Wastewater treatment sludge is a mixture of water and solid waste created during the treatment process. Primary and secondary stages produce different types of sludge, which are treated through anaerobic or aerobic digestion, composting, or dewatering before being sent to a landfill. Biosolids can sometimes be used as fertilizer or to create biogas. Safe disposal is important due to the presence of toxic chemicals and pathogens in wastewater.
Wastewater treatment sludge, or biosolids, is made up of a thick mixture of water and solid waste created as a by-product of wastewater treatment processes. Wastewater is produced when homes, industries and farms discharge waste-polluted water from toilets, by-products of manufacturing processes, showers, animal slaughterhouses and sinks. In order to prevent damage to external water quality or aquatic life, water treatment is performed to restore clarity, remove odors and remove contaminants from wastewater before it can be safely released into the environment. Usually, wastewater treatment sludge is created during the primary and secondary stages of this process.
After the wastewater has been led through pipes into a septic tank or treatment plant, the primary stage of wastewater treatment involves collecting the wastewater into a settling tank. Once in the tank, the heavy solids are allowed to settle to the bottom and the buoyant solids rise to the top, creating both an overlay and underlay of raw sewage treatment sludge, with water sandwiched between. The top layer of sludge is usually composed of fats, oils and other fats, while the bottom layer contains heavy solids such as human or animal feces. The raw primary sludge is then thickened and removed from the tank before the bacteria are able to digest too much of it.
The water from the primary stage passes to the external environment or to the secondary stage, biological wastewater treatment, where the secondary wastewater treatment sludge is produced. Since most of the solids have been removed in the first stage, the waste in the remaining water is mostly dissolved and difficult to remove with a filter. Biological wastewater systems introduce simple bacteria into the wastewater and let the organisms feed on the soluble organic wastes. As the bacteria digest and remove waste from the water, they produce a solid by-product that can be removed with a filter. After further treatments, these biosolids can sometimes be used as fertilizer.
Since wastewater often contains toxic chemicals and pathogens, safe disposal of wastewater treatment sludge is important. Disposal typically consists of treating the sludge through anaerobic digestion, dewatering the resulting biosolids to form sludge, and then sending the sludge to a landfill. Anaerobic digestion takes place in an airtight tank where the bacteria feed on the organic matter present in the sludge in an oxygen-free environment. The gases produced by this digestion process can sometimes be exploited to create biogas, a source of renewable energy used in heating or in the propulsion of vehicles.
Aerobic digestion, or bacterial decomposition in an oxygenated environment, of wastewater treatment sludge may also be used. Although faster than anaerobic digestion, aerobic digestion is more expensive to operate because it requires a lot of energy. On smaller sites, composting can also be used to treat sludge. Composting mixes waste with high carbon organic materials, such as wood or straw, and allows aerobic bacteria to digest. The process leads to the production of heat and biosolids which can sometimes be used as fertilizer, depending on the waste content and local regulations.
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