temperature.

The piping system, including dosing equipment and distributor, should be designed to provide capacity for the design peak hourly flow, including recirculation.

The trickling filter media should be resistant to ultraviolet degradation, disintegration, erosion, aging, all common acids and alkalis, organic compounds, fungus and biological attack. Such media should be structurally capable of supporting a person’s weight or a suitable access walkway should be provided to allow for distributor maintenance.

Trickling filter media should have a minimum depth of 1.8 m (6 ft) above the underdrains. Rock and/or slag filter media depths should not exceed 3 m (10 ft) and manufactured filter media depths should not exceed those recommended by the manufacturer. Ventilation needs to be provided and forced ventilation should be considered.

To ensure sufficient void clearances, media with specific surface areas of no more than 100 m²/m³ (30 ft²/ft³) are acceptable for filters employed for carbonaceous matter reduction and 150 m²/m³ (46 ft²/ft³) for second stage nitrification.

The underdrains should have a minimum slope of 1 percent. Effluent channels should be designed to produce a minimum velocity of 0.6 m/s (2 ft/s) at design average daily flow rates of application to the TF including recirculated flows. The underdrain system, effluent channels and effluent pipe should be designed to permit free passage of air. The size of drains, channels and pipe should be such that not more than 50 percent of their cross-sectional area will be submerged under the design peak instantaneous flow, including proposed or possible future recirculated flows.

Provision should be made for flushing the underdrains unless high rate recirculation is utilized.

Appropriate valves, sluice gates, or other structures should be provided to enable flooding of trickling filters comprised of rock or slag media for filter fly control.

A freeboard of 1.2 m (4 ft) or more should be provided for tall manufactured filters to contain windblown spray. At least 1.8 m (6 ft) of headroom should be provided for maintenance of the distributor on covered filters.

All distribution devices, underdrains, channels and pipes should be installed so that they may be properly maintained, flushed or drained. Mercury seals should not be permitted for rotary distribution seals. Ease of seal replacement should be considered in the design to ensure continuity of operation.

Covers should be provided to maintain operation and treatment efficiencies at cold temperatures by avoiding excessive temperature drop through the TF.

12.4.5.2 Unit Sizing

Pilot testing is recommended to verify performance predictions based upon the various design equations, particularly when significant amounts of industrial wastes are present in the raw sewage.

Trickling filter design should consider peak organic load conditions including the oxygen demands due to recycle streams (e.g. anaerobic digester supernatant, heat treatment supernatant, dewatering filtrate) due to high concentrations of BOD₅ and TKN associated with such flows. The volume of media determined from either pilot plant studies or by the use of a design equations should be based upon the peak daily BOD₅ organic loading. Trickling filters are designed based on a wetting rate and organic loading. Wetting rates will vary from 40 to 60 m³/(m²·d) (980 to 1,470 USgpd/ft²). Total organic loading rates vary from 0.4 to 1.8 kg/(m3·d) [0.025 to 0.11 lb/(ft³·d)]. These loadings are for organic removal only with media depths of greater than 3 m (10 ft). For combined organic removal and nitrification, lower loading rates are required. The performance is temperature dependent due to the impact on nitrification and cooling that occurs through the trickling filter tower.

An enhanced trickling filter process, called the trickling filter/solids contact (TF/SC) process has been used successfully in North America. This process includes a short duration aeration cell downstream of the trickling filter to change the characteristics of the effluent solids to a suspended growth-type effluent (i.e., improved quality owing to flocculent biomass) prior to secondary sedimentation and solids recycle. The contact time in the solids contact process is generally between 30 and 60 minutes. The process may also include a re-aeration of the return solids from the solids contact tank.

12.4.6 Rotating Biological Contactors

The rotating biological contactor (RBC) is a fixed-film process that may be used to provide secondary treatment and can also be operated in seasonal or continuous nitrification and denitrification modes.

12.4.6.1 Design Considerations

Considerations for the RBC process should include those relevant to biological processes and specifically the following:

• Pretreatment effectiveness including scum and grease removal;
• Maximum organic loading rate on active disc surface area; and
• Minimum detention time at design peak daily flow.

Sewage temperature affects RBC performance. Year-round operation requires that the RBC be covered to protect the biological growth from cold temperatures and the excessive loss of heat from the sewage with the resulting loss of performance.

Enclosures should be constructed of a suitable corrosion resistant material. Windows or simple louvered mechanisms which can be opened in the summer and closed in the winter should be installed to provide adequate ventilation. To minimize condensation, the enclosure should be adequately insulated and/or heated. Forced ventilation should be supplied when the RBCs are contained within a building provided with interior access for personnel.

RBCs need to be preceded by effective primary sedimentation tanks equipped with scum and grease removal devices or pretreatment devices which provide for effective removal of grit, debris and excessive oil and grease prior to the RBC units.

Solids separation is an important part of the RBC process; accordingly, downstream secondary clarification is required. (Chapter 13 – Secondary Sedimentation)

The temperature of sewage entering any RBC should not drop below 5 °C (41 °F) unless there is sufficient flexibility to decrease the hydraulic loading rate or the units have been increased in capacity to accommodate the lower treatment efficiencies and rates. Otherwise, insulation or additional heating should be provided to the plant.

Adequate flexibility in process operation should be provided by considering one or more of the following:

• Variable rotational speeds in first and second stages;
• Multiple treatment trains;
• Removable baffles between all stages;
• Positive influent flow control to each unit or flow train;
• Positively controlled alternate flow distribution systems;
• Positive airflow metering and control to each unit when supplemental air operation or air drive units are used;
• Use of air scouring, reverse rotation and chemical cleaning to control excess growth on media; and
• Recirculation of secondary clarifier effluent.

The arrangement of RBC shafts in a series of stages has been shown to significantly increase treatment efficiency, by making the process more plugflow in nature. It is recommended that an RBC plant be constructed in at least four stages for each tank. Four stages may be provided on a single unit by providing baffles within the tank or by multiple tanks. Sewage flow to RBC units may be either perpendicular or parallel to the media shafts.

RBC units may be placed in either steel or concrete tanks with baffles when required and constructed of a variety of materials. The design of the tanks should include:

• Adequate structural support for the RBC and drive unit;
• Elimination of the “dead” areas;
• Satisfactory hydraulic transfer capacity between stages of units; and
• Considerations for operator safety.

The structure should be designed to withstand the increased loads which could result if the tank were to be suddenly dewatered with a full biological growth on the RBC units. The sudden loss of buoyancy resulting from unexpected tank dewatering could increase the bearing support loadings by as much as 40 percent.

Provisions for operator protection can be included in the tanks design by setting the top of the RBC tanks about 0.3 m (1.0 ft) above the surrounding floor and walkways, with handrails placed along the top of the tanks, to provide an effective barrier between the operator and exposed moving equipment. The high tank walls will also prevent loss or damage by any material accidentally dropped in the vicinity of the units and entering the tanks.

Except under special circumstances, high-density media should not be used in the first stage. Its use in subsequent stages should be based on appropriate loading criteria, structural limitations of the shaft and media configuration. The peripheral velocity of a rotating shaft should be approximately 18 m/min (60 ft/min) for mechanically driven shaft and between 9 - 18 m/min (30 to 60 ft/min) for an air-driven shaft. Provision should also be made for rotational speed control and reversal.

A means for removing excess biofilm growth, such as air scouring or water stripping, chemical additives, rotational speed control/reversal should be provided. First-stage DO monitoring should be provided. The RBC should be able to maintain a measurable DO level in all stages. Periodic high organic loadings may require supplemental aeration in the first stage to promote sloughing of biomass.

Consideration should be given to providing recirculation of RBC effluent flow. This may be necessary during initial system start-up and when the inflow rate is reduced. If flow can be recycled through the sludge holding or treatment units and then to the RBC process, then the organic load from the sludge units can be imposed on the RBC process. This imposed load will help to maintain the biogrowth and, as a secondary benefit, help stabilize and reduce the sludge.

Load cells, especially in the first stage(s), can provide useful operating and shaft load data. Stop motion detectors, rpm indicators and clamp-on ammeters are also potentially useful monitoring instruments.

In all RBC designs, access to individual shafts for repair or possible removal should be considered. Bearings should also be accessible for easy removal and replacement if necessary. Where all units in a large installation are physically located in very close proximity, it may be necessary to utilize large off-theroad cranes for shaft removal. Consideration should be given to crane reach, crane size, and the impact of being able to drain RBC tanks and dry a unit prior to shaft removal when designing the RBC layout.

12.4.6.2 Unit Sizing

Unit sizing should be based on experience at similar full-scale installations or thoroughly documented pilot testing with the site-specific sewage. In determining design loading rates, expressed in units of volume per day per unit area of media covered by biological growth, the following parameters should be considered:

• Design flow rate and influent sewage strength;
• Percentage of BOD₅ to be removed;
• Percentage of influent BOD₅ which is soluble;
• Media arrangement, including number of stages and unit area in each stage;
• Rotational velocity of the media;
• Retention time within the tank containing the media; and
• Sewage temperature.

In addition to the above parameters, loading rates for nitrification will depend upon influent TKN, pH and allowable effluent ammonia nitrogen concentration.

Hydraulic loading to the RBCs should range between 75 to 155 L/(m²·d) (1.8 to 3.8 USgpd/ft²) of media surface area without nitrification and 30 to 80 L/(m²·d) (0.73 to 2.0 USgpd/ft²) with nitrification.

Organic loading to the first stage of an RBC train should not exceed 0.03 to 0.04 kg BOD₅/(m²·d) [0.006 to 0.008 lb BOD₅/(ft²·d)] or 0.012 to 0.02 kg BOD₅/(m²·d) [0.0025 to 0.0041 lb BOD₅/(ft²·d)]. Loadings in the higher end of these ranges will increase the likelihood of developing problems such as heavier than normal biofilm thickness, depletion of dissolved oxygen, nuisance organisms and deterioration of overall process performance. The structural capacity of the shaft, provisions for stripping biomass, consistently low influent levels of sulfur compounds to the RBC units, the media surface area required in the remaining stages and the ability to vary the operational mode of the facility may justify choosing a loading in the high end of the range when the operator can carefully monitor process operations.

For purposes of plant design, the optimum tank volume is measured as sewage volume held within a tank containing a shaft of media per unit of growth covered surface on the shaft, or liters per square metre (L/m²). The optimum tank volume determined when treating municipal sewage of up to 300 mg/L BOD₅ is 0.042 L/m² (0.0010 US gal/ft²), which takes into account sewage displaced by the media and attached biomass. The use of tank volumes in excess of 0.042 L/m² (0.0010 US gal/ft²) does not yield corresponding increases in treatment capacity when treating sewage in this concentration range.

Based on a tank volume of 0.042 L/m² (0.0010 US gal/ft²), the detention time in each RBC stage should range between 40 to 120 minutes without nitrification and 90 to 250 minutes with nitrification.

RBCs should operate at a submergence of approximately 40 percent based on total media surface area. To avoid possible shaft overstressing and inadequate media wetting, the liquid operating level should never drop below 35 percent submergence. Media submergence of up to 95 percent may be allowed if supplemental air is provided. A clearance of 10 to 25 cm (4 to 10 in) between the tank floor and the bottom of the rotating media should be provided so as to maintain sufficient bottom velocities to prevent solids deposition in the tank.

12.4.7 Integrated Fixed-film Systems

Integrated Fixed-film Systems (IFS), with or without activated sludge, are hybrid dual systems with suspended biomass and fixed-film growth processes. These systems can be configured either as single-pass processes with no recycles or with sludge recycle (i.e., RAS).

12.4.7.1 Design Considerations

The basic IFS concept involves a single-pass system and is to have continuously operating, non-cloggable fixed-film reactors with no need for backwashing or return sludge flows, low head-loss and high specific biofilm surface area. This is achieved by having the biomass grow on small carrier elements that move along with the sewage in the reactor or the attachedgrowth support media may be immobile within the reactor for some designs. In the case of free-moving carrier elements, movement is normally induced by coarse bubble aeration in the aeration zone, although fine bubble aeration systems have also been used, while mechanical mixing is utilized in an anoxic/anaerobic zone. For small plants, mechanical mixers are omitted for simplicity reasons and pulse aeration for a few seconds a few times per day can be used to move the biofilm carriers in anoxic reactors.

Free-moving biofilm carrier elements are available in various materials, densities, geometries and sizes, and are generally made of polyethylene or polypropylene. For free-moving carrier elements, a screen is placed at the outlet of the reactor to keep the biofilm elements in the reactor. Agitation constantly moves the carrier elements over the surface of the screen and the scrubbing action prevents clogging. Upstream fine screening of raw sewage should also be considered for such designs.

Almost any size or shape of tank can be retrofitted with the IFS process. The amount of carrier elements in the reactor may be decided for each case based on the degree of treatment desired, BOD₅, TKN and hydraulic loadings, temperature and oxygen transfer capability. The reactor volume is completely mixed and consequently there is no “dead” or unused space in the reactor.

Solids separation is an important part of the IFS process; accordingly, downstream secondary clarification is required. (Chapter 13 – Secondary Sedimentation)

Similar design considerations should be considered for the Integrated Fixed-Film Activated Sludge (IFAS) process, with the design based on the mixed liquor suspended solids and its mixing and aeration needs. The IFAS process includes a RAS stream to provide for activated sludge as well as fixed film biomass for biological treatment.

12.4.7.2 Unit Sizing

Organic loading rates for these reactors are typically in the order of 3.5 to 7.0 g BOD₅/m² of media surface area/d (0.0007 to 0.0014 lb/(ft²·d)) for CBOD₅ removal and less than 3.5 g BOD₅/m² of media surface area/d for nitrification (0.0007 lb/(ft²·d)) based on the protected surface area. For nitrification with the IFS process, the required media surface area will usually be dictated by TKN loading, TAN removal requirements and biological growth conditions in the reactor (e.g. temperature, pH, DO). The designer should consult vendors for design details.

12.4.8 Biological Nutrient Removal

There are many proprietary Biological Nutrient Removal (BNR) systems available and the designer should consult vendors for design details. Advantages and disadvantages of BNR system are:

Advantages

• No (or reduced) chemicals or dosage control needed;
• Reduced sludge production;
• Reduced metal concentrations in effluent and sludge;
• High phosphorus content in sludge which increases its fertilizer value;
• Improved sludge settleability and dewatering characteristics;
• Reduced oxygen requirements;
• Reduced process alkalinity requirements;
• Increased oxygen transfer efficiency in aeration basin; and
• Reduced effluent nitrogen concentration.

Disadvantages

• Effluent filtration may be necessary to achieve very low phosphorus concentrations (since effluent solids would have higher phosphorus content);
• Phosphorus release may occur in anaerobic digesters and get transferred back to the STP headworks via solids processing recycle streams;
• Foaming;
• Need for skilled operation; and
• Need for more monitoring.

12.4.8.1 Biological Phosphorus Removal

A number of process configurations for enhanced biological phosphorus removal (BPR) have been developed as alternatives to chemical phosphorus removal. Phosphorus is removed in BPR processes by incorporating phosphorus into cell mass in excess of metabolic requirements. The key to the biological phosphorus removal is the continuous exposure of the microorganisms to alternating anaerobic and aerobic conditions. Exposure to these alternating conditions provides favorable conditions for BPR organisms to proliferate in sufficient numbers. The sludge containing the excess phosphorus is either wasted or removed through a sidestream. The alternating exposure to anaerobic and aerobic conditions can be accomplished in the main biological treatment process, or “mainstream”, or in the return sludge stream, or “sidestream”. The BPR process may require chemical phosphorous removal (as a polishing and/or backup system) to achieve very low phosphorus levels. The BPR is often combined with nitrification and denitrification processes for control of nutrients.

Unit Sizing

Typical design criteria for biological phosphorus removal are provided in Table 12-3 and 12-4.

Table 12-3 – Design Criteria for Biological Phosphorus Removal

Design Parameter	Mainstream Treatment Configuration	Sidestream Treatment Configuration
Food/Microorganism Ratio (kg BOD5/(kg MLVSS·d)	0.2 - 0.7	0.1 - 0.5
Solids Retention Time (d)	2 - 25	10 - 30
MLSS (mg/L)	2000 - 4000	600 - 5000
Hydraulic Retention Time (hrs) Anaerobic Zone	1 - 3	8 - 12
Hydraulic Retention Time (hrs) Aerobic Zone	0.5 - 1.5	4 - 10
Return Activated Sludge (% of Influent Flow Rate)	25 - 40	20 - 50
Stripper Underflow (% of Influent Flow Rate)	N/A	10 - 20

12.4.8.2 Biological Nitrogen Removal

The principal nitrogen conversion and removal processes are conversion of ammonia to nitrate by biological nitrification and removal of nitrogen by biological denitrification.

Nitrification

Biological nitrification consists of the conversion of ammonia to nitrite followed by the conversion of nitrite to nitrate. This process does not increase the removal of nitrogen from the sewage over that achieved by conventional biological treatment. Nitrification is used when treatment requirements call for removal of effluent ammonia or as a first step of a total nitrogen removal system. To achieve nitrification, all that is required is the maintenance of conditions suitable for the growth of nitrifying organisms. Nitrification can be achieved in either a single stage (combined with organics removal) or in a separate nitrification stage. In each case, suspended growth, attached growth or hybrid systems can be used.

Combined Nitrification/Denitrification

The removal of nitrogen by biological nitrification/denitrification is a two-step process. In the first step, ammonia is converted aerobically to nitrate (NO₃^-) (nitrification). In the second step, nitrates are converted to nitrogen gas (denitrification). The removal of nitrate by conversion to nitrogen gas can be accomplished biologically under anoxic conditions. The carbon requirements may be provided by internal sources, such as sewage and cell material, or by an external source.

12.4.8.3 Combined Biological Nitrogen and Phosphorus Removal

A number of biological processes have been developed for the combined removal of nitrogen and phosphorus. Many of these are proprietary and use a form of the activated sludge process but utilizing combinations of anaerobic, anoxic and aerobic zones or compartments to accomplish biological nitrogen and phosphorus removal. Process design criteria are outlined in Table 12-4.

Table 12-4 – Design Criteria for Combined Biological Nitrogen and Phosphorus Removal

Design Parameter	Value
Food-to-Microorganism Ratio (kg BOD5/(kg MLVSS·d)	0.1 - 0.25
Solids Retention Time (d)	10 - 40
MLSS (mg/L)	2000 - 5000
Hydraulic Retention Time (hrs) Anaerobic Zone	0.5 - 2.0
Hydraulic Retention Time (hrs) Anoxic Zone (total)	0.5 - 10
Hydraulic Retention Time (hrs) Aerobic Zone (total)	4 - 12
Hydraulic Retention Time (hrs) Total	5 - 24
Return Activated Sludge (% of Influent Flow Rate)	25 - 100
Internal Recycle (if required) (% of Influent Flow Rate)	100 - 600

Chapter 13: Secondary Sedimentation

The suspended and attached growth processes require separation of biomass from the biological process effluent to produce the secondary quality effluent, and for return of the microorganisms to the bioreactor (i.e., in the case of the activated sludge process) and wasting of excess biomass (i.e., waste sludge from all process types). This can be achieved by secondary sedimentation. A summary of the design loadings for secondary sedimentation tanks is provided in Appendix V, which should be used in conjunction with the details in this chapter.

13.1 General

Secondary sedimentation tanks (also known as secondary or final clarifiers) should be designed for the larger surface area in accordance with either clarification or solids thickening requirements based on the appropriate surface overflow rates and solids loading rates, respectively.

The surface area requirements for clarification vary with the settling characteristics of the suspended solids in the bioreactor effluent. In the case of the activated sludge process (ASP), factors that can influence the settling characteristics are chemical addition to the mixed liquor for phosphorus removal, and nitrification.

Circular, rectangular, or square clarifiers may be used. In selecting the clarifier shape, the designer should consider the following factors:

• Effective use of the site;
• Means of future expansion;
• Head loss through the system;
• Operational and maintenance issues; and
• Economics of tank construction, including inlet and outlet piping, and sludge and scum removal equipment.

13.1.1 Number of Units

Multiple units capable of independent operation are desirable and should be provided in all plants where design average daily flows exceed 380 m³/d (0.1 mUSgd). Plants not having multiple clarifiers should include other provisions (e.g., access to portable units during maintenance) to ensure continuity of treatment and meeting required site-specific effluent quality criteria in terms of concentrations and loadings.

13.1.2 Flow Distribution

It is recommended that effective flow splitting devices and control appurtenances (e.g. gates, splitter boxes) be provided to permit even or adjustable proportioning of flow and solids loading to each clarifier, throughout the expected range of flows. Flow distribution devices should consider solids distribution and avoid creating solids imbalances when splitting the flow. One example of poor solids splitting may occur when a bend is located directly upstream of a flow split device, which causes solids to be pushed against the outside channel wall and towards one side of the flow split device. See Section 3.13.2 - Flow Distribution.

13.2 Design Considerations

13.2.1 Dimensions

It is recommended that the minimum length of flow from inlet to outlet be 3.7 m (12 ft) unless special provisions are made to prevent short-circuiting. The vertical side-water depths (SWD) should be designed to provide an adequate separation zone between the sludge blanket and the overflow weirs. Secondary sedimentation tanks should have a SWD between 3.6 and 4.6 m (12 and 15 ft).

Greater SWD are recommended for secondary clarifiers in excess of 372 m² (4000 ft²) surface area [equivalent to 21 m (70 ft) diameter for circular clarifiers] and for plants providing nitrification. Less than 3.7 m (12 ft) SWD may be considered for package plants having a design average daily flow of less than 95 m³/d (25,000 USgpd), if justified based on successful operating experience. For circular clarifiers with sludge hoppers, a 1:12 bottom slope should be considered.

Rectangular tanks should satisfy the following geometrical ratios:

• Length: width (L/W) of 4:1, or greater; and
• Width: depth (W/D) of 1:1 to 2.25:1.

13.2.2 Surface Overflow Rates

13.2.2.1 Intermediate Sedimentation Tanks

Surface Overflow Rate (SOR) for intermediate sedimentation tanks, following the fixed-film reactor processes, should not exceed 60 m³/(m²·d) (1,470 USgpd/ft²) based on design peak hourly flow (DPHF).

13.2.2.2 Final Sedimentation Tanks

Settling tests should be conducted wherever a pilot study of biological treatment is warranted by unusual sewage characteristics, treatment requirements, or where proposed hydraulic or solids loadings differ from the recommended guidelines in this section.

Activated Sludge Process

To perform properly while producing a concentrated return flow, activated sludge sedimentation tanks should be designed to meet thickening and solids separation requirements. Since the rate of recirculation of return activated sludge (RAS) from the final sedimentation tanks to the aeration or reaeration tanks is quite high in activated sludge processes, SOR and weir overflow rate should be adjusted for the various processes to minimize the problems with sludge loadings, density currents, inlet hydraulic turbulence and occasional poor sludge settleability. The size of the sedimentation tank should be based on the larger surface area determined for SOR, based on the DPHF, and peak daily solids loading rate (SLR).

Attached Growth Biological Reactors

SOR for sedimentation tanks following fixed-film processes such as trickling filters or rotating biological contactors should not exceed 50 m³/(m²·d) (1,200 USgpd/ft²) based on the DPHF.

The design criteria for activated sludge and attached growth systems shown in the Table 13-1 should not be exceeded. For flat-bottom circular clarifiers and shallower clarifiers, reduced design SOR should be used.

13.2.3 Inlet Structures

Inlets and baffling should be designed to dissipate the inlet velocity, to distribute the flow uniformly and to prevent short-circuiting. It is recommended that channels be designed to maintain a velocity of at least 0.3 m/s (1 ft/s) at one-half of the design average daily flow. Corner pockets and dead ends should be eliminated and corner fillets or channeling should be used where necessary. It is recommended that provisions be made for elimination or removal of floating materials which may accumulate in inlet structures.

With circular basins having 100 percent sludge recirculation, the inlet well should not be less than 20 percent of the tank diameter and have a depth of 55 to 65 percent of the SWD. The maximum flow velocity to the centre inlet well should not exceed 1.0 m/s (3.3 ft/s) and the outflow velocity should not exceed 0.08 m/s (0.26 ft/s). Other inlet structures and feed systems (e.g., peripheral and spiral feed circular sedimentation tanks) exist and the designer should consider site-specific design considerations for appropriate selection.

With rectangular tanks, baffled inlet ports are generally used to achieve uniform flow distribution. Maximum inlet port velocities should be in the range of 0.08 to 0.16 m/s (0.26 to 0.52 ft/s).

Table 13-1 – Final Clarifier Recommended Loading Rates

Treatment Process	Surface Overflow Rate at Design Peak Hourly Flow1 m3/(m2·d) (USgpd/ft2)	Peak Solids Loading Rate3 kg/(m2·d) (lb/(day·ft2))
Conventional ASP (CAS), Step Aeration, Complete Mix, Contact Stabilization, Carbonaceous Stage of Separate-Stage Nitrification	50 (1200)2	240 (50)
Extended Aeration, Single-Stage Nitrification	40 (1000)	170 (35)
Two-Stage Nitrification	33 (800)	170 (35)
Activated Sludge with Chemical Addition to Mixed Liquor for Phosphorus Removal	37 (900)	As above, depending on the treatment process

13.2.4 Weirs

Outlet weirs should be provided with sufficient effective length and in locations such that the clarified effluent can be withdrawn from the tank without causing excessive localized upflow resulting in solids carryover. For conventional circular tanks, a peripheral weir is generally all that is required to provide a suitable weir loading rate. With rectangular tanks, multiple weirs (e.g. more than one perpendicular or finger weirs) will generally be required and these should be located away from the area of upturn of the density current. Wall baffling can be used to reduce the likelihood of upflow causing solids carryover. The use of interior baffles and peripheral baffles (Stamford baffles) should reduce short-circuiting and enhance flocculation.

Overflow weirs should be readily adjustable over the life of the structure to correct for differential settlement of the tank. Consideration should be given to cleaning, maintenance and replacement in the design of weir troughs.

Overflow weirs should be located to optimize actual hydraulic retention time and minimize short-circuiting. It is recommended that peripheral weirs be placed at least 0.3 m (1 ft) from the wall.

It is recommended that weir loadings not exceed those shown in Table 13-2.

If pumping is required, the pumps should be operated as nearly continuous as possible to avoid flow disturbances. This is most readily provided by the use of variable speed pumps. Weir loadings should be related to pump delivery rates to avoid short-circuiting.

Table 13-2 – Recommended Weir Loading Rates for Final Clarifier

Design Average Daily Flow	Loading Rate at Design Peak Hourly Flow m3/(m·d) (USgpd/ft)
Equal to or less than 4000 m3/d (1 mUSgd)	250 (20,000)
Greater than 4000 m3/d (1 mUSgd)	375 (30,000)

Weir troughs should be designed to prevent submergence at DPHF and to maintain a velocity of at least 0.3 m/s (1 ft/s) at one-half design average daily flow.

13.2.5 Submerged Surface

The tops of troughs, beams and similar submerged construction elements should have a minimum slope of 1.4 vertical to 1 horizontal. The underside of such elements should have a slope of 1 to 1 to prevent the accumulation of scum and solids.

13.2.6 Units Out-of-Service

The ability for a unit to be dewatered or taken out-of-service should conform to the provisions outlined in Section 8.5.16 - Component Backup Requirements. It is recommended that secondary sedimentation tanks be designed to provide for distribution of the plant flow to the remaining tanks, when one tank is out-of-service and/or dewatered.

13.2.7 Freeboard

It is recommended that the walls of sedimentation tanks extend at least 150 mm (6 in) above the surrounding ground surface and be provided with not less than 600 mm (24 in) freeboard.

Additional freeboard or the use of wind screens is recommended where larger sedimentation tanks are subject to high velocity wind currents that would cause tank surface waves and inhibit effective scum removal.

13.2.8 Sludge Settleability

Sludge settleability determines the capacity of an activated sludge clarifier since it partly determines the sludge settling rate against which the effluent overflow rate acts. The common measure of settleability in the activated sludge process is the sludge volume index (SVI). Several models have been developed to relate SVI to sludge settling velocity. However, SVI is a poor procedure for mixed liquor suspended solids (MLSS) concentration of greater than 3,000 mg/L and the stirred SVI (sSVI) test should be used. Where possible, design of activated sludge clarifiers should be based on field measurement of sludge settling velocity using batch settling tests at varying initial suspended solid concentration. To control high SVI conditions, bioselectors (Section 12.2.3 - Selectors) should be considered for activated sludge plants.

13.2.9 Clarifier Enhancements (Inlet and Baffles)

Many types of clarifier enhancement are available to improve secondary clarification performance. These enhancements are used to reduce shortcircuiting and enhance flocculation. An inlet flocculation zone can be used to dissipate energy in the influent to the tank, through design of the centre feed well for circular clarifiers or as target baffles for rectangular clarifiers.

Interior (ring) and peripheral (wall) baffles have also been shown to disrupt the density currents, thus avoiding short-circuiting and solids carryover, especially at elevated flows. An interior baffle supported off the sludge collection mechanism (circular tank) or attached to the walls (rectangular tank) can be used to dissipate inlet energy and disrupt the density current. A peripheral, wall or effluent baffle can be used to deflect the density current away from the effluent weir and avoid short-circuiting and solids carryover. Two types of effluent baffles are common, the McKinney baffle which is horizontal in orientation and the Stamford baffle which is oriented at a 45° angle.

13.3 Scum and Sludge Removal

13.3.1 Scum Removal

Full-surface mechanical scum collection and removal facilities, including baffling, are recommended for all sedimentation tanks. Where freezing would cause equipment damage, provision should be made for removal or protection (e.g. by covering a clarifier) of scum collectors in the winter. The characteristics of scum, which may adversely affect pumping, piping, sludge handling and disposal, need to be recognized in design. Provisions should be made to remove scum from the liquid train of the sewage treatment plant and to direct it to the solids treatment process. Under certain conditions, such as in biological nutrient removal (BNR) facilities with separate scum/foam removal on the bioreactors, separate clarifier scum removal would be unnecessary.

13.3.2 Sludge Removal

Mechanical sludge collection and withdrawal facilities should be designed to ensure rapid removal of the settled solids or RAS, to avoid adverse effects on the sludge quality caused by anaerobic conditions. Sludge should not remain in the sedimentation basin for more than 30 minutes. Sludge scraper systems may consist of chain and flight type or traveling bridge type for rectangular sedimentation tanks. Rotary circular scraper mechanisms are used in circular tanks. Scraper mechanisms, both in rectangular and circular sedimentation tanks, need to avoid excess travel time that could lead to long sludge detention times in the sedimentation tank and possible anaerobic conditions and associated solids carryover and odour potential. The designer should consult the manufacturer for recommended rates of sludge collectors and flight speeds.

In large tanks, traveling bridge or rotary mechanisms should be equipped with suction sludge draw off pipes for rapid sludge removal. Suction withdrawal should be provided for activated sludge clarifiers over 18 m (60 ft) in diameter, especially for nitrifying ASPs. RAS rates should be adjustable for adequate control, including individual adjustments on suction collection pipes.

When the settled solids are scraped towards a hopper for removal in rectangular tanks, the hopper has normally been located at the inlet end of the tank. Other designs placing the hopper at tank mid-point, or at the effluent end, to take advantage of the density current, have also been used successfully.

13.3.2.1 Sludge Hopper

The minimum slope of the side walls should be 1.7 vertical to 1.0 horizontal. Hopper wall surfaces should be made smooth with rounded corners to aid in sludge removal. Hopper bottoms should have a maximum dimension of 0.6 m (2 ft). Extra depth sludge hoppers for sludge thickening should not be used.

13.3.2.2 Cross Collectors

Cross-collectors serving one or more sedimentation tanks may be useful in place of multiple sludge hoppers.

13.3.2.3 Sludge Removal Pipeline

Each hopper should have an individually valved sludge withdrawal line at least 150 mm (6 in) in diameter. The static head available for withdrawal of sludge should be 760 mm (30 in) or greater, as necessary to maintain a 0.9 m/s (3 ft/s) velocity in the withdrawal pipe. Clearance between the end of the withdrawal line and the hopper walls should be sufficient to prevent bridging of the sludge. Adequate provisions should be made for rodding or backflushing individual pipe runs and allowance for visual confirmation of return sludge flow. Piping should be provided to return sludge for further processing.

13.3.2.4 Sludge Removal Control

Separate secondary sedimentation tank sludge lines may drain to a common sludge well. Sludge wells equipped with telescoping valves or other appropriate equipment should be provided for viewing, sampling and controlling the rate of sludge withdrawal. A means of measuring the RAS flow rate should be provided.

Wherever possible, pipes discharging RAS or waste activated sludge (WAS) should be located to permit visual confirmation that sludge is being discharged. It is recommended that each sedimentation tank has its own sludge withdrawal lines to ensure adequate control of the sludge wasting rate for each tank.

13.4 Protective and Service Facilities

All secondary sedimentation tanks need to be equipped to enhance safety for operators. It is recommended that such features include machinery covers, lifelines, stairways, walkways, handrails and slip resistant surfaces.

The design should provide for convenient and safe access to routine maintenance items such as gear boxes, scum removal mechanisms, baffles, weirs, inlet stilling baffle areas and effluent channels.

Electrical equipment, fixtures and controls in enclosed settling basins and scum tanks, where hazardous concentrations of flammable gases or vapors may accumulate, need to meet the requirements of current Electrical Safety Code for Class 1, Zone 0 or Zone 1 (or Division 1 for existing installations as defined under the code), Group D locations (O. Reg. 164/99) made under the Electricity Act, 1998.

It is recommended that the fixtures and controls be located so as to provide convenient and safe access for operation and maintenance. Adequate area lighting needs to be provided.