Dosing Optimization of Mg(OH)2 for Side-stream Phosphorus Control at the Bozeman WRF

[b]Background[/b] Many wastewater facilities around the world that operate a combination of enhanced biological phosphorus removal (EBPR), anaerobic digestion and dewatering struggle with a high phosphate (PO₄) recycle load, uncontrolled struvite formation, and poor sludge dewaterability. Although several commercial solutions have emerged to mitigate these challenges, their installation can be operationally and cost prohibitive for smaller utilities. This paper presents a case study on the optimization of chemical addition to the digesters for low cost sidestream phosphorus control. The City of Bozeman Water Reclamation Facility (WRF) utilizes anaerobic digestion to process sludge using three digesters operating in series. Digesters 1 and 2 process thickened primary sludge (TPS) and Digester 3 incorporates thickened waste activated sludge (TWAS) into already digested TPS. Digester 3 sludge is dewatered by screw press and the pressate is returned to the WRF headworks. Since 2017, the WRF has utilized EBPR in secondary treatment. To address potential PO₄ recycle concerns, the City adds magnesium hydroxide (Mg(OH)₂) slurry into Digester 3 to sequester PO₄ as struvite. The dose rate is constant, at approximately 600 dry lbs Mg(OH)₂ per day. The onset of Mg(OH)₂ dosing coincided with implementation of EBPR, and data on the PO₄ concentrations in Digester 3 without Mg(OH)₂ dosing is not available. The City has also not conducted extensive sampling to determine residual concentrations of PO₄ and soluble Mg remaining in Digester 3, or optimize Mg(OH)₂ dosing.  Despite sequestering released PO₄, significant struvite formation continues within the screw presses, and they are cleaned on a weekly basis. Additionally, the digesters have not been taken out of service, and it is unknown if the intentionally formed struvite is accumulating within the digester.  To improve understanding of the Mg(OH)₂ dosing strategy, the City engaged a consultant to collect site data and determine if the existing strategy should be optimized or replaced. [b]Methodology[/b] The study was comprised of three main components: 1)A month long special sampling program was executed to produce supplemental information to facilitate PO₄ and Mg2+ mass balances across Digester 3 and the dewatering screw press. 2)Onsite benchtop tests were conducted to assess the amount of PO₄ being released and sequestered in Digester 3. The tests involved: (a) waste activated sludge (WAS) phosphorus (P) release testing, and; (b) acid dissolution of the struvite in the digested sludge matrix. In the acid dissolution process the amount of struvite was quantified by measuring PO₄, NH4+, and Mg2+ concentrations in the digested sludge before and after pH adjustment to 4.5 using strong acid. The presence of struvite was confirmed by comparing molar ratios and performing x-ray diffraction (XRD). 3)Process modeling was conducted to determine the sensitivity of WRF effluent total phosphorus (TP) loads to increased PO₄ and Mg2+ recycle loads. The three components of the study were analyzed together to draw conclusions on the Mg(OH)₂ dosing strategy. [b]Findings[/b] WAS P release testing and acid dissolution of struvite from Digester 3 solids provided good agreement on the amount of PO₄ released in Digester 3. Without Mg(OH)₂ dosing, it is predicted that PO₄ concentrations in Digester 3 would vary throughout the year in proportion to the TWAS solids loading to Digester 3. For the first 6 months of 2025, these load variations would have resulted in 400 to 700 mg PO₄-P/L in Digester 3 and the downstream screw press dewatering pressate (Figure 1). Mg(OH)₂ dosing to Digester 3, was sufficient to fully precipitate the phosphorus in Digester 3 for the study period of January to July 2025. On average the dose was 40% higher than was required, resulting in a high concentration of residual Mg2+ in Digester 3 and pressate recycle. The high Mg2+ concentration presents a strong driver for struvite formation, even at the low observed PO₄ concentrations (Figure 2). Process modeling predicts that lowering the average Mg(OH)₂ dosing rate to Digester 3 by 40% would result in a dewatering pressate residual PO₄ concentration of less than 60 mg PO₄-P/L and not have any significant impact on Bozeman WRF effluent PO₄ concentrations (Figure 3), or digester performance. A mass balance of Mg2+ and TP around Digester 3 (Figure 4) demonstrates that most, if not all, of the dosed Mg can be accounted for in digester effluent. This would suggest there is not a significant accumulation of struvite within the digester. Using XRD and acid dissolution, the amount of struvite contained within the plant biosolids is estimated to be between 13.5 and 21% on a dry weight basis (Figure 5). The Mg(OH)₂ dosing approach employed at the Bozeman WRF has provided effective management of the pressate PO₄ loading and aided in plant compliance. One conclusion of this study, the City implemented a TWAS load paced dosing approach for Mg(OH)₂ to stabilize and reduce residual Mg2+ concentration in Digester 3. The impact of this strategy on operating cost, nuisance struvite formation, sludge dewaterability, digester performance and plant effluent TP will be monitored. Findings from this implementation will also be included in the final paper. The City is also investigating if precipitated struvite can be separated from the digested sludge and recovered for reuse.