Strengthening Partnerships with Local Industries through a Co-digestion Program

Introduction Clean Water Services (CWS), a water resources management utility that serves Washington County, Oregon, is developing a Co-digestion Program to utilize available digestion capacity at the Rock Creek Water Resources Recovery Facility (WRRF) and increase biogas generation. This program allows CWS to better serve the district, by strengthening relationships with surrounding industries that can provide High Strength Wastes (HSW). We anticipate needing a variety of HSW sources to meet gas production goals, which requires a streamlined evaluation approach. A key focus of this program is to improve our interactions with the industries by having a fast response time. This can be accomplished by both working directly with our local contributors and by conducting in-house testing. In addition to providing a product that has consistent quality and quantity, an ideal partner would provide information to CWS on a frequent basis. In exchange, CWS can provide reliability, offer a financial incentive, and can help align company values to obtain business certifications. Identification and Evaluation of Potential Sources The initial candidates are current CWS industrial contributors that have additional biodegradable streams which treatment, disposal and transportation represent a significant cost or do not align with their sustainability goals. If a candidate is considered promising, tours of the source facilities are conducted to further the relationship, understand their practices and processing, and collect samples. A short-duration biogas potential test (BPT5) that was developed in-house is performed to determine the biogas yield of each HSW sample. The preferred HSW should be relatively easy to digest, and the gas yield should be measured within the duration of the test which is conducted for 5 days. This provides a conservative estimate of biogas generation and does not account for slowly degradable components of the material. The BPT5 procedure also helps identify toxicity impacts, and suggest feedings strategies that would be required full-scale. Figure 1 shows the results obtained with the HSW that have been tested so far. Because the evaluation can be conducted over a short period of time, multiple samples from a single source can be tested. Understanding the potential variability of the source and its impact on the digestion process is an important factor considered prior to acceptance. A basic physicochemical characterization of the HSW samples is also used to infer potential impacts to the overall treatment system, which include biosolids generation and nutrient loading return to the liquids process. Full-scale trials are also being performed to validate expectations in terms of handling concerns, as well as to reaffirm commitment with partners, and address potential operational issues. This requires the implementation of temporary receiving facilities and staff training. The information gathered throughout this evaluation is presented in Table 1. The data provides necessary background to conduct negotiation for tipping fees with the industries. Acceptance of High Strength Wastes and Negotiations Based on an evaluation of the hydraulic capacity of our full-scale digesters and biogas production goals, we have generated a gas yield criterion that is used to select HSW. As shown in Table 1, the HSW meet the gas yield criterion of 4.4 ft3/gal in various degrees, however this is not the only criteria that would be used for selection. Other criteria for acceptance include: management strategies that have minimum impacts to maintenance and operation, number of truckloads of HSW to be received, cost recovery, and staff satisfaction. The Durham WRRF currently uses Fats, Oils and Greases (FOG) for co-digestion which are highly variable and create multiple issues that represent a significant investment and a strain on staff morale. For Rock Creek WRRF, HSW that are more consistent than FOG and have less handling complexity are preferred. Beverage Industry A (which results are presented in Figure 1 and Table 1), has provided an HSW that meets our gas yield criteria and is very easy to handle. Their disposition has made them a very suitable candidate and a contract was signed in October of 2023. We have started feeding the Rock Creek digesters with this waste using temporary feeding facilities and have accepted other of their HSW. They have benefited from this program by reducing their transportation costs, and by being able to maintain sustainability certifications. Conclusions The evaluation approach provides prompt information to the team and the industries about the biogas generation potential of HSW, possible impacts to the digestion process and operation, and facility requirements. The focus on fast response times, collaboration among internal groups, and negotiations with industrial partners, exemplified by the successful contract with Beverage Industry A, highlights the proactive and comprehensive approach taken by CWS. This initiative not only aligns with our sustainability goals but also demonstrates a commitment to creating partnerships and innovation. The manuscript will include: (1) the results obtained so far, and that have supported our HSW selection, (2) the lessons learned throughout the evaluation process, and (3) examples of how the fast response and information exchange have improved our relationship with local industries.