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MODULE-5 Wastewater Management
Small Sewage Treatment Plants
Any system that exceeds a discharge volume of 14,500 gpd currently requires treatment beyond that of an on-site or shared system and must be permitted by the State Department of Ecology (Ecology) through the Waste Discharge Permit regulations (Chapter 173-226 WAC). The permit places strict restrictions on the pollutant concentration that may be discharged, and also includes provisions for monitoring and reporting requirements. Effluent limitations in a permit must be either technology-based or water quality-based. The more stringent of these two types of limits are selected for each pollutant of concern. The design of the facility, especially the disposal facilities, is closely reviewed by Ecology. Once secured, permits must be renewed or administratively extended every five years. Beginning in 2009, systems with design flows between 14,500 and 100,000 gpd will be approved and regulated by DOH.
The use of a small sewage treatment plant expands the options for higher density development as lot sizes are not controlled by the need to provide an on-site septic system. Instead of wastewater as a control, density is determined by local zoning and subdivision rules and regulations, providing flexibility in design and land use management. Some sewage treatment plants can remove up to eight times more nitrogen than a conventional on-site septic system, thereby providing the option for higher density/cluster type development with reduced water quality impacts.
The drainfield area requirements for small sewage treatment plants is significantly less compared to on-site sewage systems due to the higher treatment and removal of solids that could otherwise clog the drainfield. Disposal is often managed through subsurface trenches or fields or through open infiltration basins. With good soils, the infiltration rate can be up to six times faster than that for an on-site sewage system, requiring significantly less land for the disposal facility.
There are two primary issues to consider with the use of a small sewage treatment plant: cost and wastewater disposal constraints. New treatment plants are expensive to construct, upgrade, and maintain. The key to making a treatment plant affordable is to maximize the number of users to lower the capital cost, as well as long-term operation and maintenance costs, for those using the facilities. Appropriate design of the collection system (sewers) can also keep costs down. There are also several Federal and State grant and loan options available to local communities to support effective wastewater treatment and management. Some funding options for local communities are provided in Ecology's Criteria for Sewage Works Design ("Orange Book") Section G3-2.
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