ABSTRACT: This research demonstrated the feasibility of a two-phase anaerobic sludge digestion process. Process failure and maximum system loading capacity were also investigated. Three-dimensional plots of chemical oxygen demand (sol), volatile fatty acids (VFA), and mixed
liquor volatile suspended soils (MLVSS) profiles of both reactors during maximization and recovery periods (after failure) were made to evaluate the system feasibility. Hydrolysis-acidification and acetogenesis-methanogenesis dominated in the acid and methane phase reactors, respectively.
Most of the reactions inside the methane-phase reactor (M-UASB) occurred in the lower part of the reactor (sludge bed and blanket). A combination of hydraulic and organic overloading of the M-UASB reactor was a major cause of process failure; this was indicated by MLVSS washout, increase in
total VFA concentration, reduction of the system removal efficiency, a drop in pH, an increase in total VFA/alkalinity ratio, a reduction of methane gas production, and an increase in the effluent total VFA. Because of system overloading, there was a shift in the metabolic pathway to a less
favorable one, resulting in a shift of VFA producers (acidogens and acetogens) and consumers (methanogens, SRB, and NRB) ratio inside the reactor; this possibly led to a significant accumulation of carbon dioxide and hydrogen gas production. As a result, the partial pressure of hydrogen gas
inside the system increased to a certain level, causing a shift in the metabolic pathway and accumulation of propionic acid. The maximum hydraulic loading rates achieved were 1.6 and 0.90 m3/m3 d, equivalent to hydraulic retention times of less than 0.63 and 1.13 days,
for the M-UASB and the whole system, respectively.
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