Control of hydrogen sulfide (H2S) emissions from air degasifying towers at water treatment facilities is a high priority for municipalities across North America. H2S emissions are controlled by the Clean Air Act of 1990 and its Amendments, and thus require removal from the air prior to being discharged into the environment. The primary regulatory driving force is that of odor nuisance; public health impacts become a concern only at higher H2S concentrations. Unfortunately, H2S has a very low odor detection threshold, and as a result it has to be removed to concentration levels much lower than what is required by the Clean Air Act.
The use of H2S degrading microorganisms to treat off-gas from degasifying (stripping) towers is an important and developing application of cell immobilization technology. Biotrickling filter technology utilizes immobilized microbial cells that are attached to a medium inside the reactor. The H2S is then biologically oxidized (degraded) with microorganisms to odorless compounds. Since the process relies completely on biological means, it is environmentally-friendly and has a low operating cost compared to physical/chemical alternatives such as chemical scrubbers.
A full-scale biotrickling filter was installed at the JEA Buckman Water Reclamation Facility in Jacksonville, Florida. The objective was to determine the effect of very low empty bed residence times and high H2S loadings (> 250 g H2S/m3 .hr) on the biotrickling filter’s performance.
The biotrickling filter uses structured synthetic EcoBase™ media that has a life expectancy of more than ten years. The biotrickling filter inlet and outlet H2S concentrations were continuously measured and recorded with OdaLogs over a 9 month period. The residence time for the BF reactor was varied between 2.4 and 9.8 s, and the H2S concentration varied between 50 and 350 ppmv for the duration of the test period.
At an average empty bed residence time (EBRT) of 2.8 s, the biotrickling filter removed more than 99% of the H2S at volumetric loading rates up to 272 g/m3 .hr. The H2S removal efficiency was 97% at volumetric loading rates greater than 472 g/m3 .hr at the same EBRT.
The results show that the biotrickling filter could effectively remove H2S under very high volumetric loading rates (> 250 g/m3 .hr) and short residence times (< 2.8 s). The significance of this finding is that it is possible to economically treat off-gas from air stripping towers with biotrickling filter technology. Biotrickling filters are an environmentally-friendly technology and provide benefits over existing chemical scrubber technology such as very low operating cost, low maintenance requirements, and elimination of the use of hazardous chemicals at a similar reactor foot print.
