Conference Agenda

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Session Overview
Session 05B: Resource Recovery: Gas and Energy
Monday, 09/Sep/2019:
1:15pm - 2:45pm

Session Chair: Anthony Tartaglione, Black and Veatch;
Location: E143-144

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1:15pm - 2:00pm

Lowering Co-Digestion Costs Through an Innovative Combination of Novel Food Waste Pre-processing Technique and Strategies for Improving Solids Treatment

Ganesh Rajagopalan, Bhargavi Subramanian

Kennedy Jenks Consultants, United States of America;

Over the years, water resource recovery facilities (WRRFs) have been exploring opportunities to enhance digester gas production through addition of high strength organic wastes from food processing and other facilities. Several major challenges exist for implementing co-digestion in WRRFs including the cost of food waste (FW) pre-processing, digester stability and capacity, undesirable consequences to digester operations, downstream impacts to biosolids generation and treatment as well as the lack of operational experience. To assist WRRFs in this pursuit, two complimentary approaches were implemented in this 3-year long project performed at Silicon Valley Clean Water (SVCW), Redwood City, CA: (i) a novel pre-processing technology which selectively extracts FW from waste materials based on its viscosity rather than the more commonly used size based screening methods, and (ii) full-scale co-digestion strategies to lower the mass of dewatered cake solids requiring disposal. The strategic addition of sludge, Fats, Oil and Grease (FOG) as well as FW is intended to facilitate better “sludge-organic waste” interactions that can enhance dewatering characteristics and reduce mass of dewatered cake solids generated. This, in turn, can lower the net mass of sludge cake requiring disposal and the corresponding hauling fees, demonstrating additional benefit over the enhanced gas production. The ultimate goal of this study is to identify ways to improve co-digestion so that more utilities can implement this process. Results pertaining (but not limited to) the above-mentioned aspects of FOG and FW co-digestion, namely, preprocessing, gas production, dewatering characteristics etc. will be presented in the talk.

2:00pm - 2:45pm

Effects of Step-Wise Acclimation versus Shock Loading on the Performance of Anaerobic Co-Digestion of Fats, Oils, and Greases for Increased Methane Production

Ashley Berninghaus, Tyler Radniecki

Oregon State University, United States of America;

Anaerobic co-digestion has become a popular option to increase biogas production, thus increasing resource recovery potential. Grease trap waste, or FOG (fats, oils, and greases), is the co-substrate that has shown the highest methane production potential. However, process optimization to maximize biogas production rates is lacking, partially due to a lack of knowledge on the influence of different FOG loading regimes on co-digestion processes. To address this knowledge gap, lab-scale anaerobic digesters were operated under either a stepwise (i.e. gradual increases in FOG loadings) or shock loading (i.e. rapid increases in FOG loadings) regime. Process performance parameters (e.g. biogas yields and solids reduction) and stability parameters (e.g. pH, nutrients and volatile fatty acid profiles) were monitored throughout the experiments.

The step-wise loading study showed that an increase in OLR and subsequent decrease in HRT resulted in a linear increase in methane yield up to a point, after which the methane yield decreased significantly. The observed increase in methane yield suggests that the microbial community was able to acclimate to an increased organic load. The observed decrease in the last phase of the experiment is likely due to washout. Alternatively, the shock loading study showed that subsequent shocks of a moderate size (e.g. OLR values of 2.21-3.44 g VS/L/d) did not increase methane yield, but did increase reactor stability by causing a lesser accumulation of organic acids during shock periods. These results also suggest that a failure-inducing shock (e.g. an OLR of 10.71 g VS/L/d) will cause reactor upset and failure, such as a decrease in biogas methane content and VS reduction efficiency, as well as increased organic acid accumulation. However, when allowed to recover, the reactor methane yields observed were greater than those pre-failure. Yet, when these failure-inducing shocks are repeated, the data suggests that a decrease in methane yield will occur over time.

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