Factors that affect sewage treatment plant quality
The type of process used by a sewage treatment plant certainly affects the quality of effluent that is discharged to the receiving lake or river. It is important to keep in mind that each STP is designed and operated for a unique location, and may have special features to respond to local needs, financial constraints and the assimilative capacity of the receiving waterway. Ontario STPs are often categorized as primary, secondary or tertiary treatment facilities, and for simplicity’s sake the discussion below will follow this approach. Primary treatment is the most basic sewage treatment, involving mainly mechanical screening and settling of solids to form primary sludge and a wastewater that is more or less clear to the naked eye. At secondary treatment plants, the wastewater leaving the primary treatment is mixed with a controlled but very dense community of microorganisms and plentiful oxygen. This beneficial microbial community is able to consume dissolved organic matter and also many pathogens. Dead microorganisms are continuously cropped and settled out to form secondary sludge.
Many factors can impede the ability of the microbial community to do its job, especially inadequate oxygen levels or sudden changes in concentrations of nutrients or toxins, which can inactivate the community. Some facilities follow the secondary process with a tertiary treatment, usually custom-designed to control a single parameter that regulators have identified as a special problem for the receiving water body. Parameters that can be controlled by tertiary treatment include colour, metals, organic chemicals and nutrients, especially phosphorus and nitrogen. The quality of STP effluent depends not only on the treatment technologies at the facility, but also on the skill of the operators, the season of the year, the weather, and the types of industries that are hooked up to the municipal sewer system. In Ontario an estimated 12,000 industrial facilities are connected to municipal sewers and add a wide array of pollutants to the system. In some municipalities, such as Hamilton, landfill leachate is collected and diverted to the sewage treatment plant, even though STPs are not designed to treat this type of waste.
| This is an article from the 2002/03 Annual Report to the Legislature from the Environmental Commissioner of Ontario. |
Citing This Article
Environmental Commissioner of Ontario. 2003. "The Environmental Impacts of Sewage Treatment Plant Effluents." Thinking Beyond the Near and Now, ECO Annual Report, 2002-03. Toronto, ON : Environmental Commissioner of Ontario. 38.
