Partnering with Local Sources of High Strength Wastes Through a Co-digestion Program

Introduction Clean Water Services (CWS) is pursuing an opportunity to use available digestion capacity of the Rock Creek Water Resources Recovery Facility (WRRF) by developing a Co-digestion Program. This program serves two purposes: (1) it allows CWS to better serve the district by creating and strengthening relationships with surrounding industries that can provide High Strength Wastes (HWS) and, (2) it increases the overall biogas generation to a quantity that allows CWS to consider Renewable Natural Gas (RNG). 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, the preference is to utilize HSW from surrounding industries that are more consistent and require simpler handling strategies than FOG. Multiple groups within CWS are collaborating in this program to systematically identify, characterize and select wastes that can contribute to gas production goals for RNG without compromising digestion capacity and stability. Criteria for selection is being developed, which include management strategies that have minimum impacts to maintenance and the operation of the digesters, in addition to cost recovery, and staff satisfaction. A key aspect to the development of this program is the focus on a fast response time by both working directly with the industries, and by conducting in-house testing. The testing method was developed to provide information to the team about the biogas generation potential of HSW, possible impacts to the digestion process and operation, and facility requirements in a timely fashion. Program development 1.Defining the Capacity of the Existing System The first step of the evaluation is to determine the capabilities of the existing infrastructure to take on additional organic loading. Using values developed during the facilities planning effort, initial capacities can be estimated for HSW. Within the allowable volume defined by this effort, the biogas shortfall from the total RNG goal must be met. This has allowed us to stablish a minimum biogas yield per gallon of HSW required. The initial value of 4.4 ft3/gallon is used to select potential sources. Other operational conditions have been evaluated which allow us to understand how flexible our gas yield criterion is and the quality of the wastes we would be required to accept under those conditions. The capacity and biogas generation values will be periodically re-evaluated based on operation, and to determine if more HSW can be fed to the digesters in the future. A parallel effort to evaluate anaerobic digester stability is being used and to understand the current capacity of the digesters (Sosa-Hernandez & Schauer, 2023). With a defined capacity and yield, the HSW volumes can be forecasted to stablish volumetric requirements for permanent feeding facilities. 2. Identification and Evaluation of Potential Sources Individuals from CWS leadership, operations, business opportunities and the industrial pretreatment groups have worked together to identify potential candidates for co-digestion materials. The initial candidates are current CWS industrial contributors that have additional biodegradable streams which disposal and transportation represents a significant. If a candidate is considered promising, tours of the source facilities are conducted to further the relationship, understand their practices and processing, and obtain samples. These tours also allow the team to identify other HSW that would not have been considered as potential candidates by the industry. A basic characterization of the HSW samples and qualitative assessment of potential management strategies is performed. A short-duration biogas potential test (BPT5) that was developed in-house is performed to determine the biogas yield per Chemical Oxygen Demand (COD), volatile solids (VS) and volume of each HSW sample. Each of these tests is conducted for 5 days and provides a single data point that corresponds to a conservative estimate of biogas generation, and does not account for slowly degradable components of the material. The preferred co-digestion feedstocks should be relatively easy to digest, and the gas yield should be measured within the duration of the test. The BPT5 procedure also helps identify any toxicity impacts, and suggest handling and feedings strategies that would be required full-scale. Additionally, the gas yields obtained with the BPT5 are being compared to the results obtained with in-house materials such as primary and waste activated sludge, to understand their impact and degradability. A test that is easily implementable and can provide information in a short time frame is ideal for this program, as we can offer relatively quick feedback to the industrial contributor and start negotiations. The gas yields shown in Figure 1a, were used to estimate the average contributions to the gas production goal as well as the available volumetric capacity presented in Figure 1b. As observed in Figure 1, all the HSWs presented meet the volumetric gas yield criteria in various degrees, however this is not the only criteria that would be used for selection. The physicochemical characterization of the HSW samples is used to project potential impacts on the overall treatment system including biosolids generation and nutrient loading return to the liquids process. For example, the solids concentration and nitrogen contributions of these wastes are also being considered as presented in Table 1. 3.Assessing Impact through Full-Scale Testing Full-scale trials are also being performed to validate expectations in terms of handling concerns, as well as to reaffirm commitment with partners and start a working relationship. This requires the implementation of temporary receiving facilities and handling strategies. Another consideration for selection of HSW is staff acceptance and satisfaction, which is influenced by their knowledge of the new incoming product. Staff is included in the setup and equipment of the temporary facility, and training is provided to be able to safely handle the foreign material. The information gathered throughout this evaluation is presented in Table 1, which can help make decisions as to which are the most suitable candidates. This provides the necessary background to conduct negotiation for tipping fees with the industries. 4.Negotiations and Product Acceptance In addition to providing a product that has consistent quality and quantity, an ideal partner would provide information to CWS on a frequent basis. CWS would provide reliability, offer a financial incentive, and would help align their company values with certifications. These benefits and the information generated through this evaluation is considered during the negotiation process prior to acceptance. 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 the partnership has been solidified through a contract that 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 that were feasible even though they require higher investment. Conclusions Through a systematic evaluation, CWS has defined the capacity of the existing system, identified, and evaluated potential HSW sources, and conducted full-scale testing to assess real-world impacts. 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 sustainability goals but also demonstrates a commitment to innovation and partnership.