There are many different types of processes that could be used to clean up wastewaters depending on the type of waste and contamination. The use of septic tanks and other On-Site Sewage Facilities (OSSF) is still widespread in rural areas.
Sewage can be treated close to where the sewage is created, a decentralized system (in septic tanks, bio filters or aerobic treatment systems), or be collected and transported by a network of pipes and pump stations to a municipal treatment plant, a centralized system (see sewerage and pipes and infrastructure). Sewage collection and treatment is typically subject to local, state and federal regulations and standards.
Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment.
Primary treatment
consists of holding the sewage in a retention tank where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment.
Secondary treatment
removes dissolved and suspended biological matter. Secondary treatment is typically performed by the selected micro-organisms.
Tertiary treatment
consists of treating water by disinfecting chemically or physically through lagoons and microfiltration prior to discharge into a stream, river, bay, lagoon or used for the irrigation or for groundwater recharge or agricultural purposes.