Wastewater Treatment – ​​Safe Drinking Water Foundation (2023)


What is wastewater?

Wastewater is water that has already been used and must be treated before being discharged into another body of water so that it does not further contaminate the sources. Wastewater comes from various sources. Anything you flush down the toilet or down the drain is wastewater. Rainwater and runoff, along with various contaminants, run down street drains and end up in a wastewater treatment plant. Wastewater can also come from agricultural and industrial sources. Some effluents are more difficult to treat than others; For example, industrial wastewater can be difficult to treat, while domestic wastewater is relatively easy to treat (although household waste is becoming more difficult to treat as more pharmaceuticals and personal care products are found in the world). water from household waste). pollutants produced, see theEmerging Pollutants Brochureor read the Canadian Press article calledSee everyday chemicals in water, Ontario said).

Who is responsible for ensuring that wastewater is properly treated?

Similar to the supply of drinking water, the federal government has delegated responsibility for wastewater treatment to the provinces and territories. However, there are two federal laws that may apply to wastewater. The Fisheries Law prohibits the release of pollutants into waters where fish live. The Canadian Environmental Protection Act regulates the release of toxic substances into the environment and allows the federal government to develop regulations for the use of toxic substances.

Most provincial and territorial governments have legislation on standards and requirements for wastewater treatment. Wastewater treatment plant operators must obtain permits or licenses from the provincial or territorial government, and these permits may also require additional treatment or restrictions on wastewater discharge. For example, in British Columbia, all municipalities are required to have a Liquid Waste Management Plan; downloads without an approved plan are illegal. Provincial and territorial governments generally support local governments with funds for the construction and maintenance of infrastructure.

Municipalities directly oversee the wastewater treatment process and may enact additional legislation. For example, the Ottawa-Carleton Regional Municipality has developed a program to remove toxic substances from the wastewater treatment system that requires all industrial, institutional, and commercial facilities to limit the amount of certain contaminants that can enter drains.

How do cities treat wastewater to make it safe to discharge?

There are various stages of wastewater treatment; These are the primary, secondary and tertiary treatment levels. Most municipal wastewater treatment plants use primary and secondary treatment stages, and some also use tertiary treatment. The type and order of treatment can vary from one wastewater treatment plant to another, but this diagram of the Ottawa-Carleton wastewater treatment plant illustrates the basic components.

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The primary treatment level uses filters and settling tanks to remove most solids. This step is extremely important as solids make up about 35% of the contaminants to be removed. The screens typically have openings of about 10 millimeters, which is small enough to remove sticks, trash, and other large material from the gutter. This material is removed and disposed of in a landfill.

The water is then dumped into settling tanks (or clarifiers) where it remains for several hours to allow the sludge to settle and a foam to form on top. The bottom ash is then removed from the top, the sludge is removed from the bottom and the partially treated wastewater is sent to the secondary treatment level. Primary treatment typically removes up to 50% of biological oxygen demand (BOD; these are substances that use oxygen from water), about 90% of suspended solids, and up to 55% of fecal coliforms. Although primary treatment removes a significant amount of contaminants from wastewater, it is not enough to ensure that all contaminants are removed.

Secondary wastewater treatment uses bacteria to digest the remaining contaminants. This is done by intensively mixing the wastewater with bacteria and oxygen. Oxygen helps bacteria digest contaminants faster. The water is then fed to settling tanks, where the sludge settles again and the water remains 90 to 95 percent free of contaminants. The figure below shows the tailings ponds at the Winnipeg Wastewater Treatment Plant. Secondary treatment removes about 85 to 90 percent of BOD and suspended solids and about 90 to 99 percent of coliform bacteria.

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Some wastewater treatment plants connect with a sand filter to remove additional contaminants. The water is then disinfected with chlorine, ozone or ultraviolet light and then drained. For more information on all steps of the water treatment process, go toChlorination Data Sheet.

The sludge removed from the tailings ponds and the slag removed from the top during the first passes are treated separately from the water. Anaerobic bacteria (anaerobic bacteria do not need oxygen) feed on the sludge for 10 to 20 days at temperatures around 38 degrees Celsius. This process reduces odor and organic matter in the sludge and produces a highly combustible gas of methane and carbon dioxide that can be used as fuel to heat the treatment plant. Finally, the sludge is sent to a centrifuge, as in the photo below. A centrifuge is a machine that spins very quickly, forcing the liquid to separate from the solid. The liquid can then be processed with wastewater and the solid used as fertilizer in the fields.

A third (or advanced) treatment removes dissolved substances such as paints, metals, organic chemicals, and nutrients such as phosphorus and nitrogen. There are a number of physical, chemical and biological treatment methods used for tertiary treatment. One of the biological treatment methods is called Biological Nutrient Removal (BNR). This diagram shows the treatment steps that wastewater from Saskatoon goes through.

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In this wastewater treatment plant, wastewater first undergoes primary and secondary treatment. For third-party treatment, the BNR process is carried out in bioreactors. The BNR process uses bacteria under different conditions in different tanks to digest impurities in the water. All three tanks have unique environments with different amounts of oxygen. As the water flows through the three tanks, the phosphorous is removed and the ammonia is broken down into nitrate and nitrogen gas, something other bacterial processes cannot do. The BNR process can remove over 90% of phosphates, while conventional processes remove much less than 90%. The water spends approximately nine hours in the bioreactors before entering the secondary clarifier, which is a settling tank where bacteria-laden sludge settles to the bottom of the tank.

How do small communities treat wastewater to make it safe to discharge?

In small communities, wastewater treatment plants may consist of individual septic systems, simple collection systems that discharge wastewater directly into surface water, or municipal ponds that are emptied annually. These facilities generally treat and distribute waste as close to its source as possible, minimizing operating costs and maintenance requirements. The longer waste sits in a pond before being disposed of, the less likely it is to contaminate drinking water sources. Some communities store waste in ponds, while others dump it directly into water sources.

Ponds are reservoirs in the ground that store waste for a period of time until it is discharged to the ground or into a body of water. Shallow lagoons less than 1.5 meters deep are used for primary treatment, which allows solid waste to settle at the bottom of the lagoon in a period of 6 to 20 days. However, shallow ponds cannot effectively remove most contaminants that pose problems for ground and surface water. Deep ponds, more than ten feet deep, can allow for long-term storage and treatment for six months to a year. Many ponds in small communities are emptied once a year. Rural communities often use adjacent land to dispose of wastewater. If the soil is suitable and there are no nearby water sources, soil bacteria can remove and break down contaminants in the wastewater. Due to the availability of land in many rural areas, this can be an effective method of treating wastewater. However, there are other communities that dispose of waste in a way that endangers the quality of the lakes, rivers, or groundwater sources that provide drinking water.

The Environmental Protection Agency estimates that 10 to 20 percent of small municipal wastewater treatment plants in the United States are malfunctioning; State water quality agencies have identified faulty wastewater treatment systems as the second biggest threat to water quality (after underground storage tanks). When inadequate wastewater treatment is combined with ineffective drinking water treatment, the result is a serious pollution problem for large numbers of rural communities.

Rural communities often find it difficult to install and maintain wastewater treatment plants. And while many communities have inadequate wastewater treatment methods, there are some communities that are leading the way with innovative treatment methods and water conservation measures. In several arctic communities, including Iqaluit, Nunavut, the high cost of water has led to wastewater treatments that allow the water to be reused. Wastewater is passed through a septic tank, filtered and disinfected with ozone treatment; it is then reused for non-consumptive purposes such as baths and laundry. These saving measures allow up to 55% of wastewater to be reused and relieve wastewater treatment and storage processes. To learn more about water conservation, including how First Nations and rural communities are making efforts to reduce water use, visitWater consumption data sheet.

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How do wastewater treatment plants work?

Many people live in rural areas that do not have sewage treatment plants. In fact, according to Environment Canada, only 57% of Canadians had wastewater treatment plants in 2000, compared to 74% of Americans, 86.5% of Germans and 99% of Swedes. . Many people in rural areas use septic systems to safely store waste. The sewage goes through pipes from the house to an underground septic tank. The following diagram illustrates the basic components of a septic system.

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The following diagram shows the tank in more detail. In the septic tank, solids settle to the bottom and foam forms on top, similar to settling tanks in municipal wastewater treatment plants. After separation, the water flows into the drain field and screens and compartments retain the sludge and scum in the tank, where bacteria begin to partially digest the sludge. When partially treated wastewater enters the catchment area, it begins to seep into the ground and seeps downward. As long as the soil is suitable, soil microbes digest contaminants and remove bacteria, viruses and excess nutrients until the water reaches the groundwater source.

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If the soil is not suitable or there are nearby water sources, an alternative system can be used so that the water sources are not contaminated. The alternative system can use sand, peat or plastic instead of soil. Constructed sumps, ponds, aerators or disinfection devices are also effective in wastewater treatment.

It is estimated that around half of all rural wells are contaminated, many of them from septic tanks. The US Centers for Disease Control and Prevention suggests that wells should be at least 50 feet away from septic tanks, but that distance also varies by soil type.

A recent study of groundwater sources in southeastern Michigan by the US Geological Survey tested 38 wells between 1999 and 2001. Of the wells located near culverts, viruses were detected in only two of the 18 wells. Of the 20 wells near the septic systems, viruses were found in seven wells. This suggests that septic systems are a major source of groundwater contamination. Previous studies of contaminated wells cited by the USGS study found coliform bacteria in up to 80% of the wells, as well as some with significant numbers ofE. coliBacteria.

When using a septic system, it's important to maintain it properly, because if it's not done, contaminants will leach into the ground or water sources. The U.S. Environmental Protection Agency (EPA) recommends that you have your sewer system inspected at least every three years so it can be checked for leaks and faults, and pumped out even when full (usually once every three to five years). .

Following a few water conservation practices can significantly reduce the pressure on your sewer system. You can find more information about saving water in the information sheetwater consumption🇧🇷 Here are some things you can do to take care of your septic system:

  • Do not use the drain or toilet to dispose of garbage; Avoid pouring dental floss, diapers, coffee grounds, and paper towels down the drain, as they can clog the septic system.

  • Spread out your laundry loads throughout the week. If too much water is fed into the septic tank, you won't have time to treat the waste and could flood your sewage field with sewage.

  • Plant grass in your drainfield, but keep trees and shrubs away as roots can clog the system and cause damage.

  • Do not drive into your drainfield as this can compact the soil and damage purification system components.

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Are there "natural" ways to treat wastewater?

If nature itself can purify water, then mimicking natural processes may be the most effective and sustainable method of wastewater treatment. Much of the turnover of water occurs naturally in wetlands. Constructed swamps consist of a lined cell into which water flows. Plants are planted in the cell, and the roots filter impurities from the water. Below is a diagram of a constructed wetland. Note that many of the processes in a wetland are similar to the biological nutrient removal process described above.

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Another natural method is the so-called rapid percolation, in which a container is filled with already treated wastewater. The soil acts as a filter, removing contaminants from the water. This method is similar to what happens in a septic system. A third "natural" process is surface runoff, which is used in regions with nearly impermeable soils. The water flows down a sloping surface planted with thick grass. Since the soil is highly impermeable, water is pushed through the vegetation, effectively removing contaminants.

Slow irrigation is a process that uses a portion of the soil and allows water to flow slowly enough that the soil's ability to absorb water and remove contaminants is not overwhelmed. Forestry is similar to slow irrigation in that it uses a large amount of land to treat wastewater and plant plants or trees that thrive during the treatment process. Aquaculture uses aquatic plant and animal species to treat wastewater, similar to the process for constructed wetlands.

There are also alternative separation systems that can save water. Such a system separates black water (from toilets) from gray water (from showers and dishwashers) so that gray water can be treated and minimally used to water lawns. In addition, there are incineration, chemical or composting toilets that release garbage when it is safe to do so.

If wastewater treatment processes can remove almost all contaminants from wastewater, how can wastewater contaminate drinking water sources?

There are several ways that wastewater can cause pollution problems. First, not all waste reaches the treatment plant. A study by Sierra Legal found that more than 90 billion gallons of raw sewage is dumped into the Great Lakes each year. That's the same as dumping over 100 Olympic sized swimming pools of raw sewage into the Great Lakes every day! Hundreds of billions of raw sewage enters Canadian waters each year. This graph shows the level of municipal wastewater treatment in Canada based on the people served by municipal wastewater systems. While the percentage of people not receiving or receiving primary treatment is declining, 24% of Canadians do not have wastewater treatment plants capable of removing most contaminants.

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Another way that wastewater can contaminate lakes and streams is through the failure of municipal sewers. Many cities, especially older ones, have combined sewer systems that collect domestic wastewater in the same pipes as rainwater. When it rains a lot, the gutters collect more water than the system can hold. When this happens, a combination of raw sewage and stormwater is released directly into the environment. This is called Combined Water Overflow (CSO). In 2001, Vancouver alone dumped an estimated 22 billion liters of raw sewage from CSOs into the environment. Non-accidental discharges or drifts can also occur during maintenance and power outages. In 2001, Ontario reported 144 major wastewater treatment plant diversions.

Some cities choose to dump raw sewage into oceans and rivers because it is cheaper than effective treatment. A report published by Sierra Legal found that out of 22 Canadian cities, Victoria, Dawson City, Montreal, Saint John, Halifax and St. While not all wastewater is discharged directly into the oceans, these six cities produce 400 million liters of raw sewage every day. ! Montreal dumps about 3.6 billion gallons of raw sewage into the St. Lawrence every year, and Victoria is the only major city in Canada that dumps all of its waste into the ocean without trying to improve the system. The City of Victoria dumps over 34 billion liters of raw sewage into bodies of water each year and still claims that their actions do not harm the environment! Halifax and St. John's plan to build sewage treatment plants, but in the meantime they still dump 65.7 billion liters and 33 billion liters of raw sewage, respectively, into the Atlantic. For more information on water pollution, seeWater Pollution Brochure, or theOperation Water Pollution Resources and Lesson Plans.

What about industrial waste? Can public wastewater treatment plants really remove all these toxic chemicals?

Commercial and industrial waste does not go directly to public wastewater treatment plants because the public wastewater treatment system cannot effectively remove all contaminants. Wastewater from commercial and industrial processes generally falls into the following four categories and is treated accordingly:

1. Part of the wastewater can be treated on site and reused in the factory for various purposes.

2. There are some wastewater treatment plants designed to treat industrial effluents.

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3. Some of the wastewater is similar to domestic wastewater and can be sent to a public treatment plant. Or the water can be pre-treated and sent to the public wastewater treatment plant.

4. Wastewater from certain processes is highly toxic and must be treated on site or disposed of as hazardous waste.

There are more than 23,000 different chemicals and substances used in consumer products and industrial processes in Canada, and more are being developed. Some of these substances are difficult to remove and can cause major contamination problems. For more information on emerging pollutants, seeemerging pollutantsor read the Canadian Press article calledSee everyday chemicals in water, Ontario said🇧🇷 For more information on sources of contamination, including ways to minimize water contamination, see the fact sheet titledWater Pollution.

The Safe Drinking Water Foundation offers educational programs that can supplement the information in this fact sheet. Operation Drop of Water analyzes the chemical contaminants found in the water; It is designed for a science class. Operation Water Flow examines how water is used, where it comes from and how much it costs; has classes in social studies, math, biology, chemistry, and science. Operation Spirit of Water presents First Nations perspectives on water and related issues; It is designed to teach native studies or social studies. Operation Water Health addresses common health problems related to drinking water in Canada and around the world and is designed for collaboration in health, science, and social studies. Operation Water Pollution focuses on how water pollution is created and how it is cleaned, and is designed for collaboration between science and the social sciences. For more information on these and other educational activities, as well as additional fact sheets, visit the Safe Drinking Water Foundation website atwww.safewater.org.

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Canadian Council of Ministers of the Environment. February 2003. Linking Water Science to Policy: Wastewater Treatment for Small Communities.

Centers for Disease Control and Prevention. 2006. Reference Guide for Healthy Living: Chapter 8: Rural Water Supply and Water Quality Issues.http://www.cdc.gov/nceh/publications/books/housing/cha08.htm.

Saskatoon City. 2022. Wastewater.

eco-justice. September 3, 2020. Ecojustice report reveals that sewage pollution remains a major problem in the Great Lakes Basin.

Ecological efficiency. November 2013. Great Lakes wastewater bulletin.http://www.ecojustice.ca/publications/reports/the-great-lakes-sewage-report-card.

Environment Canada. 1992. Clean Water: Life Depends on It!

Francy, Donna S.; Bushon, Rebecca, N.; Stopper, Julie; Luzano, Emma J.; and Fout, G. Shay. 2004. US Geological Survey: Environmental Factors and Chemical and Microbiological Components of Water Quality Associated with the Presence of Enteric Viruses in Groundwater from Small Public Water Supplies in Southeastern Michigan.http://pubs.usgs.gov/sir/2004/5219/pdf/sir20045219.pdf.

Government of Canada. November 2010. Water Pollution: Causes and Effects.https://www.canada.ca/en/environment-climate-change/services/water-overview/verschmutzung-ursachen-auswirkungen.html

Government of Canada. January 2016. Water Pollution: Erosion and Sedimentation.https://www.canada.ca/en/environment-climate-change/services/water-overview/verschmutzung-ursachen-auswirkungen/erosion-sedimentation.html.

United States Department of Environmental Protection. May 2018. Septic systems (local/decentralized systems).https://www.epa.gov/septic

United States Department of Environmental Protection. 2007. Septic Technologies: Background and Technology.

United States Department of Environmental Protection. Sourcebooks on water: fact sheets.

United States Department of Environmental Protection. 2005. Manual for the Management of Local and Grouped (Decentralized) Wastewater Treatment Systems.

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