Alternate: Taking Back Control of Residuals: Flipping the Script From Disposal to Reuse

The City of Asheville is a thriving and growing city in the mountains of North Carolina. The City has three water treatment plants: William DeBruhl, North Fork, and Mills River Water Treatment plants (WTPs) that serve approximately 92,000 residents. Of the three WTPs operated by the City, two are direct filtration (North Fork and William DeBruhl) and one is conventional treatment (Mills River). The direct filtration WTPs residual production is entirely from the filter backwash, see Figure 1 below for a typical residuals handling process diagram. Each WTP utilizes the same residual handling process, storage lagoons. Each lagoon has a fixed capacity and are dredged and dewatered for disposal on an annual basis via a third party hauler. Recently the City has had an increase in turbidity during intense storm events and caused treatment challenges and an increase in residuals generation at the direct filtration plants. With fixed lagoon capacity, increased residuals production due to climate change resulted in the City acting to evaluate residual handling alternatives. In order to understand the causes of changing storm intensity to develop future predictions of raw water quality and potential increases in residuals production, an evaluation of historical precipitation and raw water turbidity was conducted to determine relationships between the two. Using these relationships, climate change projections were utilized to predict future precipitation and raw water turbidity. The time horizons for analysis were selected as 2040-2069 and 2070-2099. These two time horizons provide an understanding of the projected climate in mid-century and late-century. The future time horizons were compared to the base time period of 1980-2009. For both time horizons historical data was evaluated and trends were identified. The climate change projections indicated that air temperature and precipitation would increase over both time horizons. The precipitation data was then used to predict turbidity increases at all three of the WTPs, see Figure 2 below. The predicted raw water turbidity increased over both time horizons evaluated with an overall increase across all three WTPs of 32%. The historic dry residual production for the Mills River, William DeBruhl, and North Fork WTPs was quantified using a published empirical formula (Davis and Cornwell 2013). The empirical formula is recognized and practiced within industry, however, it is important to note that the formula provides only an estimate of dry tons produced. Both the Mills River and North Fork WTPs use an alum coagulant, while the William DeBruhl WTP uses a poly aluminum chloride (PAC) coagulant, GPAC 2070. The ratio for chemical sludge produced for each kg of coagulant added, varies. For aluminum sulfate (alum) the ratio is 0.26 and for polyaluminum chloride the ratio is 0.16. These ratios are based on the amount of aluminum hydroxide produced during the coagulation process. The ratio between turbidity and TSS, ranges from 1 to 2 (Kawamura 2000). Due to the absence of historical data, a value of 1.5 was assigned to the ratio of turbidity to TSS. The values captured from the empirical formula were used as an estimate to determine the residual production at each WTP from 2015 to 2018. The formula produced an estimate of annual and max month production of dry residuals. The estimated dry residual production provided by this formula was not used to determine sludge production rates for sizing dewatering equipment or landfill disposal cost estimates. The dry residual values used for the landfill disposal cost estimates were calculated using the historical volumes and percent solid values provided by the WTP operators. The values from the empirical formula and historical data were compared to the dry solids removed by Bionomics in 2019 in Figure 3. As part of the evaluation, dewatering technologies were considered that included: belt filter presses, centrifuges, screw presses, and volute presses. Each technology was evaluated based on footprint, polymer consumption, O&M costs, solids throughput, and detailed interviews with existing installations. Strategies considered included: permanent dewatering at each site (Alternative 1), a centralized dewatering facility (Alternative 2), and a mobile dewatering trailer (Alternative 3). These three alternatives were compared against Alternative 4, which was to continue using a third-party residuals hauler. Capital and O&M costs were compiled and analyzed using a 20-year net present value, see below in Figure 4. It was determined that Alternative 3 had the lowest 20-year net present value. Due to potential revenue streams with regionalization of the residuals disposal for surrounding municipalities, the City ultimately selected Alternative 2 - Centralized dewatering with volute presses. Following the Citys selection of Alternative 4, a pilot test was conducted of a volute press with onsite residuals at the Mills River WTP. Pilot testing was conducted in March 2021. Pilot testing resulted in a maximum dewatered cake solids of 20.4% and average cake solids of 13.7%, see Figure 5 below. The study evaluated two polymers: Praestol K111 L-NA and Praestol K650. The Praestol K111 L-NA performed the best out of the two using 16-22 lbs/ton. In addition to onsite residuals handling concerns, the local Water Resource and Recovery Facility (WRRF), decided to stop accepting and treating water residuals from surrounding utilities and eliminated the City's primary method for disposal. Historically, onsite residuals at each WTP were pumped and taken for liquid disposal to the local WRRF through third party contractors. Following the decision by the local WRRF, the costs to dispose of residuals increased significantly due to the need to dewater and dispose at landfills. While landfill options have some reliably in accepting City residuals, third party hauling and disposal fees have increased 528% in recent years and the program poses multiple long-term risks. To determine a sustainable option, an evaluation was needed on short- and long-term residuals disposal alternatives. North Carolina is one of five states that classifies water treatment residuals as biosolids. In order to obtain acceptance by the State the City completed toxicity characteristic leaching procedure (TCLP) testing to determine if any hazardous chemicals or metals were within the residuals for landfilling. The results concluded the residuals did not have hazardous elements. The results were sent to the State for evaluation and the State characterized the residuals as a class A solid. With the determination of residuals quality a market analysis of disposal options was considered. The market investigation included an assessment of local beneficial use outlets available for residuals solids. Existing program data was reviewed and representatives from outlets and local markets were identified and interviewed to understand product preferences. The market analysis included potential land application sites, biosolids brokers, soil blending end users, surrounding municipalities and composting. The market analysis consisted of a questionnaire that was sent to evaluate desirability of the residuals. The residuals were desirable as a bulking agent for a local composter and top soil broker. The market analysis and discussions with end users were utilized to conduct a dewatering demonstration which invited several potential end users and surrounding municipalities to observe the dewatering operation and potential residuals quality for comingling and soil blending. This led to site visits at surrounding interested end users including a local soil blender and composter. The information that was obtained through the study will be used by the City to discuss cost-effective residual handling and disposal methods and a decision is pending. As a part of these considerations the City is exploring a regional partnership with surrounding municipalities for residuals handling and disposal. This paper will present an in-depth description of each of the residuals handling and disposal alternatives that the City considered. Additionally, it will present a description of the evaluation process followed to reach a final selection, with an emphasis on key elements of the selection criteria, including market desirability of the residuals, equipment capital and O&M costs.