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ALCOSAN – Ravine Street Stream Removal Program

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The Allegheny County Sanitary Authority (ALCOSAN) is committed to reducing extraneous sources of stormwater that enter their interceptor system and subsequently are conveyed to the treatment plant. The entry of large quantities of stormwater contributes to combined sewer overflows (CSO's), hydraulic overloading, sediment buildup and increased treatment costs. One major source of stormwater inflow to the system occurs where natural streams have been intercepted by municipal combined sewer systems and subsequently conveyed to the ALCOSAN interceptor system. In most cases, these direct stream connections were built in an era prior to the establishment of ALCOSAN, when municipal sewer systems simply discharged to the rivers and streams. When ALCOSAN built the sewage treatment plant and interceptor sewer system in the 1950's, these stream inflow points were often distant from rivers and creeks, and therefore they were either unknown or difficult if not impossible to remove. The CSO diversion structures enabled the ALCOSAN interceptor system to take sanitary sewer flow during dry weather periods while allowing the drastically greater wet weather flow to simply overflow through flap gates.

As the regulations for CSO's have changed and become much more stringent since the construction of the ALCOSAN system, it has become necessary to seriously re-evaluate the locations where streams enter the combined sewer system and determine the feasibility of removing them and conveying them to a waterway.

This paper details the process of performing a feasibility study for removal of a direct stream connection to a municipal combined sewer system that flows into the ALCOSAN system. When the original scope of work was developed, it seemed to be a relatively simple problem to approach in that a single location had been identified where the stream entered the sewer system. Despite the availability of many sources of sewer system data, one of the most interesting discoveries was just how many discrepancies there were with regards to the existing system configuration. For example, as the system was investigated, a major highway drainage system was found to be connected to the combined sewer system, yet not shown on any maps. Ultimately, the scope of the feasibility study virtually doubled in magnitude simply due to the findings from the intense field reconnaissance work.

Throughout the development of the alternatives, the complexities of intercepting the flows and conveying them though a densely populated urban community seemed to grow with each subsequent level of design detail. Ultimately, a viable alternative was developed that fulfilled the need to remove the two major sources of stream and storm water inflow to the system and convey them to the river. However, the complexity of the developed alternative was startling in comparison to the apparent simplicity of the original plan to intercept a single stream input source and convey it to the river.
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Document Type: Research Article

Publication date: 2008-01-01

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