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ODOR CONTROL STUDY The existing scrubbing system requires that the operators manually change hypochlorite dose to maintain a set-point ORP . For automatic control , solenoid valves or flow control valves can be added to the feed pipes or new sodium hypochlorite feed pumps could be used instead of the existing hypochlorite generation system . A gas phase chlorine monitor in the last scrubber would help keep adequate hypochlorite in the scrubber sump . The addition of hypochlorite could then be controlled automatically via the ORP and/or gas phase chlorine sensors . If all of these improvements were made , the overall scrubber removal efficiency would be expected to improve to at least 90 % efficiency and perhaps greater than 95 % . The construction and engineering costs for these improvements are estimated to be approximately $ 170 , 000 . If a new hypochlorite storage and feed system is needed , the estimated costs would increase to $300 , 000 . Operating costs for 24 hour per day, 7 days per week operation are conservatively estimated at $ 100 , 000 to $ 140 , 000 per year, mostly for chemicals and power. These estimated operating costs are approximately $ 50 , 000 to $90 , 000 more than existing operating costs because of the additional run time and chemical usage . The construction costs for these improvements include approximately $ 50 , 000 for adding a permanent drain for the mechanical thickener, opening the existing dampers , and pulling more air through the Sludge Storage Tanks . This should increase the negative pressure within the tanks and reduce fugitive emissions . Alternatively, the existing RSF blower could be operated at nearly full speed for 8 hours per day, 5 days per week during sludge dewatering and operated at half speed when sludge is not being dewatered . This would require retrofitting the existing blower with a VFD . When sludge is not being dewatered , the existing blower could be -lowered to approximately half speed and a new automated damper on the exhaust duct from the Dewatering Building could be partially closed so that much of the air would be drawn from the headspace of the Sludge Storage Tanks . The amount of air drawn from the Dewatering Building would be about half the amount drawn during sludge dewatering operations . A second smaller blower could also be used in addition to retrofitting the existing blower with a VFD . This would provide more redundancy for the odor control system . The construction and engineering costs for these improvements are estimated to be approximately $ 320 , 000 . Operating costs for this mode of operation are conservatively estimated at $ 80 , 000 to $ 95 , 000 per year, which is about $ 30 , 000 to $45 , 000 more than current operating costs . Table 6 provides a comparison of estimated costs for alternative improvements to reduce emissions from the RSF . 3 . 2 .2 Sludge and Septage Storage Tanks Instead of using the existing large RSF scrubber to treat emissions from the Sludge and Septage Storage tanks , a smaller odor control device could be used to treat these emissions . A 4 , 000 cfm modular two-stage scrubber could be constructed and operated 24 hours per day, 7 days per week . A modular or custom biofilter could also be used to `' 20 Indian River County, Central Wastewater Treatment Facility PBS&J #071230, February 2003