Higher treatment rates reduce capital costs and make it easier to build large systems because the vessels needed to contain the waste are smaller. Successful anaerobic treatment requires a microbial balance between the fast-growing acidogens and the slow-growing methanogens. Because of the slow growth rate of methanogens, efficient biomass retention is required for successful high-rate anaerobic system performance (Stronach et al., 1986; Wohlt et al., 1990). To achieve rapid anaerobic treatment of liquid wastewater, researchers recognized that maintenance of a high population of biomass in the reactor is necessary in order to compensate for the various problems associated with anaerobic processes (Dague et al., 1970; Ndon and Dague, 1997).
The retention of active biomass has been achieved in advanced reactors such as the UASB, which has been used widely. More recently, the IBR, the ASBR, various anaerobic contact reactors, anaerobic bio-filters and fluidized bed reactors have become available. These reactors are often referred to as 'high-rate' anaerobic reactors. These methods are considered to be innovative process technologies (Switzenbaum, 1995). These processes all have a high concentration of biomass, which provides long SRTs while maintaining short HRTs. Many of these systems are also suited for treating diluted wastewater and have been used commercially for treatment of municipal and industrial wastewaters with high loading rates and stability (Lo and Liao, 1985; Lettinga and Hulshoff-Pol, 1991b). Anaerobic processes are sufficient for pre-treatment of wastewater only and some form of post-treatment is required.
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