Acid+ Digestion - How Columbus fixed their Acid Phase Digesters and is Researching a New Process that Intensifies Digestion and Recovers Phosphorus

Introduction Today, utilities are being increasingly asked to balance highly sustainable operations while keeping impacts to rate payers to a minimum. Solids stabilization is one area that utilities have used increasingly to achieve that balance, however, there is no integrated solution currently on the market that intensifies digestion while providing nutrient recovery in the same process. This research project will optimize acid gas digestion and develop an improved understanding of acid-gas digestion that will be leveraged to develop a novel process called, Acid+ Digestion. This research was recently selected by the Water Research Foundation (WRF) for a $100,000 matching grant for a total budget of $455,000. The Southerly Wastewater Treatment Plant (WWTP) is the larger of two WWTPs that serve the City of Columbus and the greater Central Ohio sanitary sewershed. Figure 1 shows the existing Dual Phase Digestion System at the Columbus Southerly WWTP. The Methane Phase Digesters have foamed periodically since the system was started in 2009. Foam builds up in the annular space between the digester covers and the tank wall. During severe foaming episodes, foam spills over the tank walls. Foam is a byproduct of the surface chemistry gas, liquid, and solids phases. Foaming robs a system of capacity, which in turn hinders the production of methane gas. Figure 2 shows foaming episode at a Methane Phase Digester. Research Overview Research is expected to begin in early January 2022, and we expect to have preliminary results by May 2022 to be presented for the WEF Biosolids Conference. We also performed preliminary calculations and BioWin modeling as part of the successful WRF proposal that can be presented. Our initial objective is to optimize acid gas digestion ('AG') of wastewater biosolids for clear direction for facilities currently equipped with that process. Our larger objective is to build on optimized AG to develop a novel advanced digestion process called 'Acid+ Digestion' (or simply 'Acid+'). Acid+ would offer water resource recovery facilities (WRRFs) an option to roughly double digester capacity; enhance nutrient resource recovery as struvite; protect digestion, dewatering, and centrate handling from struvite fouling; and reduce biosolids phosphorus (P) by half, with configurations that offer other as-needed site-specific benefits. The optimized-AG and Acid+ processes will each be run in separate controlled, laboratory-scale experiments with conventional Methane Digestion as a control. The research will explore each of the processes benefits as retrofits upstream of existing Methane Digesters. Benefits to both process upgrades are anticipated to include reduced foaming, reduction of H2S in biogas (by wasting acid-phase digester gas), and enhanced process stability. Acid+ is expected to additionally provide controlled production of struvite at lower chemical demand (for resource recovery, protection of methane-phase digesters and piping/equipment from struvite fouling, and reduced biosolids P) while possibly increasing existing digester capacity by 75% to 150%. Figure 3 shows a high-level overview of BC's research objective and project. Acid Phase Digestion The original promise of AG digestion was to create separate environments idealized for hydrolysis and acidogenesis and methane formation; with distinct microbial populations in separate reactors performing better than in a conventional mixed culture. While the underlying fundamentals of acid gas digestion are strong, implementation has been limited to only 10 to 20 facilities in North America due to full-scale performance not matching promised improvements. Our sponsoring utility, City of Columbus failed to realize AG's promised benefits during the initial years of AG operation at the Southerly WWTP. Because the process failed to reduce (and even increased) foaming, Waste activated sludge (WAS) had to be bypassed around digestion to prevent the methane digesters from overflowing through floating-cover annular spaces. WAS bypass cost an extra $1M annually. Two and a half years ago, Darin Wise and Columbus conducted a self-directed study based on results from a University of Wisconsin, Madison masters' thesis (Siebels, 2011) that found maintaining AG solids retention time (SRT) at 1.8 days under all sludge production conditions eliminated foaming. All Southerly WAS is now digested. Figure 4 shows a period of daily and rolling annual, monthly, and weekly average Acid-phase feed at Southerly. These data will later highlight challenges with AG digestion related to consistent SRT maintenance. The Columbus experience described above is not unique and several utilities with AG digestion either eliminated their Acid digesters or have found that the process 'only works' at lower SRTs. Optimize Acid Gas Digestion Our initial investigative track will explore AG digestion with both batch and continuous-feed testing to determine the actual kinetics, operational parameters, and biological consortia that allow a less than-1.8-day AG digester to work well and that cause greater-than-1.8-day SRTs to fail. Our AG testing plan will investigate: - Foaming potential, hydrolysis kinetics, soluble products generation, biogas quantity and quality, and microbial consortia profiles & all at three different operating temperatures. - Response to Short-Term Stressed Conditions. The AG feed volumes highlight actual peak and minimum feed volumes. If an AG tank is operated at 1.4D SRT at average feed of 240,000 gpd, what happens during peak- or minimum- week loading when SRTs become 1.2 days or 1.7 days for a sustained period? - It has been seen that exceeding a maximum SRT (currently set at 1.8 days for southerly) induces problems & but why? Our research will answer that question. - H2S Evolves in Acid-Phase Biogas. Digester gas H2S is often removed by iron sponge, activated carbon, or other finite-life media. The potential to divert the Acid-phase H2S to waste could double the life between media replacements or reduce maintenance on gas use systems like boilers which typically have limited gas treatment. - Inform Acid+ Research. Acid+ effectiveness is dependent on the effectiveness of the 1st-stage Acid digester; not just for hydrolysis and VFA production, but also for P release and solubilization of WAS P, Mg, Ca, and other cations. Acid+ Digestion Figure 5 shows how Southerly's three AG digesters could be redeployed under an Acid+ configuration. The table also summarizes the role of each tank/stage within the process. BC also developed an EnviroSim BioWin process model of Acid+ implementation for a 20-mgd WWTP with Bio-P. The existing digesters were resized to provide 20-day SRTs at average load (0.07mgd and 38,100ppd at 86% volatile solids, or 'VS'). Acid-phase (Figure 5) and Harvesting Digesters (D2 and D3, in same figure) were sized to provide 1.5-day SRTs at average, sludge feed (QF). Finally, two parallel models were also run for Methane-only (without D1 through D3) and AG (without D2 and D3), both without recycles or recuperative thickening. Each process was run to failure by increasing the feed flow and solids proportionally until the VFA to alkalinity ratio exceeded 0.20. Table 5-3 summarizes the loading at which each process failed as well as the capacity and total tank-volume increases for each process. There are a few aspects of this BioWin modeling that are inconsistent with our team's digestion experience. Because these digestion-model modules are relatively new and rarely calibrated fully to actual digester operation (by liquid-stream process engineers), we expect our Acid+ testing to provide data for first-of-its-kind, digestion-process model calibration at a variety of extreme conditions. This alone will provide tremendous value to the industry. The benefits of this new process are as follows (numbered as a continuation of the earlier AG benefits): - Cost-effective nutrient recovery through production of high-P struvite or brushite products with reduced struvite fouling on downstream piping and equipment. Most other struvite recovery systems target nutrient collection on thickening or dewatering side streams, preventing nutrient recycles that either require retreatment or that may be discharged in the WRRF effluent. - Lowers biosolids P content by 40-60%, which allows for increased per-acre land application rates with greater carbon and more-balanced N benefits for sites with agronomic P limits. - Improves biosolids dewaterability by alleviating cation imbalances, for less polymer and energy. - Increases downstream digestion capacity by up to 160%, with an only 20-25% increase in digester volume (see Table 5-3). This is comparable to capacity increases with commercially available THP systems, without complex mechanical and thermal requirements. - Reduces chemical demand over other struvite-recovery processes.