Business Case Evaluation of Co-digestion for Wasatch Resource Recovery and Wasatch Integrated Waste Management

INTRODUCTION Wasatch Resource Recovery (WRR) is an existing anaerobic digestion facility located in North Salt Lake, Utah, next to the South Davis Sewer District's South (SDSD) Wastewater Treatment Plant. WRR has anaerobic digesters that became operational in March 2019. Figure 1 is a picture of WRR. WRR receives organic waste streams, such as fats, oils, and grease; food scraps; liquid waste; pre-consumer food waste; and bottled waste. The digesters are operated at mesophilic conditions (approximately ~95 degrees Fahrenheit) and are equipped with gas mixing. The biogas produced from the anaerobic digesters is treated to renewable natural gas (RNG) standards while the digestate is dewatered to achieve 14 to 16 percent cake solids and then hauled off for land application. The centrate from the dewatering process is sent to a sidestream ammonia recovery process before it is conveyed to SDSD WWTP for further treatment. Wasatch Integrated Waste Management District (WIW) owns the Davis Material Recovery and Transfer Facility (Figure 2), which is a mixed waste receiving and processing facility located in Layton, Utah. The facility processes residential waste generated in Davis and Morgan Counties. WIW is interested in diverting the organic fraction of the mixed waste from the landfill to WRR for anaerobic digestion and biogas production. This paper evaluates the feasibility of anaerobically digesting the organic fraction of municipal solid waste (OFMSW) from WIW's Recovery and Transfer Facility using the existing digester capacity at WRR. The Study Objectives were to: -Determine the required pre-digestion processing of the OFMSW for digestion at WRR -Conduct digester capacity analysis to determine the available digester capacity at WRR to accept OFMSW from WIW -Determine biogas and biosolids production and side-stream impacts from WIW's OFMSW -Determine if the business case is economically feasible for both WRR and WIW METHODOLOGY The MSW from WIW will need major pre-processing and additional polishing steps before it will be suitable as feed to anaerobic digesters. WIW recovers metals, plastics, cardboard, etc. from MSW and operates a trommel screen that results in 2-inch minus product. The organics would be recovered from the 2-inch product using an organics extrusion press (OREX) system to produce the OFMSW wet cake for trucking to WRR. Further processing would be required at WRR where an Organics Polishing System (OPS) would be added to further polish the wet cake to remove glass and grit prior to digestion. Figure 3 shows the proposed process flow diagram for the business case evaluation. The digester capacity was evaluated based on organic loading rate and hydraulic retention time (HRT). Specific chemical oxygen demand (COD) load limit of 0.5 (.i.e mass of COD loading per day per mass of volatile solids in digester) and minimum HRT of 15 days were used as the evaluation criteria for the digester capacity analysis. RESULTS AND DISCUSSION Analysis of the digestion capacity at WRR found that there was available digester capacity for acceptance of OFMSW. All of the OFMSW from WIW could be accepted because of its higher solids content. The higher solids content was compatible with the lower solids content of the FOG and food waste currently being fed to the digesters. Adding the OFMSW increased the biomass in the digesters and improved the food to microorganism ratio for stable digestion operation. The key costs and parameters for the base case of the business case evaluation is shown in Table 1. The key parameters that had significant impact on the business case were residual distribution cost, OFMSW material quality, RNG value, and the chemical cost used in sidestream treatment at WRR. The business case assumed the residual and rejects distribution cost for all scenarios would include WIW backhauling the rejects and residual from WRR. Since the cost of trucking the cake material from WIW to WRR included the round-trip mileage and costs for going from the Davis Facility to WRR to the landfill/compost facility to the Davis Facility, it was assumed that there were not any additional costs to backhaul. A sensitivity analysis was conducted to evaluate how the business case shifts with varying key parameters. The sensitivity analysis focused on the following parameters: -HSW (assumes the quality of the OFMSW could be improved) -Sidestream chemical costs are offset by revenue from recovered ammonia as ammonium sulfate -Increased RNG value CONCLUSION As shown in Table 2, the best scenario was the one with sidestream chemical costs being offset by ammonia revenue. This suggests that a reasonable cost for digestion of OFMSW is possible, with the right conditions. Distribution costs of residuals and rejects are a major cost and back hauling of materials is a crucial element of a successful project. Additionally, a higher value for RNG, as well as revenue from ammonia recovery, to offset the sidestream chemical cost are necessary. A tipping fee is necessary for profitable WRR operation, unless WIW takes the residuals and rejects as planned.