Digester Gas Hydrogen Sulfide Control Using a Full-scale Biotrickling Filter at the Joint Water Pollution Control Plant

The Sanitation Districts of Los Angeles County conducted a full-scale test to determine if a biotrickling filter system could be used to remove hydrogen sulfide (H2S) from digester gas. Testing was done at the Districts largest wastewater treatment plant the Joint Water Pollution Control Plant (JWPCP) which is located in Carson, CA. Most of the digester gas that is generated at the JWPCP is used in a combined cycle energy recovery facility that employs gas and steam turbines to generate approximately 20 megawatts of electricity, which is sufficient to power plant operations. Digester gas that is combusted in the JWPCP’s local South Coast Air Basin is required to have H2S concentrations below 40 parts per million (ppm) to meet air pollution regulations. In order to achieve H2S concentrations below 40 ppm, ferrous chloride (FeCl2) is added to the JWPCP digesters. Addition of FeCl2 is costing the Districts approximately $4 million/year and also adds to solids that require disposal. As an alternative to FeCl2 addition a biotrickling filter technology that was developed by the Districts was tested. A full-scale unit treating gas from one anaerobic digester was evaluated at the JWPCP for 13 months achieving better than 99% removal of H2S after the initial start-up phase. Reduction in the amount of FeCl2 resulted in a savings of $500 per day with a pay-back period of six months. Based upon the test results, the Districts are currently designing a large scale biotrickling filter system to treat 1,500 cubic feet per minute of digester gas. The cost savings from reduced FeCl2 usage at the JWPCP would pay for proposed biotrickling filter system in less than two years.