BIOFILTER INSPECTION AND TESTING PROGRAM RESULTS IN LOWER BIOFILTER O&M COSTS AND IMPROVES BIOFILTER PERFORMANCE

The Clark County Water Reclamation District (CCWRD) began experimenting with biofilters in 1989 when the CCWRD staff designed and installed biofilters at the Lincoln Lift Station and the Central WRF siphon inlet structure. Since the initial installations, CCWRD has expanded the program and now operates 15 biofilters that treat odors from sewers, lift stations, and water reclamation facilities. The overall performance has been good to excellent; however, the most important result is that the biofilters are a significant improvement in both cost effectiveness and performance compared to the previous odor control systems. This paper presents the most recent biofilter designs, the design improvements over the last 13 years and the inspection and testing protocols that were important in the developing the CCWRD biofilter program.CCWRD certifies each biofilter per a testing plan approved by the Clark County Dept. of Air Quality Management. The certification process measures airflow and H2S gas and NH3 removal efficiency at several locations on the biofilter surface. The CCWRD conducts additional sampling to evaluate the performance of the biofilter at several locations beneath the surface of the biofilter. Inspectors measure pH, H2S gas removal efficiency, media condition and moisture content at various depths in the media. The additional sampling is intended to identify the biofilter components that remove the most odors, media mixes that last longest and persistent O&M problems. CCWRD used the initial findings to change the organic media to stabilized wood chips, which has resulted in lower headlosses while maintaining greater then 99% H2S gas removal efficiency. The following is a summary of the important findings:Stone air plenums typically remove 60% of the inlet H2S gas concentrations.99%of H2S gas removal occurred in the first 6 inches of organic mediaShort-circuiting is usually due to drying caused by improperly set sprinkler systems.Media consisting of stabilized wood chips provide the longest media life, the lowest headloss (2 inches WC after 1 year) and were easiest to maintain.Over watering is the most significant O&M problem.Future Improvements. CCWRD refined its biofilter design based on their inspection data; however, the district also investigated performance data from inorganic media biofilters and organic media biofilters at other municipalities. The new design utilizes thiobacilus growing on the acid resistant stone media to biodegrade H2S gas to sulfuric acid before it reaches the organic media, which degrades quickly in acid conditions. The new design includes two stone media layers intended to remove 95% of the inlet H2S gas and a relatively shallow organic wood chip layer to polish the effluent air. Since this design requires regular irrigation, it takes advantages of the over watering “problem”. The design combines the best features of inorganic media biofilters (long media life and excellent H2S gas removal efficiency) and organic media biofilters (high loading rates and excellent organic odor and H2S gas removal efficiency). Pilot testing of the new hybrid design should began in November 2003. The current pilot test uses reclaimed effluent water for irrigation water. Inlet air stream is for this test is greater then 40 ppm H2S gas.