Troubleshooting VSR: A Multi-Approach Strategy Towards Significant Operational Savings

Charlotte Water (CLT Water) is the largest public water and wastewater utility in the Carolinas, serving more than a million customers in the City of Charlotte and greater Mecklenburg County. CLT Water owns and operates five major wastewater treatment plants (WWTPs) with an average treated flow of 78.1 million gallons per day (mgd). Additionally, CLT Water oversees one of the largest class B biosolids programs in the southeastern United States, operating with 17,000 acres of permitted agricultural land in both North and South Carolina and producing over 100,000 wet tons (WT) of biosolids annually. CLT Water's largest WWTP is McAlpine Creek Wastewater Management Facility (McAlpine Creek), with an average treated flow of 41.3 mgd and 60,000 WT of biosolids produced annually. There are three major regulatory categories that are required in order to meet the Class B land application biosolids standards: (1) metals concentrations, (2) vector attraction reduction, and (3) pathogen reduction. This paper will focus on volatile solids reduction (VSR) - the vector attraction reduction alternative CLT Water uses to meet land application standards, the financial impact of a sudden failure to meet the VSR requirements, and the troubleshooting and optimization steps taken to address the issue experienced and to improve the overall solids treatment process. McAlpine Creek is a regional solids processing facility, which in addition to treating the solids produced at the plant, it also receives primary sludge (PS) and waste activated sludge (WAS) from the Sugar Creek WWTP (Sugar Creek) for treatment. McAlpine Creek is designed to receive primary solids from Sugar Creek through a dedicated force main, which connects to McAlpine Creek's primary solids gravity thickeners (GTs), while WAS from Sugar Creek, also designed to be pumped through a dedicated force main, connects to a combined WAS well at McAlpine Creek. Even though the infrastructure exists for Sugar Creek PS and WAS to enter directly into McAlpine's solids treatment train, due to a history of hydraulic overloading of the GTs, Sugar Creek's PS and WAS were being sent through gravity sewer lines and entering McAlpine Creek through the influent flow. McAlpine Creek solids treatment train currently consists of primary clarification, primary sludge gravity thickening, WAS centrifuge thickening, anaerobic digestion of thickened primary sludge (TPS) and thickened waste activated sludge (TWAS), and finally centrifuge dewatering of digested sludge. CLT Water meets the vector attraction reduction requirement by achieving a minimum of 38% VSR through the anaerobic digestion calculated on a monthly basis. Typically, McAlpine Creek achieves the minimum VSR requirement with a comfortable buffer, achieving around 45% VSR, without any major issues. In July 2022, operational staff became alarmed when they noticed a trending decline in VSR results prompting the need for immediate operational changes to be made. The efforts however were not enough to avoid a VSR failure of 36% in July 2022, which prevented CLT Water from land applying the biosolids. Instead, CLT Water had to landfill the entire July biosolids production, totaling over 4,000 WT which cost the City approximately $500,000. Operational efforts to troubleshoot the problem continued for several months past July 2022 with improved success later in the fall and through the same summer period of 2023. The main strategies implemented were: (1) digester feed optimization by improving performance of gravity thickeners, (2) VSR calculation method evaluation, and (3) data tracking and proactive sampling. Improving GT performance included adjusting pumping flow rates, throttling existing valves, and replacing malfunctioning valves from primary clarifiers to GTs to improve PS % solids from <0.5% to 1.0-1.5%. Another significant change was reverting the Sugar Creek PS and WAS transport and tie in points to their original design (PS connected to GTs and WAS connected to a combined WAS well), Sugar Creek's solids were no longer going to McAlpine Creek's influent. Additional optimization trials included actions related to polymer use, configuration of duty/standby GTs, and installation of TSS probes and VFDs on Sugar Creek's PS pumps to McAlpine Creek. While a few of the optimization trials did not cause any improvements, several of the strategies yielded positive results and improvements of PS thickening, digestion, and consequently, volatile solids reduction. CLT Water staff went beyond operational changes and started evaluating the VSR calculation method used by the CLT Water biosolids program for many years. There are two main allowable VSR calculation methods including, approximate mass balance and Van Kleeck. CLT Water uses the latter method. After initial research, staff learned the Van Kleeck calculation underestimates the VSR whenever there is grit accumulation inside digesters. Several digester cleanout projects at various WWTPs were performed over the last ten years including at McAlpine Creek confirming that significant grit accumulation does occur inside CLT Water's digesters. With this information, CLT Water started calculating VSR using both methods and tracking the results side by side. The evaluation showed that the approximate mass balance method yields a slightly higher VSR result and proved to be more representative of CLT Water's process due to the proven grit accumulation. Finally, based on the VSR calculation method evaluation, CLT Water decided to more closely monitor percent solids going into the digesters to predict any potential issues sooner, giving staff adequate time to make operational changes to avoid future failures. Additionally, CLT Water found it to be beneficial to run an EPA-approved alternative test (40-day anaerobic digestion bench test) during months that the percent solids based on historical trends were expected to decline. The 40-day digestion bench test was performed in August 2022, as an immediate response to the July 2022 VSR failure, and proactively conducted in the Summer of 2023. By continuing to perform the alternative test early in the months known to suffer from percent solids falling-off, if by the end of the month the calculated VSR results do fail, staff can use the results from the 40-day digestion bench test to meet the EPA requirements and potentially not interrupt any land application operations and incur unexpected landfill costs. This paper will detail out the multi-approach strategy taken by CLT Water staff to troubleshoot and optimize processes around thickening, anaerobic digestion, and VSR calculation which resulted in many lessons learned, benefits around the overall solids treatment processes, and significant cost savings in the biosolids beneficial reuse program.