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Abstract

Sewer and Tank Sediment Flushing: Case Studies (EPA/600/R-98/157) 1998

Past studies have identified urban combined sewer overflow (CSO) and stormwater runoff as major contributors to the degradation of many urban lakes, streams, and rivers. Sewage solids deposited in combined sewer (CS) systems during dry weather are major contributors to the CSO-pollution load. Innovative methods for cleaning accumulated sludge and debris in CSO and stormwater conveyance systems and storage tanks have emerged over the last 15 years by creating high speed flushing waves to resuspend deposited sediments. Cleansing efficiency of periodic flush waves depends on flush volume, flush discharge rate, sewer slope, sewer length, sewer flow rate, sewer diameter and population density. Maximum flushing volumes at upstream points are limited by available space, hydraulic limitations and costs. Maximum flushing rates at the downstream point are limited by the regulator/interceptor capacities prior to overflow. The relationship between cleaning efficiency and pipe length is important. The aim of flushing is to wash the resuspended sediment to strategic locations, i.e., to a point where the waste stream is flowing with sufficient velocity, to another point where flushing will be initiated, to a storage sump which will allow later removal of the stored contents, or to the wastewater treatment plant (WWTP). This reduces the amount of solids resuspended during storm events, lessens the need for CSO treatment and sludge removal at downstream storage facilities, and allows the conveyance of more flow to the WWTP or to the drainage outlet. This report will demonstrate that sewer system and storage tank flushing that reduces sediment deposition and accumulation is of prime importance to optimizing performance, maintaining structural integrity, and minimizing pollution of receiving waters.

This document is also available for purchase throught the National Technical Information Service web site. NTIS# PB99127839

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