Combined sewerage is a sewer system that receives both domestic or municipal wastewater and stormwater.  Usually these sewers are designed to receive the stormwater flow which results from a 10-year flood. This sanitation option is especially suitable for low-income coastal areas subject to regular annual flooding, but also where it is cheaper than simplified or settled sewerage and separate stormwater drainage.

There are two types of low-cost combined sewerage: (1) the Brazilian version which accepts raw (unsettled) wastewater + stormwater; and (2) the Indian and Vietnamese versions which receive settled wastewater (commonly septic tank effluent) + stormwater.

BRAZILIAN VERSION
The reference text is Projetos de Pequenos Sistemas Unitários de Esgotamento (Caixa Econômica Federal, 2004) − in Portuguese.  
►A synopsis of the system in English is given here.

INDIAN VERSION
Combined surface sewerage: a low-cost option for effective sanitation in semi-urban areas of India (Environmental Engineering and Policy, 1999)

VIETNAMESE VERSION
Decentralized sanitation implementation in Vietnam: a peri-urban case study (Water Science & Technology, 2007)
Good Practices in Sanitation Infrastructures for Periurban Poor Communities: A Case in Hanoi (Building Abroad, 2008)

See also: Low-cost stormwater drainage

►Separate and combined sewer systems: a long-term modelling approach (WST, 2009)