Optimizing Side Impacts of Using Primary Sludge Fermentate for Shortcut N removal in Chemical P Removal Facilities

Introduction Traditionally, primary sludge fermentate has been utilized in biological-P removal plants to stabilize and enhance P removal (WRF Project 4975 Report, 2023). However, there is an emerging interest in employing fermentate in chemically enhanced P removal plants to support nitrogen removal and decrease the dependency on external carbon. Especially with the development of novel nitrogen removal systems such as partial denitrification anammox (PdNA), the use of fermentate can result in significant carbon removal savings (Ali et al., 2021; Ladipo-Obasa et al., 2022). It was estimated that for a plant like Blue Plains that relies on methanol (MeOH) as an external carbon source for N removal, the use of primary sludge fermentate can replace 15% of MeOH when deployed for full denitrification or 30-40% of MeOH when deployed for PdNA. To maximize N removal with fermentate, the fermentation yield in terms of volatile fatty acids or overall soluble COD yields plays a big role. This yield is dependent on the operational conditions of the fermenter with solids retention time (SRT) being one of the most important factors. The relationship between SRT and sCOD or VFA yields is shown in Figure 1A & 1B. Typically, an SRT of 3 days is needed to maximize yields (Fig. 1A & 1B). However, it should be noted that nutrients (NH4, OP) are also released (Fig 1C & 1D). Typically, in a non-chemical P plant, a yield of around 0.0055 gNH4-N/gVSS and 0.000842 gPO4-P/gVSS is expected (Fig 1C & 1D). However, when relying on ferric dosing, it is expected that enhanced P release will be observed due to the iron reduction in the fermenter (Fig 1D). This could cause a challenge as now P first bound with Fe, needs to be recaptured to meet discharge limits. This paper provides an assessment of primary sludge fermentation side impacts that need to be accounted for fermentate use for N removal in chemical-P plants. This includes impact of P recycles loads, solids load due to settling changes and odor emissions. Material and methods: Primary sludge was collected from Blue Plains Advanced Wastewater Treatment Plant. Fermentation tests were conducted in batches during different seasons. The last two seasons (Spring and Summer) results will be included in the full paper. Fermentate underwent characterization in terms of chemical composition and was tested for settling parameters (Ngo et al., 2021). Additionally, a full-scale batch fermentation was conducted in the gravity thickener during winter conditions. Results and discussion: Side impact 1: P release yields in chemical P removal plants All tests of Blue Plains primary sludge yielded consistent results in terms of sCOD and NH4 yields. Overall, these yields were a little lower than reported literature (Fig. 1A & 1C), most likely due to the long sewer systems and significant fermentation and loss of soluble organics before the sewage reaches the plant. For P yields, a larger variability existed (Fig. 1D), which might be related to the fermentation conditions (pH variability) or Fe dosing rates in the plant. Overall, the P concentration expected in the fermentate is about 30.9+/-12.6 mg P/L or 0.74+/-0.3 mg Peq/L in the mainstream. Given our TP limit of 0.18 mg TP/L, this contribution is significant and needs to be addressed. Potential avenues to look into to resolve this are optimization of fermentation conditions, potentially operating under pH <5 (Ali et al., 2021). A second avenue is to re-oxidize fermentate from ferrous to ferric and bind this back to FePO4. Current measurements show the availability of 0.72 mol Fe/P, which would allow us to decrease P content to 0.21 mg Peq/L in mainstream/L theoretically or 0.38 mg Peq in mainstream/L at current plant stoichiometries. Side impact 2: Settling changes during fermentation Spring and summer tests show a deterioration in flocculation behavior (Fig. 2A). Even though threshold of flocculation value was different, the trends were consistent. The decreased collision efficiency resulted in increased turbidity and non-settling fraction in the fermentate. This was consistent with observations from literature (Bouzas et al., 2007). It should also be noted that TOF reached the higher levels at the highest sCOD yields and thus we need to anticipate an increase in particulates at increased yields. Typically, we are seeing solids levels of 325+/-230 mg TSS/L in fermentate. While flocculation deteriorated, hindered settling properties improved with the decrease trend of SVI30 (Fig. 2B). This illustrated the interpretation of floc morphology and behavior between flocculation and hindered settling properties (Fig. 3). Side impact 3: Odor formation During fermentation, odor is formed and its composition under current gravity thickener operation and fermentation is showed on table 1. Increased odor is a result of microbial reactions under deep anaerobic conditions. Hydrogen sulfide formation remained low due to the presence of ferrous. Understanding the odor compounds can support evaluation of odor treatment and mitigation strategies. Conclusions Overall, this study quantified the expected increase in P loading coming from fermentate, the expected impacts on settling and odor formation. These three factors must be taken into account when considering primary sludge fermentate for N removal.