Biological wastewater treatment uses live bacteria to remove contaminants from sewage so that water quality and aquatic life won’t be harmed. Homes, industries, and farms discharge water polluted by waste from toilets, by-products of manufacturing processes, showers, animal slaughters, and sinks, among other sources. In order to restore clarity to the water, neutralize odor, and remove contaminants, sewage and run-off must go through several steps before it can be safely released into the environment. Biological wastewater treatment is typically the second step in this process and focuses on reducing the amount of oxygen-demanding materials from water.
Wastewater is conducted through pipes into a treatment system either on site, such as a septic tank or biofilter, or at a water treatment plant. The location and process needed to render wastewater safe depends on the treatment regulations in a given region. Typically, the first step, or primary stage, of dealing with sewage entails collecting the wastewater in a tank and allowing the heavy solids to settle to the bottom, while the fatty solids float to the top. These solids are then skimmed off and left behind as the water moves on, either to the outside environment or to the secondary stage, biological wastewater treatment.
Once the bulk of the solid waste has been removed from the wastewater, most of the remaining contaminants are soluble waste, or waste that is dissolved in water. Because it is dissolved, it can’t be easily removed with a filter. A biological wastewater treatment system introduces simple bacteria to the sewage and lets the organisms feed on the soluble, organic waste. The bacteria remove the dissolved waste from the water, producing a solid by-product that can be removed with a filter. Bacteria also help congeal the less soluble waste that does not get eaten, turning it into floc, which can be filtered out. Floc consists of clumps of solid that are suspended, rather than dissolved, in liquid.
Biological wastewater treatment targets waste with an oxygen demand, or wastes that will lower the content of dissolved oxygen (DO) in water. Low DO can harm aquatic environments by choking out plants and animals that require a certain amount of oxygen to survive. A similar problem arises when nutrients such as nitrogen and phosphorous are released into an aquatic environment, causing eutrophication. Eutrophication occurs when the released nutrients encourage excessive growth of certain plants and algae, which consume oxygen and suffocate other animals and plants. Phosphorous, nitrogen, and most oxygen-demanding wastes are organic, so biological wastewater treatment is often efficient at removing these threats.
The types of waste that may be introduced into a biological wastewater treatment system can vary, as bacterial growth will adapt to favor cells most capable of consuming the waste at hand. Biological wastewater treatment usually occurs in an outdoor tank, making the process vulnerable to seasonal temperature changes. This is typically not a problem, as bacteria will adapt themselves quickly to variations in warmth without any need to install a heating or cooling system. Once this secondary stage is complete, wastewater may continue to a tertiary stage, which often includes microfiltration or chemical disinfection.