What is Wastewater Treatment Sludge?

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  • Written By: Caitlin Kenney
  • Edited By: Bronwyn Harris
  • Last Modified Date: 11 December 2019
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Wastewater treatment sludge, or biosolids, is made up of a soupy mixture of water and the solid wastes created as a byproduct of sewage treatment processes. Sewage is produced when homes, industries, and farms discharge water polluted by waste from toilets, by-products of manufacturing processes, showers, animal slaughters, and sinks. In order to prevent damage to the outside water quality or aquatic life, water treatment is performed to restore clarity, remove odor, 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 steps of this process.

After wastewater is conducted through pipes either into a septic tank or a treatment facility, the primary stage of dealing with sewage entails collecting the wastewater in a sedimentation tank. Once in the tank, the heavy solids are allowed to settle to the bottom and the floatable solids rise to the top, creating both an overlying and underlying layer of raw wastewater treatment sludge, with water sandwiched between. The top layer of sludge is usually composed of greases, oils, and other fats, while the bottom layer contains heavy solids such as human or animal fecal matter. The raw, primary sludge is then thickened and removed from the tank before bacteria are able to digest too much of it.


The water from the primary stage moves on either to the outside environment or to the secondary stage, biological wastewater treatment, where secondary wastewater treatment sludge is produced. Since the bulk of the solids have been removed in the first step, the waste in the leftover water is mostly dissolved and difficult to remove with a filter. Biological wastewater systems introduce simple bacteria to the wastewater and let the organisms feed on the soluble, organic waste. As they digest and remove the waste from the water, the bacteria produce a solid by-product that can be removed with a filter. After further treatment, these biosolids can sometimes be used as fertilizer.

As sewage often contains toxic chemicals and pathogens, safe disposal of wastewater treatment sludge is important. Typically disposal consists of treating the sludge through anaerobic digestion, dewatering the resulting biosolids to form sludge cakes, and then sending the sludge cakes to a landfill. Anaerobic digestion occurs in an airtight tank wherein bacteria feed on the organic matter in the sludge in an oxygen-free environment. The gases produced by this digestion process can sometimes be harnessed to create biogas, a renewable energy source used in heating or vehicle propulsion.

Aerobic digestion, or bacteria-aided decomposition in an oxygenated environment, of wastewater treatment sludge can also be used. While faster than anaerobic digestion, aerobic digestion is more expensive to operate because it is so energy intensive. On small scale sites, composting can also be used to treat sludge. Composting mixes waste with organic materials high in carbon, such as wood or straw, and allows aerobic bacteria digestion. The process results in heat production and biosolids that can sometimes be used as fertilizer, depending on the content of the waste and the local regulations.


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