HOW AUTOMATED ORTHOPHOSPHATE MONITORING CUT COSTS BY 25 PERCENT


An automated system for monitoring orthophosphate and regulating chemical feed gives the staff of the Watertown, Wisconsin Wastewater Treatment Plant (WWTP) peace of mind and saves the city between $25,000 and $40,000 per year in ferric chloride costs.


The Operators work hard to maintain monthly average effl uent Total Phosphorus (TP) levels below 1.0 mg P/L in accordance with a total maximum daily load (TMDL) for the Rock River. Some phosphorus removal occurs in all activated sludge facilities— approximately 1 mg P/L is removed for every 100 mg BOD/L that is removed due to normal biomass growth, and is discarded when sludge is wasted. An extra boost is needed, however, to achieve the necessary effl uent quality at Watertown WWTP.


Watertown WWTP staff members utilise a chemical booster, ferric chloride, which is added to mixed liquor upstream from the fi nal settling tanks. The chemical reactions that occur when ferric chloride is added to mixed liquor are complicated, involving both fast and slow reactions. Ultimately, dissolved phosphate is converted to particulate phosphorus. In this method, known as simultaneous precipitation, the particulate phosphorus co- precipitates with mixed liquor and is removed with waste activated sludge (WAS) as shown in Figure 1.


Prior to continuously monitoring the Watertown system, plant operators played it safe with conservatively large dosages of ferric chloride to ensure that TP levels would remain below the prescribed limit.


“The guys would take orthophosphate readings in the morning and afternoon and bump the ferric chloride levels up if the phosphate levels were high,” recalls Kevin Freber, Water Systems Manager for the City of Watertown. “Basically, they were guessing at the right amount to feed into the mixed liquor.”


“If the reading was still high, or higher, in the afternoon, they’d jack up to the next feed rate,” he adds. “It would run like that for seven or eight hours until the next morning. They’d fi nish up for the weekend and leave it cranked up.”


In many cases, the result was a steady over-application of ferric chloride. “It wasn’t their fault,” Freber notes. “They just didn’t have the right tools to make good decisions.”


Continuous Orthophosphate Monitoring


In 2012, Freber and his team installed their fi rst automated orthophosphate analyser in the plant, which is designed for 5.2 million-gallon-per-day (MGD) fl ow but handles an average daily fl ow of 3.0 MGD. The instrument—a fi rst generation orthophosphate analyser by WTW, a Xylem brand—took measurements of orthophosphate every 15 minutes in secondary


effl uent upstream from the UV purifi cation system. It then automatically adjusted the ferric chloride feed pumps to regulate chemical input rates based on a 0.7 mg P/L setpoint.


“Before we installed the fi rst orthophosphate analyser, we spent over $100,000 on ferric chloride per year,” says Freber. “After the fi rst year with the analyser, we saved $25,000, enough to pay for the unit. We’ve been running right about $60,000 to $75,000 a year for ferric chloride since then.” Plant staff also realised other benefi ts, such as reduced sludge production, further reducing operating costs for sludge disposal.


In 2015, Freber worked with Mulcahy Shaw Water of Cedarburg, Wisconsin, to upgrade to a new IQ Sensor Net P700 IQ orthophosphate analyser manufactured by WTW, a Xylem brand. The new P700 IQ dramatically reduced reagent costs and lengthened the solution exchange interval to every four to eight months. Among the increased capabilities of the new instrument was its wide measuring range of 0.05 to 50.00 mg P/L. The broad detection capability provides plant designers and management with the fl exibility to site the monitoring system and the ferric chloride input in a variety of places within the treatment system, points out Robert Smith, Application Engineer, Wastewater, for YSI.


AET October / November 2017 www.envirotech-online.com Figure 1 - Typical Simultaneous Precipitation ‘P’ Removal System Confi guration


“Ferric chloride or alum may be added to wastewater upstream or downstream from the biological processes, depending on whether plant staff wants to remove the phosphorus with the primary sludge or waste activated sludge (WAS),” explains Smith. “It can be removed in the primary settling tanks, fi nal settling tanks or effl uent fi lters. Once they’ve decided where the precipitation of


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