The City of Baltimore, Maryland evaluated the feasibility of converting the conventional high rate (CHR) anaerobic digesters to the two-phase (acid-gas) mesophilic anaerobic digestion process (two-phase process) at its 180-million-gallon per day (MGD) Back River Wastewater Treatment Plant (WWTP) using a dedicated 25-dry ton per day (dtpd) process evaluation facility (PEF). The evaluation of the two-phase process was performed in conjunction with an extensive upgrade of the digestion facilities. The PEF's acid phase reactor (APR) consisted of a new 200,000-gallon, tall, cylindrical vessel, the configuration of which was designed by the City. The gas, or methane, phase reactors (MPR), consisted of two existing 1.3-million gallon (MG), cylindrical, "flat," digesters. This paper provides the findings of the first 14 months of the evaluation during which time the two-phase and CHR processes were fed the same blend of thickened primary sludge (PS) and waste activated sludge (WAS). The CHR process was carried out in the plant's two existing three-MG egg-shaped digesters (ESDs).It was demonstrated that two-phase process operating conditions had been established in the PEF based on profiles through the system of the volatile fatty acid (VFA) concentration and the pH, and also by the headspace gas composition in the APR all being in agreement with two-phase process theory. Also, the VFA species composition of the APR effluent was comparable to the composition at two other municipal wastewater treatment plants using the two-phase process.At an APR solids loading rate (SLR) of 2.0 pounds of volatile solids per cubic foot per day (lbs VS/cf/day) of sludge averaging six percent solids concentration, the volatile solids destruction (VSD) performance of the two-phase process averaged 58 percent whereas it averaged 48 percent in the CHR process, as calculated by the mass-balance (MB) method. The solids retention time (SRT) in the two-phase process averaged 11-days. In terms of the thickened sludge volume fed to the digesters, digester gas (DG) production in the two-phase process averaged 3.92 cf/gal, whereas it averaged 2.99 cf/gal in the CHR process, at the SLR of 2.0 lbs VS/cf/day. At APR SLRs of 2.5 and 3.25 lbs VS/cf/day, the VSD performance of the two-phase process was not improved relative to the CHR process. The DG production as a function of VSD was nearly equal in the two-phase and CHR processes, ranging from 15 to 17 cf/lb VS destroyed.The APR was operated mesophilically in an upflow mode at a recirculation ratio of about 7.5:1, with a heated mixture of raw and recirculated APR sludge introduced to the bottom of the reactor and with the effluent being withdrawn from the top. Lithium chloride tracer tests in the APR demonstrated the flow regime approximated that of a complete mix reactor rather than a plug flow reactor. While tracer tests were not performed in the MPRs, based on the City's operating and maintenance experience, it appears that the MPRs also operated in a complete mix flow regime with hydraulic short-circu...

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