SIDE-BY-SIDE TEST OF A CHEMICAL SCRUBBER VS. THREE BIOTRICKLING TOWERS

In 2004 King County (the County) began pilot testing the control of wastewater odors using biotrickling towers (BTT)/bioscrubbers in a side-by-side comparison with a three-stage wet chemical scrubber at the South Treatment Plant in Renton, Washington. The study objective was to compare performance of a three-stage chemical scrubber to three different vendor-supplied BTTs, each followed by a carbon bed adsorber. In addition to evaluating the performance of the four units, operation and maintenance requirements were compared to determine if biotechnology could be used to adequately control odor from King County's collection and treatment systems.The chemical scrubber used sulfuric acid in its first stage, sodium hydroxide (caustic) in the second, and sodium hydroxide with sodium hypochlorite in the third. The BTTs varied in their construction and operation. Each contained one or two stages of treatment and each was rated for approximately 2,000 cubic feet per minute (cfm). All four scrubbers were connected to a common header conveying foul air from the plant's raw sewage influent channel. Makeup water was disinfected plant secondary effluent.The pilot test program consisted of three phases: start-up, performance testing, and upset testing. Start-up of the units allowed each BTT sufficient time to grow a population of microorganisms for stable odorous air treatment. Performance testing was conducted using empty bed residence times (EBRT) of approximately 16 and 6 seconds for the BTTs and approximately 1.5 seconds per chemical scrubber stage. The hydrogen sulfide (H2S) concentrations varied from about 1 to 4 parts per million by volume (ppmV) in the foul air. Bottled H2S was spiked into the foul air during a portion of the test to bring the concentration up to approximately 20 ppmV. Upset testing was performed twice, once by interrupting the foul air supply and the second time by interrupting the make-up water supply, for 24-hours each, and monitoring performance before and after each interruption. An unplanned upset occurred when two of the BTTs were accidentally impacted by a chemical release resulting in an interruption in the ability of the microorganisms to control odor. Successful reseeding of the microorganism population provided valuable data showing the recovery of odor control after such an unplanned event.The study results show that BTTs can compete with chemical scrubbers to remove residual odor, especially when coupled with carbon adsorbers. The operation and maintenance time required for the BTT systems was less than for the chemical scrubber. The variability of the data was also lower. This paper will focus on the results of the pilot test and the performance evaluation of the chemical scrubber and BTTs under the testing conditions. Results are presented in terms of H2S removal, odor removal (with associated change in hedonic tone), volatile organic compound (VOC) and reduced sulfur compound (RSC) removal, and lessons learned from the pilot study. This paper will be of interest to utilities that either are using chemical scrubbers or biotechnology for odor control or are interested in learning more about biotechnology.