Biofiltration Applications for Air Emissions Control using Natural Media: Media Stability and Design Optimization

Lift stations and wastewater treatment plants are a source of odors involving hydrogen sulfide (H2S), which results from the anaerobic decomposition of sulfates. Traditionally the control of H2S emissions includes direct air stripping, precipitation and chemical oxidation. In this study, low cost biofiltration using natural media such as wood chips and compost has been used efficiently for odor control and removal of low concentration H2S at wastewater lift stations in the semi-arid environment of the Rio Grande Valley. The City of Brownsville, Texas and the Public Utilities Board have been working with the Environmental Engineering Department at Texas A&M University Kingsville to optimize the performance of lift station biofilters for air emissions and odor control applications in residential and commercially developed areas. Low cost media selected for use included compost and wood waste chips from the Brownsville Municipal Landfill Composting Facility. The large size wood waste chips left after the compost screening have several characteristics which make them ideal for use as biofiltration media when combined with compost. The chips have already shown some resistance to degradation and can provide biofilter support and inhibit compaction, which can increase performance and increase bed useful life.The objectives of this research were to evaluate the removal efficiency of natural media biofiltration for H2S and ammonia emissions from wastewater lift stations, evaluate natural media stability, and develop a biofilter design incorporating a wood waste chip/compost ratio for optimal emission control and biofilter longevity.Three biofilters were constructed at 80/20, 50/50, and 20/80 wood chip/compost ratios. Controlled flows of H2S and ammonia from gas cylinders, and moisture saturated air were mixed to obtain a stream with 30 ppm H2S and 10 ppm NH3 which was delivered to the inlet of the biofilters. The calculated flow rates insured 120 seconds total empty bed contact time. Effluent H2S, NH3, CO2, and H2O in the gas stream were measured. H2S, CO2 and H2O from three intermediate ports were also measured to create a profile across the bed. Samples of the biofilter material were collected and extracted with dionized water and analyzed for nitrate, sulfate, pH and C/N ratios.The data were used to calculate the removal efficiency and the kinetics. Effluent H2S removal was greater than 97% for all of the biofilter mixtures almost immediately; however, within the first six inches of the 80/20 and 50/50 wood chip/compost biofilters, H2S removals started at 76% and gradually increased to 97% within four to eight days. Ammonia reduction was the highest for the 80/20 and 50/50 biofilters. Based on a multiparameter analysis the 50/50 mixture of compost/wood chips appeared optimal based on excellent H2S removal, coupled with a long term supply of compost nutrients for biomass growth, and adequate woody mass to reduce long term compaction and pressure drop.Analysis of the H2S sampling port data and the CO2 respiration data suggested that most of the activity occurs in the first section of the column (bottom 0.15m (6 inches) of the biofilter). It appeared that some threshold microbial activity may support a baseline value of approximately 10–12 ppm of ammonia emission regardless of what substrate gas is introduced into the natural media biofilter. However, the addition of ammonia gas and the baseline ammonia generation did not affect the removal efficiency of the biofilters for H2S. The acclimated microorganisms in all the three biofilters were able to survive a three day shut down and upon restart reacclimated in a very short period of time.