Chapter 10: Preliminary Treatment

This chapter describes those processes, generally located at the headworks of a sewage treatment plant, that are designed to remove debris from sewage, to protect equipment and downstream processes. The preliminary treatment processes described in this chapter are screening, comminution, grinding and grit removal. A section is also included on preaeration and flow equalization.

10.1 Screening Devices

Screens should be placed in the influent flow at the headworks of the sewage treatment plant (STP) to remove debris that may harm other process units.

Coarse screens, or trash racks, should be provided as the first treatment stage for the protection of plant equipment against blockage or physical damage. Trash racks can precede finer screens when serving combined sewer systems. Coarse screening can be provided in the form of bar screens (manually or mechanically cleaned). Table 10-1 provides the approximate screen opening sizes for various classifications of screens.

Table 10-1 Screen Size Openings for Various Screen Classifications
Screen Classification Screen Opening Size (Range)
Trash Rack > 25 mm (>1 in)
Coarse 6 25 mm (1/4 - 1 in)
Fine 1 6 mm (1/25 1/4 in)
Microscreen < 1 mm (<1/25 in)

When considering which types of screening devices should be used (whether manually or mechanically cleaned), the following factors should be considered:

  • Effect on downstream treatment and sludge disposal operations;
  • Possible damage to comminutor or barminutor devices caused by stones or coarse grit particles;
  • Head losses of the various alternative screening devices;
  • Impact of debris loading on screen (including leaf loading in the case of combined sewer systems);
  • Maintenance and labour requirements; and
  • Screenings disposal requirements.

Facilities for the removal, drainage, washing, storage and ultimate disposal of accumulated screenings should be provided when manually or mechanically cleaned screens are used.

10.1.1 Coarse Screens

It is recommended that protection for pumps and other equipment be provided by upstream trash racks, coarse bar racks, or coarse screens.

10.1.1.1 Design and Installation

In general, manually cleaned screens should be placed on a slope of 30 to 45º from the horizontal. Approach velocities should be between 0.4 m/s (1.25 ft/s) to prevent settling and 0.9 m/s (3.0 ft/s) at design average daily flow, to prevent forcing material through the openings.

Dual channels should be provided and equipped with the necessary gates to isolate flow from any screening unit. Provisions should also be made to facilitate dewatering each unit or channel for maintenance or repair. The channel preceding and following the screen should be shaped to eliminate stranding and settling of solids. The screens serving combined sewer system should be provided with a bypass or equipment to remove the screen to prevent flooding in the event of screen blinding due to excess material like leaves.

Where a single mechanically cleaned screen is used, an auxiliary manually cleaned screen should be provided. Where two or more screens are present, the capacity should be provided to treat design peak hourly flow with one unit out of service.

The screen channel invert should be 75 to 150 mm (3 to 6 in) below the invert of the incoming sewer. Entrance channels should be designed to provide equal and uniform distribution of flow to the screens.

The effect of changes in backwater elevation, due to intermittent cleaning of screens, should be considered in the location of flow measurement equipment (Section 9.2.1 - Flow Elements).

Screening devices and screenings storage areas should be protected from freezing. A convenient and adequate means for removing screenings should be provided. Hoisting or lifting equipment should be provided. Provision should also be made for washing of screenings.

Facilities are recommended to be provided for handling, storage and disposal of screenings in an acceptable manner in accordance with applicable requirements. Final disposal to landfill generally requires that the screenings meet a limit in terms of dry solids, slump, or presence of a free liquid. (Section 18.5.1 - Municipal Solids Waste Landfilling). Separate grinding of screenings and return to the sewage flow is not recommended.

Manually cleaned screening facilities should include an accessible platform from which the operator may rake screenings easily and safely. Suitable drainage facilities are recommended to be provided for both the platform and the storage area.

10.1.1.2 Access and Ventilation

Screens located in pits more than 1.2 m (4 ft) deep need to be provided with stairway access. Access ladders are acceptable for pits less than 1.2 m (4 ft) deep, in lieu of stairways.

Screening devices, installed in a building where other equipment or offices are located, need to be isolated from the rest of the building. These devices should be provided with separate outside entrances and be provided with separate and independent fresh air supply.

Fresh air needs to be forced into enclosed screening device areas or into open pits more than 1.2 m (4 ft) deep. Dampers should not be used on exhaust or fresh air ducts and fine screens or other obstructions should be avoided to prevent clogging. Where continuous ventilation is required, at least 12 complete air changes per hour are recommended. Where continuous ventilation would cause excessive heat loss, intermittent ventilation of at least 30 complete air changes per hour are recommended when personnel enter the area. The air change requirements are based on 100 percent fresh air.

Switches for operation of ventilation equipment should be marked and located conveniently. All intermittently operated ventilation equipment is recommended to be interconnected with the respective pit lighting system. It is recommended that the fan wheel be fabricated from non-sparking material. Explosion proof gas detectors need to be provided.

10.1.1.3 Safety and Shields

Manually cleaned screen channels need to be protected by guard railings and deck gratings, with adequate provisions for removal or openings to facilitate raking.

Mechanically cleaned screen channels need to be protected by guard railings and deck gratings. Consideration should also be given to temporary access arrangements to facilitate maintenance and repair. Mechanical screening equipment is recommended to have adequate removable enclosures to protect personnel against accidental contact with moving parts and to prevent dripping in multi-level installations.

A positive means of locking out each mechanical device and temporary access for use during maintenance is recommended. Floor design and drainage should be provided to prevent slippery areas. Suitable lighting should also be provided in all work and access areas.

10.1.1.4 Electrical Equipment and Control Systems

It is recommended that all mechanical units which are operated by timing devices be provided with auxiliary controls which will set the cleaning mechanism in operation at a preset high water elevation. If the cleaning mechanism fails to lower the high water, an alarm should be signaled.

Electrical equipment, fixtures and controls in the screening area where hazardous gases may accumulate needs to meet the requirements of the Electrical Safety Code for Class I, Zone 1, Group D locations (O. Reg. 164/99 made under the Electricity Act, 1998).

It is recommended that automatic controls be supplemented by a manual override at the location of the equipment.

10.1.2 Fine Screens

Fine screens should not be considered equivalent to primary sedimentation but may be used in lieu of primary sedimentation where subsequent treatment units are designed on the basis of anticipated screen performance. Selection of screen capacity should consider flow restriction due to retained solids, gummy materials, frequency of cleaning and extent of cleaning. Where fine screens are used, additional provision for removal of floatable oils and greases should be considered. Care should be taken with smaller screen size openings to avoid plugging.

Air exchanges similar to coarse screens need to be provided (Section 10.1.1.2 Access and Ventilation).

If there is a high organic content in the screenings, washing is an effective way of breaking up and reducing the amount of fecal and organic content. This will help to reduce odours within the screening area and reduce the solid content for disposal. Washers are most efficient when used in combination with compactors, since water is added to break-up organics and this water should be removed.

Volume reduction is a means to minimize the cost of disposal. Depending on the characteristics of the screenings, they can be effectively dried to 50% moisture content and reduced by up to 75% of their original volume, reducing hauling and disposal cost. Screw compactors and piston-type compactors are used to dewater and compact screenings. Compacting is an approach to save costs and in some cases disposal sites require screenings to pass specific standards for dryness.

10.1.2.1 Design and Installation

Tests should be conducted to determine BOD5 and suspended solids removal efficiencies at the design maximum day flow and design maximum day BOD5 loadings. Pilot testing for an extended time is preferred to cover key seasonal operational variations.

It is recommended that a minimum of two fine screens be provided, each unit being capable of independent operation. Capacity is recommended to be provided to treat design peak instantaneous flow with one unit out of service.

Fine screens should be preceded by a coarse bar screening device. Fine screens should be protected from freezing and located to facilitate maintenance.

It is recommended that hosing equipment be provided to facilitate cleaning. Provisions should be made for isolating and removing units from their location for servicing.

10.1.3 Microscreening

Microscreens are classified as having less than 1 mm (1/25 in) screen openings. Screens can be constructed of different types of material such as woven metal, perforated metal plates and woven cloth. This screen category is conventionally used as a polishing step, although recently some companies have explored the possibilities of providing complete pretreatment to create a more compact STP and to replace the need for primary treatment.

Similar to fine screens, microscreening may be accomplished directly or indirectly. The effectiveness of the direct method of capturing solids is largely dependent on the size of the screen openings. Indirect capture of solids will occur when a mat or film develops on the screen from previous solids retention. This will reduce the effective size of the screen opening and hence, increase the overall efficiency of the screening process. Caution should be used when indirect filtration occurs with microscreens since there is a high potential for fouling and excess headloss.

10.2 Comminutors and Grinders

10.2.1 General

Grinders and comminutors (including barminutors) represent an alternative to coarse screening. Since problems can occur due to the recombining of comminuted or barminuted materials in downstream treatment units, it is recommended that the physical removal of the coarse material from the sewage influent be used rather than the use of shredding or cutting devices with reintroduction of the material to the treated sewage.

When comminutors are used, they are commonly placed downstream of grit removal to avoid damage to the cutters caused by grit particles. On the other hand, if comminution is provided upstream of grit removal units, more efficient grit removal will be achieved. If mechanical grit removal is used, equipment protection in the form of some type of coarse screening device should be provided upstream of the grit removal facilities.

10.2.2 Design Considerations

Comminutors should be protected by a coarse screening device. Comminutors not preceded by grit removal equipment are recommended to be protected by a 150 mm (6 in) deep gravel trap.

Comminutors may be used in lieu of screening devices to protect equipment where stringy substance accumulation on downstream equipment will not be a substantial problem.

It is recommended that comminutor capacity be adequate to handle design peak hourly flow. A screened bypass channel should be provided and should be automatic for all comminutor failures. Channel gates should be provided where necessary.

Provision needs to be made to facilitate servicing units in place and for removing units from their location for servicing. Provisions for access, ventilation, shields and safety should be in accordance with Sections 10.1.1.2 and 10.1.1.3.

Electrical equipment in comminutor chambers where hazardous gases may accumulate needs to meet the requirements of the Electrical Safety Code for Class I, Zone 1, Group D locations (O. Reg. 164/99 made under the Electricity Act, 1998). Motors need to be protected against accidental submergence.

10.3 Grit Removal Facilities

10.3.1 General

Grit removal facilities should be provided for all mechanical STP especially those receiving sewage from combined sewers or from sewer systems receiving substantial amounts of grit. If a plant serving a separate sewer system is designed without grit removal facilities, it is recommended that the design include provision for future installation. It is also recommended that consideration be given to the possible damaging effects on pumps, comminutors and other preceding as well as downstream equipment and the need for additional storage capacity in treatment units where grit is likely to accumulate.

The quantity of grit removed at an STP can vary significantly depending on the sewage flow, characteristics of the service area, type of collection system and type of screen located before grit collection. Grit collection can vary from 4 to 37 mL/m3 of treated sewage (0.54 to 4.9 cu ft/mil. US gal) for separate sewer systems and 4 to 180 mL/m3 (0.54 to 24.1 cu ft/mil. US gal) for combined sewer systems.

Grit removal is normally accomplished by grit channels, detritus tanks, aerated grit tanks or vortex grit tanks. Automated grit removal equipment is preferred to avoid manual cleaning (i.e., as required with grit channels). Grit removal can also be accomplished using centrifugal type separators and stationary screens, although these are less commonly used in Ontario.

10.3.2 Design Factors

Grit removal facilities should be located ahead of pumps and comminuting devices. Coarse bar racks should be placed ahead of grit removal facilities. Grit removal facilities located outside should be protected from freezing. Heat tracing for example may be required on specific equipment or processes. It is recommended that adequate stairway access to above- or below-grade facilities be provided.

Ventilation should be provided, with recommended continuous fresh air introduction rates of at least 12 air changes per hour, or intermittently at a rate of at least 30 air changes per hour. Odour control facilities may also be warranted.

All electrical work in enclosed grit removal areas where hazardous gases may accumulate needs to meet the requirements of the Electrical Safety Code for Class I, Zone 1, Group D locations (O. Reg. 164/99 made under the Electricity Act, 1998). Explosion proof gas detectors need to be provided.

Plants treating sewage from combined sewers should have at least two mechanically cleaned grit removal units, with provisions for bypassing. A single manually cleaned or mechanically cleaned grit chamber with bypass is acceptable for small sewage treatment plants serving separate sanitary sewer systems. Minimum facilities for larger plants serving separate sanitary sewers should be at least one mechanically cleaned unit with a bypass.

Facilities other than channel-type should be provided with adequate and flexible controls for velocity and/or air supply devices and with grit collection and removal equipment. Aerated grit tanks should have air rates adjustable in the range of 4.7 to 12.4 L/(m·s) (3 to 8 cfm/ft of tank length). Detention time in the tank should be in the range of 3 to 5 minutes at the design peak hourly flow.

The design effectiveness of a grit removal system should be commensurate with the requirements of the subsequent process units.

Inlet turbulence should be minimized in channel type units. Channel-type chambers should be designed to control the velocities during normal variations in flow as close as possible to 0.3 m/s (1 ft/s). The detention period should be based on the size of particle to be removed.

All aerated grit removal facilities should be provided with adequate control devices to regulate air supply and agitation.

The need for grit washing should be determined by the method of grit handling and final disposal.

The designer should make provision for isolating and draining each unit. It is recommended that the design provide for complete draining and cleaning by means of a sloped bottom equipped with a drain sump. An adequate supply of water under pressure should be provided for cleanup.

Grit removal facilities located in deep pits should be provided with mechanical equipment for hoisting or transporting grit to ground level. Impervious, nonslip, working surfaces with adequate drainage are recommended for grit handling areas. Grit transporting facilities should be provided with protection against freezing and loss of material. Hoisting equipment needs to be provided or available that is capable to lift all mechanical equipment for servicing and repair.

10.3.3 Types of Grit Removal Systems

10.3.3.1 Grit Channels

Grit channels are usually employed in smaller plants. Grit removal is accomplished by velocity control provided by proportional weirs. Grit channels are normally manually cleaned. The design parameters for grit channels are as follows:

  • Number of channels minimum of 1; recommend at least 2 for larger sewage treatment plants (with one channel out of service there should be enough capacity in remaining units to handle the design peak hourly flow);
  • Control velocity - 0.3 m/s (1 ft/s);
  • Control weirs - proportional, Sutro (or Parshall in parabolic channels);
  • Minimum channel width - 380 mm (15 in);
  • Minimum length - that required to settle 0.2 mm (1/16 in) particle with a specific gravity (SG) of 2.65 plus 50 per cent allowance for inlet and outlet turbulence. However, analysis of grit removal data indicates that the SG ranges from 1.3 to 2.7; and
  • Grit storage - with permanently positioned weirs, the weir crest should be kept 150 to 300 mm (6 to 12 in) above the grit channel invert to provide for storage of settled grit (weir plates that are capable of vertical adjustment are preferred since they can be moved to prevent the sedimentation of organic solids following grit cleaning).
10.3.3.2 Detritus Tanks

Detritus tanks should be designed with sufficient surface area to remove the same, or smaller, particle size and density as required for grit channels at the design peak hourly flow rate. Detritus tanks, since they are mechanically cleaned and do not need dewatering for cleaning, do not require multiple units.

The grit settled in the detritus tank will have a significant organic content due to the lighter solids settling out during low flow periods. Separation of the organics from the grit before, during, or after the removal of the settled contents of the tank should accomplish in one of the following ways:

  • Compressed air can be diffused into the tank periodically to re-suspend organic material;
  • The removed detritus can be washed in a grit washer with the organic laden wash water being returned to the head of the detritus tank;
  • A classifying-type conveyor can be used to remove the grit and return the organics to the detritus tank; and
  • The removed detritus can be passed through a centrifugal-type separator.
10.3.3.3 Aerated Grit Tanks

Aerated grit tanks for the removal of 0.2 mm (1/16 in), or larger, particles with specific gravity of 2.65 should be designed in accordance with the following parameters:

  • Detention time - 2 to 5 minutes at design peak hourly flow rate (the longer retention times provide additional benefit in the form of preaeration);
  • Air supply - 4.7 to 12.4 L/(m·s) (3 to 8 cfm/ft), via wide band diffusion header positioned lengthwise along one wall of tank; (air supply should be variable);
  • Inlet conditions - inlet flow should be parallel to induced roll pattern developed in tank;
  • Baffling - minimum of one transverse baffle near outlet weir, with additional transverse baffles in long tanks and longitudinal baffles in wide tanks;
  • Outlet conditions - outlet weir oriented parallel to direction of induced roll;
  • Tank dimensions - lower limit of above aeration rates generally suitable for tanks up to 3.7 m (12 ft) deep and 4.3 m (14 ft) wide; wider, or deeper tanks require aeration rates in the upper end of the above range; long, narrow aerated grit tanks are generally more efficient than short tanks and produce cleaner grit; length-to-width (L/W) ratio normally is 1.5:1 to 2:1, but up to 5:1 is acceptable; depth-to-width (D/W) ratio 1:1.5 to 1:2;
  • Desired velocities - surface velocity in the direction of roll in tanks should be 0.45 to 0.6 m/s (1.5 to 2.0 ft/s) (tank floor velocities will be approximately 75 per cent of above);
  • Grit collectors - air lifts, pumps, mechanical conveyors or clam shell buckets may be used for the removal of grit (pretreatment in the form of screening will be required upstream of mechanical grit removal processes);
  • Grit washing - depending upon the method of removal and ultimate disposal, the grit may have to be washed after removal by devices of the type discussed in the previous section;
  • Multiple units - generally not required, or where grit removal method requires bypassing of tank (as with clam shell bucket); and
  • Tank geometry - critical with respect to location of air diffusion header, sloping tank bottom, grit hopper and fitting of grit collector mechanism into the tank structure. Consultation with equipment suppliers is advisable.
10.3.3.4 Vortex Grit Removal

The vortex grit removal systems are proprietary and rely on a mechanically induced vortex to capture grit solids in the center hopper of a circular tank. The designer should ensure that the manufacturer verifies that the appropriately sized unit has been field tested to determine performance parameters and should consider its performance during low flow periods. The designer should obtain design data from the manufacturer for appropriate entrance and exit channels and a concrete tank for installation of the grit removal equipment.

10.4 Preaeration

Preaeration of sewage to reduce septicity may be required in special cases. Preaeration can be incorporated through extended use of aerated grit process or by the provision of a separate unit process.

10.4.1 Mixing

Aeration or mechanical equipment should be provided to maintain adequate mixing. Corner fillets and hopper bottoms with draw-offs should be provided to alleviate the accumulation of sludge and grit. A spray wash down system or tipping bucket should be included to wash down tank after use.

10.4.2 Aeration

Aeration equipment should be sufficient to maintain a minimum of 1.0 mg/L of dissolved oxygen in the mixed basin contents at all times. Air supply rates should be a minimum of 0.16 L/(m3.s) (1.25 cfm/1000 US gal of storage capacity). The air supply should be isolated from other treatment plant aeration requirements to facilitate process aeration control, although process air supply equipment may be utilized as a source of standby aeration. Although coarse bubble diffusers have been used in the past, consideration for using fine pore diffusers should be made due to the higher oxygen transfer efficiency and current reliability (e.g. membrane diffusers).

10.4.3 Controls

Inlets and outlets for all basin compartments should be suitably equipped with accessible external valves, stop plates, weirs, or other devices to permit flow control and the removal of an individual unit from service. Equipment should be provided to measure and indicate liquid levels and flow rates.

10.4.4 Access

Suitable access should be provided to facilitate cleaning and the maintenance of equipment.

10.5 Flow Equalization

Full equalization of diurnal sewage flow peaks can result in a reduction in construction costs over variable flow design and can also result in reduced energy costs and improved treatment efficiency. Partial or side-line equalization minimizes pumping requirements but is less effective at equalizing pollutant concentrations. Consideration should be made for taking any tanks out of service for cleaning or maintenance either by multiple tanks or provision to bypass.

10.5.1 General

Use of flow equalization should be considered where significant variations in organic and hydraulic loadings can be expected.

10.5.2 Location

Equalization basins should be located downstream of pretreatment facilities such as bar screens, comminutors and grit chambers.

10.5.3 Type

Flow equalization can be provided by using separate basins or on-line treatment units, such as aeration tanks. Equalization basins may be designed as either in-line or side-line units. Unused treatment units, such as sedimentation or aeration tanks, may be utilized as equalization basins during the early period of design life.

10.5.4 Size

Equalization basin capacity should be sufficient to effectively reduce expected flow and load variations. With a diurnal flow pattern, the volume required to achieve the desired degree of equalization can be determined from a cumulative flow plot over a representative 24-hour period.

10.5.5 Electrical

All electrical work in housed equalization basins, where hazardous concentrations of flammable gases or vapours may accumulate, needs to meet the requirements of the Electrical Safety Code for Class I, Zone 1, Group D locations (O. Reg. 164/99 made under the Electricity Act, 1998).

10.6 Screenings, Grit Handling and Disposal

Special consideration needs to be given to the design of screenings and grit handling systems to ensure the material is easy to handle, odours are reduced and acceptable for final disposal.

The design of screenings handling equipment will be also dictated by disposal practices. Landfill practices are changing and some landfills do no accept material containing free water or fecal material. Screenings disposed of through a transfer station may require additional considerations. Screenings handling devices include:

  • Belts and Dumpsters - screenings may be moved to a dumpster by belts. The belts will need to be cleaned, so a nearby wash station should be included in the design. Because screenings in the dumpster will generate odours and attract insects, enclosing the dumpster should be considered;
  • Washers - screenings from screens with smaller openings (i.e., less than 12 mm or 0.5 in) will contain fecal material. Washers should be considered that will remove fecal material from the screenings. Most washers are combined with compactors that remove excess water from the rags; and
  • Compactors - compactors, when used with screenings, will remove excess water so that landfills will accept the waste. If the compactor is placed outside, the discharge tube should be heat-taped and insulated. Large amounts of rock in screenings will cause binding problems in the discharge tube. Flushing or an alternative means of dewatering should be considered.

Most screenings storage will produce odours, vectors problems and drainage. Odour control and proper ventilation should be addressed in all storage container siting decisions.

Dumpsters that receive screenings should have a way to be dewatered with a floor drain to the sanitary sewer, as close as possible to the dumpster. Drainage from dumpsters may damage concrete floors because of acidity, so the floor should have a protective coating. A cleanup station should be in the immediate area for cleaning when the dumpster is picked up. Redundancy or another method of screenings handling should be considered in case of equipment failure. Because screenings and storage rooms have corrosive atmospheres, all equipment should be of noncorrosive design.

Grit washing effectively removes organics from the grit. Screw and rake grit washers have proved to be reliable and usually produce a material low in organics. To ensure a low volatile content, ample dilution water may be required. Pumps normally provide sufficient dilution water, but bucket elevators may not, especially during periods of peak grit capture. Consequently, they may require supplementary liquid to function properly.

Disposal of screenings and grit is the most critical design consideration. Most landfills cannot accept waste that contains free water. Some will not accept waste with visible fecal material. The design of the dumpster box and the type of screenings/grit handling will be dictated, in most cases, by these landfill requirements.

Chapter 11: Primary Sedimentation

This chapter describes the primary sedimentation process that typically follows the preliminary treatment process of most municipal sewage treatment plants. Design considerations and descriptions of different types of primary sedimentation tanks (also known as primary clarifiers), as well as primary sludge and scum collection and removal systems are included in this chapter. A summary of the design loadings for primary clarifiers is provided in Appendix V which should be used in conjunction with the details in this chapter.

11.1 General

The need for primary sedimentation tanks may be governed by the need to remove scum and grease or other debris prior to secondary treatment. The designer of primary sedimentation tanks should consider the following factors:

  • The characteristics of the raw sewage;
  • The type of sludge digestion system, either available or proposed (aerobic digestion should not be used with raw primary sludge);
  • The type of secondary treatment following primary treatment;
  • The need for handling of waste activated sludge in the primary sedimentation tank(s); and
  • The need for phosphorus removal in the primary sedimentation tank(s).

Primary sedimentation provides low-cost suspended solids and BOD5 removal, especially in cases where the raw sewage contains a high proportion of settleable solids, as is often the case with sewage containing significant food processing waste, or similar wastes. Primary sedimentation may also incorporate ballasting or recirculation to enhance solids settling.

Primary sedimentation tanks used for phosphorus precipitation with normal strength municipal sewage typically exhibit BOD5 and suspended solids removals of 45 and 85%, respectively. Without chemical addition for phosphorus removal, the BOD5 and suspended solids reductions typically would be 35 and 65%, respectively. For suspended solids removal, the range is 60% to 90% with chemical addition and 40% to 70% without. Actual removal rates depend mainly on raw sewage characteristics and contributing sources (e.g. industrial inputs), chemical dosage (if any), mixing and clarifier hydraulics. BOD5 removal rates are affected by the proportion of soluble to particulate fractions of BOD5 in the raw sewage. The use of the secondary clarifiers for phosphorus removal has been the most common approach. This has been at least partially due to the reduced chemical requirements when the secondary units are used for phosphorus removal.

In view of the potential for increased BOD5 and suspended solids removals, there may be circumstances when consideration should be given to the use of primary sedimentation tanks for phosphorus removal rather than the secondary sedimentation tanks. Such circumstances might include the following:

  • Where existing aeration tanks and/or secondary clarifiers are overloaded;
  • Where nitrification is made a requirement for an existing secondary treatment plant;
  • Where excessive waste activated sludge production is causing anaerobic digester operating problems; and
  • Where economic evaluation shows the process to be more cost effective despite the higher chemical requirement and costs.

The use of the primary sedimentation tanks for phosphorus removal will generally permit removal down to the 1.0 mg/L level. However, if lower phosphorus levels are required, chemical addition to the primary sedimentation tanks may not be successful. This problem is at least partially due to the fact that some forms of phosphorus are more amenable to precipitation after aeration and that the phosphorus level variations are generally greater in raw sewage than experienced in the aeration tank effluent. It is therefore recommended that precipitation testing (e.g. jar testing) be carried out before a final decision is made on which plant treatment units are to be used for phosphorus removal.

Primary sedimentation tanks can be either rectangular or circular. With rectangular tanks, length to width (L/W) ratios of at least 4:1 are preferred. Width to depth (W/D) ratios of 1:1 to 2.25:1 are typical.

Factors to be considered in the selection of either rectangular or circular primary sedimentation tanks are outlined below.

11.1.1 Rectangular Tanks

  • Permit common wall construction;
  • Usually result in a thicker sludge;
  • Usually less expensive to cover, if chain and flight-type collector is used;
  • Traveling bridge type collectors may be less expensive than rotary circular collectors for large tanks, although chain and flight type collectors are often more popular and allow easier covering of tank if warranted (e.g. for odour control); and
  • Width of tank controlled by sludge withdrawal mechanism.

11.1.2 Circular Tanks

  • Rotary circular sludge collector mechanism usually less costly for small tanks and requires less maintenance than chain and flight type collectors for rectangular tanks;
  • Potential for pre-cast construction;
  • Sludge sump can be equipped with a blade to provide stirring to avoid sludge bridging;
  • Usually more susceptible to short-circuiting; and
  • For tank depths greater than 3 m (10 ft), may be less expensive than rectangular tanks.

11.2 Design Considerations

11.2.1 Number of Units

Multiple primary sedimentation tanks 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 units should include other provisions to assure continuity of treatment to meet the final effluent quality criteria.

11.2.2 Flow Distribution

Effective flow splitting devices and control appurtenances (i.e., gates and splitter boxes) need to be provided to permit proper proportioning of flow and solids loading to each unit, throughout the expected range of flows. (Section 3.13.2 - Flow Distribution and Section 8.5.8 - Flows and Organic Loadings Distribution.)

11.2.3 Dimensions

It is recommended that the minimum length from the inlet to the outlet be 3 m (10 ft) unless special provisions are made to prevent flow short-circuiting. The side water depth (SWD) should be designed to provide an adequate separation zone between the sludge blanket and the overflow weirs. Generally, primary sedimentation tanks have a SWD from 3.0 to 4.6 m (10 to 15 ft).

11.2.4 Surface Overflow Rates

Primary sedimentation tank sizing should reflect the degree of solids removal required and the need to avoid septic conditions during low flow periods. The primary sedimentation tanks surface overflow rates (SOR) are shown in Table 11-1. It is recommended that sizing be calculated for both design average daily flow and design peak daily flow conditions and the larger surface area determined to be used.

Table 11-1 shows the recommended design parameters for primary sedimentation tanks. The recommended SOR should be used for design unless the designer can demonstrate that higher SOR can be accommodated and still achieve the required treatment efficiency. Required treatment efficiency would be based on expected removal efficiency and the capacities of downstream processes. For instance, for plant expansions, it may be possible to show through full-scale testing of the existing primary treatment units that higher SOR will produce the desired results. Although not a common design parameter, the hydraulic retention time in a primary sedimentation tank generally varies from 1.5 to 2.5 hours at design average daily flow. Two hours is a typical value.

Table 11-1- Primary Sedimentation Tanks Surface Overflow Rates
Type of Primary Sedimentation Tank2 Surface Overflow Rates1 at Design Average Daily Flow m3/(m2·d) (USgpd/ft2) Surface Overflow Rates1 at Design Peak Daily Flow m3/(m2·d) (USgpd/ft2)
Tanks not receiving waste activated sludge3, 4 30 40 (740 - 980) 60 - 80 (1470 1960)
Tanks receiving waste activated sludge4 25 30 (610 740) 50 - 60 (1230 1470)

Surface overflow rates need to be calculated with all flows received at the primary sedimentation tanks. Primary settling of normal domestic sewage can be expected to remove approximately 1/3 of the influent BOD5 when operating at an overflow rate of 40 m3/(m2·d) (980 USgpd/ft2).

The following design SOR has traditionally been used in the past for the design of primary treatment plants, that is at design average and peak daily flows of < 35 (860) and < 70 (1720) m3/(m2·d) (USgpd/ft2).

Anticipated BOD5 removal should be determined by laboratory tests and consideration of the character of the wastes. Significant reduction in BOD5 removal efficiency may result when the peak daily overflow rate exceeds 60 m3/(m2·d) (1470 USgpd/ft2).

Waste activated sludge in this instance would also include biological waste sludge from other biological processes including fixed film systems.

11.2.5 Inlet Structures

Inlets and baffling should be designed to dissipate the inlet velocity, to distribute the flow equally both horizontally and vertically, to protect the sludge hopper 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. It is recommended that corner pockets and dead zones be eliminated and corner fillets or channeling be used where necessary. Provisions should be made for elimination or removal of floating materials which may accumulate in inlet structures.

11.2.6 Weirs

11.2.6.1 General

Overflow weirs should be readily adjustable over the life of the structure to correct for differential settlement of the tank.

11.2.6.2 Location

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

11.2.6.3 Design Rates

It is recommended that weir loadings not exceed values shown in Table 11-2.

Table 11-2 Recommended Weir Loading Rates
Average Plant Capacity Loading Rate at Design Peak Daily 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)

If influent pumping is required, the pumps should be operated as continuously as possible. Also, weir loadings should be related to pump delivery rates to avoid short-circuiting during pump operations, although peak flow conditions are expected to govern.

11.2.6.4 Weir Troughs

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

11.2.7 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.

11.2.8 Sedimentation Tank Dewatering

The ability to dewater a primary sedimentation tank and take tanks out of service should conform to the provisions outlined in Section 8.4.15 Component Backup Requirements. It is recommended that primary sedimentation feed channels be designed to provide for distribution of peak sewage flow to the remaining tanks, when one tank is out of service and/or dewatered. Consideration should be given to provide adequate means for dewatering of tanks, for example a sump of adequate size for temporary insertion of a submersible pump to dewater the tank.

11.2.9 Freeboard

It is recommended that the walls of primary sedimentation tanks extend at least 150 mm (6 in) above the surrounding ground surface and provide not less than 300 mm (12 in) freeboard. Additional freeboard or the use of wind screens is recommended where larger primary sedimentation tanks are subject to high velocity wind currents that would cause tank surface waves and inhibit effective scum removal.

11.3 Sludge and Scum Removal

11.3.1 Scum Removal

Full-surface mechanical scum collection and removal facilities, including baffling, should be provided for all primary sedimentation tanks. The characteristics of scum, which may adversely affect pumping, piping, sludge handling and disposal, need to be recognized in design. Scum pits may require heating to avoid freezing problems and consideration should be given to mixers. Smooth-walled pipe should be used for scum lines to minimize grease buildup. Glass-lined pipe is recommended for scum piping. Scum lines should be provided with clean outs or steam injection points to minimize blockage due to grease buildup.

Provisions should be made to remove scum from the sewage treatment process and direct it to either the sludge treatment process or an alternative treatment and disposal process. Scum treatment can be provided by digestion, but this can lead to problems in the digesters. If treated by digestion, consideration should be given to injecting the scum into the sludge recirculation line downstream of heat exchanger. As an alternative approach, scum may be transferred directly to landfill with screenings or to dewatering or incineration units, if available. Other special provisions for disposal may be necessary.

11.3.2 Sludge Removal

Mechanical sludge collection and withdrawal facilities should be designed to ensure rapid removal of sludge.

Each sedimentation tank should have its own sludge withdrawal line to ensure adequate control of sludge removal rate for each tank. Sludge removal needs to be adequate to handle the expected maximum sludge accumulation rate and avoid excessive blanket levels.

11.3.2.1 Sludge Hopper

The minimum slope of the side walls should be 1.7 vertical to 1 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 can be considered but should be designed appropriately.

11.3.2.2 Cross-Collectors

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

11.3.2.3 Sludge Removal Pipeline

Each hopper should have an individually valved sludge withdrawal line at least 150 mm (6 in) in diameter, although the need to maintain minimum pipe velocities and pump run times may require consideration of smaller diameter pipes. 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 back-flushing individual pipe runs. Piping should be provided to remove sludge for further processing.

11.3.2.4 Sludge Removal Control

Separate primary 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 sludge removal rate should be provided. Air-lift pumps are not recommended for the removal of primary sludge.

11.4 Safety

All primary sedimentation tanks should be suitably equipped to enhance safety for operators and include machinery covers, life lines, 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, should comply with the requirements contained in Section 8.9 - Safety. The explosion-proof classification may need to extend to equipment within a close envelope to an open tank. The fixtures and controls should be located so as to provide convenient and safe access for operation and maintenance. Adequate area lighting should be provided.