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ODOR CONTROL STUDY <br /> solutions have a higher oxidizing power at near neutral pH (--7 ) than at a high pH (� 12 ) . <br /> The sampling data indicates that approximately 10 % of the total reduced sulfides <br /> entering the scrubber are organic sulfides . Removal efficiencies for the existing <br /> scrubbers are approximately 50 % for total reduced sulfides and 35 to 40 % for organic <br /> sulfides . <br /> Because of the high volume of air that flows through the scrubber, the sulfur mass <br /> emission rate is quite high . The H2S and organic sulfide removal efficiency of the <br /> existing scrubbers can be increased by: <br /> • Operating the scrubbers in series , as a two-stage system . <br /> • Adding hypochlorite solution to both scrubbers . <br /> • Installing oxidation-reduction potential (ORP ) monitors on both scrubbers . <br /> • Adding a chlorine monitor to the last scrubber to ensure adequate chlorine <br /> addition . <br /> • Operating the first scrubber with mostly caustic soda . Based on our experience , <br /> the pH would be set at approximately 10 and the ORP would beset at <br /> approximately +600 mV . This stage would remove most of the H2S and some of <br /> the organic sulfides . <br /> • Operating the second scrubber with mostly -hypochlorite solution . The pH would <br /> be set at approximately 8 and the chlorine monitor would be used to pace the <br /> addition of chlorine solution . This stage would remove most of the remaining H2S <br /> and organic sulfides . The chlorine monitor measures residual gas phase chlorine . <br /> A small amount of chlorine in the gas phase indicates that almost all of the liquid <br /> phase sulfides have been oxidized . <br /> The existing ductwork would allow the system to be operated in two stages . In addition , <br /> the existing fan appears capable of overcoming the additional pressure drop required . <br /> The County may want to consider a two-speed motor or variable frequency drive (VFD ) <br /> for this fan , so that it would operate more efficiently when one stage is off-line for <br /> maintenance or when the dewatering facilities are not operating . <br /> Based on cursory analysis of the existing sodium hypochlorite feed system , it is <br /> believed that the exiting hypochlorite generation facilities can be used to provide <br /> hypochlorite solution to the existing RSF scrubbers . This will need to be confirmed by <br /> further detailed analysis during design . However, automatic control of hypochlorite <br /> addition is essential to successful operation of the scrubbers . It is recommended that <br /> ORP sensors be installed adjacent to the existing pH sensors . ORP is an indication of <br /> the oxidizing power of the chemical solution . For scrubbers intended to remove organic <br /> sulfides , it is recommended that the ORP be maintained at approximately + 700 to + 800 <br /> mV and the pH be maintained at approximately 7 to 8 . Of course these values are <br /> generic and site-specific operations will dictate the exact values . <br /> Ma19 Indian River County, Central Wastewater Treatment Facility <br /> 111 PBSW #071230, February 2003 <br />