3. Water Conservation Measures

3.1 Water Conservation at Sewage Treatment Works

This section is divided into four main areas, namely conducting a water use assessment at a sewage treatment works, information on typical water use areas at the treatment works, water efficiency measures that can be implemented, and using plant effluent to reduce potable water consumption at the sewage treatment works. Some of the areas where water use can be reduced are applicable to both water efficiency and the use of STP effluent, and are therefore discussed in both subsections.

3.1.1 Water Use Assessments

There are a number of options for sewage treatment works to reduce potable water use. However, the cost-effectiveness of each option will vary for each facility as it is dependent on whether it is a new or existing facility, and for existing facilities, where potable water is currently used and the amount of water used in each of these areas.

For existing sewage treatment works, it is recommended that a water use assessment (also known as a water audit) be carried out. Plant staff should carry out an assessment of potable water use at the treatment works before deciding if external help is required to determine which conservation measures should be implemented at the plant. The cost of using external expertise for assessing water use at a plant should not outweigh the potential economic benefits of implementing water conservation measures.

There are six main stages to a water use assessment, which are:

  • a walkthrough of the sewage treatment works to identify all processes and pieces of equipment that use water
  • identifying the water piping layout
  • identifying the water flow and quality required for each area where water is used
  • preparing a process flow diagram (including flow and pressure) of water use at the sewage treatment works
  • measuring or estimating the amount of water used in each area, either by using data from an existing water meter, using a bucket and stopwatch, or installing meters (which may be the best option for large water using areas)
  • comparing water use measured with that specified within equipment operating manuals or industry standard data (e.g. for filter backwashing).

Estimates of water use can be easily determined for some areas where water is used (e.g. filter backwashing, chemical make-up water, toilet and urinal flushing), but for other areas, estimation is more difficult (e.g. landscape irrigation, equipment and site cleaning). However, even when water use in a specific area cannot be easily determined, cost-effective water conservation measures can be identified using the information in Section 3.1.2 where typical areas of water use at sewage treatment works are provided.

Measuring or estimating the amount of water used in all areas is the baseline that should be used to determine the feasibility and effectiveness of water conservation measures. This can be done by comparing water use data after the implementation of water conservation measures with the baseline data.

3.1.2 Areas of Water Use at Sewage Treatment Works

The following subsections provide information on areas where potable water is typically used at sewage treatment works. Information is also provided on typical water quality requirements for each area. There is little information available in the literature on potable water consumption rates in sewage works.

Water is used for a number of purposes at sewage treatment works. These include water used as part of treatment processes, site services and domestic use (washrooms, showers and kitchen).

Process Water Use

There are nine main process areas in a sewage treatment works where water is used, namely:

  • for media surface washing and filter backwashing
  • for chemical make-up
  • as seal water in pumps
  • for foam control on the surface of activated sludge aeration tank contents or in other process units
  • for conveying scum and/or fats, oils and grease (FOG)
  • high-pressure spray washing of screens, disk and drum filters, drum thickeners and belt press filter cloth
  • cooling/heating water
  • scrubber or other odour control systems; and,
  • clean-in-place (CIP) systems for equipment such as mechanical thickening or dewatering processes for biosolids.
Media Surface Washing and Filter Backwashing

Filters may be used at sewage treatment works for tertiary treatment. Some sewage works use surface washing of filters to provide the shearing force necessary to clean media prior to the backwash cycle. The amount of water used for media surface washing is dependent on whether it is a single or dual-sweep system, the size and number of filters and how long the surface wash cycle is. The amount of water for filter backwashing is also dependent on the size and number of filters,length of backwash cycle, and if auxiliary surface washing or air-scour is used.

There are options for minimizing the amount of potable water used for media surface washing and filter backwashing and these are discussed in Section 3.1.3. Plant effluent can be used as an alternative to potable water for media surface washing and/or filter backwashing provided the total suspended solids concentration is below the level that can block the spray nozzles, typically around 10 mg/L. The option of using plant effluent for filter cleaning is discussed further in Section 3.1.4.

Chemical Make-Up Water

Chemical make-up water is used for polymer solutions for sludge or biosolids thickening and dewatering, and for metal salt solutions for phosphorus removal at some sewage treatment works. It is also used for gas chlorinators or hypochlorite solutions for chlorine disinfection processes. The amount of water used for chemical make-up cannot be reduced as the amount used is typically the minimum required to dilute the chemical for addition to a process. Therefore, options to reduce consumption through water-efficiency measures are not discussed further.

Potable water quality is not necessary for chemical make-up water, but the suspended solids level of the make-up water must be low enough to ensure there is no blockage of nozzles for polymer injection systems and chlorinators. The option of using plant effluent for chemical makeup water is discussed further in Section 3.1.4.

Pump Seal Water

Water is used as seal water for pumps at sewage treatment works. The amount of water used for this purpose should be minimized to conserve water. It is not practical to use plant effluent for pump seal water at many existing facilities due to the network of effluent pipes that would need to be installed to feed to pumps around the plant. However, use of plant effluent could be considered for new STPs, and this is discussed in Section 3.1.4.

Foam Control

Foam can form in aeration tanks or other process units due to the excessive growth of certain filamentous bacteria in the floc, or as a result of soaps, detergents and surfactants found in wastewater. Spray nozzles along the edge of aeration tanks or hand-held hoses are used to control foaming. The amount of water used for foam control is dependent on the number and size of nozzles and the frequency and degree of foaming problems at the plant. Plant effluent can be used for this in place of potable water provided the total suspended solids concentration is below the level that can block the spray nozzles, typically around 10 mg/L. The option of using plant effluent for foam control is discussed further in Section 3.1.4.

Conveying Scum and FOG

Water may be used to convey scum and FOG from primary and secondary sedimentation tanks for further processing. Typical uses include flushing from a scum trough, and using water sprays to move scum to a collection trough instead of a mechanical method. Options for minimizing water use for conveying scum and FOG are discussed in Section 3.1.3. Reusing plant effluent for conveying scum and FOG is discussed in Section 3.1.4.

High-Pressure Spray Washing

High-pressure spray washers may be used at a sewage works to clean screens, disk and drum filters, drum thickeners, centrifuges and belt press filter cloth. The amount of water needed for spray washing these pieces of equipment is dependent on whether hot or cold water is used, if it is a closed-loop system, the pressure used and the cleaning chemicals used. Options for minimizing water use for high-pressure spray washing are discussed in Section 3.1.3. Reusing plant effluent in spray washing equipment is discussed in Section 3.1.4.

Cooling/Heating Water

For facilities using cogeneration to produce electricity and recoverable heat, water is used for cooling the engines and as the medium for heat recovery. It is possible to use plant effluent for cooling/heating water, and this is discussed in Section 3.1.4.

Odour Control

Wet chemical scrubbers and biological filters used for odour control at sewage treatment works require water. Effluent can be used in place of potable water for odour control, which is discussed further in Section 3.1.4. The main effluent quality criterion for this is low total suspended solids to prevent blockage of nozzles used to spray water and to ensure good circulation of water for biological filters (typically 10 mg/L or less of total suspended solids is acceptable).

CIP

Automatic cleaning of process equipment, such as biosolids mechanical thickeners and dewatering centrifuges, involves the use of CIP systems at some sewage treatment works. There are options to optimize water use by CIP systems, which are discussed in Section 3.1.3. Using plant effluent in CIP systems is also an option and this is discussed further in Section 3.1.4.

Site Services Water Use

Site service water is required for the following areas at many sewage treatment works:

  • boiler feed
  • landscape irrigation
  • fire protection
  • equipment and site cleaning.
Boiler Feed

Softened, potable water is used for boiler feed water. There are a number of options for optimizing the amount of water used for boiler feed, and these are discussed in Section 3.1.3. Due to the high quality of water needed for boiler feed water, plant effluent use is not a cost-effective option and is not discussed further.

Landscape Irrigation

Many sewage treatment works have landscaping around the facility and potable water may be used for irrigation of these areas. Potable water used for irrigation can be reduced by watering less often or through more efficient watering methods, which are discussed in Section 3.1.3. It is possible to use plant effluent or collected rainwater for landscape irrigation instead of potable water and these measures are discussed in Section 3.1.4.

Fire Protection

A high flow water supply is required on an emergency basis for fire protection at sewage treatment works.

Equipment and Site Cleaning

Water is used to wash down platforms and other areas around the plant, as well as tanks and plant equipment. Hose connection points are used at various locations around the facility for this purpose. The amount of water used for equipment and site cleaning is dependent on the diameter of the hose used and the frequency of site cleaning carried out. Changes to the hose diameter and operational practices can reduce water consumption for cleaning, which is discussed further in Section 3.1.3.

Implementation of plant effluent use for on-site cleaning may not be practical in existing plants due to the requirement for the effluent to be pumped to numerous locations around the site. In new or expanded facilities, provision of a separate effluent water system for use for site cleaning in the plant should be considered, which is discussed in Section 3.1.4. A health and safety assessment should be carried out when considering this option due to the potential exposure of plant staff to various effluent constituents that can adversely affect health, which is also discussed in Section 3.1.4.

Domestic Water Use

Many sewage treatment works have washrooms, showers and a kitchen area for plant staff. The amount of water used for these facilities varies according to the plant size and number of plant staff, as well as the types of fixtures used, although there are limited data on how water use varies with the treatment capacity of sewage works. There are options to reduce water consumption in these areas, which are discussed in Section 3.1.3.

Potable water quality is required for drinking water fountains, washroom sinks, showers and kitchen use. In order to achieve potable water quality, plant effluent would likely require tertiary and quaternary treatment in most cases which would be cost-prohibitive. Therefore, use of plant effluent for drinking water fountain, sinks, showers and kitchen use at sewage treatment works is not considered further in this document.

Potable water quality is not necessary for toilet and urinal flushing due to the lower exposure rate but the microbial quality of the water used is important. Sewage treatment works effluent may be used for toilet and urinal flushing if microbial quality is acceptable, which may require further treatment, such as additional chlorination. U.S. EPA guidelines (U.S. EPA, 2004) specify a non-detect limit for fecal coliform for toilet flushing and further information on this is provided in Section 4.4.6. Installation of a new line to existing washrooms for toilet and urinal flushing is unlikely to be practical from an economic viewpoint at an existing sewage works and is therefore not considered further. However, use of effluent for toilet and urinal flushing and other uses should be considered in the design of new or expanded facilities when the implementation of separate effluent distribution systems can be economical.

3.1.3 Water Efficiency Measures and their Technical and Cost Impacts

There are a number of water efficiency measures available for sewage treatment works. Water efficiency measures are those that can reduce the amount of potable water used through changes to equipment and operational practices. These options are discussed below, together with information on determining if these measures are practical and cost-effective for specific facilities. A summary of this information is provided in Table 3.1 at the end of this section, to help identify cost-effective water efficiency measures for a sewage treatment works; however, site specific factors need to be taken into account for each individual plant to determine which water conservation measures are feasible.

Process Water Use
Media Surface Washing and Filter Backwashing

Optimization of the filter backwash procedure may lead to a reduction in water use at some plants. The required backwashing rates and percent of filter bed expansion are determined by the type of filter media, grain size and its uniformity coefficients (Murphy, 2010). Adequate expansion of the filter bed means that the entire filter media is fluidized and all individual particles are suspended. If the bed is not properly expanded and fluidized, trapped floc can remain in the filter media and result in more frequent backwash cycles. There are a number of devices that can be used to measure filter bed expansion. Most of these involve inserting a temporary measuring device into the filter during backwashing, e.g. disks, floats, graduated tubes. Alternatively, an instrument can be used that employs acoustic technology to determine the height of the filter bed both during the filtration process and during backwashing (Murphy, 2010).

The installation of an on-line turbidimeter on the filter backwash effluent water can be used to reduce the volume of backwash water. The backwash pump can be shut off as soon as the backwash effluent turbidity reaches a certain value.

Pump Seal Water

The amount of potable water used for pump sealing should be checked against the pump manufacturers' information to make sure the minimum amount required is used.

Conveying Scum and FOG

The amount of potable water used for conveying scum and FOG can be optimized by installing high-pressure, low volume nozzles, where applicable. The flow used can also be adjusted to meet the minimum required for effective conveyance.

High-Pressure Spray Washing

The amount of potable water used for spray washing can be optimized by installing high-pressure, low volume nozzles on spray washers, where applicable. The flow used can also be adjusted to meet the minimum required to clean effectively.

CIP Water

There are three types of CIP systems: single-use, reuse and multi-use systems, which are shown schematically in Figure 3.1. Single-use systems discharge CIP water after the cleaning cycle is complete. A reuse system uses the same cleaning solution for a large number of cleaning operations. A multi-use system returns the cleaning solution after use to a collection tank, from where it is subsequently reused for prewashes and rinses before being discharged. It is also possible to combine a reuse and multi-use system in one CIP plant. Reuse and multi-use systems are more water efficient than a single-use system and use fewer chemicals (Palmowski et al., 2005). However, they typically have higher capital costs than single-use systems and may not be appropriate for all potential process applications (e.g. sludge management processes), due to the potential for high solids in CIP water after first use. It is not considered to be economically feasible to pretreat first-use water to remove solids, if this is required. However, it is recommended that a reuse or multi-use system be considered for sewage treatment works that are installing a CIP system.

Figure 3.1 Schematic of Types of CIP Systems

This schematic depicts the three types of clean-in-place systems, each having a clean water input and a dirty water output stream. The first diagram represents a Single-Use Clean-in-place System and simply shows cleaning water input and dirty water discharge. The second diagram shows a Reuse Clean-in-place System where a portion of the dirty water stream is recycled back to the Clean-in-place system for reuse, after blending with cleaning water input, and with the remaining portion of the dirty water discharged. The third diagram shows a Multi-Use Clean-in-place System where the effluent from the Clean-in-place system is introduced into a collection tank and subsequently used as prewash or rinse water before being discharged as dirty water.

Site Services Water Use
Boiler Feed

There are a number of potential changes to the operation of a boiler that can minimize the amount of water used for boiler feed, which are discussed below. All of these changes will also result in a reduction in energy use for boiler operation.

  • Install a condensate return system if it is absent. As more condensate is returned, less feed water is required. Return of high purity condensate reduces chemical use for boiler feed water and reduces energy losses due to boiler blowdown.
  • Repair any steam distribution and condensate return system leaks from existing condensate return systems.
  • Minimize the boiler blowdown rate through improved feed water treatment to remove carbonates and dissolved solids that can build up in the boiler.

It is recommended that a boiler assessment be carried out by a qualified professional to determine site-specific boiler operation and feed water requirements before any changes are made. This assessment should identify what measures are cost-effective for the facility.

Landscape Irrigation

More than 50 percent of the water applied to landscaped areas can be lost due to evaporation or to run-off because of over watering (Natural Resources Canada, 2010). As a general rule, most lawn and garden areas require no more than 2 to 3 cm of water per week. Watering early in the morning (after any dew has dried) will help to reduce losses due to evaporation. This can be arranged using timers. However, overwatering can still occur using timers and, therefore, linking watering to soil moisture levels will be more effective at reducing water use for landscape irrigation than using timers.

The type of sprinkler used can affect evaporation rates. Sprinklers that lay water down in a flat pattern are better than oscillating sprinklers which lose as much as 50 percent of what they disperse through evaporation. The use of irrigation controls that respond to soil moisture conditions at the site can prevent over-watering of landscape areas (Natural Resources Canada, 2010). Water use for landscape irrigation can be reduced further by the use of xeriscaping, i.e., drought-tolerant landscaping.

Rainwater harvesting, i.e., collecting and storing rainwater from roofs and/or the ground (using underground storage tanks), can be used as an alternative to potable water for irrigation. This type of system will require an investment in the collection and storage of rainwater. The use of rainwater harvesting at a sewage treatment works shows leadership in water conservation by the municipality. The City of Guelph (City of Guelph, 2009a) and American Rainwater Catchment System Association (ARCSA, 2009) are a good source of further information on rainwater harvesting.

Equipment and Site Cleaning

The flow rate of water from a hose is dependent on the diameter of the hose and pressure of the water system and significant water savings can result from using smaller diameter hoses. For example a 16 mm hose can discharge approximately 70 L/min at 414 kPa (60 psi), and a 19 mm hose approximately 120 L/min at 414 kPa (60 psi). As an alternative to reducing the diameter of the hose used for equipment and site cleaning, flow restrictors can be easily and inexpensively fitted onto a hose to reduce water flow.

All hoses should be equipped with spring-loaded shutoff nozzles to ensure hoses are not left running when not in use. In addition, plant staff should be instructed to use hoses sparingly and to use dry cleaning practices as an alternative, where possible.

Domestic Water Use

Water conservation devices or fixtures can be used to reduce water consumption by washrooms and kitchen areas at sewage treatment works. These measures would be the same as for a residence, except that commercial-grade fixtures that meet or exceed current building code requirements should be used.

There are a number of measures that can be used to optimize domestic water use, which include the following:

  • fixing leaks
  • installing low-flow showers
  • fitting taps with aerators
  • installing dual-flush or low-flow toilets and urinals.
Fixing Leaks

A tap that leaks one drip per second will waste a couple of thousand litres of water per year. A toilet that continues to run after flushing, if the leak is large enough, can waste up to 200,000 litres of water per year. Therefore, washroom and kitchen taps should be checked for leaks and fixed if leaks are detected. This measure is a simple and inexpensive method of reducing domestic water consumption at sewage treatment works and elsewhere.

Low-Flow Showers

Low-flow showerheads use up to 70 percent less water than standard showerheads (Natural Resources Canada, 2010). Showerhead flow restrictors, which may affect delivered water pressure, can reduce water use by approximately 60 percent. Either type of head will also reduce energy used to heat water for showers. The flows from water-efficient showerheads range from as low as 3.8 L/min up to 9.5 L/min, with the most common around 5 L/min. These compare favourably to older units at around 14 L/min. This measure is a simple and inexpensive method of reducing domestic water consumption at sewage treatment works and elsewhere.

Tap Aerators

Aerators restrict the flow of water from a tap without reducing water pressure (Natural Resources Canada, 2010). Installation of tap aerators is very easy and inexpensive and can reduce the amount of water used by more than 50 percent. Aerators come in a number of sizes and varying flow rates, for example, 5 L/min is typical for washroom sinks and 7.6 L/min for a kitchen with an aerator installed.

Low-Flow Toilets and Urinals

Standard, older model toilets can use up to 18 litres of water per flush. The amount of water used for toilet flushing can be reduced by either installing a water-saver flush kit inside the toilet tank (such as a toilet dam) or replacing it with a low-volume 6 litre model, or better (Natural Resources Canada, 2010). Installing a water-saver flush kit is simple and inexpensive. If this is not possible due to the type of toilet at the facility, consideration should be given to replacing toilets with an ultra-low-flow model. The Unified North American requirement for Toilet Fixtures (Alliance for Water Efficiency, 2009) and the U.S. EPA’s WaterSense group (U.S. EPA, 2010a) provide information on water efficient toilets that have been assessed for performance.

Table 3.1 Summary of Water Efficiency Measures for Sewage Works

Process Water Use
Water Efficiency Measure Relative Payback Period1
Media surface washing and filter backwashing Medium
Optimizing water used for pump seal Short
Optimizing water use in high-pressure spray washers Medium
Installing reuse or multi-use CIP system for new sewage works Long
Site Services Water Use
Water Efficiency Measure Relative Payback Period1
Optimizing the boiler feed system Medium
Optimizing the landscape irrigation system Medium
Minimizing water use by hoses for equipment and site cleaning Short
Domestic Water Use
Water Efficiency Measure Relative Payback Period1
Fixing leaks Short
Installing low-flow showers Short
Installing tap aerators Short
Installing low-flow toilets and urinals Medium

1 "Payback" is based on estimated capital and O&M costs of implementing the measure and the estimated cost savings that will result:

Short
no or very low cost to implement the measure (i.e., immediate payback).
Medium
estimated payback of less than 5 years.
Long
estimated payback of 5 years or more.

3.1.4 Plant Effluent Use Applications and their Technical and Cost Impacts

Technically, final effluent from a sewage treatment works can be used on-site for all areas where potable water is used for process water and site services. However, the treatment required to achieve an acceptable water quality for reuse is cost-prohibitive for a number of reuse applications. The additional treatment required for water reuse is dependent on effluent characteristics and the reuse application and associated quality requirements.

The areas described below are those considered to be potentially feasible for effluent use application at existing sewage treatment works. For new sewage treatment works, consideration should be given to installing an effluent use system so that plant effluent is used in as many areas as possible, including toilet and urinal flushing, pump seal water, for equipment and plant cleaning, and for landscape irrigation at the plant site.

Consideration needs to be given to potential exposure of plant staff to bioaerosols that can contain pathogenic microorganisms and endotoxins (i.e., toxins released during bacterial cell lysis) when considering using effluent at the sewage treatment works. The risk of exposure to plant staff to bioaerosols should be assessed prior to any water reuse scheme and the need to minimize exposure through the use of equipment or processes that reduce the contact workers have to the reuse water, and also the need for supplemental disinfection of the reuse water. Plants that have an effluent disinfection process may need to install a secondary ultraviolet (UV) or chlorine disinfection system to ensure that the coliform count is acceptable for the intended use of the effluent. Plants using chlorination to disinfect effluent could consider increasing the chlorine contact time (if it is an option) or adding more chlorine to plant effluent if a lower effluent coliform count is required for water reuse. Plants that have a seasonal effluent disinfection system may need to disinfect continuously when effluent is being used. Labelling and signage are important when using plant effluent to ensure it is clear that it is for non-potable use. Also, there is the potential for deposition of scale and other particulate matter in the distribution system, and as a result, the pipe work will need to be cleaned periodically and the design of the system should allow for this.

The information provided in this section is generic. Further information on effluent quality requirements and treatment technologies for the use of STP effluent is provided in Sections 4.4 and 4.5, respectively.

Process Water Use
Media Surface Washing and Filter Backwashing

Plant effluent can be used for filter media surface washing and backwashing of filters at sewage treatment works if it has a relatively low total suspended solids (TSS) concentration (typically less than 10 mg/L). However, consideration must be given to the impact of the backwash water temperature on backwash requirements. Cold water has a higher viscosity, and therefore can fluidize or expand the filter bed better than warmer water. At the higher water temperature of plant effluent, a higher backwash flow rate may be required to achieve the optimum bed expansion.

Chemical Make-Up Water

Plant effluent can be used for chemical make-up water with little or no additional treatment for many sewage treatment works. The higher temperature of plant effluent compared with potable and surface water sources can be beneficial for chemical make-up due to the increase in chemical efficiency at higher temperatures.

Facilities that have secondary treatment should produce effluent of adequate quality for this purpose. Additional chlorination may be required where plant staff are exposed to make-up water systems to reduce the fecal coliform count to a non- detectable level, based on requirements in some U.S. States for unrestricted urban reuse (Metcalf & Eddy and AECOM, 2007; U.S. EPA, 2004). Information on this is provided in Section 4.4.

For sewage treatment works that do not currently chlorinate plant effluent (i.e., have no disinfection or use UV irradiation) but require the reuse water to have residual chlorine, this option will need to be given special consideration.

When using plant effluent for chemical make-up water, it is recommended that the efficacy of this process be tested to ensure there are no issues with chemical interactions, as a result of specific ions in the effluent, with the chemical or polymer.

Foam Control

Effluent from a sewage treatment works can be used for foam control spraying. Health and safety considerations due to the potential exposure of plant staff to pathogens and endotoxins should be considered and assessed for this option. In addition to an assessment of the risk of exposure of plant workers to bioaerosols, an assessment should be carried out to determine the requirements for supplemental disinfection of reuse water for this option to minimize the risk of human exposure to effluent microbes.

Conveying Scum and FOG

Effluent from a sewage treatment works can be used for conveying scum and FOG, and as effluent is being sprayed into primary or secondary sedimentation tanks, the potential for human exposure to pathogens from plant effluent is no greater than from the tanks themselves. Therefore, additional disinfection to reduce the quantity of effluent microbes is not considered to be necessary for this reuse option.

High-Pressure Spray Washing

Plant effluent can be used with high-pressure spray washers if hose connections are available within the vicinity of the equipment that the washer is used to clean, such as screens, disk and drum filters, drum thickeners and belt press filter cloth. Health and safety considerations due to the potential exposure of plant staff to pathogens and endotoxins should be considered and assessed for this option. A study by Visser et al. (2006) indicated that exposure to endotoxins when using plant effluent for spray washing/washing down is influenced by the area being cleaned (e.g. exposure was higher for cleaning a filter press compared with sludge buffer tank), and by the type of nozzle being used (i.e., using a fully-open fire hose nozzle increased the risk of exposure 3-fold compared with partially opened). In addition to an assessment of the risk of exposure of plant workers to bioaerosols, an assessment should be carried out to determine the requirements for supplemental disinfection of reuse water for this option to minimize the risk of human exposure to effluent microbes.

Cooling/Heating Water

Plant effluent can be used in place of fresh water for engine cooling in cogeneration systems, and for the heat exchanger medium for heat recovery. An analysis of the cooling capability of plant effluent should be carried out to make sure it is sufficient for the engines as effluent temperature will typically be higher than for potable water.

Odour Control

Plant effluent use in odour control systems is a relatively common practice and is recommended for sewage treatment plants that use chemical scrubbers or biological filters. The amount of water required for odour control is dependent on the type of odour control system and size of the process. However, it may not be suitable for facilities that have moderate-to-high TSS levels in their effluent (e.g. greater than 10 mg/L). Where required, an assessment of the economic feasibility of installing a filter to pretreat effluent prior to use in odour control should be carried out.

CIP

Potable water can be replaced with plant effluent as the source of CIP water, which may be a more economical option for sewage treatment works than a reuse CIP water system. This option will require effluent to be pumped to the CIP area. The amount of water saved for this option is dependent on the size of the CIP system and frequency of CIP cleaning. As CIP systems are typically enclosed, the risk of exposure to effluent microbes from reuse water is lower when compared with manual cleaning of equipment or processes.

Site Services Water Use
Landscape Irrigation

The use of STP effluent for on-site landscape irrigation may require additional disinfection of the effluent to achieve levels suitable for potential human exposure to the irrigation water. Based on regulations for irrigation water in certain U.S. States (Metcalf & Eddy and AECOM, 2007; U.S. EPA, 2004), the total coliform level should be between 2 and 23 most probable number (MPN) / 100 mL for this option. If plant effluent coliform levels are greater than this, additional chlorine dosage may be required at facilities using chlorine for disinfection if additional contact time is not available. As the concentration of residual chlorine in irrigation water may adversely affect soil and plants, dechlorination may be required after chlorination. Alternatively, UV disinfection could be used for supplemental disinfection.

If required, for plants that use UV disinfection and/or practice seasonal disinfection of effluent, an assessment of the cost-effectiveness of installing a supplemental UV or chlorination/dechlorination system for effluent use in landscape irrigation should be carried out.

Equipment and Site Cleaning

There is the potential for site workers to be exposed to pathogenic microorganisms and endotoxins during equipment and site cleaning. Disinfection can be used to minimize the risk of exposure to pathogenic microorganisms.

A study carried out by Visser et al. (2006) identified that exposure to endotoxins during cleaning at a sewage treatment works is higher when using plant effluent compared with potable water and surface water sources. The study indicated that the type of nozzle used for spray cleaning can affect the exposure rate, whereby "fire hose" nozzles resulted in the greatest exposure levels.

3.1.5 Case Study Examples

City of Peterborough, Ontario

The City of Peterborough started using plant effluent to clean out tanks in a small section of their sewage treatment plant 25 years ago. The effluent use system was upgraded three years ago to allow plant-wide access to effluent for water reuse. Secondary effluent is disinfected using UV irradiation. Two submersible pumps convey disinfected effluent for water reuse from a collection chamber to a pressure vessel, from where three booster pumps transport effluent around the plant. Currently, plant effluent is used for cleaning all tanks and as CIP water for centrifuge cleaning. The City estimates an annual saving of $30,000 in potable water cost with their reuse scheme. There are plans to use plant effluent for polymer make-up water during wintertime as City potable water is colder at that time of year, which can lead to inefficiencies in the polymer make-up process.

City of London, Ontario

The Oxford Pollution Control Plant in the City of London uses effluent from a membrane bioreactor (MBR) process for landscape irrigation at the plant. Effluent from the MBR is disinfected using UV irradiation. The portion of disinfected effluent that is to be used is then pumped through a separate dedicated UV system for supplemental disinfection before it is used for irrigation. Landscape areas around the plant are irrigated using effluent for one hour every night during the summer and fall.

Region of Waterloo, Ontario

Effluent from the Kitchener Wastewater Treatment Plant (WWTP) will be used by several areas in the sludge dewatering building, which is located approximately 3 kilometres from the WWTP. Approximately 350 m3/d of effluent will be used for centrifuge flushing and cooling, sludge dilution water, conveyor flushing, anti-foam spraying and in the biofilter for odour control. The system, which will be operational in July 2011, consists of a forcemain from the WWTP to a storage tank and pumps at the dewatering site. The estimated payback for using effluent in place of potable water at the dewatering building is 4 years, based on an approximate capital cost of $1.3 million and annual O&M cost of approximately $6,000.

3.2 Upstream Flow and Load Management

3.2.1 Benefits of Upstream Flow and Load Management

Reducing the volume and strength of influent to a sewage treatment works can have significant benefits. The need to increase the capacity of a collection system and/or sewage treatment works may be avoided if the influent flow can be decreased by the use of flow reduction programs. In addition, the frequency and severity of overflow events may be reduced through lowering the influent flow to a sewage works.

Energy use can be lowered with a reduction in the influent flow to a sewage works, as less volume is required to be pumped through the collection and treatment processes. There is also the potential for energy use reduction at a sewage treatment works with a reduction in the influent strength as a result of reducing the organic loading from ICI facilities. Lower strength influent through at-source reductions in contaminant loads, in terms of solids and oxidizable organic and inorganic material, will typically result in less energy requirement for biological treatment processes and solids management, as well as less chemical use for phosphorus removal and chlorination, where used.

3.2.2 Sources of Influent

Municipal wastewater is comprised of discharges from residences and ICI facilities. Typically, domestic wastewater from residences makes up the majority of the flow discharged to sewage works. While discharges from ICI facilities may represent a lower percentage of the flow than residential wastewater, it can, in some cases, represent a significant contaminant mass load to a sewage treatment works, especially for certain industry types. In some major urban areas, ICI facilities can also represent a significant portion of the wastewater flow. A reduction in the flow and/or mass loading from ICI sources to a municipal sewage works can result in a decrease in energy use at the sewage works.

The focus of this section with regard to source control is on ICI dischargers to municipal sewers. Typically, the per capita flow from commercial and institutional facilities is lower than from residences, although water use is primarily domestic for these facilities and therefore the strength and composition is similar.

For industrial facilities, wastewater flow, composition and strength can vary significantly. This variation not only occurs between different industry types, but also amongst similar industries. This is due to differences in housekeeping and process water reuse practices, as well as variations in the production processes. Most municipalities have a sewer use bylaw in place to ensure discharges to sewers from industrial facilities are treatable at the receiving municipal sewage treatment works.

3.2.3 General Approaches to Source Control

Source control can be used to reduce the flow and/or strength of wastewater discharged to sewer. For the purpose of this Guidance Manual, source control refers to ICI dischargers only.

There are four main approaches to source control for ICI dischargers to municipal sewers, namely:

  • incentive programs
  • water use assessments and buy-back programs
  • education programs
  • sewer use bylaws.

Incentive programs can be used to reduce water consumption, which in turn results in lower flows to the sewer and therefore to the sewage treatment works. In addition to providing incentives, education of ICI facility owners and managers can be used to reduce water consumption and/or wastewater strength at source. Municipalities can also provide assistance or funding for a water use assessment (or audit) at ICI facilities. After the assessment is completed, a buy-back program can be used to pay the ICI facility owner for specific water use reductions that lead to a lower effluent discharge volume to the sewer.

As an alternative to the use of such programs, which rely on a voluntary approach to source control, sewer use bylaws can be used to specify source control measures required at an ICI facility. These measures would provide an opportunity for load management in terms of lower mass loads of treatable materials, such as organic matter and nutrients.

Source Control Measures and Considerations

Measures that can be taken by an ICI facility to reduce wastewater flows and loads can be determined by conducting a water use assessment and preparing a pollution prevention plan. Information is available on the MOE website on simple measures for reducing water consumption at ICI facilities (MOE, 1999) that can be used as a resource. Flow monitoring may be required at some facilities that use large volumes of water to identify the most cost-effective water conservation measures. This may be done using installed water meters, if available, or by conducting a water monitoring program.

It should be noted that a lower flow to the sewer by an ICI discharger through water conservation measures may result in increased concentrations of treatable and non- treatable parameters, which may exceed sewer use bylaw limits. In these cases, pollution prevention programs may be required to reduce mass loadings to levels below set limits, and/or the ICI owner can pay a sewer surcharge for treatable parameters that exceed sewer bylaw limits.

3.2.4 Source Control Programs

There are a number of potential programs that can be implemented to reduce the flow and load from ICI dischargers to a municipal sewage works. When implementing any such scheme, consideration should be given to any potential adverse impact of reducing the wastewater flow to a sewage works, such as odour and solids deposition problems in the collection system, and a potential increase in odour at the sewage treatment works. A reduction in load to a sewage treatment works may impact the capacity requirements of certain treatment processes (e.g. secondary aeration and solids management), which should be reviewed to determine if there is a potential for process and/or equipment sizing reductions as a result of lower loads. Further information on options to reduce energy use at sewage treatment works is provided in Section 5.

As the focus of this Guidance Manual is sewage works, outdoor water efficiency programs (such as for landscape irrigation) at ICI facilities are not considered.

Incentive Programs

Incentive programs provide an economic incentive for facilities to reduce water consumption through either a payback scheme for water saved or rebates for the installation of water saving devices or equipment. The following subsections describe options for incentive programs for reduction of domestic water use for ICI facilities and process water use by industry.

Domestic Water Use

Rebate programs can be applied to the replacement of older model toilets with ultra- low-flow models. This type of program can also be applied to low-flow shower heads. Further information on the use of these water-saving fixtures is provided in Section 3.1.3.

If a municipality applies a sewer charge on water bills, the investment in low-flow toilets or other water saving devices can translate into a reduction in both the water and sewer components of the water bill for ICI facilities.

A tiered rate for water use for ICI facilities can be a good incentive for facilities to conserve water. For example, the City of Toronto’s Industrial Water Rate Structure provides a lower water rate for eligible industrial facilities that use in excess of 6,000 m3/year (City of Toronto, 2010a). In order to be eligible for this lower rate, industrial facilities must prepare a comprehensive water conservation plan to be submitted to the City, and must submit regular progress reports.

Industrial Process Water Use

A report prepared by the MOE (2009) indicates that a feasible option for conserving water is for municipalities to offer a capacity buy-back rebate to industrial facilities that implement the water savings recommendations identified in an approved water audit (i.e., water use assessment). The report recommends the following areas should be considered:

  • once through cooling
  • cooling towers
  • water used for cleaning
  • process water
  • boilers and steam.

For once through cooling systems, where water is supplied from municipal utility sources, consideration should be given to reduce the need for once through cooling used for air conditioning, refrigeration, and medical systems. Water used for cooling can be reduced by reusing water from another process for cooling make-up, installing a chiller or cooling tower as an alternative, or replacing water-cooled equipment with air-cooled equipment. Water use by cooling towers can be reduced by minimizing blowdown (i.e., deliberate purges to prevent excessive buildup of total dissolved solids in the cooling water system).

Good housekeeping practices can be used to reduce water consumption for cleaning, including sweeping floors instead of hosing with water, vacuuming or dry sweeping material spills, and using squeegees and scrapers first to remove residuals from machines.

Process water use can be reduced through a number of measures, which are site- specific. Examples include using chiller water for cooling water make-up, and filtering and recycling washwater.

Measures that can be used to optimize water use by boilers and for steam include regularly checking steam traps and lines for leaks and repairing leaks as soon as possible, recovering and returning steam condensate, installing an automatic control system to turn off the unit when not in use during nights and/or weekends, and installing automatic blowdown control for boilers.

The MOE report (2009) recommends that rebates be considered for commercial laundry operators who purchase and install a water efficient clothes washer in their operations, and that the installation of low-flow pre-rinse spray valves is considered for commercial kitchens.

The use of a tiered water rate structure, as discussed above under "Domestic Water Use" is also a good option for encouraging industries to reduce process water.

There are a number of examples of municipalities currently operating incentive programs to encourage water conservation in industry, some of which are discussed in Section 3.2.5. These programs are designed to encourage the replacement or upgrade of equipment to reduce the amount of water used in industrial processes, which in turn decreases the wastewater flow to sewers.

Water Use Assessments and Buy-Back Programs

Providing assistance with water assessments, either through technical assistance or an economic incentive to hire a water assessment professional, is likely to be a cost- effective option for ICI facilities that are large water users, e.g. food processing plants. For smaller ICI facilities, providing educational information on water conservation may be sufficient.

A reduction in water use by an ICI facility as a result of implementing an approved water use assessment can be converted to a "buy-back" rebate to the facility. This involves providing a rebate based on water costs and sewer capacity.

Education Programs

Information on incentive programs needs to be disseminated to ICI facilities and this can be done either through materials mailed to facilities, holding seminars or workshops and through web-based access to information.

Information should be provided on general measures that facilities can implement themselves (e.g. fixing leaks, fitting with tap aerators), rebate programs that are available and contact details for more information, if required. For some areas, it may be necessary to have materials specific to water conservation measures for different industries or commercial types of facilities.

Sewer Use Bylaws

Municipal sewage is typically made up of wastewater from residential and ICI facilities. Wastewater from industry can contain significantly higher levels of suspended solids and organic matter than wastewater from residences and commercial and institutional facilities. As a result, many municipalities will charge an over strength fee to treat industrial discharges that exceed bylaw limits for specific treatable parameters.

To encourage a greater reduction in wastewater strength, a municipality could consider having a tiered rate for surchargeable parameters.

The Canadian Council of Ministers of the Environment (CCME) has published a model sewer use bylaw guidance document and template (Marbek, 2009) that municipalities can use to develop their own bylaws to regulate industrial discharges and implement source control measures.

Some municipalities have imposed pollution prevention planning requirements for industrial dischargers as part of a sewer use bylaw (e.g. the City of Toronto). There is also the option to implement a pretreatment program as an alternative to pollution prevention. In most cases, it is more effective to eliminate the source or treat contaminants of emerging concern at source than treating the diluted stream at the downstream municipal sewage treatment works.

3.2.5 Case Study Examples

The City of Toronto currently has a "Water Saver Program" for ICI facilities (City of Toronto, 2010b). This program provides a rebate to ICI facilities of 30 cents for every litre of water saved and also has rebates to help offset the cost of installing water saving hardware and equipment. Toronto’s WaterSaver website (City of Toronto, 2010c) provides a summary of the program and includes links to case study examples, as well as a phone number and email contact at the City for more information.

The Region of Peel has a "Water Smart Businesses" program (Region of Peel, 2010) that includes financial assistance for ICI facilities to conduct a water audit, rebates on installing low-flow toilets, a spray valve replacement program for food service businesses, and tips on minimizing water use for irrigation.

The Region of Waterloo has a range of programs aimed at water use reduction, including toilet rebates, a rain barrel program, information and seminars on xeriscaping (i.e., dry landscaping, including use of indigenous drought tolerant plants), and a bylaw that restricts lawn and garden watering during the summer for residences. The Water Efficiency Technology (WET) program is aimed at reducing water consumption by ICI facilities by providing funding and access to tools that can be used to determine water use and identify and implement efficiency measures. The Region provides free low-flow shower heads, faucet aerators and pre-rinse spray valves, free water use review by Regional staff and co-funding for a third-party in-depth audit. Capital funding is available to help with any upgrades made from the recommendations of the in-depth audit (Region of Waterloo, 2010).

The City of Guelph has a capacity buy back scheme and provides engineering services to ICI facilities for conducting water audits (City of Guelph, 2009b). It also offers toilet and washing machine rebates for ICI facilities.

In British Columbia, the Capital Regional District (CRD, 2010a) is operating a rebate program to replace once-through cooling systems that are designed to encourage the replacement of once-through cooling condensers and ice makers with more water-efficient options.

The Metropolitan Water District of Southern California "Save a Buck" program (MWDSC, 2010) promotes water-saving devices for ICI facilities by providing rebates for specific devices, such as replacing hoses for cleaning with high pressure washers and replacing water-cooled ice making machines with air cooled versions. The program also allows ICI facilities to obtain rebates for installing water-saving washing machines, toilets and urinals.

The City of Austin, Texas produces an ICI water conservation newsletter and offers ICI workshops on how these facilities can conserve water (City of Austin, 2010).