Principles of Collection

Combined

A combined sewer is a sewage collection system of pipes and tunnels designed to simultaneously collect surface runoff and sewage water in a shared system. Under certain wet weather conditions, however, these sewer systems can become overloaded and release some of the untreated combined waste streams, these are known as combined sewer overflows (CSOs). This type of gravity sewer design is no longer used in almost every instance worldwide when constructing new sewer systems.

Seperate

Separate sewer systems have two separate pipes. One pipe carries stormwater (rain water) from storm drains to local streams. Pollution and trash in stormwater flows to local streams with little or no treatment. A second pipe carries sanitary sewage to the wastewater treatment facility.

Separate sewerage consists in the separate collection of municipal wastewaters (blackwater from toilets, greywater and industrial wastewater) and surface run-off (rainwater and stormwater). The separate collection prevent the overflow of sewer systems and treatment stations during rainy periods and the mixing of the relatively little polluted surface run-off with chemical and microbial pollutants from the municipal wastewater. The design of the sewers and the (semi-)centralised treatment stations thus needs to consider the volume of the wastewater only and the surface run-off and rainwater can be reused (e.g. for landscaping or agriculture) after a simplified treatment.

Principles of Transportation

Gravity

Pressurized

Vacuum

©Biao Huang @Ningbo University