Drain PipeMunicipal wastewater effluents are the liquid wastes that come out of a community’s sewer system and municipal wastewater treatment plants. There are two types: sanitary sewage, which comes from homes, businesses, institutions, and industries; and stormwater, which comes from rain or melting snow that drains off rooftops, lawns, roads, and other urban surfaces. Sanitary sewage usually receives some level of treatment before being discharged into a receiving body of water. Stormwater, on the other hand, is usually discharged without treatment. Municipal wastewater effluents are of concern not only because of the many pollutants that they normally contain, but also because of their sheer volume.

Municipal wastewater effluents can contain:

• Grit, debris, and suspended solids, which can discolour the water, make it unfit for recreational, domestic, and industrial use, and eventually smother and contaminate plant and animal life on the bottom of the receiving water body;
• Disease-causing pathogens (e.g., bacteria and viruses), which can make the water unfit for drinking, swimming, and other recreational uses and can contaminate shellfish;
• Decaying organic wastes, which use up the water’s dissolved oxygen and threaten the survival of fish and other aquatic life;
• Nutrients, which over stimulate the growth of algae and other aquatic plants, giving rise to odours and other aesthetic problems, diminished biodiversity, and, in some cases, toxic contamination of shellfish; and
• Approximately 200 different identified chemicals, many of which may be either acutely or chronically toxic to aquatic organisms and may pose a health risk to humans. Many of these chemicals may have long-term environmental effects, as they are not easily broken down and tend to accumulate in aquatic or terrestrial organisms through the food chain.