Impact of Primary Sludge Dilution on Fermentation Hydrolysis Rate and Odor Mitigation

[b]Introduction[/b] Gravity thickeners (GTs) are commonly used for primary sludge (PS) thickening. To control odors, operators often minimize blanket height and dilute PS. Beyond thickening, GTs are sometimes repurposed as fermenters to support biological phosphorus and enhance nitrogen removal (Ladipo-Obasa et al., 2023). Effective conversion requires adequate sludge retention time (SRT), sludge blanket formation, and processes such as elutriation to enhance soluble COD and volatile fatty acid transfer to the liquid fraction. The latter conditions enhance odor formation and might require odor treatment compared to conventional PS thickening. In chemical phosphorus (ChemP) plants, fermentation is challenged by iron-mediated phosphorus release that risks phosphorus effluent compliance (Islam et al., 2024). PS dilution was hypothesized to potentially decrease blanket concentrations and increase pH which might help with P-Fe precipitation and minimization of odor formation, two of the side impact to be mitigated for successful application of fermentate for nutrient removal support. This paper presents the findings from a full-scale implementation. [b]Materials and Methods[/b] A full-scale gravity thickener (GT) at Blue Plains AWWTP was converted to fermenter by controlling sludge blanket height targeting SRT of about 1 day (Table 1; Fig. 1). Three TSS feed ranges were defined: low (<3.5 g/L), medium (3.5–5 g/L), and high (>5 g/L). Grab samples were collected from the primary sludge influent, GT overflow, and three depths of sludge blanket and analyzed for a variety of organic and nutrient compounds. Fermentation yields were calculated using a weight-based average concentration in the fermenter and thus assuming 100% elutriation. A gas analyzer (Odalog) provided real-time H₂S monitoring onsite. [b]Results and Discussion[/b] Impact of PS dilution on sCOD and VFA fermentation yields and rates Feed dilution reduced PS TSS and COD concentration and increased pH in the fermenter but did not improve sCOD and VFA yields, or VFA compositions (Table 1; Fig. 2B). sCOD production rates showed no statistical differences indicating no impact of the operating conditions, including feed TSS concentration or solids loading rates variations, on fermentation kinetics. Although elevated pH or elutriation may enhance downstream recovery (Filipe et al., 2001), the observed yield 0.21±0.09 g sCOD/g VSS already exceeded typical literature values (~0.15 g sCOD/g VSS) under comparable SRTs (Islam et al., 2023), indicating the system was near maximum potential with little room for improvement. [b]Impact of PS dilution on Odor emission[/b] By reducing the amount of soluble organic compounds in the sludge blanket, primary sludge (PS) dilution considerably decreased hydrogen sulfide (H₂S) emissions from the gravity thickener (Zhou et al., 2014; Mutegoa et al., 2023). Dilution provides an efficient and practical odor-control lever without affecting sCOD or VFA fermentation yields, as demonstrated by the fact that operating at lower PS TSS kept blanket sCOD below 700 mg/L and maintained H₂S below the typical nuisance threshold of 7 ppm (Fig 2C & 2F). Additionally, reduced TSS elevated pH, increasing efficiency of iron sulfide (FeS) precipitation and further mitigating odor (Jiang et al., 2013). These effects were consistent across 4, 12, and 24-hour average and peak H₂S measurements, indicating that dilution can effectively control odor without compromising COD or volatile fatty acid (VFA) yields. [b]Impact of PS Dilution on P Release[/b] As feed TSS decreased, iron (Fe) release also declined (Fig. 2D, E; Table 1), this reduction was steeper than the corresponding decrease in COD, indicating that Fe–P interactions were modified under lower-solids conditions. Reducing PS TSS via dilution created ideal conditions, (high pH) which promoted precipitation of Fe–P minerals (Fe:P molar ratio = 1.13 ± 0.29), including vivianite (Guo et al., 2022). At medium and high PS TSS the fermentate orthophosphate (OP) averaged 36.5 mg OP-P/L, which corresponds to a projected mainstream effluent OP of 0.98 mg OP-P/L, well above the Blue Plains limit of 0.18 mg TP/L whereas under low PS TSS the fermentate OP dropped below 17.5 mg P/L, translating to mainstream OP <0.4 mg OP-P/L. Although this still does not meet the discharge requirement, it demonstrates that operating GTs at lower PS TSS can substantially suppress orthophosphate release by both reducing soluble OP loads and enhancing Fe-driven precipitation, making dilution a low-cost, full-scale operational lever to partially decouple COD recovery. [b]Conclusion[/b] The dilution of primary sludge in gravity thickeners significantly decreased PS total suspended solids, weight-based COD, and volatile fatty acid concentrations in the sludge blanket, while maintaining fermentation yields and production rates of sCOD. Dilution effectively reduced hydrogen sulfide emissions, with decreases in sCOD concentrations directly associated with lower odor levels. The findings indicate that PS dilution serves as an effective and economical method for odor control in full-scale fermentation systems, while maintaining the production of sCOD and VFAs for subsequent biological nutrient removal. However, seasonal fermentation kinetics remain under evaluation as monitoring continues across the winter season.