Designing for the Future of Solids Management Optimizing Biodrying and Pyrolysis Treatment Design

The presence of PFAS in biosolids has resulted in significant challenges for the municipal wastewater industry, especially for those utilities who land apply biosolids for beneficial use. In addition, the State of California and serval municipalities across the country have regulations in practice that have developed programs to reduce Short Lived Climate Pollutants (SLCP), which include the diversion of organic waste from landfills and reduce methane emissions. With climate change and a greater focus on a circular economy, these practices are very likely to become more common throughout the country. With these challenges comes opportunities making it an exciting time for solids treatment in our industry. The Windsor Water District (District) in California owns and operates a 2.25 MGD Water Reclamation Facility (WRF) that currently uses sludge ponds for storage and stabilization of the waste activated sludge (WAS) and sludge generated by the advanced wastewater treatment (AWT) clarifiers. The District currently contracts with an outside service provider that specializes in sludge dredging, dewatering, and disposal for beneficial use. Figure 1 shows the existing solids treatment process flow diagram. The District established specific goals for their future solids management, including 1) eliminate current reliance on outside contractors for biosolids disposal, 2) increase beneficial use of biosolids, 3) reduce truck traffic and the associated carbon footprint by increasing the solids content of biosolids hauled offsite, and 4) provide management option in the future for surrounding municipalities. To achieve these goals, the District selected Hazen to evaluate technologies and advanced treatment options for producing Class A biosolids without considering anaerobic digestion due to size and only producing WAS and AWT sludge. All options require mechanical thickening and mechanical dewatering prior to implementation. The following technologies were considered and evaluated 1) thermo-chemical hydrolysis (TCH), 2) lime stabilization, 3) composting 4) drying (thermal drying, solar drying, and biodrying), and 5) biodrying followed by pyrolysis. Thermal drying and biodrying options were selected for further evaluation on a life cycle cost basis, which showed that the options were similar. The District selected the biodrying followed by pyrolysis. This option meets the District's needs for the future and eliminates the reliance on land application should land becomes scarce or future regulations for PFAS are in place. In addition, biodrying requires approximately 30% the energy needed for thermal drying. Figure 2 shows the new solids treatment process flow diagram. The new process thickens combined WAS and AWT sludge prior to dewatering, biodrying and pyrolysis to produce a biochar for beneficial use. Rotary drum thickening (RDT) was selected for continuous operation, while the selected centrifuge dewatering operation will be limited to one shift per day Monday through Thursday. Therefore, storage of thickened sludge (THS) prior to dewatering is required. In addition, the biodrying is a batch process and pyrolysis operation is continuous. To integrate these unit processes to operate as one continuous system, it was important to size the unit processes for the sequence of operation and to optimize performance. Since mechanical thickening and dewatering equipment is unavailable at the existing WRF, bench tests on the WRF's WAS sludge samples were conducted to understand the performance of the selected thickening and the dewatering technologies along with their required polymer dosage. The bench test results showed 3 to 6% total solids (TS) with greater than 92 % solids capture rate for the RDT and 16% TS with greater than 95% solids capture for the dewatering centrifuge. The biodrying batch time is inversely proportional to the dewatered cake concentration. After working closely with the BioDryer manufacturer, BioForceTech (BFT), it was determined that 8 units would be needed for a feed concentration of 16% TS. A dewatered cake feeding and sequencing schedule to the BioDryers was developed to optimize biodrying performance. The BioDryers are sized to dry 16% TS cake to 75 to 80% TS feed into the pyrolysis under future average annual conditions with one unit out of service. Under normal operation, all 8 units will be fed at a reduced load. The system is designed with a supplement fuel source beyond the energy provided by the pyrolysis for biodrying under future maximum month conditions. Under this condition 35% of the thermal energy need is provided by the supplement fuel, which is natural gas for our system. When the pyrolysis is not operational the BioDryers will produce Class A biosolids with TS greater than 90%. With the system sized for 16% TS dewatered cake, it was important to optimize the dewatering centrifuge performance to ensure a minimum of 16% TS was achieved. This required that the centrifuge feed be well blended, have a concentration range between 2 and 3%TS, and that phosphorus release not acquire within the sludge storage tank (SST) during storage. This was achieved by utilizing a jet aeration and mixing system within the SST and diluting the thickened sludge within the SST with unthickened combined WAS and AWT sludge. In addition, the chopper pumps were used for the mixed pumps to limit the effect of the thickening polymer conditioning on the upstream dewatering performance. The SSTs were sized to provide sludge blending and storage when dewatering is not in operation and one of the SSTs is out of service under future annual average conditions. The pyrolysis unit was sized to process the dried cake under future average annual conditions without redundancy. The pyrolysis energy balance and biochar production were based upon the results of sludge sampling for proximate, ultimate and heat value analysis. The new solids treatment system will have odor emissions control systems for new processing including the pyrolysis. This paper discusses a performance optimization and energy saving case study utilizing an integrated design involving the modification of solids treatment at a water resource recovery facility (WRRF). All stakeholders realized a novel outcome they all achieved their objectives. This paper will benefit utility managers, operation and maintenance (O&M) practitioners, and engineers by providing: -a highlight of the novel biodrying and pyrolysis system sizing, -optimization of solids handling prior to biodrying, and -the lessons learned during the detailed design process