The Results of Replacing a Class A Biosolids Stabilization Technology to Gain Capacity and Save Money

INTRODUCTION Due to historical and continued population growth, the Town of Erie Colorado's North Water Reclamation Facility (NWRF) needed to expand. The NWRF's existing solids treatment process utilized an alkaline biosolids stabilization process to achieve Class A cake. Frequent shutdowns of the dewatering system occurred due to significant scaling in its screw press, caused by the high dosage of lime. Acid addition was consequently introduced as a permanent solution to reduce scaling, but as a result, the stabilized solids did not meet Class A fecal coliform reduction requirements. Due to expansion needs and the current system issues, a new or greatly modified system was required. The Town became interested in evaluating a new solids processing system that would both meet the requirements of the desired Class A end-product, as well as reduce the total annual operating and maintenance costs. HDR was tasked with a comprehensive alternatives evaluation for several biosolids stabilization technologies to determine which option would best suit the Town. The evaluation included establishing design conditions, analyzing the existing system's capacity, performing a preliminary screening of all alternatives, conducting a non-economic evaluation for the narrowed alternatives, and developing an economic evaluation including capital and operation and maintenance costs. This approach incorporated both monetary and non-monetary considerations to ensure that the selected technology would meet the Town's short-term and long-term interests. APPROACH The initial step was to compare the capacity of the existing biosolids handling system against the projected 10- and 20-year solids flows and loads, and determine the scope of expansion required to keep the existing system in operation. The next step in the evaluation process was to perform a screening level alternatives analysis for other biosolids stabilization technologies. Several considerations were included in the analysis including the number of installations of each technology within the United States, feasibility of construction, extensive annual costs for operation or maintenance, and process safety. After completing the preliminary screening of biosolids stabilization alternatives, HDR and the Town eliminated multiple technologies from further consideration. The following stabilization systems were shortlisted for further evaluation: - Alkaline Biosolids Stabilization: Existing System Expansion - Autothermal Thermophilic Aerobic Digestion (ATAD) - BCR Neutralizer System The first alternative, which is to expand the existing lime stabilization system, assumed no changes to the plant's current biosolids stabilization process other than increased capacity. The second alternative, Autothermal thermophilic aerobic digestion (ATAD), examined changing the process to a two-step aerobic digestion system patented by Thermal Process Systems (TPS) that utilizes high temperatures to trigger microbial activity by high carbon consumption thermophiles. The last alternative, the BCR Neutralizer system, is a process in which sodium chlorite and sulfuric acid are combined to make chlorine dioxide, which is then added to thickened WAS. It is a batch process rather than a continuous flow process, and it uses three additional chemicals to obtain Class A biosolids: sodium nitrate, ferric sulfate, and sodium hydroxide. The non-economic evaluation played a key role in the selection process. The non-economic factors included: - Operator Attention - Operator Familiarity - Maintenance Requirements/Complexity - Flexibility to meet Future Flows and Loads - Footprint - Implementation - Redundancy - Sustainability - Safety The last step in further evaluating the three alternatives was to conduct a monetary comparative analysis by developing the opinion of probable construction costs and a net present value (NPV) analysis using labor, energy, and chemical costs for a 10-year and 20-year evaluation. RESULTS The non-economic evaluation demonstrated that the ATAD alternative was preferred to the BCR Neutralizer alternative following the non-economic evaluation. After operating a chemical lime system for biosolids stabilization and handling several operational and maintenance-related difficulties associated with it, the Town favored a treatment process that did not require heavy chemical use. Additionally, the Town preferred an alternative that would be easily implemented within the plant's existing process, and that would sustainably and reliably produce a desirable biosolids end-product, so that they could reduce their dependence on third-party hauling companies. The figure below shows ATAD alternative as the favorable stabilization process versus the BCR Neutralizer. Figure 1: Non-Economic Evaluation of Narrowed Alternatives (Town of Erie NWRF Expansion Preliminary Design Report, HDR, 2019) The economic evaluation showed that the ATAD alternative was twice the capital cost of the other two alternatives, however, after review of the 10 and 20 year net present values, the ATAD alternative was the most favorable. Table 1 below provides a summary of those costs. Table 1: NPV Analysis for Keep Existing and ATAD Alternatives (Town of Erie NWRF Expansion Master Plan, HDR, 2019) Construction started towards the end of 2020 and the ATAD process was in operation by the start of 2023. The biosolids system is currently processing an average of 2800 PPD. With a design capacity of 11,700 PPD, the treatment system will have capacity to meet future growth. By year end of 2023, the new ATAD process has provided annual cost savings to the Town including a reduction in chemical cost by $75,000, reduction in hauling costs by $100,000, 10% reduction in electrical costs and $40,000 reduction in gas usage compared to previous years using the existing lime stabilization system. CONCLUSION After the evaluation was completed, the ATAD alternative was selected by both non-economic and economic selection criteria. The selection would provide the Town with a more sustainable biosolids stabilization process and the best value given review of the NPV analysis. As a result of this decision, design and construction of a new ATAD solids treatment facility was built. The Town is currently producing Class A Biosolids consistently. Actual cost savings have been apparent compared to the existing system, with first year annual cost savings equaling nearly a quarter million dollars. The outcome of this analysis highlights the importance of reviewing the big picture and not just the initial capital cost for selecting a biosolids stabilization process. This approach can be implemented for facilities looking to improve their solids handling system by applying a similar evaluation process to ultimately achieve their goals.