FIXED FILM AND GRANULAR SLUDGE REACTOR SYSTEMS FOR BIOGAS PRODUCTION

Anaerobic digestion is now being used to treat a steadily widening range of wastes. This has prompted experimentation with a diverse collection of reactor designs that are intended to enhance efficiency, digestion speed, endurance of flow rate fluctuations, or other desirable qualities. Digestion technology now appears to be reaching a point where it provides a range of options among which a designer may select according to local conditions.As a contribution toward this development, this paper compares design features and experimental results from several recent studies on upflow anaerobic reactors. The particular types considered are the upflow anaerobic fixed-bed (UAFB) reactor, the expanded granular sludge bed (EGSB) reactor, the anaerobic fluidized-bed reactor (AFBR), and both the conventional upflow anaerobic sludge blanket (UASB) reactor and a partitioned variation of the UASB, designed to maintain a good hydraulic regime under changing inflows.Each of these types is a different solution to the problem of increasing the reactor biomass to enhance digestion by making the solids retention time greater than the hydraulic retention time. The UASB and EGSB rely on the formation of sludge granules, while the UAFB and AFBR provide inert solid support materials for biofilm growth. On the other hand, the EGSB and AFBR are distinguished by maintaining upflow velocities high enough to suspend the particles of their beds, to maximize metabolic interactions with the feedstock. The experimental results provide indications of the strengths and weaknesses of each type, but additional direct comparison experiments would be desirable.