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Threats to Sources of Drinking Water and Aquatic Ecosystem Health in Canada

9. Municipal Wastewater Effluents

Municipal wastewater effluents are composed of suspended solids, human waste, debris and chemicals from residential, industrial and commercial sources.Mark Servos,1 Patricia Chambers,1 Ron Macdonald2 and Glen Van Der Kraak3

1Environment Canada, National Water Research Institute, Burlington, ON
2Greater Vancouver Regional District, Vancouver, BC
3University of Guelph, Department of Zoology, Guelph, ON


Current Status

Municipal wastewater is a complex mixture of human waste, suspended solids, debris and a variety of chemicals derived from residential, commercial, and industrial sources. It represents the largest source of effluent discharge to Canadian waters, totaling approximately 4300 million cubic metres in 1991 (Statistics Canada 1994). The volume of the wastes, the pollutants they contain, and potential for impacts to water quality make municipal wastewater a concern in Canada.

Historically, sewage treatment was undertaken as a public health measure to prevent the transmission of waterborne diseases. Since the early 1900s, the need to adopt sewage treatment has been recognized for the prevention of negative environmental impacts where municipal wastewater treatment plants (MWTP) effluents are discharged. The identification of the role of phosphorus (P) in causing eutrophication of freshwaters in the 1960s spurred the adoption of P-removal technology at many MWTPs and this was followed in the 1960s to 1980s by adoption of disinfection of municipal effluents to reduce the discharge of pathogens to receiving waters. Traditional wastewater treatment systems are designed to remove settleable solids and floating scums (primary treatment), fine suspended solids and oxygen-consuming material (secondary treatment, lagoons), and nitrogen (N) and/or phosphorus (tertiary treatment). Along with these constituents, MWTPs remove other contaminants including some heavy metals and organic chemicals. These materials come from either human waste or the residential, commercial and industrial products discharged to sewer collection networks. The materials removed from wastewater end up in the organic biosolids or chemical sludges. These may be disposed of in landfills or by incineration. Alternatively high quality biosolids are frequently applied to land as an organic enrichment material.

In 1996, 74% of the Canadian population was serviced by municipal sewers. The remaining 26%, mostly in rural areas, relied on individual septic tanks or private treatment systems. Within the municipal population on sewers, 94% were served by some level of sewage treatment while the remaining 6% discharged raw sewage directly to Canadian waters (Environment Canada 1996).


Population growth and continued urbanization will continue to increase the quantity of wastewater discharged to MWTPs. Public expectations will increase demand on municipalities to provide greater levels of treatment for wastes, on the premise that improved receiving water quality will benefit human and environmental health. These expectations for wastewater treatment are likely to evolve faster than municipalities are able to respond through infrastructure programs. Additionally, MWTPs were designed to remove solids, oxygen-demanding material and, in some cases N or P but may not adequately remove all constituents identified in wastewater. New technologies are becoming available, but may be very expensive.

Many municipalities face problems related to aging infrastructure. Sewer collection networks deteriorate allowing non-wastewater inflow into sewers, thereby adding to the volume of water that must be treated. MWTPs may not have been expanded to keep up with population growth and may be hydraulically overloaded particularly during wet weather. Significant expenditure may be required simply to maintain existing levels of treatment, let alone meet higher levels of treatment. For municipalities to justify substantial investments in infrastructure renewal or upgrades to higher treatment levels requires an understanding of the environmental implication of these discharges on water quality health and the benefit to be gained by the investment.

Emerging Issues

Some negative effects of municipal wastes have been documented for over a century. Some of these issues remain relevant to this day. New issues continue to become apparent as the science of understanding the environment advances, and the tools and technology of environmental monitoring develop.


MWTP effluent, if not disinfected, contains bacteria and pathogens. Many beaches in urban areas are frequently closed for several days during and immediately after rainfall events because of microbial contamination caused by MWTP overflows, stormwater and CSO discharges. Disinfection of MWTP effluent reduces the risk of microbial pollution although undisinfected discharges still occur. The public health risks of microbial pollution from MWTP effluent to Canadian recreational waters is not well understood. Shellfish harvesting may be restricted due to bacterial contamination from municipal effluent, stormwater, agricultural waste, or leakage from septic systems. In 1992, approximately 3018 km2 were closed to harvesting due to bacterial contamination along the three Canadian coasts (Chambers et al. 1997). Isolated incidences of microbial contamination of drinking source water in Canada from CSOs, stormwater and inadequately treated MWTP effluent have been reported. Because municipalities treat and disinfect water used for drinking, outbreaks of waterborne disease are rare in Canada (Health Canada 1995a,b).

Oxygen Depletion

Discharge of MWTP effluent with high BOD loads can cause reductions in dissolved oxygen (DO) in the receiving water. DO threats to fish and other organisms often occur during summer months. However, in colder climates where rivers and lakes are ice-covered for many months, DO depletion can occur due to ice cover preventing re-aeration. Acute effects of low DO are normally avoided in Canada as a result of municipal licensing conditions, though little information is available on the effects of chronic DO stress on aquatic organisms, particularly when other stressors are also present.

Toxicity of Effluents

The toxicity of municipal effluents is dependent on a variety of factors, including the size and characteristics of the sewershed, the type and efficiency of treatment and disinfection processes and the physical, chemical and biological characteristics of the receiving waters. In many cases the acute toxicity of MWTP effluent is due to un-ionized ammonia or, in the case of chlorinated effluents, to total residual chlorine. Other contaminants including cyanide, sulfides, phenols, surfactants and heavy metals, such as copper, zinc and chromium, also contribute to acute or chronic toxicity (Chambers et al. 1997). Many factors can moderate the toxicity in the effluent or receiving environment including pH, hardness, dissolved organic carbon and temperature. Despite considerable investment in treatment systems, acute and chronic toxicity remains a concern in many sites receiving municipal effluents.

Many chemicals detected in municipal effluents are hydrophobic and may tend to adsorb to particles in the effluent or sediments in the receiving environment, rather than remain in the water phase. The distribution of these chemicals may therefore differ considerably from more soluble compounds which will tend to move with the effluent plume. Hydrophobic chemicals may also tend to bioaccumulate in organisms and move through food webs. The distribution and fate of contaminants in the environment is extremely complex and dependent on the physical and chemical characteristics of the chemicals as well as the physical, chemical and biological characteristics of the receiving environment.

Under the Canadian Environmental Protection Act a number of Priority Substances were identified (PSL-1,2) and risk assessments conducted to evaluate their potential to cause harm in the Canadian environment. Municipal effluents have been identified as major sources of many of these chemicals, some of which have been declared "CEPA toxic." Chlorinated wastewater effluents on the PSL-1 were declared CEPA toxic. Four substances on PSL-2 (ammonia, chloramines, nonylphenol and its ethoxylates and textile mill effluents) are associated with municipal effluents and have been proposed to be declared CEPA toxic. Considerable information on their distribution, treatability, fate and effects are needed in order to develop and implement appropriate risk management strategies.

Eutrophication of Receiving Waters

MWTP effluents contribute nutrients, primarily N and P, to receiving water bodies and thus may cause eutrophication. Nutrients can accrue in the bottom sediments and be released into the water at a later time, and thus have a long-lasting impact on water quality. Nutrient addition to aquatic ecosystems can increase growth of primary producers (algae and rooted aquatic plants) to levels that result in impairment of the ecosystem (e.g., changes in food web structure, changes in habitat, loss of species, infestations of nuisance species). These ecological changes can affect human use of aquatic resources (including water-based recreational activities and fisheries) and impair water quality for municipal, industrial and agricultural users. With the recognition of the role nutrients play in eutrophication, many MWTPs discharging to inland waters have upgraded to reduce P loading. Nevertheless, eutrophication continues to be a pervasive problem due to slow response time of ecological systems to reductions in loadings and uncertainties in establishing ecologically appropriate loading limits.


MWTPs generate sludge as a result of decomposition and settling of wastewater as it undergoes treatment (Warman 1997). Biosolids are the organic portion of the sewage sludge that has been stabilized through digestion to meet suitable criteria for application to land (Webber and Bates 1997). Biosolids are rich in inorganic and organic materials and plant nutrients and are therefore a desirable additive to agricultural land. Most provinces have guidelines for the management of land application of biosolids designed to match biosolid nutrient content with the nutrient demands of the crop, while limiting accumulation of heavy metals and potentially toxic constituents. MWTPs generate significant quantities of sewage sludge; the City of Toronto sewage sludge production for 1999 was about 70,000 tonnes dry weight. In Canada, sewage sludge is incinerated, landfilled, or applied to land. Land application of biosolids is expected to increase in the future.

Intermittent Discharges

Discharges from stormwater and combined sewer systems are intermittent in nature, occurring during rain events, and often for brief periods of time. Similarly, wastewater treatment systems, while releasing a continuous discharge of treated effluent, during wet weather may release untreated wastewater (MWTP bypasses) or poorly treated wastewater (reduction of treatment efficiency). Limited information exists to evaluate the effects of intermittent discharges of untreated effluents on water quality, and the ecological significance of these discharges. There is laboratory evidence that short-term exposure to some contaminants may have ecological relevance. Translating this understanding to in situ conditions needs development.

By-products of Treatment

The physical and chemical processes of wastewater treatment may transform wastewater constituents. For example: (i) secondary treatment with activated sludge processes may increase ammonia concentrations in final effluent by converting organic material into ammonia nitrogen; (ii) nitrification to reduce ammonia levels will result in increased nitrate and nitrite levels in the effluent; (iii) degradation of certain components may result in different forms which are not necessarily less toxic (nonylphenol poly ethoxylates degrade to 4-nonphenol, a more toxic material); and (iv) disinfection of effluents with chlorine which results in residual chlorine which is toxic to fish.

Endocrine Disrupting Substances

Municipal effluents are a source of chemicals that may alter endocrine function, thereby adversely affecting reproduction or development in animals (Jobling and Sumpter 1993; Ternes et al. 1999). Natural and synthetic hormones and certain industrial chemicals capable of estrogenic effects have been identified in sewage effluents. These chemicals have been causally linked to changes in reproductive endocrine function in laboratory tests on fish. The extent to which these will have adverse effects on aquatic species or humans is not fully understood. Evidence suggests that these effects may occur even at low concentrations and/or from transient exposure.

Pharmaceuticals and Personal Care Products

Pharmaceuticals and personal care products (such as antibiotics, blood lipid regulators, analgesics, anti-inflammatory drugs, and beta-blockers, fragrances [musks], skin care products, disinfectants and antiseptics) have been detected in municipal effluents and associated surface waters (Daughton and Ternes 1999). Traditionally, drugs were not viewed as environmental pollutants but their potential to cause a variety of physiological responses raises concerns for effects on organisms in the aquatic environment. The array of pharmaceuticals in use will continue to diversify and grow with an aging population and rapid developments in the pharmaceutical industry. We have limited knowledge of the environmental transformation, fate or effects of these compounds. The implications of exposure to these complex mixtures remains unknown.

Pollution Prevention and Treatment Processes

Many industry sectors have embraced pollution prevention to address traditional "end-of-pipe" discharge issues. Understanding the complete picture of a plant or system operation allows innovative ideas for cost cutting, waste reduction, and energy and resource efficiencies to be developed. Increasing demand on water will result in a variety of water conservation measures which will have implications for wastewater management. These efforts are made more difficult for MWTPs due to the complex nature of municipal wastewater. Large municipalities are able to dramatically reduce discharges of undesirable materials through source control, industry education, and municipal infrastructure modifications (e.g., inflow/infiltration control). These can achieve substantial benefits in terms of reduced discharge loading and MWTP bypassing without upgrading treatment facilities. Smaller municipalities may have limited resources that they can devote to these efforts. Facilitation and information sharing of these opportunities is key to reducing wastewater contaminants at source, rather than at the treatment plant.

Ecological Risk Assessment

MWTP effluents have traditionally been regulated based on "specified level of technology" or "end-of-pipe" effluent quality criteria. This form of permitting neither guarantees environmental protection nor assures compliance with all environmental regulation. Ecological Risk Assessment (ERA) is an approach to managing municipal liquid wastes which addresses the site-specific constraints of different receiving environments. However, ERA frequently requires extensive environmental information and interpretation skills not routinely possessed by municipal waste collection and treatment operators. Guidance and development of environmental tools and a regulatory framework for municipalities to conduct ecological risk assessment is needed. Development of these tools and framework could draw from the combined experiences of other industry sectors. This approach would have to ensure that recommendations are based on unambiguous science and provide regulatory clarity for municipalities.

Knowledge and Program Needs

Many of the existing and emerging issues require further development on a policy and regulatory level. However, substantial uncertainties on the sources and treatment and environmental fate and effects still exist and require knowledge and understanding of:

Sources and Treatment

  • Improved strategies for control of un-ionized ammonia and other acutely toxic chemicals in sewer systems and MWTPs.
  • Sources and understanding of the fate of priority and toxic substances in treatment systems.
  • Sources, distribution and fate of environmental estrogens, pharmaceuticals and personal care products in municipal treatment systems.
  • Pollution prevention strategies as a complimentary approach to wastewater treatment.
  • Waste treatment and disposal quality criteria, objectives and standards based on the environmental and assimilative capacity of the receiving water.

Environmental Fate and Effects

  • Sub-lethal effects of dissolved oxygen depletion on aquatic organisms and its contribution to cumulative stress.
  • The role of municipal waste effluents and septic system discharges in causing bacterial contamination of shellfish beds and recreational waters.
  • Sources, fate and effects of toxic priority substances (e.g., PSL-1,2) in receiving environments.
  • The potential impact on groundwater and surface waters of agricultural application of biosolids.
  • The role of MWTPs in causing eutrophication and the occurrence of nuisance and toxic algal blooms.
  • The ecosystem consequences of by-products of treatment such as residual chlorine, ammonia and metabolites or organic contaminants.
  • The relevance of regulatory bioassays to ecosystem integrity.
  • The distribution, fate and effects of endocrine disrupting substances on the growth, reproduction and development of aquatic biota in the environment exposed to municipal effluents.
  • The potential interactive effects of low-level exposure to complex mixtures of biologically active compounds.
  • The implications of potential transfer of antibiotic resistance to the environment.
  • Acute and chronic toxicity, bioaccumulation, and biomagnification of contaminants in intermittent discharges.
  • The current and long-term trends in water quality associated with MWTP effluents.
  • Indicators of ecological effects to assess the potential impacts of municipal wastewater effluents.

Treatment facilities will continue to play an important role in managing municipal wastewater. Continued research is required to ensure that new knowledge and technologies are incorporated into municipal wastewater management and environmental assessments. Beyond end-of-pipe controls, other solutions such as water conservation, infrastructure renewal, or pollution prevention, need to be encouraged. The following actions are recommended to ensure the protection of water quality from the discharge of municipal wastewater effluents into the Canadian environment.

  • Encourage infrastructure planning, including technological advances, to ensure that improved treatment and environmental protection measures are not diminished by development or population growth.
  • Foster partnering opportunities with federal, provincial and municipal governments for monitoring programs and ecological impact assessments.
  • Incorporate municipal wastewater planning with integrated watershed management to account for the cumulative effects of numerous environmental stressors.
  • Develop tools and indicators for assessment of environmental impacts of MWTP effluent and a framework for ecological risk assessment.
  • Develop and adopt pollution prevention approaches to municipal wastewater planning to minimize influent loadings of undesirable substances.


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  • Daughton, C. and T. Ternes. 1999. Pharmaceuticals and personal care products in the environment: agents of subtle change? Environ. Health Perspect. 107: 907-938.
  • Environment Canada. 1996. Municipal water use database. Ottawa, ON.
  • Health Canada. 1995a. Notifiable disease summary. Canada Communicable Disease Report 21-18: 166.
  • Health Canada. 1995b. Foodborne and waterborne disease in Canada. Annual summaries. Health Protection Branch, Ottawa, ON.
  • Jobling, S. and J.P. Sumpter. 1993. Detergent components in sewage effluent are weakly oestrogenic to fish: an in vitro study using rainbow trout (Oncorhynchus mykiss) hepatocytes. Aquat. Toxicol. 27: 361-372.
  • Nelson, H. 1994. Fecal coliform contamination in Georgia Strait. Environment Canada, Shellfish and Aquaculture Program, Pacific and Yukon Region, Vancouver, BC.
  • Statistics Canada. 1994. Human activity and the environment 1994. Statistics Canada Catalogue No. 11-528, Ottawa, ON.
  • Ternes, T., M. Stumpf, J. Mueller, K. Haberer, R.-D. Wilken and M. Servos. 1999. Behavior and occurrence of estrogens in municipal sewage treatment plants--1. Investigations in Germany, Canada and Brazil. Sci. Total Environ. 225: 82-90.
  • Warman, P.R. 1997. Alternative amendments in agriculture: overview of their characteristics and use. In P.R Warman (ed.), Alternative amendments in agriculture. Proceedings of the Joint Symposium on Alternative Amendments in Agriculture. Agricultural Institute of Canada, Truro, NS.
  • Webber, M.D. and T.E. Bates. 1997. Municipal sewage sludge use on agricultural land. In P.R. Warman (ed.), Alternative amendments in agriculture. Proceedings of the Joint Symposium on Alternative Amendments in Agriculture. Agricultural Institute of Canada, Truro, NS.