Practical Experience Applied to Biofiltration When a Composting Facility Just Can't Stink

The Rancho Las Virgenes Solids Handling and Composting Facility (Rancho), located in western Los Angeles County, provides biosolids treatment for customers of Las Virgenes Municipal Water District (District) and Triunfo Sanitation District. Treatment includes anaerobic digestion, dewatering, in-vessel composting, and related processes. The resulting product is a soil amendment that meets all federal, state, and local requirements for unlimited distribution (Class A). The plant is adjacent to Malibu Creek State Park, dedicated wildlife corridors, and upscale suburban housing. These surroundings have dictated plant designs that emphasize pleasant appearance and positive control of any odors.The means of treatment for up to 56,000 cubic feet per minute (cfm) of odorous air collected from the in-vessel composting, dewatering, and various storage wetwells is wood media biofiltration. The original biofilter, placed service in November 1994, was constructed inground. Media has typically been roughly-ground soft wood waste, occupying 15,000 square feet, 3 to 3½ feet thick, providing approximately 50 second contact time. This configuration has generally provided excellent removals of odorous gases (primarily ammonia and hydrogen sulfide), particularly during the first one to two years of media life. However, experience during the first six years of operation has indicated a number of maintenance concerns and opportunities for design improvements. These have become a priority as surrounding residents have become very concerned about off-site odors during periods of operational difficulties, near-exhausted media, and maintenance operations.Over several years, the District has worked together with Black & Veatch Corporation to address these concerns and develop improvements. Most recently, this has led to a 1 million project to provide a major expansion and renovation of the biofilter. Design factors addressed include:Facilitating access for media replacementProviding redundancy to maintain full operations during maintenanceImproving drainage and air distribution to mediaMaintaining optimum media moisture, including humidification of odorous air prior to treatmentOptimizing continuous odor removal and facilitating improved performance monitoringReducing operations and maintenance costsBlack & Veatch originally proposed a revised design where the foundation of the media would be concrete, with integral air plenums and distribution channels. Value engineering of this concept with District staff led to a plan to complete a number of improvements to the existing inground facility, while providing the expanded area with the new concrete design. Following bidding of this dual facility plan, further negotiations with the contractor and vendors led to modification of the concrete design. The completed installation utilizes the patented BacTee aeration system on top of a concrete slab-on-grade. This lowered construction costs, due to the simplified concrete construction, and enhanced air and moisture distribution, improving treatment efficiency, extending media life and reducing energy requirements.