A sewerage or sewer system collects wastewater. Sanitary sewers were designed and built to carry wastewater from domestic, industrial and commercial sources. (EPA 2004) Gravity is used to convey the wastewater wherever possible. Thus, it is not surprising that natural stormwater drainage is usually used. The principle of using gravity as the driving force for conveying wastewater in a sewerage system should be applied wherever possible, because this will minimize the cost of pumping. Sewerage systems can be mainly classified into combined sewerage and separate sewerage. Combined sewerage carries both stormwater and wastewater, while separate sewerage carries stormwater or wastewater separately. Recent trends have been for the development of separate sewerage systems. The main reason for this is that stormwater is generally less polluted than wastewater, and that treatment of combined wastewater and stormwater is difficult during heavy rainfalls, resulting in untreated overflows. In practice, there is usually ingress of stormwater into wastewater sewerage pipes, because of unsealed pipe joints, and unintentional or illegal connections of rainwater run-off. Conversely, there may be unintentional or illegal wastewater connections to stormwater sewerage. (UNEP 2000)

The graphic on the right illustrates the difference between a combined and a separated sewerage system and the table at the bottom shows the advantages and disadvantages of these two systems. Further types of sewer systems can be found at the end of this page.

In many developing countries, more than 70% of industrial wastes are dumped untreated into waters where they pollute the usable water supply. (WWAP 2009) In many countries, the responsibility for industrial wastewater treatment falls back on ordinary taxpayers. In the absence of a user pays system for pollution control, large volumes of contaminated industrial wastewater end up in municipal sewage treatment plants, which are expensive to construct, operate and maintain. The Netherlands introduced a series of incentives to polluters to reduce pollution at source, rather than opting for the more expensive end-of-pipe solution of public sewage treatment. This approach has been cost-effective in reaching water quality targets. (Corcoran et al. 2010) The treatment of industrial wastewaters before discharge to the sewer is also extremely important in preventing pollutants from entering the wastewater and stormwater systems. (UNEP 2000)

Combined Sewers	Separated Sewers
Advantages
Stormwater and wastewater can be managed at the same time	Stormwater and wastewater can be managed separately
Convenience (minimal intervention by users)	Convenience (minimal intervention by users)
Lower water demand	Limited or no risk of sewage overflow
Moderate operation and maintenance costs	Moderate operation costs
	No problems related to discharging industrial wastewater
	Surface run-off and rainwater can be reused (e.g. for landscaping or agriculture)
Disadvantages
Need a reliable supply of piped water	Needs a reliable supply of piped water
High capital costs	High capital costs, more expensive than combined sewer system
Difficult to construct in high-density areas	Difficult to construct in high-density areas
Requires skilled engineers and operators	Requires skilled engineers and operators
Adequate treatment and/or disposal required	Adequate treatment and/or disposal required
Problems associated with blockages and breakdown of pumping equipment	Problems associated with blockages and breakdown of pumping equipment
Recycling of nutrients and energy becomes difficult	Need for pumping on flat ground
Risk of sewage overflow	Difficult and costly to maintain

(adopted from Seyoum 2013)

• Further types of sewer systems