Pairing Table-top Analysis with Bench-Scale Testing to Assess Food Waste Co-digestion Feasibility

Introduction Water Resource Recovery Facilities (WRRFs) have increasingly been investigating and implementing co-digestion of food waste to generate revenue and maximize biogas production. In California, an additional regulatory driver is Senate Bill (SB) 1383, which requires organics diversion from landfills to mitigate short-term greenhouse gas emissions. To meet these goals, the City of Oxnard applied for and obtained an EPA grant to study food waste pre-processing and co-digestion implementation at the Oxnard Wastewater Treatment Plant (OWTP). Carollo developed a feasibility study for co-digestion implementation, and Dr. Matt Higgins (Bucknell University) conducted a bench-scale study involving co-digestion, dewatering, and odor testing of the City's sludge and source-separated food waste from the City's Del Norte Materials Recovery Facility (MRF). The results of the bench-scale study were then used to refine the design criteria and operational assumptions used in the feasibility study. This session will cover the approach and key findings of the bench-scale and feasibility studies. The City and the Carollo/Bucknell team hope that this study can serve as an example to other WRRFs interested in implementing food waste co-digestion. Bench Scale Study Three months of samples of primary and waste activated sludge from the OWTP and source separated food waste from the Del Norte MRF were sent to the Bucknell Anaerobic Digestion Laboratory (see photo in Figure 1) at Bucknell University. The food waste was diluted and blended before delivery. Four anaerobic digesters were operated, Digester 1 as a control with no food waste addition, and Digesters 2- 4 at increasing food waste loading rates. The target food waste VS loading rates to each digester expressed as the mass of VS from food waste divided by the total mass of VS in the digester feed (food waste plus feed sludges) were: Digester 2: 25%, Digester 3: 35%, Digester 4: 50%. In addition to evaluating digestion performance, this study also evaluated the impacts of food waste co-digestion on foaming potential, dewatering, and cake odor production. A summary of the findings from the bench-scale study is provided below: -Food waste characteristics: --The food waste slurry TS and VS content averaged 5.8 %TS and 81 %VS, respectively. -Digester performance: --The food waste did not impact the digesters pH, alkalinity, and nitrogen and phosphorus concentrations in any appreciable way at the loading rates in this study, except for the digester upset. --As expected, food waste co-digestion increased the volatile solids reduction (VSR), biogas yield (see Figure 2 for a graph of biogas production for the 4 Digesters), and methane content of the biogas. The overall VSR for the control digester averaged 45.9%, while the VSR for the test digesters was 52.4-59.5%. From this, the VSR for the food waste itself was estimated as 72-77%. --A digester upset in Digester 4 occurred when it was fed a food waste VS loading of 60% relative to the total VS loading. Based on this, a conservative maximum food waste loading for stable digestion of 45% was selected for the feasibility study. This limit allows for significant food waste addition while maintaining digester stability. -Foaming potential: --The viscosity and foaming potential increased as food waste loading increased. --No clear pattern was observed for volume expansion. The lower two loading rates of food waste (Digesters 2 and 3) had a greater volume expansion than the control, while at the highest food waste loading (Digester 4), the volume expansion was less than the control. -Dewaterability: oAs the food waste load increased, the polymer demand increased and the cake solids concentration decreased. The lower TS concentration could have been due to a lower concentration of divalent cations, which was observed in the higher food waste loading digesters. -Cake odor potential: oFood waste addition did not negatively impact cake odor potential. As a result of these bench scale results, the design team was able to refine site specific design and operational criteria for the full-scale co-digestion cost and energy feasibility study, including identifying the maximum design food waste VS ratios for stable digestion and quantifying anticipated changes in biogas production and revenue, polymer demand and costs, dewaterability and biosolids production post co-digestion. These criteria were adopted for the following desktop feasibility study. Feasibility Study The purpose of the feasibility study was to determine the required facilities, costs, and other impacts of food waste co-digestion and whether the benefits from increased biogas production and cogeneration power generation are sufficient to cover the implementation costs of a co-digestion program. A summary of the findings from the feasibility study is provided below: The capacity assessment determined that the OWTP has sufficient capacity in all their processes to handle the increased loads, except when one digester is out of service. In that instance, the City could divert a portion of the food waste to the Agromin composting facility rather than building an additional digester. The capacity assessment also found that, with food waste co-digestion, the OWTP would approximately double its biogas production. -The condition assessment found that the City has already made or plans to make many of the improvements identified in the condition assessment. Most notably, the City plans to refurbish a digester which is currently out of service. -This feasibility study evaluated a wide range of technologies for pre-treatment of source separated food waste to produce a bioslurry suitable for co-digestion. Three alternatives were selected for a more detailed life-cycle cost, site layout, and non-financial evaluation. A hammer-mill at the Del Norte MRF with additional polishing with a sludge screen at the OWTP was the preferred technology selected for this study (see Figure 3 for a process schematic). -The co-digestion program alternative was evaluated against a baseline alternative, whereby the City continues their current operations. The City's food waste is currently managed by a third party which composts it at the Agromin composting facility. -The hourly energy analysis allowed the City to accurately estimate energy savings from increased biogas production from food waste co-digestion by taking into account fluctuations in biogas production as well as varying electricity usage charges during the winter and summer and during on-peak and off-peak hours. -The energy analysis found that food waste co-digestion would increase cogeneration power generation by 79 percent compared to the baseline alternative. However, the net plant power usage for the co-digestion alternative is only 23 percent lower than the baseline when accounting for additional power loads for food waste processing. -The results from the energy analysis were fed into the cost/benefit analysis which consisted of a 20-year life-cycle cost analysis including capital and O&M costs to compare the baseline and co-digestion alternatives (see Figure 4 for cost results). The analyses found that based on the unit cost assumptions made, the co-digestion alternative is more costly both in capital and O&M costs compared to the baseline, even when accounting for the increased biogas production and cogeneration power generation of the co-digestion alternative. -The analysis identified that one of the major O&M costs is the cost for food waste slurry hauling from the MRF to the OWTP. A sensitivity analysis indicated that the hauling cost can greatly impact the affordability of food waste co-digestion compared to the baseline scenario. In conclusion, this bench scale study and feasibility study found that, while the City of Oxnard has sufficient capacity for food waste co-digestion, the co-digestion alternative is more costly compared to the status quo of continued composting of the City's food waste. These results are likely due to the fact that the biogas is used for cogeneration and to the high hauling costs assumed in this study. While the results are specific to the City of Oxnard, the approach taken in this project of combining feasibility studies with bench-scale studies can serve as an example for other WRRFs. This project demonstrated how WRRFs interested in implementing food waste co-digestion can use cost-effective bench scale studies to gain confidence in the techno-economic evaluation of full-scale food waste co-digestion.