BROOKFIELD, WISCONSIN
The City of Brookfield covers almost 27 square miles and is home to 40,000 residents. It is a major business, retail and industrial hub in southeastern Wisconsin. Brookfield is geographically unique because the city straddles the sub- continental divide. The western portion of the city flows to the Fox River Water Pollution Control Center (FRWPCC), operated by the City of Brookfield. The Challenge The FRWPCC is an activated sludge plant with tertiary filtration designed for an average flow of 12.5 million gallons per day (MGD) and a peak wet weather flow of 50 MGD. The plant complies with a 1.0 mg total phosphorus (TP)/L discharge limit by adding alum to secondary treatment (simultaneous precipitation). The problem with this method is that treatment required an average dosage of over 8,000 gallons per month of alum at a cost that often exceeded $10,000! FRWPCC needed a solution that would reduce chemical usage and ultimately, save the city money. The facility was also looking for an overall cost-effective solution that would allow them to meet a more stringent limit of 0.075 mg TP/L in the future. FRWPCC sought ways to reduce chemical usage through better control of the chemical feed. Manual adjustment of the chemical pumps based on infrequent grab sampling tended to err on the conservative side to ensure compliance. However, the result was overdosing and high chemical costs. Continuous monitoring was crucial in determining the correct dose and proper timing of the alum. The Solution Rick Wenzel, Process Supervisor for the Control Center, considered many options before recommending the IQ SensorNet P700 Orthophosphate Analyzer (figure 1) from YSI. Low reagent consumption and user-defined automatic calibration to a standard were the key differentiators from other options. The P700 was installed in the filter building to continuously monitor the orthophosphate concentration in filtered effluent. The monitoring system consists of a lightweight 0.45 micron sample filter on a slide rail (figure 2), sample tubing, a 2020 XT controller, and a handrail-mounted cabinet which houses the sample pump, photometer assembly, and chemistry. The phosphate measurement is displayed as PO4-P and the measurement value is reported to the central SCADA system via a 4-20 mA output. The output signal is used to directly control the dosage of alum to the secondary treatment system by adjusting the speed of chemical metering pumps.
Objective: Reduce Chemical Costs by Monitoring Orthophosphate
Rick Wenzel holding the 0.45 micron filter membrane for the P700 Orthophosphate Analyzer
Acknowledgements: Many thanks to Rick Wenzel, Process Supervisor at the Fox River Water Pollution Control Center, for providing his experience with the IQ SensorNet system and sensors.
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