Master Planning for Near-Term and Long-Term Residuals Management for a Multi-WRRF Utility

Background Renewable Water Resources (ReWa) operates nine (9) wastewater resource recovery facilities (WRRFs) in the upstate of South Carolina. Four facilities stabilize biosolids to achieve Class B pathogen reduction through anaerobic digestion. One (1) WRRF employs aerobic digestion for stabilization, and the four (4) remaining facilities transfer biosolids to other WRRFs for stabilization. Four facilities have dewatering equipment onsite to allow for dewatering and dewatered cake disposal at the landfill, but the remainder of the WRRFs must transfer liquid solids to these facilities for dewatering prior to landfilling. The primary method of biosolids management is land application of thickened, liquid Class B biosolids. The land applied solids range from 2% to 6% total solids. ReWa has over 10,900 acres of farmland permitted to receive biosolids. The contingency biosolids management method is dewatering and landfill disposal. Hazen and Sawyer was retained by ReWa to provide Biosolids Master Planning Services to develop near-term and long-term recommendations for a more flexible and sustainable biosolids management program. Evaluation of centralization of biosolids management was also considered. Development of the master plan will be primary focus of the presentation. ...Near-Term Evaluation Near-term solutions which could be implemented quickly and alleviate treatment and disposal constraints were evaluated. Low-capital intensity solutions were prioritized. Near-term solutions recommended for implementation included the following: - Procurement of a mobile belt filter press (BFP) to create operator flexibility at WRRFs with no onsite dewatering capabilities or supplement dewatering capabilities at facilities where dewatering infrastructure already exists. - Transition to a dewatered cake Class B land application program to provide management flexibility and to reduce volume of biosolids managed using dewatering infrastructure already present at selected facilities. - Construction of dewatered cake storage to create additional storage during periods of inclement weather when sludge storage tanks are full and biosolids cannot be land applied Implementation of each near-term solution was considered at appropriate WRRFs across ReWa's system. Benefits of a dewatered cake land application program include: - Improved logistics with fewer transport activities to move same dry mass of material - Reduced liability and risk exposure with fewer trucks on the road - Reduced GHG emissions from reduced transit mileage requirement - Reduced need for CDL drivers due to approximately four-fold reduction in number of trips - System wide O&M cost benefit Construction of a 45,000 sf regional covered storage facility at a centrally-located WRRF was also recommended. The WRRF was selected as the location for the regional cake storage facility due to the available land onsite, remote location and proximity to permitted land application fields. The 45,000 sf regional covered storage facility would provide approximately 25 days of storage systemwide based on 2020 sludge production and flow estimates and is consistent with what other Carolina utilities with robust dewatered cake land application programs have installed. Regulatory and Market Analysis In preparation for the facility making a Class B dewatered cake product in the near-term and potentially a Class A product in the long-term, a regulatory review, benchmarking with other SC utilities and a Class A market analysis was completed. Results of the market analysis were used to shape discussions regarding potential Class A alternatives investigated. It was recommended that for long-term planning, solids reduction in product mass and/or volume be strongly considered to reduce downstream management challenges either with land application or alternative management options. Long-term planning should also include trigger points that would dictate a change in biosolids management pathway to address a changing regulatory environment or sustainability initiative for ReWa. Figure 1 details a potential long-term biosolids management roadmap with regulatory trigger points for ReWa. Long-Term Evaluation Long-term improvement opportunities were based on evaluating advanced processing technologies, and beneficial use of digester gas. Thermal hydrolysis (THP), Lystek, CleanB and thermal drying were the advanced processing technologies considered at various facilities across the system; a site layout was developed for each advanced processing technology for facility buildout. In addition to these decentralized alternatives, several system wide alternatives were also evaluated to consider opportunities for centralization. In the long-term, it was recommended that ReWa transition to a thermally dried product to reduce volume of material handled and increase end use outlets. A centralized drying facility was recommended to take advantage of economies of scale and reduce staffing requirements across facilities. Locating a regional dryer facility at ReWa's largest facility was recommended. This facility has the largest amount of available digester gas to supplement thermal dryer fuel demands. There are several types of thermal dryers which should be investigated more thoroughly once ReWa is ready to progress with this phase of the biosolids management program. For the purposes of CIP development and planning, a dryer with the capacity to handle solids from the three largest WRRFs on an annual average basis through 2040 was assumed. Based on the current flow projections, maximum month solids loading and greater would need to be handled by a secondary outlet after 2034. This secondary outlet could either be through the dewatered cake land application program or installation of a second thermal dryer. By installing a thermal dryer to manage solids from the three largest facilities in 2030, ReWa could manage approximately 60% of the solids through the thermal drying pathway and the balance via the Class B dewatered cake land application program as shown on the left side of Figure 2. Additionally, moving from a 15% dewatered cake to a 90% dried pellet would significantly reduce the total mass and volume of solids managed by ReWa as shown on the right side of Figure 2. The recommended course of action developed can be advanced in phases as indicated in Table 1. The timeline shown is flexible and may be adjusted to address a changing regulatory environment or sustainability initiative for ReWa. This paper and presentation will describe the evaluations conducted as part of the master planning effort and present the selected biosolids management path forward as detailed above.