Evaluation Of Two-Phase Anaerobic Co-Digestion Of Wastewater Sludge With High-Strength Waste To Increase Solids Processing Capability At The F. Wayne Hill Water Resources Center

The F. Wayne Hill Water Resources Center (FWHWRC) in Gwinnett County, GA, currently uses five egg-shaped mesophilic anaerobic digesters for wastewater sludge stabilization and receives high-strength waste such as fats, oils, and grease (FOG) that is co-digested with sludge. A future need for an increase in solids handling capacity is anticipated that warrants intensification of sludge management including the anaerobic digestion (AD) process. The purpose of this study was to evaluate the potential transitioning of the current digesters to two-phase AD, to determine if lower solids retention times (SRT), and thus a possible increase in the solids handling capacity of the plant could be achieved, as well as assess operational stability of the modified digestion process. This study evaluated the efficacy of two-phase AD in terms of methane production, and COD and solids reduction for FWHWRC. This was done through experiments conducted in two different phases of bench-scale work listed as follows: - Phase I - Biomechanical methane potential (BMP) tests to evaluate the effect of volumetric FOG/HSW loading on the performance of co-digestion. - Phase II - An extended trial with six semi-continuous bench scale digesters operated to investigate the impacts of single-phase compared to two-phase digestion under a range of loading conditions for municipal sludge and FOG/HSW. Phase I – BMP Testing Biochemical methane potential tests (BMP) tests were performed with feed collected from FWHWRC to evaluate the effect of volumetric FOG/HSW loading on the performance of co-digestion. One of the main goals of this approach was to determine the FOG/HSW loading at which methane production could be impacted and to identify bottlenecks that cause the limitation. BMP tests were performed with 5-20% FOG/HSW feed by volume for co-digestion and compared against control BMP tests without FOG/HSW addition. Methane yields increased from 0.25 L/g VS for the control tests to >0.4 L/g VS for tests where FOG/HSW was added at 10% of the feed volume. As can be seen in Figure 1, higher FOG/HSW additions led to significant lag phases in methane production, even though the eventual methane yields were higher than the control. Analysis of long-chain fatty acids (LCFAs) and short-chain fatty acids (SCFAs) showed that palmitic and stearic acids accumulated under higher FOG/HSW volumetric loadings as shown in Figure 2, and concurrent observation of a lack of acetate accumulation suggests that the bottleneck in inhibited digesters co-digesting FOG/HSW with sludge is likely to be the conversion of LCFAs to acetate and not direct complete inhibition of methanogenesis. These results also provide an indication that for adopting two-phase AD, it would be critical to ensure the conversion of LCFAs to acetate in the acidogenic phase. Phase II – Bench Scale Digester Trials Six semi-continuously fed digesters – R1 through R6 – were set up and operated in the laboratory for a duration of 300 days. Two digesters, R1 and R2, served as controls, with R1 representing a non-co-digesting single-phase digester, and R2 a co-digesting single-phase digester. R3 and R4 represented a two-phase system with no FOG/HSW addition, while R5 and R6 represent a two-phase system with FOG/HSW addition. Full details of the reactor configurations, operating conditions and results from microbial community analysis will be provided in the final paper.
Key Outcomes The results from operating these six digesters over a series of conditions including various FOG/HSW loadings, SRTs, and buffering regimes, provided several key observations:
1. Two-phase AD was found to provide no added benefits in increasing solids handling or methane production when not co-digesting FOG/HSW. This is contrary to the argument that two-phase AD provides kinetic advantages for acidogenesis and methanogenesis and could here be a result of thermodynamic limitations governing overall methane yields and thus COD and solids reduction.
2. Successful co-digestion, even in single-phase operation, as currently practiced at FWHWRC, required operating conditions (particularly pH) that support a microbial community that allows for the conversion of LCFAs such as palmitic and stearic acids to acetate, and further to methane. Although it is common to classify co-digestion performance for a given FOG/HSW loading, this observation along with the results from the BMP tests supports the use of LCFA concentrations as a more accurate parameter to classify FOG/HSW loading with.
3. Two-phase AD with co-digestion failed to achieve the desired outcome because of a lack of LCFA conversion in the acidogenic phase. This is because the low pH in acidogenic digesters likely inhibited the growth of syntrophic bacteria responsible for beta-oxidation of LCFAs. Considering the above results, it was concluded that two-phase AD, especially, when co-digesting with FOG/HSW provides no notable advantage over single phase digestion. One additional benefit of the study was a deeper understanding of factors that affect successful co-digestion with FOG/HSW in single-phase operation. Most plants that incorporate co-digestion with FOG/HSW do not have set metrics for how much FOG/HSW loading is acceptable. We identified here the components of FOG/HSW, viz., LCFAs, and particularly individual LCFAs such as palmitic and stearic acid, which we recommend be tracked in full-scale digesters to ensure optimum performance during co-digestion with FOG. Such tracking at a regular frequency of 1-2 times per SRT using techniques described in this report would allow identifying overloaded conditions well in advance of deteriorating digester performance and could signal the need for operational adjustments that would allow syntrophic LCFA degrading bacteria to grow and thrive in the digesters. In addition to further results from the BMP testing, the final paper will provide full results from the bench scale digester trials including methane production, COD and VS conversion, pH, VFA and LCFA concentrations. This will be supplemented by analysis of the microbial community undertaken using 16s rRNA sequencing which led to a deeper understanding of the role of microorganisms involved and the impact of operating conditions on the requirements for LCFA conversion.
Learning Outcomes: This paper will provide valuable information for utilities who are considering or currently operating co-digestion with FOG/HSW in either a single-phase or two-phase configuration. In particular, the improved understanding of the operational impact of LCFA conversion (or lack thereof) and the conditions required to achieve this provides a useful insight into the underlying mechanisms of digester operating issues that may be experienced by utilities. The poor performance of two-phase digestion with FOG/HSW could be critical information for utilities who are potentially considering this solution for co-digestion systems.