When one considers the source of water pollution, industrial operations quickly come to mind. However, municipal wastewater effluents are also a significant source of contamination. In fact, some pollutant levels in these effluents have been found to be many times higher than those in untreated industrial wastewaters. Although many municipalities utilize advanced processes to treat wastewater, others provide insufficient or even no treatment at all.
Wastewater is a mix of many different substances such as: grit, debris, suspended solids, pathogens, organic wastes, nutrients, and a mixture of approximately 200 known chemicals. Their varied negative impacts affect not only the health of aquatic and terrestrial organisms, but human and economic health as well.
In addition to the environmental costs, there are also many socio-economic costs involved with effluent discharges. Beaches and fisheries may be closed, tourism revenues lost, and there may be higher costs associated with treating the contaminated water before it can be used for other purposes.
When one adds all these effects together, it becomes readily apparent that municipal wastewater effluents represent one of the most significant sources of pollution. Wastewater treatment removes many of the contaminants, however, concentrations are still high enough to produce many negative impacts.
Environmental Impacts
Nutrient Pollution
Municipal wastewater effluents contain nutrients such as nitrogen and phosphorus. Although they are beneficial to plant life, high concentrations can result in adverse effects. Excess plant growth, especially algae, leads to eutrophication of the receiving surface water bodies. This condition of excess growth leads to degradation of the aquatic ecosystem.
When large algae blooms die and decompose in freshwater ecosystems, a great deal of dissolved oxygen is used up, which places fish and other aquatic organisms at risk. The excess plant material also smothers the bottom, placing additional stress on bottom-dwelling organisms. Additionally, some algae produce toxins that present environmental and human health risks.
Marine waters are also affected by nutrient pollution. Excess growth of phytoplankton and large types of algae near the water surface blocks light from reaching the seagrasses living on the bottom. When the grasses die, the bottom sediments are no longer stabilized, which leads to increased water turbidity and loss of habitat for fish and other marine life.
Many lakes and rivers are suffering from the effects of eutrophication resulting from excess nutrients in municipal wastewater discharges. Coastal areas are also under stress. Fish kills periodically occur due to nutrient levels in discharged raw sewage.
Eutrophication results in a decrease in the biodiversity of plants and aquatic organisms, thereby placing stress on the ecosystem. Declining sport fish numbers due to eutrophication also has serious impacts on an area’s economy. Tourism dollars are lost along with employment and commercial opportunities. Excess plant and algae growth around beaches makes them undesirable for swimming and other water activities, further impacting the local economies.
Dumping municipal wastewater effluents into water bodies can result in deoxygenation of the water, which can lead to aquatic life mortality.
Oxygen Depletion
In addition to the eutrophication of water bodies, wastewater effluents also contribute to another process of deoxygenation. The biological (bacterial) breakdown of organic solids in the effluent also consumes dissolved oxygen – the biological oxygen demand (BOD). In addition, the degradation of chemicals in the effluent removes oxygen from the water through chemical reactions – the chemical oxygen demand (COD).
As was previously mentioned, the loss of dissolved oxygen in the water can result in serious immediate, short-term, or long-term consequences to aquatic life. Fish survival is particularly affected by low oxygen levels. Decreased disease resistance, reduced growth, altered swimming behaviour, feeding, migration, and reproduction, increased threat of predation, and even rapid death are some of the effects of low dissolved oxygen concentrations.
Low oxygen levels can also alter the kinds of species present in the ecosystem. With a change in food supply, some populations of species decrease, while others increase. Fish such as whitefish, walleye, and pike may disappear, while bottom-feeding fish such as suckers and carp may increase in number.
Toxicity Impacts
Due to the presence of many toxic substances in wastewater effluents, and the uncertainty about how they affect living organisms separately and/or in combination with each other, it’s difficult to assess the true environmental costs associated with municipal discharges. Impacts may be acute and occur very quickly, or they may be cumulative and happen over a long period of time. One thing is known, however, municipal wastewater discharges represent a significant threat to aquatic and terrestrial life.
Ammonia is the wastewater constituent that causes the most toxic effects, and municipal treatment plants are one of the biggest sources of ammonia pollution. Ammonia affects fish such as rainbow trout, freshwater scud, walleye, and mountain whitefish, as well as bottom-dwelling invertebrates such as fingernail clams. Effects include reduced reproductive capacity and growth. Acute impacts can also occur with high ammonia levels.
The toxic impacts of wastewater discharges can be seen a large distance away from the treatment plant outlets. Toxic effluents can still do harm 10-20 km away from the discharge point. In addition, it may take 20-100 km before bottom conditions return to normal.
Bioaccumulation and Biomagnification
When living organisms are unable to excrete or breakdown substances, accumulation occurs in their tissues and high concentrations can result. This is bioaccumulation. Some compounds can also be passed up the food chain in a process called biomagnification.
Predators eat the contaminated organisms and absorb all the toxins they accumulated over their lifetime. The toxins are further passed up the food chain when others consume those predators. The species at the top of the chain can have extremely high toxin concentrations. For example, beluga whales can have concentrations that are 10 million times greater than in the water.
Municipal wastewater effluents are one of the largest sources of bioaccumulative toxic substances. Low levels of wastewater effluent constituents such as PCBs, dioxins, furans, organochlorine pesticides, mercury, and other heavy metals are still dangerous due to these accumulative processes. Studies have shown that bioaccumulation of these toxins causes reproductive problems, deformities, tumors and lesions, reduced growth, and central nervous system impairment.
Physical Environmental Alterations
Wastewater discharges can alter physical characteristics in a receiving body of water, such as alterations in temperature, flow rates, and the amount of suspended solids. Since effluents tend to be warmer than the receiving surface waters, temperature changes can occur. Temperature increases may result in changes in the kinds and numbers of species present, and may also cause increased algae growth.
Changing the flow rates of streams can cause bank erosion, flooding, and streambed changes, which result in habitat alterations, changes in the food web, and species loss. Increased suspended solids reduce sunlight penetration (affecting plant life), and smother spawning areas.
Municipal wastewater treatment plants sometimes discharge raw sewage into water bodies, which leads to an accumulation of floating debris.
Floating Debris
Municipal wastewater treatment plants generally screen out large debris, however, sometimes they may discharge raw sewage without treatment, such as when inflows exceed treatment capacity. Some municipalities without treatment facilities discharge raw sewage all the time. The net result is a great deal of debris floating on surface water bodies. If this debris is ingested by fish and wildlife, or they become entangled in it, death is often the result.
Human Health and Socioeconomic Impacts
Water pollution causes many human health problems. Contaminants may be ingested in drinking water, absorbed while bathing or swimming, inhaled while showering, or ingested in contaminated fish or shellfish. Treating these water pollution-related health issues costs hundreds of millions of dollars per year. Beach and fisheries closures resulting from contaminated water also results in great economic losses.
Drinking Water Contamination
The presence of microbial pathogens in wastewater effluents creates a human health risk. If one ingests water contaminated with bacteria, viruses, or other microorganisms such as giardia and cryptosporidium, serious illnesses or even death can result. Many water-related health problems result from human sewage contamination. There have also been cases of giardiasis – a gastrointestinal disease – that were linked to sewage contamination.
Impacts on Recreational Uses
Contamination of beaches and other recreational areas by pathogens can be a health risk to the users of those areas. Gastrointestinal conditions, and skin, ear, eye, nose, and throat infections are some of the problems arising from contact with contaminated water. Other more serious diseases that may result are hepatitis B, enteritis, cholera, and typhoid fever.
Beaches are monitored for microbial contamination, and if levels exceed the guidelines, the area is closed. This creates economic impacts through the loss of tourist dollars. Beach closures do frequently occur.
Excessive algae growth is also a problem in recreational areas. The aesthetics of the area are reduced making its use less attractive. Increased water turbidity, discolouration, and odours also affect the suitability of the area for recreational use. Again, this negatively impacts the local economy.
The presence of floating debris is very unpleasant to beach users and boaters. The dumping of raw sewage into the water results in many unpleasant sights for tourists and locals, such as condoms, sanitary napkins, tampons, and toilet paper floating on the surface. This results in a direct loss of tourist dollars that costs the local economies plenty.
Raw sewage discharges have been blamed for the decline of the recreational fishing industry. The decline of recreational fish stocks results in a loss of millions of dollars that would have come from recreational fishing.
Contamination of surface waters from municipal wastewater effluents, can negatively impact the commercial fishing industry, which results in income and employment losses.
Impacts on Shellfish Industry
The closure of shellfish (lobsters, crabs, clams, mussels, oysters) harvesting areas due to sewage contamination affects an industry worth billions of dollars, and closures are common. Municipal wastewater effluents contribute to the contamination of shellfish, which can present serious human health risks.
Gastroenteritis, salmonellosis, typhoid fever, cholera, and hepatitis may result from the consumption of contaminated shellfish. Sewage discharges can also promote the growth of toxin-producing algae that can accumulate in the shellfish. If even a single contaminated clam is consumed, the result can be death.
Impacts on Fisheries
Substances such as mercury, PCBs, mirex/photomirex, the pesticide toxaphene, and dioxins are all constituents of municipal wastewater effluents. All of these compounds accumulate in fish, which has resulted in the routine issuing of safe consumption advisories by authorities. When effluents cause the degradation or alteration of aquatic habitats, fish stocks decline and the commercial fishing industry is significantly affected. This results in a loss of income and employment.
Conclusions
The effects of municipal wastewater effluents on environmental and human health are definitely significant. The prevention of these problems, or at least their reduction is crucial. Municipalities need to upgrade treatment facilities to lessen environmental impacts. However, even a plant operating at peak efficiency with the most up-to-date technology releases contaminants into the environment.
Everyone can help reduce these impacts by conserving water, which will decrease the volume that needs to be treated, thus resulting in better plant efficiency. One can also be careful with the disposal of toxins in the sewer system. It’s easier to keep contaminants out of the system in the first place, then to remove them later. The costs of facility upgrades will obviously be very large, however, the environmental, human, and socioeconomic costs are immeasurable.
Rick de Vries is the Director of Development for the Fresh Outlook Foundation. He has a background in research and environmental sciences, and has many years of experience writing and editing for environmentally related media.