Typical city sewer systems are designed to collect for treatment or disposal both waste water from homes and businesses including liquid sewage from kitchens, baths, toilets, showers, and sinks, as well as rainwater that runs off roads and parking lots, or drains from downspouts or roofs.
Sewer systems depend largely on gravity where water is gathered as it naturally flows downhill and redirected to a body of water or treatment facility, except in very low areas where pumping stations may be required to move water uphill to larger sewers for collection.
Rainwater and sewage are often separated using a storm drain system with catch basins to prevent them from mixing together, and are then channeled through a network of pipes and pump stations to holding ponds as is often the case with storm water and some surface runoff, or to waste water treatment plants for processing.
Some older combined systems include storm water in with the waste water, but this situation is not preferred and generally avoided due to wide variations in rainfall levels that can overload the system, reduce efficiency of waste water treatment plants, and require unnecessary costly processing of storm water that might otherwise be directly redeposited untreated or minimally treated into local rivers or retention basins.
In urban areas or regions where there is a high amount of contaminant particles such as heavy metals, bacteria from animal waste, fertilizers and pesticides, sediments from soil, airborne pollutants, organic compounds, and vehicle grease and oil that rainwater may pick up from rooftops, bridges, the air, ground and roads, it may be a requirement that most storm water also be treated.
Some towns alternatively install grit chambers, buried vaults with media filters, vortex separators to eliminate coarse solids, retention basins and holding ponds that allow the pollutants to settle out naturally, or wetland environments that act as a natural filter to reduce pollutants.
Sanitary sewer mains frequently follow streams or are installed along main streets, especially in downtown areas where storm runoff averages about nine times higher than in wooded areas, and include covered manholes for purposes of maintenance access. In some towns, grey water or wash water, which includes all waste water except toilet waste and food waste, is being retained separately from black water to be recycled for use by residents in watering gardens or in toilet flushing.
Storm sewer pipes which handle rainwater are generally larger in diameter because massive amounts of water can be produced during major storms, while the sanitary sewer system often uses smaller pipes to carry waste water from homes, buildings, and industrial facilities to a treatment plant. The pipes tend to get progressively larger in diameter as the sewage and storm water move toward the waste water treatment facility to prevent or minimize overflows, backups, and flooding during periods of peak rainfall.
Pipes from residences that serve to collect rainwater runoff and sewage are referred to as building sewers and the section of piping that runs from the house to the property line is considered a private-side connection, whereas the city-side connection covers the area from the property line to the sewer main.
Once the waste water reaches a low point as it nears its destination and the flow slows, it usually must be pumped through force mains to the treatment facility where it is initially filtered with grates or by other means to separate out larger components before processing. To further treat the discharge, solids are removed for incineration by allowing them to separate and settle out, bacteria is then temporarily added to assist in removing nutrients and organic material, and lastly nitrogen and phosphorous are removed and chlorine is added to disinfect the remaining water.
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