EVALUATION OF INNOVATIVE FOUL AIR TREATMENT YIELDS COST EFFECTIVE LOW RISK SOLUTION

The City of San Diego (City) owns and operates the Metro Biosolids Center (MBC) that processes raw solids pumped approximately 4.25 miles from the North City Water Reclamation Plant (NCWRP) and digested biosolids pumped approximately 17 miles from the Point Loma Wastewater Treatment Plant (PLWTP). The stabilized solids are dewatered and stored in silos and loaded onto trucks for offsite disposal.Foul air withdrawn from various process areas and from process equipment is treated in two odor control systems (OCS): one central odor control system and one at the plant wastewater/centrate pump station. The original design for each of the OCS provided for three stages of foul air treatment using an acid packed bed scrubber for ammonia removal, a caustic/hypochlorite packed bed scrubber for hydrogen sulfide and other reduced sulfur compounds removal and carbon adsorbers for volatile organic carbon compound removal and final polishing. The treated air is discharged to the atmosphere via exhaust stacks. While chemical scrubbers at the two OCS facilities are capable of treating foul air, they require the use of hazardous chemicals and operation of several ancillary devices such as chemical storage, metering pumps and significant instrumentation at considerable expense to the City.During initial startup of these facilities, a control malfunction occurred that caused a chemical spill from one of the chemical day tanks, damaging valves and electrical equipment located in the containment area. To avoid possible future damage to this equipment, the City wanted all this equipment raised out of the containment area. Because retrofitting these facilities would be very costly, innovative foul air treatment technologies were evaluated as an alternate solution that could eliminate the use of chemicals avoiding the costly retrofit and reducing operating costs.The paper evaluates the feasibility of implementing innovative technologies for odor control at MBC. Replacement of chemical scrubbers with innovative technologies could eliminate the use of hazardous chemicals, improve worker safety, reduce foul air treatment costs and eliminate the risk of possible chemical spills that could damage equipment. Bioscrubber and biofilter technologies were evaluated. Lifecycle costs for these two technologies were compared to conventional technologies to determine the most cost effective treatment method. The conclusions presented are based on an average foul air H2S concentrations at the two odor control facilities below 5 ppm, as observed during the foul air sampling. A sensitivity analysis was performed for higher concentrations to determine the level at which one technology became more cost effective. As a result of this study, the City has received approval from the Air Pollution Control District to modify their permit to operate for their odor control system at the wastewater/centrate pump station allowing them to eliminate the use of chemicals.