The City of Waxahachie, TX required odor control for the Preliminary Treatment Area of the Waxahachie Wastewater Treatment Plant. A lava rock based biotrickling filter was installed to treat hydrogen sulfide gas present in the Preliminary Treatment Area via a bio-oxidation process.
When the Waxahachie Wastewater Treatment Plant expanded their plant to 8 MGD, they added a biotrickling odor control system to treat the Hydrogen Sulfide, H2S, generated in their Preliminary Treatment Area. This paper focuses on the design and operation of the biotrickling filter and also discusses the benefits of utilizing a lava rock media for treatment of H2S.
LAVA ROCK SYSTEM DESIGN AND OPERATION
The biotrickling filter technology installed at the Waxahachie Wastewater Treatment Plant utilizes an inorganic lava rock media for the removal of H2S. The system configuration allows the gaseous phase to flow counter-currently through the bacteria oxidation ecosystem consisting of a recirculation solution flowing over the porous rock media. Figure 1 depicts a P&ID of the bioscrubber system.
Because the inorganic media, lava rock, does not contain nutrients found inherently in organic media, it must be inoculated at system startup to provide the proper fixed-film biomass necessary to support the sulfur oxidizing bacteria. The inoculant is typically a mixture of primary effluent and non-potable water, which is fed into the vessel sump and then recirculated to the top of the scrubber vessel, in order to inoculate the entire media bed. The recirculating solution is continuously injected via PVC spray nozzles over the media to optimize the growth of the sulfur oxidizing bacteria. After initial inoculation and system start-up, secondary effluent is the most common source of makeup water. Makeup water is added as needed to react to one of two scenarios. The first is if the system pH drops below 2, the second if the sump level drops to the low-low point due to evaporative losses. In some circumstances a nutrient solution must be added when the effluent does not contain the proper minerals required. The nutrient solution is a mixture of basic minerals which approximate the mineral content of secondary effluent.