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Evaluating Influence of Struvite Inhibitor on Struvite Crystallization Process and Results from Laboratory WAS P Release Studies at Truckee Meadows Water Reclamation Facility (TMWRF)

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The Truckee Meadows Water Reclamation Facility (TMWRF) serves the Cities of Reno and Sparks, together with portions of Washoe County, Nevada. The facility has a design treatment capacity of up to 46.5 MGD and currently receives an actual flow of approximately 30 MGD. The treatment process is configured to biologically remove both ammonia and phosphorus. Solids are anaerobically digested and centrifuge dewatered. Metal salt ferric chloride is used to precipitate phosphorus from centrate sidestream to minimize the phosphorus load retuning back through the sidestream on to the plant. Struvite formation and Vivianite deposition in process piping and equipment in and around solids digestion process has increased operation and maintenance costs. The centrate has an average flow of approximately 250,000 GPD with an average ammonia concentration of 973 mg/L, and ortho-phosphate of 217 mg/L, encompassing sampling from August 2007 to February 2008 conducted by the facility. TMWRF also doses a proprietary struvite inhibitor into its dewatering centrate wet well, to control struvite formation in centrate piping.

Laboratory jar tests were conducted at Old Dominion University's (ODU) Civil and Environmental Engineering (CEE) from the centrate collected from TMWRF. Results from the jar tests were used to determine indicative operational set points for the fluidized bed reactor targeting removal of phosphorus and ammonia from centrate sidestream. Jar tests also involved evaluating different sources of magnesium (magnesium chloride and magnesium hydroxide) and phosphorus (tri-sodium phosphate and phosphoric acid) to increase ammonia removal. Around 7 gallons of centrate from TMWRF was shipped to ODU' CEE lab for these studies. Centrate received were stored under 4°C before the jar tests were conducted.

Further, a pilot study was also conducted during summer of 2008 at TMWRF's dewatering building. Pilot testing on Ostara's proprietary technology “Pearl” process was conducted in two phases: the first phase of testing consisted of feeding centrate to the Ostara reactor before dosing struvite inhibitor, while the second phase consisted of feeding the reactor with centrate drawn after the struvite inhibitor was dosed on to the centrate wet well. This evaluation was necessary to estimate any impact of the struvite inhibitor on the crystallization process, nutrient removal or product quality. It was suspected that struvite inhibitor might tie up some magnesium and would increase magnesium consumption through the crystallization process. Pilot test results are reported in Table 1, average phosphorus removal as 93.6% and ammonia as 10.96% reinforcing earlier pilot results at various treatment plants. Also, data generated during the pilot study indicate that there is no impact on the nutrient removal from the struvite inhibitor used during the second phase of the study.

Laboratory bench scale tests on WAS (waste activated sludge) P release were conducted on site during the pilot study. WAS collected from secondary clarifiers were stressed under anaerobic conditions to re-release phosphorus and magnesium. The rate of phosphorus release with and without volatile fatty acids (VFAs) in form of acetic acid or acid phase digested sludge was evaluated. Results from these studies will be used for the implementation of struvite recovery by feeding the supernatant from the WAS P or “phos-strip” tanks before thickening to the Ostara crystallization process. This type of process arrangement is expected to result in reduced or eliminated nuisance struvite deposits in the sludge handling system, digestion, and dewatering equipment, as well as reduced nutrient loads on the liquid train, and improved biological nutrient removal process stability. Table 2. shows the matrix of test conditions focusing on with and without VFA to accelerate phosphorus release. Results (Figure 3) from the preliminary studies showed that with addition of VFA to WAS, both the rate of phosphorus release and the total amount of phosphorus released are higher than without the addition of VFAs.

Keywords: WAS P release; sidestream nutrient removal; struvite

Document Type: Research Article


Publication date: January 1, 2009

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