1. Introduction

 

The formation of anaerobic granular sludge can be considered as the major reason of the successful introduction of the Upflow Anaerobic Sludge Bed (UASB) reactor concept for anaerobic treatment of industrial effluents.This granulation process allows loading rates in UASB reactors far beyond the common loading rates applied so far in conventional activated sludge processes.The resulting reduction in reactor size and required area for the treatment leads to lower investment costs in addition to the reduced operating costs due to the absence of aeration.

Two main factors made these high loading rates possible:

(a) The superior settling characteristics of granular sludge.Settling velocities of granular sludge of approximately 60 m/h are common, whereas the superficial upflow velocities in UASB reactors are  usually kept below 2 m/h, in practice.This allows an extreme uncoupling of the hydraulic retention time from the solid retention time (or sludge age). Soild retention times of over 200 days can be achieved at hydraulic retention times of only 6 h.