Co-digestion of Organic Wastes in Anaerobic Digesters: Potential Benefits, Unintended Consequences and Lessons Learned from Multiple Studies

Background Water reclamation facilities (WRFs) have increasingly been investigating and implementing co-digestion of organic materials as a method to generate revenue through tipping fees and to increase biogas (and energy) production. Co-digestion provides an excellent opportunity to divert organic wastes from landfills thereby reducing greenhouse gas (GHG) emissions. Further, co-digestion helps beneficially reuse this organic matter through renewable energy production and facilitates recycling nutrients and organic matter to soils, creating a sustainable reuse cycle. However, successful implementation of co-digestion program requires a clear understanding of organic waste characteristics and pre-processing requirements, digester stability during co-digestion, upstream and downstream process impacts, operational challenges, biogas quantity and quality, permitting requirements and economics. This presentation will share the author's experience from multiple bench scale, pilot and full-scale studies on potential benefits, unintended consequences and lessons learned for implementation of co-digestion program. Methods/Approach The study will present results from studies performed for multiple clients as well as research studies using variety of organic wastes (e.g. food waste, FOG, poultry, creamery, whey wastes). The client projects range from full-scale demonstration of food-waste extraction to bench scale studies using industrial organic wastes. The research projects include those funded by Water Research Foundation to evaluate un-intended consequences of co-digestion as well as evaluation of biogas quality during co-digestion. The presentation will address extraction of organic materials from Municipal Solids Wastes (MSW), comparison of source separated organics (SSO) and MSW extraction approaches for co-digestion, polishing requirements for the MSW extracted organics, digester stability/performance at pre-determined volatile solids loading rates (e.g. pH, volatile solids reduction, COD, alkalinity, foaming potential), biogas quantity and quality at various loading conditions using different organic wastes, dewatering performance of co-digester sludge (e.g polymer demand, percent solids), centrate quality, and dewatered cake characteristics. Further, the presentation will include key lessons learned (operations, regulatory, etc.) from a three-year full-scale food waste extraction and co-digestion demonstration project. Most of the co-digestion studies were performed in mesophilic anaerobic digesters. While the scope of evaluation varied among the studies, in general, a holistic evaluation was performed in these studies to better understand the impact of co-digestion. Digesters without organic wastes (sludge-only) and with pre-determined amounts of organic wastes were operated to selectively identify the impact of organic waste addition. Results Select Results from these studies are present below. Pre-processing of Organic Wastes An organic extrusion process (OREX, Anaergia Corporation) that uses differences in viscosities of organic and inorganic materials was used in the extraction of organic materials from MSW during a full-scale demonstration study at Silicon Valley Clean Water (SVCW) facility, Redwood City, CA. The OREX extracted organics had a solids content of approximately ~35%. The volatile solids content and COD were approximately 83% and 325,000 mg/L, respectively. The soluble COD was approximately 7,000 mg/L. The extracted organics was then polished using a paddle finisher to remove residual impurities (plastic gloves, bags, spoons, etc.) and dilute the pulp to a percent solids content of ~6% to facilitate transfer to the digester. The percent volatile solids and total COD of the polished waste were approximately 88% and 60,000 mg/L, respectively. A preliminary economic evaluation to compare SSO program and MSW extraction of organics indicated that, MSW extraction is more economical for this facility. However, the results can vary at other facilities based on site-specific conditions. Figure 1. (a) Municipal Solid Waste, (b) OREX extracted organic waste and (c) polished organic waste. Organic Waste Loading and Biogas Yield The volatile solids loadings varied from ~0.04 to 0.2 lb VS/cu.ft./day in the studies performed using a variety of organic wastes. The ratio of organic waste to sludge VS loading varied from ~10 to 100%. Results from these studies indicated that, in most cases addition of organic wastes to the digesters not only increased the total biogas production, but it also increased unit biogas production. In most cases, the digesters did not experience any process upsets. The pH did not vary by more than 0.2 to 0.5 units in most cases. The volatile solids to alkalinity ratios were well below 0.1 (a general criteria for digester stability), except in one case using poultry blood waste. However, the high ratio due to the addition of poultry blood waste did not upset the digester performance. Figure 2 shows the biogas yield in two bench scale studies using pre-processed food waste and raw organic waste. Figure 2. Specific methane yield during co-digestion of (a) pre-processed organic waste and (b) crushed raw organic wastes in sludge from two WRRFs. X-axis shows ratio of organic waste VS to sludge VS. Biogas Quality As part of a Water Research Foundation study, biogas quality during co-digestion of variety of organic wastes were evaluated for six different treatment plants Figure 3. Summary of select VOC levels in biogas from sludge and selectively from organic wastes from participating WRRFs In general, co-digestion increased the concentration of VOCs in biogas using some of the organic wastes, lowered the siloxane levels, and increased or decreased ammonia and H2S levels depending on the organic waste. The extent of these changes varied from plant and the type of organic wastes. Dewatering, Cake Odor and Centrate Quality In general, at an organic waste VS to sludge VS ratio of up to 25 to 30 %, a decrease in the net mass of dewatered cake was observed. When the amount of organic waste added increased more than 30%, the mass of dewatered cake increased compared to sludge-only digestion. The polymer demand generally increased during dewatering of co-digested sludge. Headspace total volatile organic sulfur compounds (TVOSCs) levels were measured to identify cake odor production. In general, addition of organic wastes at up to 25% of sludge VS appeared to increase cake odor. At higher organic waste loading the cake odor production significantly decreased compared to sludge-only digestion. In many cases, addition of organic wastes increased the levels of mono-valent salts and ammonia. The bivalent cation levels decreased due to organic waste addition. Lessons Learned Key lessons learned include: -A discussion with local permitting agencies prior to initiation of co-digestion will help tailor the scope and expedite the program -A clear communication with generators of organic wastes is critical to ensure the quantity and quality of organic waste supply -A Technology Impact Analysis to document and implement various process and infrastructure improvements needed prior to implementation of full-scale co-digestion program is required -A hot water system is required to effective pumping of FOG waste and prevent its deposition in grease pits.