A major problem associated with the upflow anaerobic sludge blanket (UASB) reactors is the long start-up period required for the development of anaerobic granules. In cases where the inoculation is done with municipal digester flocculant sludge, it usually takes 3 to 4 months or even a much longer period before the process can be put in operation. In view of the longer start-up period, enhanced granules formation is highly desirable in order to reduce space-time requirements of various bioreactors leading to cheaper treatment of high-strength wastes. The improvements can also lead to better treatment efficiency with greater capacity to handle large volumes of wastewater with more compact reactor design. It is therefore possible to economize on the capital investment and subsequent cost of daily operation.

Use of granular sludge from in-operating UASB reactors as the seed material has the advantage of being able to achieve high organics removal within a short start-up period. However, the availability of granular seed sludge is limited, and the costs for purchase and transportation of the inoculum are extremely high (approximately US$ 500-1000 per ton wet weight) (Liu et al., 2002). Consequently, technologies for enhanced and rapid production of anaerobic granules are sought after. While the approach for rapid production of anaerobic granules is being improved, a review of information on the selection pressure influencing anaerobic granulation can serve as a useful reference and guide. The selection pressure may result from environmental conditions (e.g. temperature, pH, and feed), process operating conditions (e.g. hydraulic retention time, upflow liquid velocity, characteristics of seed and substrate, and organic loading rate), and chemical conditions (e.g. effects of cations and polymers).

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