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MWMC FACILITIES PLAN near the bottom within a layer of river rock. Below the rock is a sloped membrane for capturing the moisture. Above the river rock is a layer of media approximately 5 feet thick. Keeping the media moist is critical for proper operation of the biofilter. This is accomplished by supplying a spray nozzle in the ductwork upstream of the biofilter and by providing an irrigation spray system on top of the biofilter. Moisture sensors are provided for controlling the irrigation system. The footprint would be large because of a required loading rate of 4 to 6 cubic feet per minute per square foot (ft3/minute/ft2) and contact time of 60 seconds through the organic media. Alternative 2--Conventional Open-Space Soil IVledia Biofilter This system consists of an above- or partially belowgrade configuration with retaining wall or soil berm filled with an inorganic media, such as selected native soil, to grow sulfur- reducing bacteria. The media allows the cultivation of a fixed-film growth that consumes the odors as air travels upward through the filter bed and into the atmosphere. The odorous air is evenly distributed throughout the biofilter by distribution piping located near the bottom within a layer of river rock. Below the rock a sloped membrane captures the moisture. Above the river rock is a layer of media, approximately 5 feet thick. A layer of washed rock is applied over the top of the soil media for aesthetics and weed control. Keeping the media moist is critical for proper operation of the biofilter. This is accomplished by providing a spray nozzle in the ductwork upstream of the biofilter and by supplying an irrigation spray system on top of the biofilter. Moisture sensors are provided for controlling the irrigation system. The footprint would be large because of a required loading rate of 2 to 3 ft3/minute/ft2 to allow a contact time of 60 to 90 seconds through the soil media. Alternative 3--Bioscrubbers (Biotrickling Tower) In a biotrickling tower, odorous air is blown into the bottom of the tower and flows up through the media material. Treated air migrates out of the filter bed and into the atmosphere. The media may be a synthetic material or an inorganic material such as lava rock. The media allows the cultivation of a fixed-film growth that consumes the odors that pass through it. The bacteria also use other odor compounds as a food source, including ammonia and various organic reduced sulfur compounds. Recirculation pumps provide a continuous stream of water that keeps the media wet, provides nutrients, and carries away waste products. A source of water, preferably non-chlorinated secondary plant effluent, is needed for the unit. If potable water is used, a supplemental nutrient feed system is required. The recirculated water is continually blown down to a drain to control pH and remove waste products. Drain water will be low in pH and should be routed to a flow stream where it can be diluted. Design bed velocities for biotrickling towers are 50 feet per minute (f-pm) maximum. The design head loss through the media is generally about 0.3 inches per foot of bed depth. A scrubbant recirculation pump is required to keep the media moist, add necessary nutrients, and maintain pH. The footprint for the bioscrubber would be smaller than for the conventional filters because of the tower configuration. Results The order-of-magnitude capital cost comparison and non-monetary comparison are summarized in Table 6.2.5-3. 6-18 MWblC_6.0_REV11.DOC