review concerning the development, applicability and possible future application of the an-
aerobic baed reactor for wastewater treatment is presented. The reactor design has been developed
since the early 1980s and has several advantages over well established systems such as the up¯ow an-
aerobic sludge blanket and the anaerobic ®lter. These include: better resilience to hydraulic and organic
shock loadings, longer biomass retention times, lower sludge yields, and the ability to partially separate
between the various phases of anaerobic catabolism. The latter causes a shift in bacterial populations
allowing increased protection against toxic materials and higher resistance to changes in environmental
parameters such as pH and temperature. The physical structure of the anaerobic baed reactor enables
important modi®cations to be made such as the insertion of an aerobic polishing stage, resulting in a
reactor which is capable of treating dicult wastewaters which currently require several units, ultimately signi®cantly reducing capital costs. # 1999 Elsevier Science Ltd. All rights reserved