Design considerations for sewage treatment plants

Chapter 8: Design Considerations for Sewage Treatment Plants

This chapter describes design considerations as they relate to sewage treatment plants. Topics covered in this chapter include plant location, effluent quality requirements, design issues and details, outfalls, essential facilities and some general aspects of safety.

8.1 Plant Location

8.1.1 General

A new sewage treatment plant site or an expansion of an existing sewage treatment plant will be evaluated through the Municipal Engineers Association’s Municipal Class Environmental Assessment (MEA's Municipal Class EA) process and will be documented in the Environmental Study Report (ESR). Some of the factors which should be taken into consideration in this evaluation include:

• Locations of drinking water sources, surface water intakes and groundwater wells;
• Adequacy of isolation from residential areas and land use surrounding plant site;
• Prevailing wind directions;
• Susceptibility of site to flooding;
• Suitability of soil conditions;
• Adequacy of site for future expansion and/or provision for additional treatment stages;
• Suitability of site with respect to access to receiving body of water or other means of treated sewage effluent disposal;
• Assimilation capacity of receiving water body;
• Acceptability of site with respect to sludge disposal/utilization options on site or access to areas off site; and
• Design capacity (see Section 3.10 - Sewage Treatment Plant Capacity Rating).

8.1.2 Flood Protection

The susceptibility of the site to flooding should be investigated and, if necessary, measures need to be taken to prevent flooding damage as may be directed by the local Conservation Authority or the Ontario Ministry of Natural Resources (MNR). The treatment plant structures, electrical and mechanical equipment should be protected from physical damage by the 100year flood event. This requirement applies to new construction and to existing facilities undergoing expansion. Flood plain regulations of provincial and federal agencies and the municipal requirements related to flood plain protection need to be followed. The designer should also consider if high receiving water levels would impact the discharge of treated sewage effluent.

8.1.3 General Plant Layout

The general arrangement of the treatment plant within the site should take into account the subsurface conditions and natural grades to provide the necessary facilities at a minimum cost.

In the layout of the plant, the designer should orient the buildings to provide adequate allowances for future linear expansions of the various treatment stages and process units and orient the plant so that the best advantage can be taken of the prevailing wind and weather conditions. The building orientation can be used to minimize effects of odours, misting and freezing problems and energy usage (heating). The plant layout should also allow for the probability of snow drifting to minimize its effects on operations.

Within these constraints, the designer should work towards a plant layout where the various processing units are arranged in a logical progression to avoid the necessity for major pipelines or conduits to convey sewage, sludge, or chemicals from one module to the next and also to arrange the plant layout to provide for convenience of operation and ease of flow splitting for proposed and future treatment units.

Where site roadways are provided for truck access, the road design should be sufficient to withstand the largest anticipated delivery and disposal vehicles with due allowance for vehicle turning and forward exit from the site.

In order to avoid the dangers of high voltage lines crossing the site, a high voltage pole should be located at the property line. Depending on the distance from this pole to the control building, the step-down transformer could be located at the terminal pole. If the distance between the terminal pole and the building is excessive, the transformer should be located adjacent to the control building. The high voltage connections should be brought by underground cable to the pothead at the transformer. In this way, the primary and secondary terminals of the transformer are fully enclosed and no fence is required around the transformer.

Sewage treatment works sites should be adequately fenced and posted to prevent persons from gaining unauthorized access. The perimeters of open tanks or excavations should be adequately safeguarded. Gates and buildings should have locks. Consideration should be given to security enhancements such as alarm systems with access areas monitored using motion detectors and cameras.

8.1.4 Provisions for Future Expansion

In addition to the site size needed to physically accommodate future treatment plant expansions, it is necessary for the designer to include provisions to accommodate the future expansion and/or process changes.

On site sewage pumping stations should be designed such that their capacity can be increased and/or parallel facilities constructed without the need for major disruption of plant operation. The layout and sizing of channels and plant piping should be such that additional treatment units can be added in the future or increases in loading rates can be accommodated hydraulically. The location of buildings and tanks should allow for the location of the next stages of expansion. Buffer areas should be provided.

The need for septage and landfill leachate receiving facilities should be evaluated and appropriate space and provisions allocated (Chapter 19 Co-treatment of Septage and Landfill Leachate at Sewage Treatment Plants).

Within buildings, space should be provided for the replacement of equipment with larger capacity units such as pumps, blowers, boilers and heat exchangers. Adequate working space should be provided around equipment and provision made for the removal of equipment for replacement, or major maintenance operations.

In sizing inlet and outlet sewers, the ultimate plant capacity should be considered. Provided that problems will not occur with excessive sedimentation in the sewers, these sewers should be sized for the ultimate condition. With diffused outfalls, satisfactory port velocities can often be obtained by blocking off ports which will not be required until subsequent expansion stages.

8.2 Establishment of Effluent Quality Requirements

The ministry Guideline B-1, Water Management - Policies, Guidelines and Provincial Water Quality Objectives provides numerical and narrative ambient surface water quality criteria as provincial water quality objectives (PWQO). It then sets effluent requirements for sewage treatment plant discharges by introducing the concept of a mixing zone whereby PWQO are to be met at the boundary of the mixing zone.

The Procedure B-1-5, Deriving Receiving-Water Based, Point Source Effluent Requirements for Ontario Waters provides the framework within which the ministry sets effluent requirements in terms of contaminant loadings and concentrations and incorporates these requirements into a Certificate of Approval (C of A) for a sewage treatment plant.

It is the responsibility of the proponent to conduct a site specific receiving water body assessment in order to determine the effluent requirements based on the assimilative capacity of the receiver. If the effluent requirements determined by the receiving water assessment are less stringent than those stipulated in federal or provincial effluent regulations or guidelines, then the most stringent of the latter will be imposed. The Ontario effluent guidelines are provided in Guideline F-5, “Levels of Treatment for Municipal and Private Sewage Treatment Works Discharging to Surface Waters”. Guideline F-5 states:

“The normal level of treatment required for municipal and private sewage treatment works discharging to surface waters is secondary treatment or equivalent.”

Secondary treatment is provided by biological processes (e.g. activated sludge process and its variations, fixed film processes) or physical-chemical processes producing an effluent quality of CBOD₅ and TSS of 15 mg/L or better. In Ontario, the compliance limits for secondary treatment are typically set as not to exceed monthly average concentration of 25 mg/L for each of CBOD₅ and TSS.

Sewage treatment lagoons producing an effluent quality of CBOD₅ of 25 mg/L and TSS of 30 mg/L are considered as providing secondary equivalent treatment. In Ontario the compliance limits for lagoons are set as annual (or period of discharge) average concentrations of CBOD₅ of 30 mg/L and TSS of 40 mg/L. The compliance limits for seasonal discharge lagoons with batch chemical dosing for phosphorus removal are usually CBOD₅ of 25 mg/L and TSS of 25 mg/L.

Sewage treatment works that provide only primary settling of solids and the addition of chemicals to improve the removal of total phosphorus and/or solids are not considered as secondary treatment, or equivalent.

Sewage treatment works should also be able to produce final effluent quality that does not exceed monthly average total phosphorus (TP) concentration of 1 mg/L when phosphorus removal is required and a monthly geometric mean density of 200 E. coli organisms per 100 mL when disinfection is required.

Treatment beyond the norm of secondary or equivalent level for various watersheds may be necessary due to limited assimilation capacity and/or critical downstream uses being made of the receiving body of water. Many sewage treatment plants in Ontario are required to meet more stringent effluent quality requirements than associated with secondary treatment.

Some sewage treatment plants may also be required to produce a nitrified effluent, due to either concerns with un-ionized ammonia toxicity or nitrogen related oxygen demands in the receiving waters.

The receiving water-based effluent requirements should be confirmed by ministry regional staff. Once confirmed, the effluent quality requirements should serve as terms of reference for the design of the sewage treatment plant.

The most important decision that the designer should make, prior to designing sewage treatment works, is to set the design effluent quality objectives required to consistently achieve the compliance limits. It should be noted that the design objectives should reflect the specified time-averaged terms (weekly average, monthly average and annual average) used in defining effluent compliance limits.

The receiving water-based effluent requirements form the basis of the ministry technical review of the proposed sewage treatment works and are incorporated into the C of A as effluent compliance limits with appropriate effluent quality objectives in terms of concentrations and loadings.

Depending on the effluent requirements, there are a number of suitable alternative sewage treatment processes that can be considered. Table 8-1 lists some of the treatment processes and the expected effluent quality produced by well designed and operated plants for treating municipal sewage.

Other factors, in addition to expected effluent quality which will affect the choice of treatment processes are:

• Ultimate sludge disposal options;
• Available land area;
• Operator skills;
• Soil conditions;
• Need for retention of treated sewage during periods of the year where receiving streams experience insufficient flows or where downstream recreational water uses make summer effluent discharges undesirable;
• Future loads such as hauled septage and/or landfill leachate handling; and
• Capital and operation and maintenance (O&M) costs.

Before deciding upon the sewage and sludge treatment processes, the designer should evaluate the alternatives available, in terms of treatment capability and overall capital and O&M costs, to ensure that the most appropriate treatment system is selected.

Notes:

1. The above values are based on raw sewage with BOD₅ = 150-200 mg/L, Soluble BOD₅ = 50% of BOD₅, TSS = 150-200 mg/L, TP = 6-8 mg/L, TKN = 30-40 mg/L, TAN = 20-25 mg/L.
2. TAN (total ammonia nitrogen) concentrations may be lower during warm weather conditions if nitrification occurs.

8.3 Definitions of Terms

8.3.1 Biochemical Oxygen Demand Test

The standard 5-day Biochemical Oxygen Demand test measures the oxygen utilized during a 5-day period for the biochemical degradation of organic material and to oxidize inorganic material such as sulphides and ferrous iron. Significant nitrogenous oxygen demand can be exerted during the testing when a sufficient population of nitrifying bacteria (nitrifiers) and quantity of ammonia and/or nitrites are present in the test samples with low organic content (such as in many secondary effluents). In such cases, an inhibitor may be used during the testing to suppress the nitrogenous oxygen demand. The oxygen demand exerted by the oxidation of inorganic material in sewage is usually not significant.

If the nitrogenous oxygen demand is suppressed by using an inhibitor, the test results are referred to as Carbonaceous Biochemical Oxygen Demand (CBOD₅). If both CBOD₅ and nitrogenous oxygen demands are measured (without using an inhibitor), the resulting oxygen demand is simply referred to as Biochemical Oxygen Demand (BOD₅) which is also known as Total Biochemical Oxygen Demand (TBOD₅).

8.3.1.1 CBOD₅

CBOD₅ should be used for the assessment of secondary (or higher) sewage treatment works performance and as an indicator of their effluent quality.

Effluents from sewage treatment plants (STP) exhibiting partial nitrification (with both nitrifiers and ammonia present) may have higher BOD₅ values than those with no nitrification (with no nitrifiers present) or complete nitrification (with no ammonia present). Since most factors that are conducive to improved effluent quality from secondary STP are also conducive to nitrification, the effluent BOD₅ values can erroneously indicate poorer quality when, in fact, both the effluent quality and the plant performance are indeed good. This is often the case during warm weather periods or in newer treatment plants that are organically under loaded.

8.3.1.2 BOD₅

The designer should use BOD₅ for the assessment of raw sewage and primary effluents in estimating design parameters such as organic loadings and process air requirements of the secondary treatment process.

Although both BOD₅ and CBOD₅ are expected to be the same in raw sewage and primary effluents, there are cases where CBOD₅ has consistently underestimated the organic strength of these sewage streams.

8.3.2 Sewage Treatment Plant Design Capacity

The sewage treatment plant design capacity should be such that the treated effluent would continuously meet the established quality criteria in terms of concentrations and loadings during the design period (Section 8.2 Establishment of Effluent Quality Requirements). For plants serving combined sewers subject to excessive wet weather flows or overflow detention pump-back flows, the design maximum day flows that the plant is to treat on a sustained basis should be taken into consideration. (Chapter 21 Control and Treatment of Combined Sewer Overflows)

8.3.3 Sewage Treatment Plant Rated Capacity

Rated Capacity of a sewage treatment plant for Municipal Engineers Association Class Environmental Assessment (MEA Class EA) requirements generally means the design average daily flow for the limiting process stage (e.g., secondary treatment stage). For more information on details for establishing STP rated capacity and STP re-rating refer to Section 3.10 Sewage Treatment Plant Capacity Rating.

8.3.4 Combined Sewer System

A combined sewer system is a sewage collection system which conveys sanitary sewage (domestic, commercial and industrial wastewaters) and stormwater runoff through a single-pipe system to a sewage treatment plant. Combined sewer systems which have been partially separated and in which roof leaders and/or foundation drains contribute stormwater inflow to the sewer system conveying sanitary flows are still defined as combined sewer systems in the ministry Procedure F-5-5, “Determination of Treatment Requirements for Municipal and Private Combined and Partially Separated Sewer Systems”.

8.3.5 Sanitary Sewer System

A sanitary sewer system is a separate sewer system which conveys sanitary sewage (domestic, commercial and industrial wastewaters), infiltrated groundwater and limited amounts of stormwater where an adjoining separate storm sewer system exists as the primary collection system to receive stormwater flows from catch basins and other sources of stormwater.

8.3.6 Dry Weather Flow

Sewage flow resulting from sanitary wastewater (combined input of domestic, commercial and industrial flows) and infiltration and inflows from sewer joints and service connections, during periods with an absence of rainfall or snowmelt.

8.3.7 Wet Weather Flow

Sewage flow resulting from sanitary wastewater (combined input of domestic, commercial and industrial flows) infiltration and inflows from sewer joints and service connections, during periods of rainfall or snowmelt; or stormwater generated by either rainfall or snowmelt that enters the sanitary sewer system or combined sewer system.

8.3.8 Average Daily Flow

The average daily flow is the average of the daily volumes to be received in a calendar year expressed as a volume per unit time. The average daily flow for sewage works having critical seasonal high hydraulic loading periods (e.g., recreational areas, campuses and industrial facilities) should be based on the average of the daily volumes to be received during the seasonal period.

8.3.9 Peak Daily Flow

The peak daily flow is the largest volume of flow to be received during a one day period expressed as a volume per unit time. This flow is also referred to as maximum daily flow or maximum day flow^{footnote 1[1]}.

8.3.10 Peak Hourly Flow

The peak hourly flow is the largest volume of flow to be received during a one-hour period expressed as a volume per unit time. This is also referred to as maximum hourly flow or maximum hour flow.

8.3.11 Peak Instantaneous Flow

The peak instantaneous flow is the instantaneous maximum flow rate as measured by a metering device.

8.3.12 Minimum Daily Flow

The minimum daily flow is the smallest volume of flow to be received during a continuous one-day period expressed as a volume per unit time. This is also referred to as minimum day flow. Initial low flow conditions in new sewage works should be evaluated in the design to minimize operational problems with freezing, septicity, flow measurements and solids dropout.

8.3.13 Design Flows

The designer should select appropriate flow rates for the design of specific process units in sewage works. These flows would be designated as design flows (e.g., design average daily flow, design peak daily flow and design peak hourly flow). For more information on recommended design flows for various STP components refer to Table 8-2.

8.3.14 Bypass

Bypassing of any treatment processes within a sewage treatment plant with the associated sewage flows being returned to the sewage treatment plant flow and discharging to the environment through the final effluent outfall of the sewage treatment plant.

8.3.15 Overflows

• Combined Sewer Overflow means a discharge to the environment from a combined sewer system;
• Sanitary Sewer Overflow means a discharge to the environment from sanitary sewer system; and
• Sewage Treatment Plant Overflow means a discharge to the environment from a sewage treatment works at a location other than the final effluent outfall or downstream of the sampling point in the final effluent outfall.

8.3.16 Emergency

A condition that if not mitigated, could result in personal injury or loss of life, structural damage to the sewage works, basement flooding or other health hazard.

8.3.17 Unavoidable Condition

A condition beyond the reasonable foresight or control of the owner and operator of the works and includes exceptional acts of nature, third party actions (e.g., vandalism), or structural, mechanical or electrical failure.

8.4 Basis of Process Selection

The selection of an appropriate process(es) for a sewage treatment plant is unique to each site and conditions. Subsequent chapters in these design guidelines outline options for each process and associated advantages and disadvantages for each. Many issues need to be considered during process selection, such as:

• Influent characteristics;
• Influent flows, including average, minimum and peak flows;
• Effluent requirements (Section 8.2 - Establishment of Effluent Quality Requirements);
• Compatibility with other processes;
• Local conditions;
• Local resources;
• End use for byproducts; and
• Economics.

8.4.1 Value Engineering Approach

The procedure to develop alternative options for a sewage treatment plant should involve a review of the needs and limitations through a comprehensive evaluation process. One method could include a workshop or value engineering approach, followed by detailed characterization of the needs and abilities of each option and an overall weighted evaluation to determine the preferred alternative.

The value engineering (VE) approach is an intensive workshop during which the project design is analyzed for optimization of cost, energy, operation and maintenance.

The VE Job Plan is important because it provides an organized approach to identifying high initial capital, energy and life-cycle costs. The functional requirements needed to operate and maintain the facilities are analyzed to ensure performance. Where the essential functions are not being furnished by the design, there is a lack of “value”. The VE team identifies alternative approaches that will provide the needed value. Portions of the project not functionally required or carrying major parts of project costs are likely targets for team evaluation. Developing recommendations to reduce these high cost areas is an important aspect of the workshop.

The workshop is conducted in six phases in this specific order:

• Orientation Phase;
• Information Phase;
• Creative Phase;
• Judgment Phase;
• Development Phase; and
• Presentation Phase.

An alternatives development workshop (facilitated workshop) can be used to identify treatment process options that require further evaluation to determine the preferred options. Once the alternative options have been conceptually developed, an evaluation matrix approach can be used to review the alternatives identified.

8.4.2 New and Innovative Technologies

The designer should refer to Section 3.9 - Technology Development for details.

8.5 Major Design Criteria

The following major design criteria should be applied to the design of new sewage treatment plants, plant expansions or upgrades:

8.5.1 Sanitary Sewer Systems

• Sanitary sewer systems should be designed with the objective of conveying all the flows to be treated at the sewage treatment plant. Any overflows within the sanitary sewer systems and overflows/bypasses at treatment works should be designed for emergency and unavoidable conditions only;
• The biological treatment processes at the treatment plant should be designed to meet effluent quality requirements over a wide range of flows including design minimum, average and peak flows for the projected design period. Ontario experience for medium to large sewage treatment plants demonstrates design peak flows in the range of 2 to 3 times the design average daily flows;
• Where treatment process units need to accommodate any peak flows the design criteria should be the design peak hourly flows unless indicated otherwise;
• In cases where peak hourly flow to average daily flow ratio is exceptionally high, which may result in washout of the biomass necessary for treatment, or in unfavorable operating conditions at minimum flow rate (e.g. small STP), judicious design peak hourly flows should be established by the designer to be best suited for the operation of the treatment process units. Peak flow mitigation efforts such as equalization techniques or standby units to be brought on-line when necessary (i.e., during wet weather conditions), may form part of the decision making process by the designer;
• Any bypasses within a sewage treatment plant should be reintroduced into the outfall prior to final effluent sampling; and
• Every effort should be made to treat all flows received at the STP within the sewage treatment capability of the individual unit processes using measures to provide the highest possible treatment. This could be facilitated by:
  • Reducing infiltration and inflow (I/I) to the collection system;
  • Maximizing the storage capacity of the collection system for equalization; and
  • Providing off-line storage in the collection system.

8.5.2 Combined Sewer Systems

Combined sewer systems should comply with ministry Procedure F-5-5:

• For an average year, only 10% of the wet weather flow during the seven-month period of concern that are above the dry weather flows from combined sewer systems may be allowed to overflow. During wet weather, the minimum level of treatment required for flows above the dry weather flows from combined sewer system is primary treatment;
• In cases where one area is served by combined (and or partially separated) sewers and the other area in the same sewer shed is served by sanitary sewers, Procedure F-5-5 applies only to the flows from the area served by the combined sewer systems; and
• For sewage collection systems consisting of both sanitary sewers and combined sewers, the design should be based on a distinction between all sanitary sewage flows from the entire system which should be conveyed to and treated at STP and combined sewer wet weather flows that are above the dry weather flows which are subject to Procedure F-5-5 requirements.

8.5.3 Design Period

Factors which will have an influence on the design period of sewage treatment plants include the following:

• Population growth rates;
• Sewershed boundaries;
• Heavy water use industries;
• Inflation and financing interest rates;
• Ease of expansion of facilities; and
• Time requirements for design and construction of any expansion.

Wherever possible, sewage treatment plants should be designed for the flows expected to be received during the next 20 years, under normal growth conditions. In certain cases, where it can be shown that staging of construction will be economically advantageous, lesser design periods may be used.

8.5.4 Sewage Flows

Wherever there are existing sewers and/or existing sewage treatment plants, the flow rates and sewage characteristics should be determined using real data, in both wet and dry weather conditions. Data collected should be analyzed to estimate the following:

• Average and peak flows of sewage generated within buildings serviced by the sewer system exclusive of any extraneous flows; and
• Average and peak infiltration and inflow for the design year.

During investigations to determine peak extraneous flows, it may be found that such flows are excessive and that measures should be taken to reduce these flows rather than provide flow equalization and/or treatment facilities to accommodate such excessive flows. It is often difficult to determine when measures to reduce infiltration will be cost effective. North American experience has indicated that if infiltration, based upon the highest weekly average within a 12-month period, is less than 0.14 L/(mm·d)/m (litres per millimetre of pipe diameter per day per linear metre of sewer length) [(0.28 US gal/(in·d)/ft)] rehabilitation of the sewer system will not be economical. The issue of extraneous flows and mitigating measures should be addressed during the MEA Municipal Class EA process.

Where it is not possible to base estimates of sewage flows and characteristics upon actual field measurements, the flow records and sewage characteristics of similar serviced communities may provide data upon which estimates can be based.

In estimating sewage flows, it is recommended that no less than 225 L/(cap·d) [59 US gal/(cap·d)] be used for average domestic sewage flows, exclusive of extraneous flows.

To estimate peak sewage flows, the average domestic flow rate (sewage flows from residential sources) should be multiplied by the Harmon factor, then the peak extraneous flows should be added. Industrial and commercial sewage flow rates should be calculated separately and added to the above sewage flow rates. (Section 5.5.2 - Design Sewage Flows)

8.5.5 Organic Loadings

Organic loadings for sewage treatment plant design should be based on actual data for the facility or a mass balance of the expected contributors. The effects of other high strength liquid wastes, such as septage and leachate, which may be accepted at the plant, should be evaluated and appropriate facilities should be included in the design (Chapter 21 Control and Treatment of Combined Sewer Overflows).

Where it is found that sewage characteristics vary significantly over the year due to excessive infiltration/inflow, population variations and/ or seasonal changes in industrial or commercial operations, estimates should be made of the expected average, maximum and minimum BOD₅ and suspended solids concentrations in the sewage for each month of the year. If nitrification is required, short and long term variations in TAN (total ammonia nitrogen) and TKN (Total Kjeldahl Nitrogen) concentrations should also be estimated.

As part of the assessment of the influent sewage characteristics, the designer should consider the restrictions established by the local municipal sewer use bylaw.

The shock effects of high concentrations and diurnal peaks for short periods of time on the treatment process, particularly for small treatment plants, should be considered.

Typical organic loading rates for domestic sewage are 75 and 90 g/(cap·d) [2.6 and 3.2 oz/(cap·d)] for BOD₅ and SS, respectively.

8.5.6 Bypasses and Overflows

8.5.6.1 General

If sewage entering the treatment plant is to be pumped into the treatment units, an emergency overflow for the pumping station should be provided. This overflow should be routed through the disinfection facilities and plant outfall sewer. If this is not possible, provision should be made for separate disinfection of such overflows.

The overflow elevation and the method of activation should ensure that the maximum feasible storage of the sewage collection system and wet well would be utilized before the controlled overflow takes place. The overflow facilities should at least be alarmed and equipped to indicate frequency and duration of overflows and provided with facilities to permit flow measurement. Automatic flow measurement and recording systems may be required in certain cases where requirements dictate.

To allow maintenance operations to be carried out, each unit process within the treatment plant should be provided with bypass capability around the unit.

Where two or more similar treatment units are considered and one unit is out of operation for repairs, the remaining units should be capable of treating the design peak sewage flow rates or be provided with bypass capacity equal to the excess hydraulic flow of the operating units.

Bypass systems should also be constructed so that each unit process can be separately bypassed (i.e., no need to bypass more unit processes than necessary).

All flows bypassing secondary and/or tertiary treatment processes should be measured.

If a bypass for the chlorine contact chamber or other disinfection process (e.g. UV) is needed, provisions may be necessary for emergency disinfection of flows in the bypass channel.

8.5.6.2 Bypasses at Sewage Treatment Plants Serving Sanitary Sewers

Bypassing of treatment processes should not occur during dry-weather flow conditions, or during wet-weather flows equal to or lower than the design peak flows of the treatment works associated with sanitary sewer systems. It is recognized that the ability to bypass some processes should be incorporated into the design of the treatment works to minimize process washouts that may cause prolonged episodes of poor treatment performance.

Bypassing of one or more treatment process occurs when the volume of flow exceeds the maximum capacity of the treatment works associated with sanitary sewer systems.

The sanitary sewer systems while not designed to receive the bulk of stormwater flows, may experience increased flows during wet weather conditions as a result of increased infiltration and inflow through pipe joints and other sources of extraneous flows, including service connections, as well as direct and indirect sources of stormwater. These additional flows may, under certain circumstances, cause the capacity of the treatment plant to be exceeded.

Other reasons which can cause the capacity to be exceeded include, but are not limited to, structural, mechanical, or electrical failure, exceptional acts of nature and third party actions such as vandalism.

8.5.6.3 Overflows at Sewage Treatment Plants Serving Sanitary Sewers

Overflows from sewage treatment works associated with sanitary sewer systems should not occur. Overflow facilities may be incorporated into the design of STP to prevent or mitigate an emergency caused by unavoidable conditions.

8.5.6.4 Bypasses and Overflows at Sewage Works Serving Combined Sewers

Bypasses and overflows at sewage works serving combined sewers are addressed in ministry Procedure F-5-5, Determination of Treatment Requirements for Municipal and Private Combined and Partially Separated Sewer Systems.

8.5.7 Pumping of Sewage

Raw sewage and any intermediate pumping stations within sewage treatment plants should be capable of conveying the peak sewage flow rates to the downstream treatment units. Pumping equipment should also be designed so that downstream treatment units receive a steady flow (with minimal flow rate variations). To achieve this, the pumping system can be provided with variable capacity, or multiple fixed capacity pumps, so that pump discharge rates will closely match the sewage inflow rate.

The designer should refer to Chapter 7 - Pumping Stations for the recommended design criteria.

8.5.8 Distribution of Flows and Organic Loadings

There will invariably be situations within STP where flow splitting is necessary. To ensure that the organic load splits in the same proportion as the flows, the suspended solids should be homogeneously dispersed throughout the liquid and the relative momentum of all particles should be approximately equal at the point of diversion. Some turbulence is therefore desirable before each point of diversion. Channel or pipe bends upstream of flow division resulting in uneven solids distribution should be avoided. The following methods can be used to provide homogeneity:

• Mechanical mixers;
• Diffused aeration;
• Bottom entrance into splitting box;
• Bar racks or posts in channels;
• Hydraulic jumps; and
• Straight section of conduit 6 to 8 diameters (or channel width) upstream of point of diversion.

Flow division control facilities should be provided as necessary to ensure organic and hydraulic loading control to plant process units and should be designed for easy operator access, adjustment, observation and maintenance. The use of upflow division boxes equipped with adjustable sharp-crested weirs or similar devices is recommended. The use of valves for flow splitting is not acceptable. Appropriate flow measurement facilities should be incorporated in the flow division control design. The designer is referred to Section 3.13 Hydraulics for more details.

8.5.9 Plant Hydraulic Gradient

The hydraulic gradient of all gravity flow and pumped sewage streams within the sewage treatment plant, including bypass channels, should be prepared to ensure that adequate provision has been made for all head losses. In calculating the hydraulic gradient, changes in head caused by all factors should be considered, including:

• Head losses due to channel and pipe wall friction;
• Head losses due to sudden enlargement or contraction in flow cross section;
• Head losses due to sudden changes in direction such as at bends, elbows, wyes and tees;
• Head losses due to sudden changes in slope or drops;
• Head losses due to obstructions in conduit;
• Head required to allow flow over weirs, through flumes, orifices and other measuring, controlling or flow division devices;
• Head losses caused by flow through comminutors, bar screens, tanks, filters and other treatment units;
• Head losses caused by air entrainment or air binding;
• Head losses incurred due to flow splitting along the side of a channel;
• Head increases caused by pumping;
• Head allowances for expansion requirements and/or process changes; and
• Head allowances due to maximum water levels in receiving waters.

Designers are cautioned to consider the consequences of excessive or inadequate allowances for head losses through sewage treatment plants. If pumping is required, excessive head loss allowances result in energy wastage. If inadequate head loss allowances are made, operation will be difficult and plant expansion would be more costly.

8.5.10 Sludge Pumping

The flow characteristics of sludge will vary according to the types and concentrations of organic solids and added chemicals. Some sludge flow characteristics will be similar to those of water while others may have pseudo plastic flow characteristics. The friction losses associated with sludge pumping applications vary greatly. Dilute sludge has losses similar to those experienced with clean water, whereas, thickened sludge has greater losses up to 15 times those of clean water. With sludge pumping, velocities of 0.9 to l.5 m/s (3.0 to 4.9 ft/s) should be developed. For heavier sludge and grease, velocities of 1.5 to 2.4 m/s (4.9 to 7.9 ft/s) are needed. To avoid blockages, a minimum line size of 150 mm (6 in) should be used. Mixing primary scum and grease with sludge lines may result in excessive headlosses and plugging and should be avoided.

8.5.11 Design Basis for Various Plant Components

The plant design should provide the necessary flexibility to perform satisfactorily within the expected range of influent sewage characteristics and flows.

All components of sewage treatment plants should be hydraulically capable of handling the anticipated peak sewage flow rates without overtopping channels and/or tanks. From a process point-of-view, design of the various process units or components of sewage treatment plants should be based upon the hydraulic, organic and inorganic loading rates shown in Table 8-2.

8.5.12 Flow Equalization

Sewage flows can be fully or partially equalized. Full equalization may result in a small reduction in construction costs over variable flow design and in addition can result in reduced energy costs and improved treatment efficiency. Alternatively, partial equalization of sewage flows is an option that has decreased benefits with slight savings in construction costs.

8.5.13 Conduits

All piping and channels should be designed to carry the design peak instantaneous flows. The incoming sewer should be designed for unrestricted flow. Channels should be designed to convey the initial and ultimate range of expected flows. To avoid solids buildup, the following scouring velocities should be developed in normally used channels at least once per day:

• Sewage Containing Grit - 0.9 m/s (3.0 ft/s); and
• Floc Suspensions - 0.45 to 0.60 m/s (1.5 to 2.0 ft/s).

Where the above velocities cannot be obtained, channels should be aerated to prevent solids deposition. Bottom corners of the channels should be filleted. Conduits should be designed to avoid creation of pockets and corners where solids can accumulate.

Suitable gates or valves should be placed in channels to seal off unused sections which might accumulate solids. The use of shear gates, stop plates or stop planks may be considered where they can be used in place of gate valves or sluice gates. Non-corrodible materials should be used for these control gates.

8.5.14 Flow Measurement

Flow measurement facilities should be provided for the following flows:

• Plant influent or effluent flow. If influent flow is significantly different from effluent flow, both should be measured. This would apply for installations such as lagoons, sequencing batch reactors and plants with excess flow storage or flow equalization;
• Excess flow treatment facility discharges (e.g. equalization tank effluents);
• Overflows and bypasses to be monitored as required by the ministry; and
• Other flows such as return activated sludge, waste activated sludge and recycle flows required for plant operational control.

Indicating, totalizing and recording flow measurement devices should be provided for all mechanical plants. Flow measurement facilities for lagoon systems should not be less than elapsed time meters used in conjunction with pumping rate tests or else should be calibrated weirs. All flow measurement equipment should be sized to function effectively over the full range of flows expected and should be protected against freezing.

Flow measurement equipment including approach and discharge conduit configuration and critical control elevations should be designed to ensure that the required hydraulic conditions necessary for accurate measurement are provided. Conditions that need to be avoided include turbulence, eddy currents and air entrainment that upset the normal hydraulic conditions that are necessary for accurate flow measurement.

8.5.15 Sampling Equipment

Effluent composite sampling equipment should be provided at all mechanical plants with a design average daily flow of 380 m³/d (0.1 mUSgd) or greater and at other facilities where monitoring effluent quality criteria parameters are necessary. Composite sampling equipment should also be provided as needed for influent sampling and for monitoring of plant performance. The influent sampling point should be located prior to any process return flows.

8.5.16 Component Backup Requirements

The designer should refer to Section 3.8 - Reliability and Redundancy for details on how to carry out the reliability and redundancy analysis of specific process trains, units or equipment.

The components of sewage treatment plants should be designed in such a way that equipment breakdown and normal maintenance operations can be accommodated with a minimal deterioration of effluent quality.

To achieve this, critical treatment processes should be provided with multiple units so that with the largest unit out of operation, the hydraulic capacity of the remaining units should be sufficient to handle the appropriate design sewage flows outlined in Table 8-2. There should also be sufficient flexibility in operation so that the normal flow into a unit that is out of operation can be distributed to all of the remaining units. It should be possible to distribute the flow to all of the units in the treatment process downstream of the affected process. In addition, where feasible, it should be possible to operate the sections of treatment plants as completely separate process trains to allow full-scale loading tests to be carried out.

With some processes such as mechanical screening or comminution, the backup facility can be provided with a less sophisticated unit such as a manually cleaned screen. The designer should provide a bypass around a manual screen to avoid flooding.

Sewage and sludge pumping systems should always be provided with a back up pump of equal capacity to the largest duty pump. In certain cases, particularly with sludge pumps, one duty pump may serve as a back up for more than one set of pumps (e.g. a raw sludge pump could back up a sludge transfer pump). Standby capacity for sludge return pumps may be determined on a case-by-case basis.

Depending upon the size of the sewage treatment plant and the sensitivity of the receiving waters, some unit processes may not require duplication. For instance, if the equivalent of primary treatment would be satisfactory under emergency conditions, one aeration basin may be sufficient.

Aeration systems will require facilities to permit continuous operation, or minimal disruption, in the event of equipment failure. The following factors should be considered when designing the back-up requirements for aeration systems:

• Effect on the aeration capacity if a piece of equipment breaks down, or requires maintenance (for example, the breakdown of one of two blowers will have a greater effect on capacity than the breakdown of one of four mechanical aerators);
• Time required to perform the necessary repair and maintenance operations;
• The general availability of spare parts and the time required for delivery and installation. Preferably the vital spare parts should be stored on site;
• Means other than duplicate equipment to provide the necessary capacity in the event of a breakdown (e.g. using oversized mechanical aerators with adjustable weirs to control power draw and oxygenation capacity, or using two-speed mechanical aerators); and
• Diffused aeration systems require a standby blower, however mechanical aeration systems may not require standby units, depending upon the number of duty units and availability of replacement parts.

Chemical feed equipment for phosphorus removal and disinfection should be provided in multiple units so that the chemical requirements can be supplied with one unit out of service.

For sludge digestion facilities at small plants, the need for multiple units can often be avoided by providing two-stage digestion along with sufficient flexibility in sludge pumping and mixing so that one stage can be serviced while the other stage receives the pumped raw sludge. When such an approach is proposed, the designer should outline the alternate methods of treatment and disposal that could be used during periods of equipment breakdown. For larger treatment plants, the provision of multiple primary and secondary digestion units can usually be economically justified.

Depending upon the receiving stream sensitivity, type of filtration equipment and the maintenance requirements of the filter units, provision of multiple effluent filtration units is often necessary.

For sludge handling and dewatering equipment, multiple units will generally be required unless satisfactory sludge storage facilities or alternate sludge disposal methods are available for use during periods of equipment repair. Often the need for full standby units will be unnecessary if the remaining duty units can be operated for additional shifts in the event of equipment breakdown.

8.5.17 Unit Bypasses

Properly located and arranged bypass structures and piping should be provided so that each unit of the STP can be removed from service independently. The bypass design should facilitate plant operation during unit maintenance and emergency repair so as to minimize deterioration of effluent quality and ensure rapid process recovery upon return to normal operational mode.

Treatment during bypassing may be accomplished through the use of duplicate or multiple treatment units in any stage if the design peak instantaneous flow can be handled hydraulically with the largest unit out of service.

The actuation of all bypasses should require manual action by operating personnel. All power-actuated bypasses should be designed to permit manual operation in the event of power failure. They should also be designed so that the valve will fail as is, upon failure of the power operator.

A fixed high water level bypass overflow to the bypass channel should be provided in addition to a manually or power actuated bypass. The designer should refer to Section 8.5.6 - Bypasses and Overflows for additional information.

8.5.18 Unit Dewatering, Flotation Protection and Plugging

Means such as drains or sumps should be provided to completely dewater each unit and to discharge to an appropriate point in the treatment process. Due consideration should be given to the possible need for hydrostatic pressure relief devices to prevent flotation of structures. Pipes subject to plugging should be provided with means for mechanical cleaning or flushing.

8.5.19 Construction Materials

Materials should be selected that are appropriate under conditions of exposure to hydrogen sulphide and other corrosive gases, greases, oil and other constituents frequently present in sewage. This is particularly important in the selection of metals and paints. Contact between dissimilar materials should be avoided or other provisions made to minimize galvanic action.

8.5.20 Installation of Mechanical Equipment

The specifications should be so written that the installation and initial operation of major items of mechanical equipment will be inspected and approved by a representative of the manufacturer.

8.5.21 Operating Equipment

A complete outfit of tools, accessories and spare parts necessary for the plant operator’s use should be provided.

Readily accessible storage space and workbench facilities should be provided. Consideration should be given to provision of a garage for large equipment storage, maintenance and repair.

8.5.22 Erosion Control During Construction

Effective site erosion control should be designed for implementation during construction.

8.5.23 Grading and Landscaping

Upon completion of the plant, the ground should be graded and then sodded or seeded. All-weather walkways should be provided for access to all units. Steep slopes should be avoided to prevent erosion. Surface water should not be permitted to drain into any unit. Particular care should be taken to protect sludge beds and intermittent sand filters from stormwater runoff. Provision should be made for landscaping, especially when a plant needs to be located near residential areas.

8.6 Plant Outfalls

The proper site and design of the plant outfall structure is important in minimizing the impact on receiving water quality. In many cases it may be a controlling factor in ensuring protection of nearby water supplies, recreational beaches or fisheries habitats.

Outfalls should be designed and located so as to obtain the greatest possible dilution of the plant effluent during periods of greatest susceptibility of nearby water uses to adverse impacts.

Dilution is a product of initial mixing of the effluent with surrounding water and subsequent dispersion due to water movement.

Entrainment of ambient lake or stream water into the effluent is generally enhanced by extending the outfall away from the shore into deeper water and often by incorporating a multi-port diffuser to spread the discharge over a larger area and to increase turbulent mixing. Similarly, dispersion is aided by maximizing the separation of the discharged plume from boundary effects of the shoreline and lake or streambed.

Reference should be made to the implementation procedure for defining mixing zones contained in the most recent version of the ministry Guideline B-1, Water Management - Policies, Guidelines and Provincial Water Quality Objectives.

Dispersion predictions require knowledge of effluent concentration, discharge rates, effluent buoyancy, jet velocity, ambient current velocity, depth of water over the outfall, ambient thermal regime (vertical temperature profile) and background water quality.

For all extended outfalls, outfall capacity should be sufficient to handle not only the treated effluent but also all flows received at the plant so as to eliminate overflowing of untreated or partially treated flows at shore.

8.6.1 Discharge Impact Control, Protection and Maintenance

The outfall sewer should be designed to discharge to the receiving stream in a manner acceptable to the reviewing authorities. The designer should contact the local Conservation Authority, the Ministry of Natural Resources (MNR) and review the federal Navigable Waters Protection Act for site specific requirements. Consideration should be given in each case to the following:

• Preference for free fall or submerged discharge at the selected site;
• Utilization of cascade aeration of effluent discharge to increase dissolved oxygen; and
• Limited across-stream dispersion as needed to protect aquatic life movement and growth in the immediate reaches of the receiving stream.

The outfall sewer should be so constructed and protected against the effects of floodwater, tide, ice or other hazards as to reasonably ensure its structural stability and freedom from stoppage. A manhole should be provided at the shore end of all gravity sewers that extend into receiving waters. Hazards to navigation should be considered in designing outfall sewers.

8.6.2 Sampling Provisions

All outfalls should be designed so that a sample of the effluent can be obtained at a point after the final treatment process and before discharge to the receiving waters.

8.7 Essential Facilities

8.7.1 Emergency Power Supply Facilities

The need for standby power and the extent of equipment requiring operation by standby power should be individually assessed for each sewage treatment plant and pumping station. Some of the factors that will require consideration in making the decisions regarding standby power and the processes to be operated by the standby power facility are as follows:

• Reliability of primary power source;
• Number of feeder lines supplying the grid system, number of alternate routes within the grid system and number of alternative transformers through which the power could be directed to the sewage treatment plant;
• Whether sewage enters the plant by gravity or is pumped;
• Type of treatment provided;
• Pieces of equipment which may become damaged or overloaded following prolonged power failure;
• Assimilation capacity of the receiving waters and ability to withstand higher pollution loadings over short time periods; and
• Other uses of receiving water body.

Standby generating capacity may not be needed for aeration equipment used in the activated sludge process in cases where there is no history of long-term (4 hours or more) power outages. However, full power generating capacity is needed for sewage discharges to critical areas of surface water receiver such as upstream or near bathing beaches and water supply intakes.

Continuous disinfection, where required, should be provided during all power outages. Continuous dechlorination is required for those systems that dechlorinate.

Where standby power is not needed for pumping or treatment, the designer should include the provision of a small [typically 25 kW (33.5 hp)] generator set having sufficient capacity to provide the power for lighting and instrumentation, so that in the event of transient power outages, the plant will have sufficient power available for safe operation and to maintain instrumentation.

The standby power equipment should be located so that it connects conveniently into the electrical distribution system of the plant. It should also be close to other potentially noisy equipment, so that adequate acoustic measures need only be taken over small areas. Sufficient fuel storage should be provided, taking into account the historical data on length of power outages in the area and any weather or other conditions that may preclude fresh deliveries of fuel. Where a diesel generator is used, a minimum of:

• 450 L (120 US gal) fuel tank should be provided for generator set capacities of up to 25 to 30 kW (34 to 40 hp);
• 900 L (240 US gal) fuel tank for set capacities from 30 to 100 kW (40 to 134 hp);
• 1135 L (300 US gal) fuel tank for set capacities from 110 to 160 kW (147 to 214 hp); and
• 2 x 1135 L (2 x 300 US gal) fuel tanks for set capacities from 160 to 300 kW (214 hp to 402 hp).

Equipment suppliers should be contacted for actual fuel consumption of their generator units. Fuel storage for both portable and permanent engine generators should be adequate to operate the pump station for a minimum of 12 and preferably 24 continuous hours without refueling. Alternative fuelled standby power equipment (e.g. natural gas) could also be considered.

Either underground or inside fuel storage tanks may be used. In considering which type to use, factors such as corrosion potential, consequences of leakage, required storage volume and the need for fuel pumps should be evaluated.

The designer should refer to Section 7.7 - Standby Power and Emergency Operation for additional details.

The location of the standby power system should generally be such that site perimeter noise levels will be in compliance with the ministry Model Municipal Noise Control By law and also located so that contaminant levels at the nearest point of impingement due to stack emissions are in compliance with the requirements of Section 9 of the Environmental Protection Act. (Section 3.11 - Emissions of Contaminants to Air).

8.7.2 Water Supply

An adequate supply of pressurized water should be provided to ensure general cleanliness around the plant. Chemical quality of the water supply should be checked for suitability for its intended uses such as in heat exchangers and chlorinators.

Where a sewage treatment works obtains water from a municipal potable water supply, the supply needs to be protected with a CSA rated, reduced pressure principle backflow preventer^{footnote 2[2]} at each point of connection with the municipal system. Potable water from a municipal or separate supply may be used directly at points above grade for the following hot and cold supplies:

• Lavatory;
• Water closet;
• Laboratory sink (with vacuum breaker);
• Shower;
• Drinking fountain;
• Eye wash fountain; and
• Safety shower.

Hot water for any of the above units should not be taken directly from a boiler used for supplying hot water to a sludge heat exchanger or digester heating unit.

Where a potable water supply is to be used for any purpose in a plant other than those listed above, a break tank, pressure pump and pressure tank should be provided. Water should be discharged to the tank through an air gap at least 150 mm (6 in) above the maximum flood line or the spill line of the tank, whichever is higher.

Wherever possible, to conserve energy and minimize operating costs, effluent water should be used for water uses not requiring potable water. Such uses as chlorinator-injector water, lawn sprinkling, foam control, flushing water, screening, thickening and dewatering process units wash and incinerator off-gas scrubbing can be supplied with STP final effluent. Fixtures supplied with non-potable water should be clearly marked as such.

Where effluent water is used, a sign should be permanently posted at every hose bib, faucet, hydrant or sill cock located on the water system beyond the break tank to indicate that the water is not safe for drinking.

8.7.3 Plant Piping

All piping to be used in sewage treatment plants should be manufactured in accordance with the most recent version of the standards from Canadian Standards Association (CSA), Canadian General Standards Board (CGSB), American Society for Testing and Materials Standards (ASTM), or other internationally recognized organizations. Piping for digester gas, propane, fuel oil and natural gas should further comply with the requirements of Canadian Gas Association (CGA) as discussed in Chapter 16 - Sludge Stabilization.

In the design of the piping, due allowance should be made for future capacities and also for ease of extending this piping without major disruption of the plant operation. In the general piping arrangement, sufficient space should be provided for piping to be removed and should provide for the proper isolation of pipe sections through valves to enable them to be repaired or replaced.

In larger plants, galleries are often used for the location of process piping and for the passage of operating staff between buildings and tank units. Tunnels may be formed by using the walls of adjacent tank structures and the floor slab may be common to all structures. Although galleries will generally cost more than buried piping systems, their use may be justified due to the more convenient plant operation and maintenance access.

The designer should allow for the possibility that piping could be installed during construction when temperature conditions could be substantially different from the design condition. For example, piping could be installed in temperatures anywhere between -20 °C to +40 °C (-4 °F to 104 °F) and substantial differences in pipe lengths could occur. For this reason the use of PVC pipe with cast iron mechanical joint fittings is not recommended. Where piping is cast in place, due allowance should be made for differential expansion between the pipe material and structures.

Piping should be arranged so that all valves, flow meters and other items which may require regular inspection or maintenance are conveniently accessible. Piping should be provided with drains at all low points and air release valves at all high points. Sludge and scum piping should be provided with cleanouts and facilities to permit water and/or steam cleaning. Scum piping should be smooth walled pipe, preferably glass-lined.

The design of the piping should allow for proper restraint under all anticipated conditions, particularly where surges may occur and high transient pressures could result, or where different temperatures occur seasonally.

Where piping connections are made between adjacent structures, at least one flexible coupling should be provided if there is any possibility that differential settlement could occur. Particular attention should be given to pipe bedding in areas adjacent to structures to avoid settlement damage.

Under the Industrial Establishments Regulation (O. Reg. 851) made under the Occupational Health and Safety Act (OHSA), piping identification as to flow direction and contents is mandatory only for piping systems containing hazardous substances. However, it is recommended that all piping be adequately identified as to contents and direction of flow so that the operation of the process units is simplified. Piping identification by complete painting of the pipe line and by use of colour banding is recommended. Where there is no existing standard colour coding, it is suggested that the following code be used for pipe colour.

Clearly visible lettering to indicate the actual pipe contents (e.g. raw sludge and waste activated sludge) should be shown on colour bands along with the flow direction arrow. To comply with CSA Standard CSA B53-58: Code for Identification of Piping Systems, the bands should be coloured as shown in Table 8-3.

The use of paints containing lead or mercury should be avoided. In order to facilitate identification of piping, particularly in the large plants, it is suggested that the different lines be color coded. The following color scheme is recommended:

• Raw sludge line - brown with black bands;
• Sludge recirculation suction line - brown with yellow bands;
• Sludge draw off line - brown with orange bands;
• Sludge recirculation discharge line - brown;
• Digester gas line - orange (or red);
• Natural gas line - orange (or red) with black bands;
• Nonpotable water line - blue with black bands;
• Potable water line - blue;
• Chlorine line - yellow;
• Sulfur dioxide - yellow with red bands;
• Sewage line - gray;
• Compressed air line - green;
• Water lines for heating digesters or buildings - blue with a 150 mm (6 in) red band spaced 760 mm (30 in) apart;
• Fuel oil/diesel - red;
• Plumbing drains and vents - black; and
• Polymer - purple.

The contents and direction of flow should be stenciled on the piping in a contrasting colour.

The designer should ensure compliance with Code for Digester Gas and Landfill Gas Installations CAN 1-B105-M81 which states that “all gas piping and controls should be painted or colour coded with high visibility paint and each system of piping should be labeled every linear 3 m (10 ft) with the name of the gas being conducted and the direction of flow”.

In sizing, material selection and pressure requirements of piping for use in sewage treatment plants, the following factors should be considered:

• Likelihood of blockage and size of line required;
• Line size required to produce scouring velocities and thus minimize solids deposition and grease buildup;
• Nature of material to be conveyed and suitable piping materials for the application;
• Flow characteristics of material to be conveyed and head requirements of pumps or differential head required for gravity flow;
• Possible settlement and need for support;
• Need for future repair; and
• Need for future removal of pipe sections.

The recommended minimum diameters of piping for various purposes are shown in Table 8-4.

8.7.4 Personnel Facilities

The necessity for personnel facilities will be largely dictated by the number of operation and maintenance staff required and the time periods during which the plant is staffed.

As a minimum, it is recommended that provision be made for storage lockers, preferably two for each employee (one for work clothes and one for clean clothes) and a washroom with shower. As the size of the plant and number of staff increases, there will be a requirement to provide more locker space, possibly in a separate change room, a lunchroom which should be of adequate size to serve as a meeting or instruction room for plant staff and a suitable office for plant supervisory staff and record keeping.

Whenever possible, these personnel facilities should be separated from the plant facilities, but with convenient access to the plant. All requirements of OHSA should be included.

8.7.5 Building Services

Adequate heating facilities of a safe type should be provided, with control levels depending on the type of area being heated. In many areas of the plant, sufficient heat need only be provided to prevent freezing of equipment or treatment units.

Buildings should be well ventilated by means of windows, doors, roof ventilators, or other means. All rooms, compartments, pits and other enclosures that are below grade and which need to be entered should have adequate forced ventilation provided when it is necessary to enter them.

Rooms containing equipment or piping should be adequately heated, ventilated and dehumidified, if necessary, to prevent undue condensation. Switches should be provided to control the forced ventilation.

Buildings should be adequately lighted throughout by means of natural light, artificial lighting facilities, or both. Control switches where needed should be conveniently placed at each entrance to each room or area.

As discussed in Section 9.6 - Security, it may be advantageous to provide intercom systems between the control centre and other buildings or locations throughout the plant site. In certain circumstances television monitoring may be warranted. Public telephone service should at least be provided to the control centre and other manned centres throughout plant. Empty conduit systems may be provided for future telephone or intercom lines.

Power outlets of suitable voltage should be provided at convenient spacing through plant buildings to provide power for maintenance equipment and extension lighting. Power outlets should also be located at outside locations to permit servicing of such equipment as scraper drive mechanisms, flow meters and comminutors.

Potable water service will be required for most buildings. Reference should be made to Section 8.7.2 - Water Supply for requirements relating to backflow prevention for potable water supplies.

8.7.6 Sanitary Facilities

Washrooms with showers and locker facilities should be provided in sufficient numbers and at convenient locations to serve the plant personnel. All requirements of OHSA should be included.

8.7.7 Stairways

Stairways should be installed in lieu of ladders for access to units requiring routine inspection and maintenance, such as digesters, trickling filters, aeration tanks, clarifiers and tertiary filters. Spiral or winding stairs should be used only for secondary access where dual means of exit are provided.

8.8 Operator Licensing

8.8.1 General

Sewage works are to be operated by persons holding a valid operator’s license of the same type as the type for the facility. At least one operator needs to hold a license of the same class or a higher class than the class of the facility and the license needs to be prominently displayed. More detailed information on licensing for operators can be found in the Licensing of Sewage Works Operators (O. Reg.129/04) made under the Ontario Water Resources Act and the ministry document “Licensing Guide for Operators of Wastewater Facilities”.

8.9 Safety

8.9.1 General

The following is only a general description of some safety considerations. The designer should refer to all applicable safety codes and regulations, including the Occupational Health and Safety Act (OSHA), Building Code (O. Reg. 350/06) under the Building Code Act, 1992, Fire Code (O. Reg. 388/97) under the Fire Protection and Prevention Act, 1997 and the Workplace Safety and Insurance Act (WSIA). Adequate provision should be made to effectively protect plant personnel and visitors from hazards. The designer should consider the following to satisfy the particular needs of each plant:

• Enclosure of the plant site with a fence and signs designed to discourage the entrance of unauthorized persons and animals;
• Hand rails and guards (e.g. kick plates) around tanks, trenches, pits, stairwells and other hazardous structures with the tops of walls less than 1070 mm (42 in) above the surrounding ground level;
• Gratings over appropriate areas of treatment units where access for maintenance is required;
• First aid equipment;
• “No Smoking” signs in hazardous areas;
• Protective clothing and equipment, such as self-contained breathing apparatus, gas detection equipment, goggles, gloves, hard hats and safety harnesses;
• Portable blower and sufficient hosing;
• Portable lighting equipment complying with the requirements of Electrical Safety Code, (O. Reg.164/99) made under the Electricity Act, 1998;
• Gas detectors listed and labeled for use in Class I, Division 1, Group D locations;
• Appropriately placed warning signs for slippery areas, non-potable water fixtures, low head clearance areas, open service manholes, hazardous chemical storage areas and flammable fuel storage areas;
• Adequate ventilation in pump station areas in accordance with Section 7.2.10 - Safety Ventilation;
• Provisions for local lockout on stop motor controls;
• Warning signs should be provided for appropriate areas including excessive noise areas and confined spaces;
• Provisions for confined space entry in accordance with the Confined Spaces Regulation (O. Reg. 632/05) under the OHSA; and
• Adequate vector control.

Equipment suppliers and chemical suppliers should also be contacted regarding particular hazards of their products and the appropriate steps taken in the facility design to ensure safe operation.

8.9.2 Hazardous Chemical Handling

The materials utilized for storage, piping, valves, pumping, metering and splash guards should be specially selected with consideration to the physical and chemical characteristics of each hazardous or corrosive chemical that they may come into contact with. Chemical buildings or storage areas should be provided with adequate warning signs, conspicuously displayed where identifiable hazards exist and a storage area for filing Material Safety Data Sheets (MSDS) as set out under the federal Hazardous Products Act and associated Controlled Products Regulations. An MSDS should be available for each chemical. All storage containers should be conspicuously labeled in accordance with the Workplace Hazardous Materials Information System (WHMIS) (O. Reg. 860) under OHSA. The WHMIS label includes: the product name, the supplier name, hazard symbol(s), risk, precautionary measures and first aid measures.

8.9.2.1 Secondary Containment

Chemical storage areas should be enclosed in dikes or curbs capable of containing the stored volume until it can be safely transferred. Liquid polymer should be similarly contained to reduce areas with slippery floors, especially to protect travel ways. Non-slip floor surfaces are desirable in polymer-handling areas.

8.9.2.2 Liquefied Gas Chemicals

Properly designed isolated areas should be provided for storage and handling of chlorine, sulfur dioxide and other hazardous gases. Gas detection kits, alarms, controls, safety devices and emergency repair kits should also be provided.

8.9.2.3 Splash Guards

All pumps or feeders for hazardous or corrosive chemicals should have guards, which will effectively prevent spray of chemicals into spaces occupied by personnel. The splash guards are in addition to guards to prevent injury from moving or rotating machinery parts.

All connections (flanged or other types), except those adjacent to storage or feeder areas, should have guards which will direct any leakage away from space occupied by personnel. Pipes containing hazardous or corrosive chemicals should not be located above shoulder level except where continuous drip collection trays and coupling guards will eliminate chemical spray or dripping onto personnel.

8.9.2.4 Piping Labeling

All piping containing or transporting corrosive or hazardous chemicals should be identified with labels every 3 m (10 ft) and with at least two labels in each room, closet, or pipe chase. Color coding may also be used (see Section 8.7.3 Plant Piping), but is not an adequate substitute for labeling.

8.9.2.5 Protective Clothing and Equipment

The following items of protective clothing or equipment should be available and utilized for all operations or procedures where their use will minimize injury hazard to personnel:

• Self-contained breathing apparatus recommended for protection against chlorine;
• Chemical worker’s goggles or other suitable goggles (safety glasses are insufficient);
• Face masks or shields for use over goggles;
• Dust mask to protect the lungs in dry chemical areas;
• Rubber gloves;
• Rubber aprons with leg straps;
• Hearing protection;
• Rubber boots (leather and wool clothing should be avoided near caustics); and
• Safety harness and line.

8.9.2.6 Warning System and Signs

Facilities should be provided for automatic shutdown of pumps and the sounding of alarms when failure occurs in a pressurized chemical discharge line. Warning signs requiring use of goggles should be located near chemical stations, pumps and other points of potential frequent hazard.

8.9.2.7 Dust Collection

Dust collection equipment should be provided to protect personnel from dusts injurious to the lungs or skin and to prevent polymer dust from settling on walkways which can become slick when wet.

8.9.2.8 Eyewash Fountains and Safety Showers

Eyewash fountains and safety showers supplied with potable water should be provided on each floor level or work location involving hazardous or corrosive chemical storage, mixing (or slaking), pumping, metering, or transportation unloading. These facilities are to be as close as practical to points of chemical exposure. They are to be fully operable during all weather conditions.

The eyewash fountains should be supplied with water of moderate temperature 15 to 32 °C (60 to 90 °F) suitable to provide 15 to 30 minutes of continuous irrigation of the eyes. The emergency showers should be capable of discharging 1.9 to 3.2 L/s (30 to 50 USgpm) of water at moderate temperature and at pressures of 140 kPa to 345 kPa (20 to 50 psi).

8.10 Laboratory

8.10.1 General

All treatment plants should have a laboratory for making the necessary analytical determinations and operating control tests, except for those plants that utilize only processes not requiring laboratory testing for process control and where satisfactory off-site laboratory provisions are made to meet regulatory monitoring requirements. For plants where a fully equipped laboratory is not required, the requirements for utilities and fume hoods may be reduced. The laboratory should have sufficient size, bench space, equipment and supplies to perform all self-monitoring analytical work and to perform the process control tests necessary for proper management of each treatment process included in the design.

The facilities and supplies necessary to perform analytical work to support industrial waste control programs will normally be included in the same laboratory. The laboratory arrangement should be sufficiently flexible to allow future expansion should more analytical work be performed there in the future. Laboratory size and instrumentation should reflect treatment plant size, staffing requirements and process complexity. Experience and training of plant operators should also be assessed in determining treatment plant laboratory needs.

8.10.2 Categories

Treatment plant laboratory needs may be divided into the following three general categories:

1. Plants performing only basic operational testing; this typically includes pH, temperature and dissolved oxygen;
2. Plants performing more complex operational laboratory tests including biochemical oxygen demand, suspended solids and fecal coliform analysis; and
3. Plants performing more complex operational, industrial pretreatment and multiple plant laboratory testing.

Expected minimum laboratory needs for these three plant classifications are outlined in this section. However, in specific cases laboratory needs may have to be modified or increased due to the industrial monitoring needs or special process control requirements.

8.10.3 Category I: Plants performing only basic operational testing

8.10.3.1 Location and Space

A floor area of up to 14 m² (150 ft²) should be adequate. It is recommended that this be at the treatment plant site. Another location in the community, utilizing space in an existing structure, owned by the involved authority, may also be acceptable.

8.10.3.2 Design and Materials

The facility should provide for electricity, water, heat, sufficient storage space, a sink and a bench top. The lab components need not be of industrial grade materials. Laboratory equipment and glassware should be of types recommended by American Public Health Association (APHA), American Water Works Association (AWWA) & Water Environment Federation (WEF), Standard Methods for the Examination of Water and Wastewater, 21^st Edition, as amended.

8.10.4 Category II: Plants performing more complex operational laboratory tests including biochemical oxygen demand, suspended solids and fecal coliform analysis

8.10.4.1 Location and Space

The laboratory size should be based on providing adequate room for the equipment to be used. In general, the laboratories for this category of plant should provide a minimum of 28 m² (300 ft²) of floor space. Adequate bench space for each analyst should be provided. The laboratory should be located at the treatment plant site on ground level. It should be isolated from vibrating, noisy or high-temperature machinery or equipment which might have adverse effects on the performance of laboratory staff or instruments.

8.10.4.2 Design and Materials

Floor surfaces should be fire resistant and highly resistant to acids, alkalis, solvents and salts. The cabinets and shelves selected may be of wood or other durable materials. Bench tops should be of acid resistant laboratory grade materials for protection of the underlying cabinets. Glass doors on wall-hung cabinets are recommended.

Fume hoods should be provided for laboratories in which required analytical work results in the production of noxious fumes. Air intake should be balanced against all exhaust ventilation to maintain an overall positive pressure relative to atmospheric in the laboratory. A laboratory grade sink and drain trap should be provided. Laboratories should be air conditioned. In addition, separate exhaust ventilation should be provided.

An analytical balance of the automated digital readout, single pan, 0.1 milligram sensitivity type should be provided. A heavy special-design balance table which will minimize vibration of the balance is recommended.

Laboratories should provide the following: first aid equipment, protective clothing and equipment (e.g. goggles, safety glasses, full face shields and gloves), fire extinguishers, chemical spill kits, posting of “No Smoking” signs in hazardous areas and appropriately placed warning signs for slippery areas, non-potable water fixtures, hazardous chemical storage areas and flammable fuel storage areas.

Eyewash fountains and safety showers supplied with potable water should be provided in the laboratory (Section 8.9.2.8 - Eyewash Fountains and Safety Showers).

8.10.5 Category III: Plants performing more complex operational, industrial pretreatment and multiple plant laboratory testing

8.10.5.1 Location and Space

The laboratory should be located at the treatment plant site on ground level, with environmental control as an important consideration. It should be isolated from vibrating, noisy, high-temperature machinery or equipment that may have adverse effects on the performance of laboratory staff or instruments.

The laboratory facility needs for Category III plants should be described in the engineering report or facilities plan. The laboratory floor space and facility layout should be based on an evaluation of the complexity, volume and variety of sample analyses expected during the design life of the plant including testing for process control, industrial pretreatment control, user-charge monitoring and effluent quality criteria and monitoring requirements.

Consideration should be given to provide separate (and possibly isolated) areas for some special laboratory equipment, glassware and chemical storage. The analytical and sample storage areas should be isolated from all potential sources of contamination. It is recommended that the organic chemical facilities be isolated from other facilities. Adequate security should be provided for sample storage areas. Provisions for the proper storage and disposal of chemical wastes should be provided.

8.10.5.2 Design and Materials

Floor surfaces should be fire resistant and highly resistant to acids, alkalis, solvents and salts.

Two exit doors should be located to permit a straight exit from the laboratory, preferably at least one to the outside of the building. Panic hardware should be used. Exit doors should have large glass windows for easy visibility of approaching or departing personnel.

Wall-hung cabinets are useful for dust-free storage of instruments and glassware. Units with sliding glass doors are recommended. A reasonable proportion of cupboard style base cabinets and drawer units should be provided.

All cabinet shelving should be acid resistant and adjustable. The laboratory furniture should be supplied with adequate water, gas, air and vacuum service fixtures, traps, strainers, plugs and tailpieces and all electrical service fixtures.

Bench tops should be constructed of materials resistant to damage from normally used laboratory reagents. Generally, bench-top height should be 915 mm (36 in). However, areas to be used exclusively for sit-down type operations should be 760 mm (30 in) high and include kneehole space.

Fume hoods should be located where air disturbance at the face of the hood is minimal. Air disturbance may be created by persons walking past the hood, by heating, ventilating, or air-conditioning systems and by drafts from opening or closing a door.

One sink should be provided inside each fume hood. A cup sink is usually adequate.

All switches, electrical outlets and utility and baffle adjustment handles should be located outside the hood. Light fixtures should be explosion-proof.

Twenty-four hour continuous exhaust capability should be provided. Exhaust fans should be explosion-proof. Exhaust velocities should be checked when fume hoods are installed.

Canopy hoods should be installed over the bench-top areas where hot plate, steam bath, or other heating equipment or heat-releasing instruments are used. The canopy should be constructed of heat and corrosion resistant material.

The laboratory should have a minimum of two sinks (not including cup sinks). At least one of them should be a double-well sink with drain boards. Additional sinks should be provided in separate work areas as needed and identified for the use intended.

Sinks and traps should be made of epoxy resin or plastic materials highly resistant to acids, alkalis, solvents and salts and should be abrasion and heat resistant, non-absorbent and light weight. Traps should be made of glass, plastic, or lead when appropriate, and easily accessible for cleaning. Sewage openings should be located toward the back so that a standing overflow will not interfere.

All water fixtures on which hoses may be used should be provided with reduced zone pressure backflow preventers to prevent contamination of water lines.

Laboratories should be separately air conditioned, with external air supply for one hundred percent make-up volume. In addition, separate exhaust ventilation should be provided. Ventilation outlet locations should be remote from ventilation inlets. Consideration should be given to providing dehumidifiers.

An analytical balance of the automatic, digital readout, single pan, 0.1 milligram sensitivity type should be provided. A heavy special-design balance table which will minimize vibration of the balance is needed.

Consideration should be given to providing line voltage regulation for power supplied to laboratories using delicate instruments.

Reagent water for analytical requirements using an all-glass distillation system should be supplied to the laboratory. Some analyses require deionization of the distilled water. Consideration should be given to softening and/or deionizing the feed water to the still.

Natural or LP gas (liquefied petroleum gas - propane) should be supplied to the laboratory. Digester gas should not be used. Adequately-sized vacuum lines should be provided, with outlets available throughout the laboratory.