A wastewater treatment process consists of progressive combinations of unit operations. The arrangement of channels, basins, pipelines, equipment, power, land, etc., used in a treatment process is called a treatment works. Each unit operation is designed to provide a specified level of contaminant reduction. The treatment works is operated and maintained under a permit. The performance of the treatment works must be in compliance with the conditions of the permit or enforcement actions can be taken against the owner and operator.
Municipal wastewater, or sewage, is defined as water carried wastes flushed to sewers or collection systems from households, commercial buildings, restaurants and some small water using industrial operations (bakery) together with extraneous other water introduced to the collection system (infiltration and inflow). Municipal collection systems are typically designed to transport a quantity of flow expected to be generated by the contributing population (100 gallons per day per person) and other established connections (dry weather flow). Industrial wastewater consists of discharges from processing and manufacturing facilities and operations.
As industrial wastewaters may contain a wide variety of conventional and non-conventional pollutants, separate treatment processes are usually provided prior to discharge to the municipal collection system (pre-treatment) or directly to the stream. The objective of wastewater treatment is to process the collected influent such that the treated effluent is in compliance with permit requirements and may be discharged from the treatment works without either, endangering public health, or damaging the quality of the environment, beyond an acceptable risk level. In addition waste residuals consisting of removed solids that may be mixed with wastewater, must be properly managed. Acceptable risk may be established through complex analytical modeling or may simply be determined by a perceived level of safety from harm. Most municipal (publicly owned) treatment works are designed to provide a certain treatment capacity for the average pollutant levels (loadings) contained in the design dry weather flow. When storm water enters the collection system, permit violations may result from:
- overflows (sanitary sewer overflows) when the hydraulic flow capacity is exceeded in the collection system and
- by-passing of raw sewage and a deterioration of treatment works performance due to extreme influent flow volumes.