Bronte Wells
Sewage plants, also known as wastewater treatment facilities, are designed to clean billions of gallons of wastewater every day to reduce the amount of pollutants such as nitrogen, phosphorus, heavy metals, toxic chemicals, and disease-causing microbes. However, aging infrastructure, insufficient treatment methods, and weather events can lead to sewage overflow and leaks, resulting in the release of untreated sewage into nearby water bodies, causing harmful algal blooms and endangering human health and aquatic ecosystems.
| Characteristics | Values |
|---|---|
| Nitrogen and phosphorus | From human waste, food, and certain soaps and detergents |
| Pathogens | Salmonella, hepatitis, dysentery, cryptosporidium, many other infectious diseases |
| Heavy metals | Arsenic, mercury |
| Microplastics | |
| Chemicals | Pesticides, solvents, oil, grease |
| Synthetic organic chemicals | |
| Inorganic chemicals | |
| Radioactive substances | |
| Sediments | |
| Disease-causing microbes | Coliform bacteria |
| Putrescible organic materials | |
| Heat | |
| Fertilizers | |
| Stormwater runoff |
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What You'll Learn
Nitrogen and Phosphorus
Domestic sewage, or wastewater from residential sources, often contains nitrogen and phosphorus compounds. These compounds are removed at wastewater treatment facilities through processes such as bacterial sulfate reduction and nitrification. However, the effectiveness of nitrogen and phosphorus removal varies among treatment plants, depending on their equipment and methods.
The presence of nitrogen and phosphorus in sewage can result from the use of nitrogen- and phosphorus-based synthetic fertilizers, which have become increasingly common. While phosphorus is a non-renewable resource facing depletion risks, the excessive release of reactive nitrogen into the environment has raised concerns for human health and ecological well-being.
To address the issue of nutrient pollution, strategies are being pursued to reduce nitrogen and phosphorus loads from wastewater treatment plants. One approach, known as optimization, involves adjusting operations and repurposing existing equipment to remove additional nutrients at a lower cost compared to full system upgrades. This can include employing technologies such as ion exchange/adsorption processes, bioelectrochemical systems, and membrane separation techniques for nitrogen recovery, as well as physical filtration and membrane processes for phosphorus removal.
Furthermore, septic systems, if not properly maintained, can contribute to nutrient pollution by releasing elevated levels of nitrogen and phosphorus into local water bodies or groundwater. This highlights the importance of regular inspections and proper maintenance of septic systems to prevent failures that may lead to the discharge of untreated wastewater.
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Microplastics and Heavy Metals
Sewage plants are a major source of water pollutants, including pathogenic organisms, oxygen-demanding wastes, plant nutrients, synthetic organic chemicals, inorganic chemicals, microplastics, sediments, radioactive substances, oil, and heat.
Microplastics
Microplastics are a significant pollutant in sewage plants, and their presence in freshwater systems is largely attributed to municipal sewage treatment plants (STPs). These plants receive wastewater from homes and businesses, which often contains microplastics from everyday plastic products. While most STPs are effective at removing microplastics, with removal rates ranging from 88% to 99.9%%, the high volume of wastewater means that these plants are still a significant entry point for microplastics into aquatic environments. The microplastics that are not removed during treatment can accumulate in sewage sludge, which can have implications for terrestrial ecosystems if the sludge is reused through land application.
The dominant shapes of microplastics found in STPs are fragments and fibers, with thermoplastics (polyethylene and polypropylene) and polyester being the predominant materials. The size distribution of microplastics in STPs likely follows a power law, indicating that studies with different size cutoffs can be compared. However, further research is needed to confirm this.
Heavy Metals
Heavy metals are another pollutant found in sewage plants, with the presence of silver, cadmium (Cd), lead (Pb), copper (Cu), and zinc (Zn) being of particular concern due to their potential environmental and human health impacts. In a case study from Limpopo Province, South Africa, it was found that the levels of these heavy metals in sewage sludge often exceeded recommended guidelines, especially in certain municipalities. Similarly, wastewater and soil samples from Nairobi, Kenya, showed the presence of mercury (Hg), lead (Pb), cadmium (Cd), chromium (Cr), nickel (Ni), and thallium (Tl), with lead and cadmium being the most prevalent.
The accumulation of heavy metals in sewage sludge and their potential impact on the environment and human health is a significant concern, especially if the sludge is applied to agricultural fields or used for landfill cover. While there are guidelines in place, such as the DWAF guidelines in South Africa, the lack of regulations regarding specific heavy metals, such as silver, can pose challenges in managing this type of pollutant.
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Pathogens and Bacteria
The presence of pathogens and bacteria in sewage is a global issue. According to the United Nations, over 80% of the world's wastewater is discharged back into the environment without proper treatment, posing risks to both human health and the environment. This untreated wastewater can contaminate water bodies, leading to the spread of diseases and the degradation of aquatic ecosystems.
Wastewater treatment plants play a crucial role in mitigating the impact of pathogens and bacteria in sewage. These facilities employ various treatment methods to reduce the concentration of pathogenic microorganisms. For example, rural wastewater treatment facilities have demonstrated lower levels of pathogenic bacteria and fungi in their effluent compared to the influent. However, it is important to note that even treated sewage and biosolids can contain high concentrations of pathogens, which can infect humans directly or indirectly through environmental exposure.
The regrowth of pathogenic bacteria in treated sewage is a significant challenge. These bacteria have the ability to regenerate, leading to potential recontamination. Therefore, effective treatment methods aim to destroy pathogen cells and remove nutrients to prevent their regrowth. Advanced technologies, such as temperature-phased anaerobic digestion and auto-thermal aerobic digestion processes, have been developed to address this issue and produce safer biosolids for agricultural use.
The characterization of bacterial communities in wastewater is an important area of research. By using advanced techniques like full-length 16S rRNA sequencing, scientists can identify and understand the microbial dynamics within wastewater treatment plants. This knowledge helps in the development of more targeted treatment processes to effectively reduce the presence of pathogenic bacteria and improve environmental and public health outcomes.
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Chemical Compounds
Sewage plants treat wastewater, which contains a wide range of chemical compounds. These chemical compounds are pollutants that can be harmful to the environment and public health if not properly treated and disposed of.
Nitrogen and Phosphorus Compounds
Nitrogen and phosphorus are the most common nutrients in wastewater, coming from human waste, food, and certain soaps and detergents. Excessive amounts of these compounds in lakes and rivers can cause algae blooms, which deplete the water of oxygen and harm aquatic life. This process is called eutrophication.
Pathogens
Pathogens are microorganisms that can cause diseases, such as bacteria, viruses, and parasites. They can come from human and animal waste, food processing, and industrial activities. Sewage treatment plants use disinfectants, such as chlorine, to kill these pathogens before discharging the treated water.
Organic Matter
Organic matter includes any material that comes from living organisms, such as food scraps, grease, and oils. The decomposition of organic matter in sewage can lead to the depletion of oxygen levels in water bodies, creating undesirable odours and harming aquatic life.
Heavy Metals and Toxic Chemicals
Industrial wastewater can contain heavy metals such as arsenic, mercury, and lead, as well as toxic chemicals like pesticides, solvents, and sludge. These chemical compounds can have severe health impacts if they contaminate water supplies.
Other Chemical Compounds
Other chemical compounds found in sewage include pharmaceuticals, hormones, and endocrine-disrupting compounds. These can be excreted in urine or feces if not fully metabolized by the human body. Additionally, stormwater runoff can introduce various contaminants, including road salts, chemicals, and debris, into the sewage system.
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Solid Particles
The solids in sewage can include a range of materials, such as food waste, toilet paper, soaps, detergents, and municipal solid waste. Sewage also contains macro-pollutants and micro-pollutants, which can include pathogens, heavy metals, oils, and grease. These solid particles are measured to assess the sewage strength and quality, as well as the treatment options required. Bar screens and primary treatment methods are used to remove large solid debris and floating and settleable matter, reducing the solids content in the sewage.
One of the main concerns with solid particles in sewage is the presence of pathogens. Pathogenic bacteria, viruses, protozoa, and helminths can be found in sewage, posing risks to human health and the environment. Sewage treatment plants aim to reduce these pollutants and kill pathogens before disposing of or reusing the treated sewage.
In addition to the solid particles originating from wastewater, sewage plants themselves can contribute to solid particle pollution. Mishaps during the transport or disposal of sludge waste can result in the release of untreated sewage, requiring cleanup and potentially causing environmental damage. Furthermore, ageing infrastructure can also lead to the leakage of liquid raw sewage, as cracks in pipes or sewers may develop over time, polluting nearby water bodies.
The disposal of sewage sludge is another challenge for sewage plants. Landfilling can prevent the release of sludge-borne pollutants by concentrating them in a single location. However, there are risks associated with landfill disposal, including the production of methane gas, a greenhouse gas, and the potential contamination of local groundwater and surface water if the landfill liner fails.
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Frequently asked questions
Sewage plants treat wastewater, which contains pollutants such as nitrogen and phosphorus from human waste, food, and certain soaps and detergents. Sewage can also contain pathogenic organisms, synthetic organic chemicals, inorganic chemicals, microplastics, sediments, radioactive substances, oil, and heat.
Nutrient pollution, caused by excess nitrogen and phosphorus, is the number one threat to water quality worldwide. This type of pollution can cause algal blooms, which can be harmful to people and wildlife and can lead to changes in biodiversity.
Wastewater treatment plants use complex machinery and human operators to safely remove solid particles, reduce pollutants, and restore oxygen to the water.
Sewage pollution is a danger to human health and can cause waterborne illnesses. It also has a significant impact on the animals and plants living in waterways, as it can reduce oxygen levels in the water, killing fish and other organisms.
Sewage overflows can be prevented by investing in infrastructure upgrades, such as updating wastewater treatment facilities and expanding natural areas to prevent stormwater from rushing into the sewer.
