High-rate anaerobic treatment systems rely on biomass settling or membrane technology to achieve longer solids retention times compared with HRTs for satisfying performance under high volumetric loading rates. This results in smaller reactor volumes, and thus lower construction costs than low-rate systems, because the volume of anaerobic digesters is sized based on the HRT, while the performance of all digesters is dependent on the sludge reten­tion time. Full-scale high-rate anaerobic digesters have shown excellent and stable perfor­mance for over 30 years, but have been used mainly for low-solids industrial and domestic

wastewater treatment (Lettinga 1995; Hulshoff Pol et al. 1997; Seghezzo et al. 1998). For high-solids wastewater or slurries, such as diluted swine waste, existing technologies (e. g., up-flow anaerobic sludge blanket and its derivatives) showed problems with solids separa­tion and solids accumulation in the reactor (Zeeman et al. 1997; Elmitwalli et al. 1999; Kalogo and Verstraete 1999). The anaerobic sequencing batch reactor (ASBR) was devel­oped to treat a high — solid influent with a high-rate technology (Figure 4.2 ; Dague et al. 1970; Dague and Pidaparti 1992). Both the ASBR and an alternative technology—the plug — flow anaerobic baffled reactor—have made it possible to treat high-solids waste (e. g., animal waste) by high-rate systems (Zhang et al. 1997; Boopathy 1998).