Decoupling Carbon and Phosphorus Release During Primary Sludge Fermentation in Chem-P Plants

Introduction: Many wastewater treatment plants use chemical phosphorus removal (CPR) to meet discharge limits[sup]1[/sup]. CPR commonly uses iron (Fe) to remove phosphorus (P), and though it is reliable, the chemical can be costly[sup]2[/sup]. Plants with high enough volatile fatty acid (VFA) concentrations can often use biological P removal (BPR), and additional carbon can be supplied from primary sludge (PS) fermentation. There is also opportunity to use PS fermentate to support nitrogen (N) removal in plants which are generally dependent on external carbon addition[sup]3[/sup]. During fermentation, nutrients are released in addition to VFAs and COD. N concentrations can be managed through PdNA or reoxidation after denitrification. P is also released through hydrolysis, but with CPR there is an additional release of Fe-bound P due to anaerobic Fe reduction. This can be a significant issue for plants with low P limits, such as Blue Plains. The objective of this study was to evaluate conditions for decoupling COD and P release by managing Fe reduction of Fe-P precipitation within the fermenter. Studies show that vivianite, an Fe-P mineral, may precipitate at a pH above 6[sup]4[/sup]. Therefore, it was hypothesized that pH adjustment may be able to support CPR optimization. Methods: Two 60-L fermentation pilot reactors were fed with Blue Plains PS at 5 g/L and controlled to a SRT target of 1-d. The system was operated under several scenarios including a control: mechanical treatment, low and high microaeration, and lime slurry addition (100 or 200 mg/L CaCO₃). Fe overdosing in full-scale occurred during testing but was not controlled as a scenario. Measurements were performed on influent and decant to calculate yields. A full-scale gravity thickener (GT) at Blue Plains was converted to fermenter conditions by allowing an SRT between 0.2-2 days and was operated under full-scale conditions for sludge composition and solids loading. Influent and effluent sludges were analyzed, and data was categorized by TSS: low (<3.5 g/L), medium (3.5–5 g/L), and high (>5 g/L). Yields were calculated assuming 100% elutriation in the fermenter by measuring concentration profiles. Results and Discussion: Full-scale fermenter data from 2024 and 2025 show an estimated yield of 0.0332 mg P/mg sCOD, while literature data from BPR plants show lower P yields per mg of sCOD, from 0.0046 to 0.0022 (Fig 1). It was hypothesized that all additional release at Blue Plains was a result of Fe dosing and thus could be decoupled from sCOD release. Separating the data into low (<6) and high (>6) pH scenarios show that a high pH leads to ~20% less P yield per mg sCOD, at 0.0238 mg P released per mg sCOD, compared to 0.0298 mg P/mg sCOD at a low pH (Fig 1). Several options were explored to raise fermentate pH above 6, into a range suitable for vivianite precipitation, including alkalinity dosing, Fe overdosing, and PS dilution. In pilot control scenarios, fermentation reduced sludge pH from 6.28±0.12 to 5.51±0.26, which is below the range for vivianite formation. The average control OP-P was 13.50±2.82 mg/L. Less P was found at a pH greater than 5.83, and when iron was available for removal (Fig 2). High fermentate pH was a result of either a high lime dose (200 mg/L CaCO₃), or overdosing of FeCl3 in primary treatment, which led to high levels of Fe(OH)3 in the PS. The low alkalinity scenario (100 mg/L CaCO₃) had a pH of 5.71 with 12.62 mg/L OP-P in the fermentate, while the high alkalinity scenario (200 mg/L CaCO₃) had a pH of 6.36 and 2.72 mg/L OP-P. Overall, the high lime dose effectively removed P, while the lower dose may not have achieved a high enough pH. Periods of higher background Fe also saw effective P removal, with pH values of 6.06 and 6.01, and OP-P at 4.76 ± 3.7 and 2.65 ± 2.4 mg/L. Within the GT, PS elutriation resulted in a lower incoming TSS, and lower blanket sCOD, pH, and OP-P. At a pH greater than 5.75, the average OP-P was below 17.5 mg/L, while operating at a lower pH led to an average OP-P of 47.6 mg/L. Fe concentrations followed a similar trend to OP-P, showing that the chemical Fe-P dynamics were modified with the change in pH (Fig 3). An estimate of 0.0046 mg P/mg sCOD (Fig 1) was used to calculate P released solely from hydrolysis within the GT operating at an SRT of 0.8 to 1.5 days. Additional P release, assumed to be a result of Fe reduction, was significantly different between scenarios with a pH below and above 6 (T-test, p = 0.1126) (Fig 4). This indicates that raising the pH above 6 is a viable solution for reducing excess P release. With elutriation, the GT achieved a fermentate OP-P release of less than 17.5 mg/L, and projected mainstream P was less than 0.4 mg/L. The lower P release at higher pH showed decoupling of OP-P and sCOD, however the projected mainstream P was not yet low enough to meet plant discharge limits of 0.18 mg P/L. This demonstrates that dilution is a solution to achieve target pH conditions without losing sCOD, however additional research is needed to understand changes in buffer capacity over different seasons and sludge characteristics. Conclusions: Results of full-scale GT and fermentation pilot studies demonstrate that pH management is a viable strategy for decoupling COD and P release at CPR plants. Higher pH can be achieved through alkalinity addition, Fe overdose, or PS dilution. These strategies maintain good sCOD yields while decreasing mainstream P.