Insights from Full-scale Primary Sludge Fermentation for Carbon Management and Nutrient Removal

Background and Objectives
Nitrogen and phosphorus removal from municipal wastewater require sufficient organic carbon in the form of readily biodegradable carbon (rbCOD) and volatile fatty acids (VFA), which are often limited in influent. Primary sludge (PS) fermentation has been applied for decades in a variety of configurations (Table 1) to produce rbCOD and VFA while also providing sludge reduction benefits1. Despite their widespread application, there is a lack of unified guidance for PS fermenter design, operation, and monitoring and limited information on tradeoffs of different elutriation mechanisms (e.g. solids vs dilute feed). While fermenters are ultimately intended to increase nutrient removal performance, unoptimized fermenters may act as internal sources of additional nitrogen and phosphorus that outweigh the benefits of increased carbon availability. This work will synthesize findings from full-scale PS fermenters (Table 2) to
1) Compare fermentation performance and key performance indicators of different types of PS fermenters and elutriation mechanisms
2) Provide guidance for monitoring PS fermenters to maximize understanding of carbon and nutrient contributions to the mainstream An example case study is provided for Eagles Point Water Resource Recovery Facility and similar analyses will be performed for the facilities in Table 2.

Eagles Point Case Study
#Eagles Point Water Resource Recovery Facility in Cottage Grove, Minnesota is operated by Metropolitan Council Environmental Services (Met Council). This facility, which utilizes biological phosphorus removal in an A2O configuration, has an average design flow of 10 MGD and a 1 mg/L total phosphorus (TP) effluent permit limit. Eagles Point has operated a PS fermenter (~90,000 gal gravity thickener with recirculating elutriation pump) since 2004. As part of a recent transition to low DO operation in the mainstream, staff collected weekly NH4, PO4, and filtered COD samples from the gravity thickener overflow (fermentate). During this period, influent BOD:TP ratios averaged 36, meeting recommended levels without the PS fermenter addition. The PS flow to the thickener and thickened sludge pumping were stable this sample collection period at 0.22 ± 0.03 MGD and 23 ± 2 gpd, respectively. Fermenter sludge blanket levels ranged from 1.5ft to over 9ft during this period with an HRT of ~0.4 d.

In recent years, staff noted that mainstream P removal performance did not seem to correlate with fermenter operation, in that effluent P concentrations did not increase during maintenance periods when the fermenter was offline. Data collected during the low DO transition provides additional context for these observations. On a mass basis, fermentate contributed 22 ± 5 lbs/day PO4-P to the anaerobic zone, while influent wastewater accounted for 166 ± 7 lbs/day. As expected, there was a strong positive correlation between fermentate filtered COD and fermentate PO4 at a consistent ~32:1 COD to P ratio (Figure 1). Assuming a VFA to sCOD ratio of 0.6:1 in PS fermentate2, the VFA to P ratio (19:1) in the produced fermentate is theoretically sufficient to uptake more P in the mainstream anaerobic zone than is recycled from the fermentate itself. However, fermentate P concentrations were positively correlated with effluent P (Figure 1). While effluent P was maintained at low levels regardless of the fermenter contribution, this result suggests a minimal or even net negative effect of the PS fermenter on mainstream P removal.

An additional consideration is temperature, which can have competing effects on fermenter performance and nutrient removal. Fermenter temperature was not measured, but influent wastewater temperature was positively correlated with both fermentate COD and effluent P. Higher ambient and water temperatures improve fermentation, resulting in increased COD outputs1, but also tend to favor microbes that compete with phosphate accumulating organisms (PAO) for rbCOD, leading to worse P removal3,4. Surprisingly, no relationship was found between fermentate COD and sludge blanket levels, suggesting that temperature was the main driver of fermenter productivity rather than SRT (Figure 2). Further sampling to be performed this year will provide a more complete accounting of carbon contributions from the fermenter. This case study highlights the value in routine nutrient monitoring of fermentate and demonstrates a gravity thickener PS fermenter that provided minimal or net negative effects to mainstream nutrient removal performance.

Significance for audience
PS fermenters are widely adopted and provide an 'internal' source of carbon for nutrient removal facilities. However, as demonstrated at Eagles Point, PS fermenters may introduce nutrient loadings back to the mainstream that are not neutralized by increased carbon availability. As facilities continue to meet lower effluent nutrient limits, accounting for internal nutrient recycles in addition to internal carbon recycles is critical. This presentation will use multiple case studies to evaluate the value proposition of PS fermenters, tradeoffs of PS fermenter configurations, and recommendations for monitoring strategies. We will also facilitate knowledge sharing of PS fermenter operation and monitoring through open discussion with participants.