The major objective of maintaining storm or combined sewer systems is to provide maximum transmission of flows to treatment and disposal, while minimizing overflows, bypasses, and local flooding conditions. This objective can be achieved by maintaining system facilities at peak capacity.
The significance of collection system maintenance as a best management practice is that when properly applied, extraneous solids and debris are removed in a controlled manner, not accumulated as pollutant sources to be flushed into receiving waters under storm conditions.
The basic part of a maintenance program is regular system inspection. Specific tasks include: (1) catchbasin maintenance; (2) cleaning (both deposits and root infestation) and flushing of pipes; (3) removal of excess shrubbery and debris from flood control channels and ditches; and (4) control of inflow and infiltration sources.
Sewer cleaning involves routine inspection of the sewer system. All plugged or restricted lines should be cleaned. Major problems in large-diameter sewers are siltation and accumulation of large debris like shopping carts and tree branches. In small-diameter sewers, siltation and penetration of tree roots are major problems. Benefits of sewer cleaning include reducing local flooding, emergency repairs, and pollutant loading. Increased carrying capacities and reduced blockages in interceptor/regulator works may directly reduce overflows.
Many types of sewer cleaning equipment are used, including hydraulic, mechanical, manual, and combination devices. The cleaning tool is pushed or pulled through the sewer to remove obstructions or cause them to be suspended in the flow and carried out of the system. However, large sewer and interceptor cleaning involves unique problems because several feet of sludge blanket can accumulate.
Regular flushing of sewers can ensure that sewer laterals and interceptors continue to carry their design capacity, as well alleviate solids buildup and reduce solid overflow.
Sewer flushing can be particularly beneficial in sewers with very flat slopes. If a modestly large quantity of water is periodically discharged through these flat sewers, small accumulations of solids can be washed from the system. This cleaning technique is effective only on freshly deposited solids.
Internally automatic flushing devices have been developed for sewer systems. An inflatable bag is used to stop flow in upstream reaches until a volume capable of generating a flush wave is accumulated. When the correct volume is reached, the bag is deflated with the assistance of a vacuum, releasing impounded water and cleaning the sewer segment.
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