An Evaluation of Dual External Carbon Source Strategies for the Full Denitrification Process

Introduction Achieving low TIN levels in wastewater effluent often necessitates the addition of an electron donor (Fu et al., 2022). The selection of an appropriate carbon source is a critical decision, influenced by considerations of cost, availability, and effectiveness in terms of biomass yield (Peng et al., 2007). In the United States, methanol is a preferred choice due to its economic viability and low yield (WRF, 2019). However, recent literature suggests fermentate as a potential alternative (Ladipo-Obasa et al., 2022). One limitation of fermentate is its contribution of ammonium to the effluent. This study proposes an intermediate strategy wherein a portion of methanol is replaced with fermentate, aimed at reducing operational costs while easing the transition towards PdNA methodologies. The primary goal is to elucidate the denitrification stoichiometries under a dual carbon source, emphasising is placed on formulating operational strategies conducive to achieving desired effluent standards, along with an evaluation of reductions in methanol consumption. Methodologies A 360 L mainstream pilot system was operated for three phases. During the initial phase, methanol was utilized as the sole carbon source, with its dosing governed by a feedforward control. In the next phase, fermentate, derived from batch fermentation of primary sludge from full-scale primary clarifiers. The third phase introduced a dual carbon source strategy to facilitate a FdN process. Fermentate was dosed continuously, while methanol, serving as an additional carbon source, compensated for the remaining chemical oxygen demand necessary. Results and Discussion Phase 1: Baseline condition with methanol as external carbon source Over 153 days in this pilot operation the effluent targets were effectively met (1.91 mg N/L,Table 1). A stoichiometry factor of 4.9 g sCOD added/g NO3-N removed and a yield coefficient of 0.42 g COD/g COD were observed (Fig.1a,c); these are similar to expected literature values for methanol-driven denitrification (Mokhayeri et al., 2009). Phase 2: Nitrate removal with available primary sludge fermentate Fermentate dosed was of high quality and reached a yield of 0.21 g sCOD/g VSS (Table 2), which was on the higher end of previously reported yields (Ali et al., 2021). Throughout the operation of 125 days, a nitrate removal of 2.6 mg N/L was achieved (Table 1). The stoichiometry and yield coefficients of fermentate, recorded at 5.4 g sCOD added/g NO3-N removed and 0.47 g COD/ g COD, respectively (Fig.1a,c). Phase 3: Denitrification with dual methanol and fermentate addition a.Overall performance and impact on overall methanol needs Over a 137-day period utilizing dual external carbon sources, a feedforward control strategy was effective at achieving the average effluent TIN at only 1.91 mgN/L. (Table1). Despite significant variations in fermentate quality, particularly in terms of soluble COD due to primary sludge solids concentration variability the strategic incorporation of methanol to address these variations ensured the attainment of favorable TIN levels in the effluent (Table 1&2). It was observed that employing both carbon sources concurrently did not modify their individual behaviour. This was evident by the predicted nitrate removal using the stoichiometries from phase 1 and 2 being similar to the observed nitrate removal (Fig.1a). Even though the predicted nitrate removal showed a slightly improved efficiency of carbon use in phase 3, this was not statistically significant, as shown in (Fig.1b). The amount of external MeOH needed in this phase was 3.2+/-1.4 g sCOD added/gNO3-N removed or showed a MeOH saving of 35% compared to phase 1. b.Microbial selection and functionality under dual substrate addition Even though overall N removal was as expected under dual substrate dosing, a change in kinetic behavior was observed (Fig.2). Nitrite accumulation was observed in denitrification activity tests, irrespective of whether fermentate or methanol was the carbon source. Nitrite accumulation was never observed when doing activity tests with full-scale MeOH adapted biomass or during phase 1 (Fig.2a,d,e). The nitrite accumulation behaviour was also confirmed in the reactor by profiling tests (Fig. 3). Notably, the PdN efficiency was higher with fermentate at 76% and methanol at 52 % (Fig. 2b,c), mirroring findings from the pilot profiling experiment, which reported a PdN efficiency of 47%, as shown in (Fig.3). This result was surprising as methylotrophs do not tend to accumulate nitrite easily. The extended 262-day period of employing fermentate as a carbon source in the reactor potentially enhanced the diversity of nirS type denitrifiers, consequently altering the community-level dynamics in response to different electron donors (Hallin et al., 2006). Nitrate levels needed to be pushed below 1.97 mg N/L to avoid nitrite coming out of the denitrification zone. Conclusions This study at the Blue Plains AWTP underscores the efficacy of dual carbon sources-methanol and fermentate-in enhancing TIN removal in wastewater treatment. Employing a sequential methodology, the research highlighted the stoichiometric and operational efficiency of combining these carbon sources. These findings provide a strategic pathway for the implementation PdNA processes, signifying a substantial advancement in cost-effective and efficient nitrogen removal strategies.