Combined Sewers-Quick Definition and History

 Combined sewers systems (CSS) are a system of pipes which carry both sanitary wastes and rainfall runoff. Most cities in the US have separate sewers where sanitary wastes are managed in a separate system of pipes as storm water runoff. 

CSS are an artifact of older sewers systems typically built before sewer treatment plants started to be built circa 1900. In the old days (ie 150,000 years ago to 100 years ago) most humans lived in small towns or rural areas with no organized systems of drainage nor to manage human waste except privies (outhouses). As towns developed from small villages, systems of pipes were developed to replace ad hoc collection of ditches along streets and typically drained these ditches (then gutters as streets are paved) away from developed areas to the nearest point of discharge, typically the closest stream, river, bay. These systems made a lot of sense, a single pipe under the street would carry both household/municipal sewage and rainfall to outfalls that discharge the waste/runoff into "receiving bodies" (final destination of the wastes, typically local rivers, bays or ocean).

Before 1870 or so, indoor plumbing was relatively rare. Most households used privies, typically outdoor but also in basements, or chamber pots which frequently wold be emptied into rough ditches along the street after use. The primary waste carried was runoff from streets including urban pollution from sources such as horses (manure), chamber pots, privy overflow, etc. during storms. As indoor plumbing became more common, households started connecting to the drainage pipes. As cities grew and more household wastes, sanitary wastes and other municipal effluent was discharged directly to rivers, etc. the problems associated sewage (odors, sediment, public health, etc) became more noticeable as systems discharged all waste/runoff directly into receiving bodies without treatment.  

Early attempts to deal with these problems typically consolidated early outfalls and moved them to less noticeable locations. Famous historic efforts include Rome (Cloaca Maxima), London , Chicago (Sanitation of Chicago,Chicago River). The website "Sewer History" includes other examples in a   historical review. Around 1900 cities started to provide some sewage treatment starting with primary treatment only (basically, removing large objects before discharge) and cities with combined sewers treatment plants would provide enough treatment for sewage flows during dry weather. During rain storms runoff would quickly exceed the capacity and sewage and rain runoff would be flushed out the original outfalls.

There are approx. 860 combined sewer systems throughout the United States (as compared to approx. 65,000 separate sewer systems regulated by the EPA) and many more in Europe and other countries. Most are in the older cities of the east coast and mid-west. On the west coast, portions of Seattle, a few cities in Washington state, Portland, San Francisco and a small part of Sacramento (downtown) are combined. San Francisco is primarily combined but areas currently being redeveloped are being separated. The EPA regulates discharges into lakes, rivers, bays and oceans based on the 1972 Clean Water Act (CWA) . In EPA regulatory terminology, the discharged water is called Combined Sewer Overflows (CSOs). See also EPA "FAQ".  Various states also have developed regulatory frameworks for regulating discharges into rivers, etc. in California the first laws were the "Porter-Colgone Act" (current code as of 2021). California's laws are enforced by the states Regional Water Quality Boards (RWQB), in the SF Bay area it is the San Francisco Bay Area Regional Water Quality Board.

For the most part combined sewers are seen as a problem since wastes including sanitary/human wastes are flushed untreated into bays and rivers. This is especially true for cities that discharge into rivers or other water bodies which are also used for drinking water. Both regulatory pressure and public distaste have led cities to either attempt to separate the sewer system or improve treatment both improving sewage treatment to include secondary treatment (increased removal of smaller pollutants) and improved treatment of wet weather discharges. During wet weather, treatment plants can increase their treatment capacity and water can be better stored and conveyed to increase the amount of water treated before being discharged. 

In dense urban cities, combined sewers might actually be considerably cleaner then separate sewers. Urban storm water can carry a considerable amount of pollution, both macro (ie large trash) and micro (oils, sanitary wastes in streets, etc). In some cases the costs associated with building new separate systems, attempting to remove pollutants from separate storm systems and the resultant water quality are better optimized by improving existing combined sewers instead of spending money on separate systems.