A two-phase anaerobic digestion process (UASB-UASB) for simulated sewage sludge
Creators: name: Fongsatitkul, Prayoon
Publisher: name: University of British Columbia
Year: 2008

description: The objectives of this research program were to demonstrate the feasibility and effectiveness of a “two-phase” anaerobic sewage sludge stabilization (UASB-UASB) process. A bench-scale experiment, consisting of two completely mixed sealed upflow anaerobic (A-UASB and M-UASB) reactors, designed to operate inside a walk-in temperature controlled room at 350 C, was employed. The system was first seeded and acclimatized, and then used in a series of different experimental runs, emphasizing the effects of influent Sludge Ratio (SR) of primary to secondary sludges and Recycle Ratio (RR) of fluidized sludge from sludge blanket portion of the reactor, on process performance. The vicinity of “best known” running condition was located by an application of 2 by 2 factorial design and Response Surface Method (RSM). Maximum system loading capacity, optimum operating conditions, and system failure/recovery process were further investigated. The results of this research study showed that a two-phase UASB-UASB process appeared to be feasible and effective in stabilizing sewage sludge at high organic loading rates, while maintaining an acceptable level of supernatant quality and CH 4 gas production. The system had a high potential to recover effectively, after a serious failure, by applying a step-loading reduction and internal recirculation (RR) approach. The “Two-phase” concept has proved to be successful in treating sewage sludge. Hydrolysis acidification predominated in the A-UASB, while acetogenesis-methanogenesis dominated in the M-UASB. Most of the reactions occurred at the lower parts (sludge blanket and bed) of both reactors. A combination of system hydraulic and organic overloading in the M-UASB reactor was a major cause of process failure. This was indicated by a washout of MLVSS, an increase in the total VFA concentration, a reduction in the system removal efficiency, a cessation of CH4 gas production, a drop of pH, and an increase in total VFA/alkalinity ratio. Applying a two-step approach to increase the HRT of the M-UASB by 1.5 and 5.8 times that of the process failure HRT (M-UASB), the system COD (sol) removal efficiency recovered exponentially with an increase in HRT, while the CH4 gas production recovered logarithmically (r2=O.81-O.99). The optimum operating HRTs for the M UASB, regarding COD (sol) removal efficiency and CH4 gas production, were 2 and 2.7 days during the maximization and recovery period, respectively. For design purposes, the optimum operating HRTs of 1 and 2 days as well as RR of 2 and 3 times that of the intluent flow rate are recommended for the A- and M-UASB reactors, respectively. A reactor diameter to height ratio of 1:8 to 1:10, and an organic loading rate of 19 kg COD(total)/m3-d at 35 ° C, with the feed sludge ratio of 4 to 1 (80/20) are also recommended. Modified design criteria, start-up and acclimatization processes, and system operation, for the two-phase anaerobic digestion of sewage sludge (UASB UASB), were finally developed.