Waters that are used for drinking, manufacturing, farming, and other purposes by residences (toilets, baths, showers, kitchens, sinks), institutions, hospitals, commercial and industrial establishments are degraded in quality as a result of the introduction of contaminating constituents. Organic wastes, suspended solids, bacteria, nitrates, and phosphates are pollutants that commonly must be removed.
To make wastewater acceptable for reuse or for returning to the environment, the concentration of contaminants must be reduced to a non-harmful level, usually a standard prescribed by the Environmental Protection Agency.
Sewage can be treated close to where it is created (in septic tanks, bio-filters or aerobic treatment systems), or collected and transported via a network of pipes and pump stations to a municipal treatment plant.
Sewage Treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff (effluents) and domestic. The task of designing and constructing facilities for treating wastewaters falls to environmental engineers. They employ a variety of engineered and natural systems to get the job done, using physical, chemical, biological, and sludge treatment methods. Its objective is to produce a waste stream (or treated effluent) and a solid waste or sludge suitable for discharge or reuse back into the environment. This material is often inadvertently contaminated with many toxic organic and inorganic compounds.
The features of wastewater treatment systems are determined by (1) the nature of the municipal and industrial wastes that are conveyed to them by sewers, and (2) the amount of treatment required to preserve and/or improve the quality of the receiving bodies of water. Discharges from treatment plants usually are disposed by dilution in rivers, lakes, or estuaries. They also may be used for certain types of irrigation (such as golf courses), transported to lagoons where they are evaporated, or discharged through submarine (underwater) outfalls into the ocean. However, outflows from treatment works must meet effluent standards set by the Environmental Protection Agency to avoid polluting the bodies of water that receive them.
Sewage Treatment plant is basically characterized as below system based on usage of Oxygen / Air in Secondary Treatment Stage (Biological Decomposition of organic matter).
Here Oxygen/Air is continuously supplied to the Biological (Aeration) Reactor either by direct Surface Aeration system using Impellers propelled by Pumps or Submerged Diffused Aeration system using Air Root Blowers for Air supply through diffusers. Aerobic condition leads to complete oxidation of Organic Matter to Carbon Dioxide, Water, Nitrogen etc. thus eliminating Odor problem caused due to incomplete oxidation. Also Air supply aids in uniform and efficient mixing inside the tank.
Here sewage is partially decomposed in closed Biological Reactor in absence of Air which leads to reduction of Organic Matter into Methane, Hydrogen Sulfide, Carbon Dioxide etc. It is widely used to treat wastewater sludge and organic waste because it provides volume and mass reduction of the input material to a large extent.