Implementation of Raleigh Water's Bioenergy Recovery Project

Raleigh Water has made a significant investment in a Bioenergy Recovery Project at the 75-mgd Neuse River Resource Recovery Facility (NRRRF) to transform management of the City's biosolids. The Project is delivering a new system that will produce a high quality Class A biosolid product for beneficial use as a soil conditioner and renewable natural gas to offset fuel usage in the City's bus fleet. The new process will include sludge thickening, sludge screening, pre-dewatering, thermal hydrolysis, sludge cooling, anaerobic digestion, final dewatering, steam boilers, upgrading of digester gas to produce renewable natural gas (RNG), a sidestream deammonification system and potential future plans to accommodate potential receiving of fats, oils and grease in future. This presentation will focus on the following aspects of the project: - Design phase decision making related to selection of core technology -Maintenance of plant operations during construction -Planning for start-up and commissioning of the new system Design While the core process selection involving advanced digestion with thermal hydrolysis was made early in the project, several other key decisions needed to be made during the design phase to find the best-fit technical solutions to meet Raleigh Water's needs. Some of these key decisions are summarized in Table 1. The full paper will provide details regarding how these key decisions were made to ensure selection of cost-effective and efficient solutions to meet Raleigh Water's goals. Maintenance of Plant Operations (MOPO) During Construction The existing solids process and end use strategy at NRRRF included aerobic digestion of waste activated sludge (WAS) for liquid class B land application, lime stabilization of blended undigested sludge to produce a Class A product and third party composting of primary/WAS dewatered cake. It was vitally important that Raleigh Water maintained the ability to reliably process residuals during the construction period. A detailed MOPO planning exercise was undertaken to overcome various challenges associated with construction phasing and plant operation. Key 'pinch points' included: -The need to continue to process WAS while rehabbing the existing gravity belt thickener building -Continuing the ability to load out dewatered cake despite moving and re-purposing two cake bins to their new location for pre-THP cake storage -The need for continued final dewatering while rehabbing the belt filter press building -Conflicts between the existing lime stabilization system and plans for temporary dewatering Planning to overcome all of these challenges was achieved by the whole team including Raleigh Water staff, the design team of Hazen / B&V and the CMAR contractor, Crowder Construction. Schematics were prepared showing both the existing and new process with each step clearly marked up to show the solids processing route and key modifications to the process. This approach ensured that the whole team understood and was able to consider risks associated with the proposed approach. An example showing moving of the cake bins is provided in Figure 1. The full paper will provide details of the full strategy and key considerations for MOPO during construction, as well as lessons learned through the process. Proactive Planning for Commissioning and Operations Planning for commissioning of the Bioenergy Recovery Project has been a collaborative ongoing effort with monthly commissioning meetings held between the Raleigh Water O&M staff, the design team and the CMAR contractor. A key consideration has been the method for seeding and startup of the digesters and THP process. Because THP heats the feed sludge to a high temperature (killing off microorganisms in the sludge), seed sludge is required. To ensure that the digesters quickly meet Class A requirements, the plans included trucking of seed sludge from the existing THP digestion facility at Hampton Roads Sanitation District. Seeding with both liquid sludge and re-wetted cake were considered as viable options. Ultimately the team selected seeding with 1.0 MG of liquid sludge into Digester 2, to avoid the need for re-wetting of the sludge and the addition of alkalinity lost during dewatering at HRSD. The seeding approach involves the following steps: -Filling Digester 2 to 25 % full with disinfected reuse water -Heating the digester contents using steam injection into the recirculation line -Seeding with 1.0 MG of Class A liquid digested sludge from HRSD -Gradually ramping up feed from THP, based on a target volatile solids organic loading rate (lb of VS added per day per lb of VS in the digester) -Transferring 50% of the sludge in Digester 2 to Digester 1 when Digester 2 becomes full -Continuing to ramp up based on loading rate Projections showing the feed solids and digester levels are provided in Figure 2 and Figure 3 respectively. The final paper will provide an update on plant status based on activities completed in early 2023. Another key aspect of the proactive planning process has been to plan for additional staffing requirements and training needs for the facility, given the increased complexity of the new system in contrast to previous operations. The final paper will outline this transition process and planned staffing and training for the new facility.