Precast Solutions - January/February 2008 - (Page 22)

TECH BRIEF TMI: TOO MUCH INFILTRATION! AN ANALYSIS OF INFLOW AND INFILTRATION FROM UNSEALED SEPTIC TANK ACCESS PORTS. BY ERIC BARGER Watertight tanks are generally overlooked as a solution to inflow and infiltration (I&I) problems in municipal and decentralized wastewater systems. The use of watertight tanks can dramatically reduce treatment costs, reduce overflow events and help contain the ever-increasing problem of inflow and infiltration. Ideally, the amount of treated sewage effluent should be less than the amount of total purchased water. Activities such as watering the lawn, washing the car and consuming water all account for effluent wastewater being less than purchased water. If the opposite is true, then the sewage collection system is experiencing infiltration. The two systems analyzed here are decentralized onFigure 1 site wastewater collection systems. The collection and treatment processes for both systems are handled by a recirculating sand filter (RSF). Both systems are comparable in physical size, customers served, weather patterns and geographical location, and both are serviced by a public utility that provides the public drinking water and sewer collection for each house. The drinking water is metered at each house and monitored monthly for billing purposes. The sewage effluent is pumped to a central treatment facility, where the total treated effluent is also metered. During a six-month period, water meter readings and treated sewage effluent were carefully monitored for Systems 1 and 2. At the end of the six-month period, the total purchased water and treated effluent for each system were evaluated to determine the treated effluent as a percentage of water purchased. This data provides a basis for predicting future performance while giving a monthly performance snapshot, which allows the utility to be proactive in combating I&I along with other issues that may arise. SYSTEM EVALUATION Since recirculating sand filters are not significantly impacted by rainfall events, it is assumed that all treated effluent originates from the septic tanks. System 1 showed an excess of sewage effluent that entered and was treated in the recirculating sand filter (see Table 1). In other words, the total water exiting the recirculating sand filter was greater than the purchased water amount. During the same period, System 2 showed the total 22 PRECAST SOLUTIONS | JAN/FEB 2008

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Precast Solutions - January/February 2008 Contents Precast Concrete Short-Span Bridges Precast Micro-Reinforced Concrete A Winning Edge The Creative Edge TMI: Too Much Infiltration! Architectural Details Cast in Stone

Precast Solutions - January/February 2008

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