From July through mid-October, wastewater from vegetable and cheese processing facilities caused phosphorus spikes at the Beaver Dam (Wis.) Wastewater Treatment Plant. Operators sampled phosphorus just before the effluent outfall three times per week and adjusted the liquid ferric chloride feed accordingly.

"We were chasing our tail a little because the adjustment was always after the fact," says utility foreman Rob Minnema. "If we were high one week, we overfed ferric to make sure the average for the month was below our 1.0 mg/L total phosphorus limit." The average dose was 300 gpd at 12.5 gph.

An upgrade completed in spring 2011 increased the plant's average design flow from 3.5 mgd to 5.6 mgd. It also moved the ferric feed points from between the three primary clarifiers and aeration basins to ahead of the secondary clarifiers, increased the size of the insulated ferric storage tank from 8,000 to 12,000 gallons, and replaced the two alternating chemical transfer pumps with new models from ProMinent.

“But we still didn’t have a system that automatically controlled the feed rate,” says Minnema. “When our Hach representative, Jim Burke, contacted us looking for a plant willing to do a three-month pilot project on their real-time phosphorus control system, we said yes.”  The study ran from December 2011 through February 2012. “Based on our effluent flow and influent orthophosphate that varies from 23 to 27 mg/L, we programmed the effluent orthophosphate setpoint to 0.60 mg/L to ensure that we stayed below our limit,” says Minnema. “Our average ferric dose of 5.55 gph resulted in a 56 percent reduction in chemicals.” After the study, the utility purchased the equipment.

Computer in control

The extended aeration activated sludge plant treats an average of 3.5 mgd from 16,000 residents and discharges to the Beaver Dam River. Two to three times per month, a 4,500-gallon tanker truck delivers ferric to the storage tank. The acidic liquid flows through a PVC feed system.

The Hach phosphorus control system included the RTC101 P-module, Filtrax filtration unit, PHOSPHAX sc phosphate analyzer, and sc1000 digital controller. “The analogy Jim used for the RTC is that it’s like cruise control for the plant,” says Minnema.

The RTC calculates the ferric dose in real time based on plant flow and orthophosphate level, then sends a 4-20 mA signal to the PLC controlling the transfer pump. If the phosphate or flow input is disrupted, the RTC refers to previous profiles to ensure that effluent compliance is maintained until the signal is restored.

Every five minutes, the Filtrax draws a sample and the PHOSPHAX measures it with ± 0.05 mg/L accuracy and reproducibility. The sc1000 controls RTC parameters, signal validation and telecommunications. Operators use a touch screen to program the controller.

Hach wastewater specialist Bob Dabkowski was instrumental in the setup of the new system and training the plant team to use it. “We installed the hardware with Bob assisting us, and he worked with our programmer,” says Minnema. “The RTC mounts in the PLC cabinet and integrates to our PLC/SCADA systems for additional redundancy.” The touch screen mounts above the PHOSPHAX unit next to the sample tub.

Operators tapped off the effluent sample line and ran it to the tub with the Filtrax probe. They installed and configured the system and had it working in two days without disrupting plant operations.

 

Meeting the limit

Dabkowski introduced the operators to the software, showing them how to open certain fields and adjust setpoints. "The touch-screen menu was easy to navigate," says Minnema. "Everyone caught on quickly."

The test period also included software adjustment, quarterly instrument maintenance, remote performance monitoring, and monthly performance reports from Hach. Now Minnema can request reports or ask Hach technicians to log in for technical support if needed.

Twice a month, operators drain and rinse the sample tub and clean the filter probe. They change the two reagents quarterly. During months of higher loading, the ferric feed rate may increase from 5.55 gph to 8 gph. "At our current ferric cost of $1.15 per gallon, we saved $17,388 during the test period," says Minnema. "Estimated annual savings of $50,000 to $70,000 have more than paid for the system."

While feeding less ferric should produce fewer solids, the plant has too many variable industrial wastewater streams for Minnema to analyze a reduction in handling costs.
"I'm sure there are some savings, but I can't quantify them," he says.

Besides affordability, a major benefit was peace of mind. Previously, Minnema and staff worried about whether the plant was over or at its limit for the month. "Now, the RTC controls the dose and I know we will be within our limit," he says. "We've had the system for a year and it has worked flawlessly."