Sewers and Sewer Systems

Sewers are pipes laid in the ground connecting household and industrial wastes and street gutters to the local waste water treatment plant. Sanitary sewers carry household, industrial and commercial waste, while storm sewers carry waste from street gutters. There are combined sewers that contain both waste from sanitary and storm sewers, however, these are older systems. Combined sewers are no longer being constructed because during large storms, treatment plants cannot handle the large amounts of material coming through, and usually part of the inflow must be diverted. If a storm sewer is present, this flow will simply be diverted. However, if only combined sewer systems are present, then part of the combined stream is diverted, carrying some sanitary waste into the river or stream. Sewage is the term used to describe the actual waste stream that flows through the sewers, and sewerage is the system of sewers that connects to the treatment plant.

Other important terms in sewer design are infiltration and exfiltration. Infiltration is the leakage of groundwater into the sewer system through pipe joints, cracked pipes, or manholes. Exfiltration is the leakage of sanitary waste out of the sewer by means of pipe joints and cracks.

Sewers need to be laid deep enough to avoid breakage due to pressure from cars, etc. They also should be laid low enough to avoid freezing in the winter months, and should be able to drain the lowest area. To determine this, we need to collect such data as physical (topography, soil conditions, water table, location of streets, other sewer networks, and underground structures), developmental (water use data, population projection), political, and financial characteristics. Common depths for sanitary sewers are 3" below basement or 11" below foundation floors. The size of the sanitary sewer can be determined by many methods, one of which is the Harmon formula, which gives a ratio of the peak flowrate to the average flowrate that is dependent on the current population in thousands, P.

Storm sewers can be sized by studying storms of a specified frequency or with the unit hydrograph that relates rainfall intensity to storm duration. The typical storm frequencies used are as follows: 2-5 years for residential areas, 10-25 years for commercial districts, and 50-100 years for major drainage systems.

Manholes are required in every sewer system for easy access to the sewer for cleaning and maintenance. They are usually found at every junction or change in slope of a sewer system, and at minimum of 300' to 400' apart (for pipes up to 24" in diameter).

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