| Summary: | 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. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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