THE ORANGE COUNTY SANITATION DISTRICT GOES FOR HIGH-RATE BIOTOWERS IN A BIG WAY

The Orange County Sanitation District (District or OCSD) of Fountain Valley and Huntington Beach, California has experimented with pilot-scale and full-scale biotowers (also called biotrickling filters) for several years with the goal of reducing chemical consumption while providing superior odor control. Studies have evolved from the experimental stage to the design stage. Three separate major design projects are underway to treat headworks and primary odors at the District's two wastewater treatment plants, including odors from the headspaces of the influent trunklines. When operational, these odor control systems will treat in excess of 560,000 cubic feet per minute (cfm) of foul air on a continuous basis. In the proposed two-stage systems, high performance biotowers packed with polyurethane foam (PUF) media will perform the bulk of the hydrogen sulfide (H2S) removal allowing second-stage chemical scrubbers to be optimized to control the remaining reduced sulfur compounds (RSCs) prior to atmospheric discharge. When used in polishing modes, the chemical scrubbers will consume small fractions of the chemicals that they currently consume in single-stage applications. This treatment train was selected to support the District's long-term goal of reducing odors while minimizing chemical usage.In this paper, design of the 12 new biotowers and eight chemical scrubbers to be installed to treat foul air from the District's Plant No. 1 headworks and primary clarifiers, and the seven new biotowers and five new chemical scrubbers being installed to treat foul air from the District's Plant No. 2 primary clarifiers are presented and discussed. The focus is on two important design aspects: the design of the biotowers and establishing the foul air flow rates to provide sufficient odor capture. The unique demands of these systems could not be met with off-the-shelf equipment or control systems. Modifications of commercially available equipment are required to meet the demands of high-rate biological treatment systems. In particular, scrubbers need to be modified to provide multiple, shallower lifts of media to reduce packing compaction and associated pressure drops; more access ports are built in to facilitate media removal and replacement. Makeup water will be conditioned to reduce hardness and chlorine residuals to prevent media scaling and shocking of the biomass, respectively. Water trickling rates will be controlled to provide sufficient water for biological treatment without flooding the media. Sump blowdown rates will be controlled to maintain the pH in the optimal range of 1.5 to 2.5 pH units. Variable frequency drives will power fan motors to maintain constant air flows under variable media pressure drops.