Maximizing Biosolids Upgrades Through Operational Flexibility and Intensified Processes

APPLICABILITY Digestion requires significant investment in tankage, heating and mixing systems, and ongoing heating and pumping expenses. By optimizing upstream thickening and using multi-phase digestion, utilities can save capital and O&M costs associated with solids treatment. This presentation will focus on the thickening and digestion improvements currently under construction and scheduled for completion in February 2029 at Inland Empire Utilities Agency's (IEUA) RP-1 facility to handle the expected 61 million gallons per day (mgd) solids treatment equivalent. The improvements include a two-prong thickening approach with dissolved air flotation thickeners (DAFTs) and rotary drum thickeners (RDTs) to reduce the liquids sent to the digesters, which increases solids retention time (SRT) and reduces heating and pumping demands. Additionally, a three-stage digestion process, including acid phase anaerobic digestion (APAD), will improve biogas and biosolids quality and reduce capital expenditures required for new tankage volume. The purpose of this presentation is to highlight IEUA's unique approach to improving thickening of three diverse streams and present practical alternatives to conventional anaerobic digestion to improve solids processes. The presentation will examine drivers for improved thickening and viable options for getting the greatest utility out of existing digester tankage. DEMONSTRATED RESULTS AND OUTCOMES Background and Goals: RP-1 handles solids from the onsite 40-mgd liquid process and a 21-mgd upstream plant that does not have solids treatment facilities. The existing system includes a gravity thickener (GT) for primary sludge (PS) and three DAFTs for WAS and scum followed by seven anaerobic digesters operating in parallel. Historically, IEUA operated the digesters in a three-stage process that included one acid and two methane phase stages to achieve Class-A biosolids with a site-specific approval. However, the APAD tank was originally designed as a methane phase digester and was too large, resulting in a SRT longer than the desired 2 days and causing unwanted methane formation. Because of these issues, IEUA stopped operating in the three-stage configuration. The existing thickening and digestion systems are aging and require improvements to address capacity and performance constraints. Figure 1 shows a high-level schematic of the recently designed process that achieves the following keys goals: 1.Upgrade thickening system to add capacity and improve reliability. 2.Return to three-stage digestion. 3.Provide operational flexibility. Designing for Operational Flexibility and Intensified Processes: Nine new RDTs will thicken PS and WAS and two existing DAFTs will thicken scum prior to digestion. RDTs can thicken sludge to greater than five percent solids, an improvement from the existing GT and DAFT system which achieves about four percent. Thicker feed sludge will increase digester SRT and reduce pumping and heating costs. The RDT system is designed for either co-thickening or separate thickening of PS and WAS giving IEUA flexibility to optimize the RDTs and polymer for each sludge stream with separate thickening or produce a homogenized digester feed with co-thickening. Generally, mechanical thickeners like RDTs are not well suited to handle scum and, therefore, scum will not be primarily sent to the new thickening facility. Instead, existing DAFTs will be rehabilitated to thicken scum. However, the system provides flexibility for scum to bypass the DAFTs and go to the RDTs or digestion directly. The thickened sludge and scum goes to digestion which will typically operate in three stages (Figure 2): APAD followed by thermophilic and mesophilic methane phase stages. APAD occurs at mesophilic temperatures (~95 to 100 degrees Fahrenheit (F)) with a high volatile solids loading rate (VSLR) of 1-2.5 lb/CF/day and a short SRT of 1-2 days. These conditions are ideal for the acid forming bacteria that perform hydrolysis and volatile acid fermentation. Then, the methane phase occurs at thermophilic temperatures (~127 to 132 degrees F) for ~12 days followed by a two-day mesophilic stage where sludge naturally cools, accomplishing Class B treatment of the solids. The system was designed with flexibility to bypass APAD and feed directly to the methane phase digesters. The intensification of the solids processes with thicker feed sludge and APAD reduces the total required digester volume. Table 1 shows the estimated max month (MM) VSLR of the existing digesters without APAD is 0.21 lb VS/CF/day. This exceeds 0.15 lb VS/CF/day, the typical sustained max VSLR recommended to maintain healthy methane phase digesters and avoid process upsets. However, the new APAD tanks (Figure 3), which have a total volume of 1.09 million gallons (MG), will be able to handle the high VSLR. Without APAD, 3.07 MG of new tankage would be required to limit the VSLR to 0.15 lb VS/CF/day. This ~2 MG in tank reduction saves on the order of $10M in capital costs. Table 1 also indicates that the estimated MM buildout SRT is 12.7 days across both phases. IEUA can send sub-Class B biosolids to an off-site compositing facility so does not need to meet the 15-day SRT required for Class B. IEUA can further expand their digestion facility in the future to extend this SRT. RELEVANCE TO AUDIENCE This presentation benefits operators and engineers interested in co-thickening and alternatives to conventional anaerobic digestion to improve solids processes. The audience will learn about thickening technologies, APAD, and designing for value and operational flexibility through a case study of IEUA's RP-1 solids upgrades. The presentation will also provide an overview of IEUA's unique three stage digestion process and highlight how it allows hydrolysis, volatile acid fermentation, and methane formation to occur in different digesters with environments best suited for the microorganisms to execute the specific reactions.