A NEUTRAL pH BIOTRICKLING SCRUBBER REPLACES THERMAL OXIDIZER SAVING 600,000 ANNUALLY

The Louisville and Jefferson County Metropolitan Sewer District (MSD) uses recuperative thermal oxidizers (RTOs) to reduce hydrogen sulfide (H2S) and organic sulfur compounds (OSCs) from the 120 MGD Morris Forman Wastewater Treatment Plant, solids handling processes. The existing thermal oxidizer uses 77 million cubic feet (cu ft) of natural gas at a cost of more than 600,000 per year. Gas prices have more than tripled in the past 5 years. Therefore, MSD desired to test a new technology that would achieve equal or better odor removal efficiency for less cost.A pilot study was completed in June 2004 on a biotrickling scrubber operating at a neutral pH for the purpose of replacing the existing oxidizer. The primary advantage of the neutral pH process is that simultaneous biodegradation of H2S and OSCs occurs at a neutral pH because of the types of Thiobacillus bacteria present. Most other types of biotrickling scrubbers operate at low pH for the purpose of H2S removal only. The pilot study successfully proved that this unique biological process would achieve equal or better removal efficiencies as the RTOs for odor and for all reduced sulfur compounds.The District subsequently designed and constructed a full scale 9,200 cubic feet per minute (cfm) biotrickling scrubber system, allowing 30 seconds Empty Bed Residence Time (EBRT), using two 14 foot (ft) diameter vessels with three stages of media. The 1.12 million full scale system will be started up and tested in March of 2006. The District will benefit because the high cost of natural gas fuel for the thermal oxidizer will be eliminated and the cost of maintenance will be reduced resulting in an annual savings of more than 600,000. In fact, natural gas prices increased 100% from the time of inception of the idea to completion of the full scale unit. A payback period of about one (1) year is estimated for the project.Specific pilot project data and operating conditions will be presented, including removal efficiencies for each reduced sulfur compound such as: hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyl disulfide. The pilot project results were achieved in about 26 seconds EBRT when inlet hydrogen sulfide concentrations were about 43 parts per million (ppm) and total OSCs averaged 2.0 ppm.