New Georgia treatment plant designed to meet high clean water standards
About an hour south of Atlanta, the city of Barnesville, Ga., is a town of shady, tree-lined streets, a fondness for celebrating its history and a sharp eye on the future. In 2013, as its 30-year-old wastewater treatment plant was reaching the end of its useful life, Barnesville found itself planning for a future of growth, progress, and a new, state-mandated <0.6 mg/L total phosphorus (TP) wastewater discharge limit.
The city contracted with Stevenson & Palmer Eng. Inc. to design and build a $12.5-million, 2.4-million-gal-per-day (mgd) wastewater treatment plant with the capacity to meet the needs of Barnesville's 10,000 residential and business customers. Also, since the majority of its treated wastewater would discharge into nearby Tobeesofkee Creek, the system was required to meet the new state-mandated TP limits. Heyward Inc., located in Atlanta, helped secure the contract as Evoqua’s manufacturers’ representatives.
To meet the new TP standards, the design engineer specified an additional stage of chemical phosphorus removal, which included a rapid mix and floc tank for precipitation, followed by filtration. Barnesville selected Evoqua’s Forty-X disc filter for the plant’s tertiary filter, based on its compact footprint and simple, cost-effective operation.
Part of the engineering effort included an important extra step: a lab test to determine the right chemical for optimum filtration. In collaboration with Barnesville and operating partner ESG Operations Inc., Evoqua used the plant’s award-winning onsite lab facilities to test three different precipitation chemicals: poly-aluminum chloride (PAC), potassium aluminum sulfate (alum) and ferric chloride. In the test, baseline samples of effluent were collected from each anticipated precipitation point, analyzed, and then put through an artificial rapid mix and flocculation stage for a calculated period of time to simulate plant conditions. Once the time elapsed, each sample was filtered through 10-μ Forty-X cloth filter media and results were analyzed.
Based on this test, alum yielded the best overall results for phosphorus removal. pH adjustments and the addition of polymer also were analyzed for each chemical tested, but neither treatment was required to meet the <0.6 mg /L limit.
Once the best chemical was determined, dosage was evaluated in the online treatment system. Both single-point injection and multiple-point injection were tested and the results documented. It was determined that multiple-point injection was the optimum chemical injection. Alum was fed at the clarifier influent with a dosing range of 15 to 40 mg/L. It also was fed in front of the rapid mix and floc tank at the disc filtration influent at a range of 5 to 20 mg/L.
Once the correct dosage was determined, the Forty-X disc filter’s effluent was monitored, and the filter’s operational settings were adjusted to achieve peak performance. Although the chemical reaction between the alum and phosphorus resulted in a slightly higher solids loading on the Forty-X filter, this loading remained well within the filter’s capabilities.
This figure illustrates the phosphorus levels as the chemical removal stage begins. Floc is formed and captured by the Forty-X disc filters, with the effluent leaving the filters well below the plant TP limit.
As tested, the chemical removal process was able to achieve 85% reduction in TP and yield an effluent quality of <0.3 mg/L of phosphorus, 50% below the allowable limits for this facility. Barnesville’s next-generation treatment plant, with its Forty-X disc filter as the tertiary treatment stage, is currently meeting its permit requirements reliably and cost-effectively, with room to grow.
As a side benefit, the test provided a set of data that documents the performance of the Forty-X disc filter in phosphorous removal. Detailed data for this test is available on request.