This article was updated on the 11th September 2019.
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Human activities create wastewater that can be catastrophic to the environment and also cause loss of water (in the form of wastewater which is 99% water by weight) in places where water is scarce. When wastewater contaminates rivers and groundwater tables, it renders the water resource unusable. Therefore, it is imperative that wastewater is treated before it is released into the environment and, if possible, treated it to make it potable.
The objective of wastewater treatment is to reduce the pollutants to less than maximum permissible limits to prevent the threat to the environment and human health. To achieve this, wastewater is collected and treated in large plants before it is permitted to be released back into the environment. All the water used in homes that flows into drains or the sewage system is referred as wastewater. Industries and businesses frequently contribute large volumes of wastewater to sewage collection systems.
Wastewater follows a determined treatment path in order to achieve water quality standards, regardless of whether conventional treatment or advanced treatment systems are used. Wastewater is normally called influent as it passes through the wastewater treatment facility. Wastewater treatment plants help nature to defend water from excessive pollution. The degree and type of wastewater decides the nature of treatment and the engineering scale of the plant. Most wastewater treatment plants consist of primary and secondary treatment.
Primary treatment involves the separation and removal of solid matter and homogenization of the remaining liquid waste. This solid matter will either float or readily settle out due to gravity. Physical processes such as screening and grit removal may be used during primary treatment.
Large objects, such as sticks or stones, which could block tank inlets or plug lines are removed during the screening process. Grit chambers are used to slow down the wastewater flow and allow grit to fall out. Solids that can settle in a sedimentation tank are pumped away.
The secondary treatment involves a biological process. Wastewater is exposed to aerobic bacteria where the biological oxygen demand (BOD) is reduced. Aerobic bacteria are used to break down pathogens, other contaminants and suspended organic matter into carbon dioxide, water and biosolids. Aerobic bacteria are naturally supplied in wetland habitats. Baffles with a special coating of aerobic bacteria are often used by sewage treatment plants.
Municipal wastewater is normally disinfected with the use of chlorine or other disinfecting compounds. Occasionally, ultraviolet light or ozone is used.
Tertiary treatment is carried out to improve the ‘final look’ of water, making it indistinguishable from any freshwater source. It is done to deodorize, decolor and further oxidize if required. There has been an increase in the number of wastewater treatment facilities that employ a tertiary treatment process. Tertiary treatment involves removing nutrients such as phosphorus and nitrogen from wastewater. The removal of nutrients is an important step in restricting downstream effects such as algal blooms and eutrophication, which destroy ecosystems and habitats.
Based on the influent, wastewater treatment plants may be a sewage treatment, municipal wastewater treatment, industrial wastewater effluent treatment or agricultural wastewater plant.