SOMETHING OLD…SOMETHING NEW: A MULTI-FACETED APPROACH TO ODOR CONTROL FOR THE CITY OF CLEARWATER

The City of Clearwater has successfully implemented several odor control technologies at its East Advanced Pollution Control Facility (APCF) which is located directly on the shores of Old Tampa Bay, Florida. Prior to 2002, odor control measures in place at the plant consisted of the injection of hydrogen peroxide (H2O2) in the collection system just upstream of the influent pump station and the injection of an enzymatic solution into a wet scrubber servicing the sludge holding tank. InNovember 2001, an assessment was made of the sludge holding tank, influent pump station wet well,headworks and anaerobic/fermentation basin influent channel for odor potential. In addition, liquidphase sulfide and gas phase hydrogen sulfide (H2S) data were analyzed. An evaluationwasconducted of all available data for the odor potential on the site, and it was determined thatthe source of odors was mainly H2S. The primary sources of the H2S were fromthe influent pump station and the headworks.For 17 years, the City of Clearwater had utilized chemical injection of a 50 percent H2O2 solutioninto the forcemain from the south/west to help control H2S generation and corrosion in the East APCF collection system. The injection point was approximately four miles from the treatment plant. The H2O2 was injected at a maximum rate of 30 gallons per day (gpd)by two chemical metering pumps. In 1991, a mist (wet) scrubber-type odor control system was installed at the East APCF which utilized sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) for absorption of the odorous air from the sludge holding tank and sludge handling building. Due to thedangers and environmental risks involved in handling and storing large volumes of chemical compounds required for thescrubber operation, the unit was taken out of service as a wet chemical scrubber. At the time of theodor evaluation, the City was injecting an enzymatic masking agent into the scrubber air flow.Data were collected in July and August 2001 in one week intervals from H2S gas analyzing equipment temporarily installed in the influent pump station and also during a 24-hour interval at the grit removal units/headworks. Baseline gaseous H2S and temperature data were collected by temporarily halting the injection of H2O2 into the collection system. Testing at this time of year was expected to produce peak testing results due to high air and wastewater temperatures in the 70 to 90 ° F range which are conducive to increased generation of H2S gas in the system. During the 24-hour testing period at the grit removal units, the maximum recorded level of H2S was 34 parts per million (ppm) with routine H2O2 application upstream and 45 ppm without. The maximum recorded level of H2Sat the influent wet well during the monitoring period was 131 ppm with routine H2O2 application upstream and 144 ppm without.Subsequently, three methods of odor control were examined: an organic media biofilter, a wet scrubber system and an inorganic media biofiltration system. An explanation of the advantages and disadvantages of each technology and a cost comparison were completed. Based on the qualitative and quantitative analysis, the inorganic biological filtration system was determined the best, most cost-effective option. The City determined that injection of the H2O2 was no longer feasible or required and has since ceased its use.Since the odor evaluation was completed, two new inorganic media biological treatment units were installed; at the headworks and at the influent pumping station. These units are functioning well with few maintenance requirements, and citizens' complaints have stopped. The existing wet scrubberservicing the sludge holding tank was recently refurbished. The unit scrubs odorous air from the thickened waste activated sludge (TWAS) holding tank and sludge handling building with approximately 35 gallons per day (gpd) 12 percent NaOCl solution. As a polishing tool, approximately 20–25 gallons of Bioxide® is added on a daily basis to the liquid in the 130,000 gallon sludge blending tank at six percent waste activated sludge (WAS) and 65 percent volatile solids (VS). In addition, when the weather becomes hot and dry, the H2S generated in the system increasessignificantly. At these times, the City injects a proprietary ferrous sulfate (FeSO4) solution at the plant influent for 24 to 48 hours until the biofiltration systems fully acclimate. It is this combination of proactive efforts and the implementation of multiple odor control technologies that has been successful. The City has found that tackling the odor problem by applying multiple odor control methods and technologies has essentially eliminated odor complaints in the EastAPCF area.The City currently plans to install the same type inorganic media odor control systems at the two other APCFs.