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Older sewer systems experience excessive wastewater flows due to groundwater infiltration and stormwater inflow. Considerable resources have been spent as communities attempt cost-effective elimination of extraneous flows in relation to the cost of interception and treatment. It is difficult to determine just when this point is reached.

The City of Enumclaw in western Washington has spent time and money for 25 years in smoke testing, storm separation, pipe joint grouting and lining. The City is now planning to upgrade their wastewater treatment plant. How much more extraneous flow can be excluded, and at what cost, is a critical question. Population is 11,180. Wet weather peak day flow is 5.3 mgd while dry weather flow averages 1.4 mgd. This 3.8 to 1 ratio of peak day to dry day flows is a vast improvement from past years, and demonstrates what City efforts have achieved.

The Clovercrest basin appeared to have the most severe remaining wet weather flows with a 6:1 ratio of peak day to dry day flows. Total project costs for Clovercrest pipe system rehabilitation is 620,000. During wet weather the excluded infiltration may average about 25,000 gpd, plus an additional 14,000 gpd induced by rainstorms for a 39,000 gpd total. Excluding of this flow would cost about 15.90 per gpd. The new treatment facilities will be designed for 3.7 mgd at an estimated project cost of 16,200,000, or 4.38 per gpd. Annual operating cost is 0.0009 per gallon. Total life cycle costs over 50 years are 8.20 per gallon, which is less expensive than rehabilitation.

Spring 2002 flow monitoring indicates that parts of the Water Yard basin have higher I/I rates than Clovercrest. It is not clear that these are sufficiently higher to make rehabilitation cost-effective. However, rehabilitation may be a good public investment as maintenance replacement of older pipes so capacity is available for future growth and development.
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Document Type: Research Article

Publication date: 2002-01-01

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