Many small communities use septic tanks as a method of partial sewage treatment in situations where effluents may be satisfactorily discharged into the soil. These systems are also suitable for use in many situations where intermittent occupation occurs. Where effluents are discharged into streams in which serious pollution is likely, further treatment by secondary processes is necessary, and in many situations tertiary treatment processes are added as a final safeguard. Prefabricated or package-plant systems using variants of the activated-sludge process or special types of percolating filter, have become available and can be designed by the manufacturers to satisfy all of the purposes of primary and secondary treatment in specific situations. Maintenance requirements of most of these processes are small, but no system may be operated for long periods without a minimum of maintenance.

Some of the simplest systems of sewage treatment can provide excellent sanitary conditions and a high level of protection from health hazards and environmental deterioration if properly designed and operated. Package plants are compact and convenient but not necessarily more effective. They are usually dependent on a reliable power-supply, but have many advantages on restricted sites or sites requiring rapid development or expansion.