In 2017, an Alliance client in southeast Missouri was faced with a 2.2 million-dollar decision. The city’s water treatment plant had reached the maximum allowed flow rate for their fourteen filters, while the rest of the water plant treatment processes were capable of higher production.
The water plant had six original filters that were idled when the new fourteen filters were placed into service in 2002. The city’s engineering consultant recommended rehabilitation of the original six filters at an estimated cost of $2.2 million, increasing the number of operating filters to twenty, and producing the needed filter production. However, Alliance staff believed that the current filters could attain a higher filtration rate and still achieve the desired water quality.
The first step was to get approval from the Missouri Department of Natural Resources (MDNR) for a filter trial to confirm that increased flow rate per filter was possible. After receiving MDNR’s approval, Alliance then determined the appropriate filter test parameters to study, such as individual filter flow rate, turbidity, iron concentration and time between needed backwashing. Throughout the trial, the measurements demonstrated good filter performance at higher flow rates, therefore confirming increased flow viability.
The current filters had been rated at 380 gallons per minute, but the trial allowed for increasing the maximum flow to 570 gallons per minute – a fifty percent increase. Two filters at a time were evaluated over five-day periods, and all fourteen filters that were involved were tested to ensure uniformity of operation. Testing periods of five days per filter was chosen because that was also the filter run time at the lower flow rate.
Although turbidity (clarity) measurements are not required for treatment of the plant’s groundwater source, turbidity was monitored by continuous analyzers during the test to ensure optimal filter performance, since the filters’ primary purpose is to catch particles that make the water cloudy or impart color. Under normal conditions, filter effluent turbidimeter readings range from 0.05-0.10 NTU (Nephelometric Turbidity Units) with results of very clear, no color. Experience has demonstrated that color in the finished water occurs at 0.25 NTU.
The last parameter monitored was the iron concentration. Effluent samples were analyzed every four hours and needed to be less than 0.3 parts per million for aesthetic purposes. Filters were tested at various flow rates of 450, 500, and 570 gallons per minute. All requirements were met and water quality was maintained throughout the trial by reducing filter run times from five days to three while treating at 570 gallons per minute. MDNR approved the findings of the trial and allowed Alliance to operate the filters at a maximum flow rate of 570 gallons per minute with some additional data gathering requirements. The filter trial, initiated by Alliance, increased the water plant production capacity to meet peak demand with existing equipment, and the client saved $2.2 million in capital project costs.