Effects Of Hydrodynamic Shear Force On Granular Sludge In Upflow Anaerobic Reactor

As so far most of high-rate anaerobic reactors belong to upflow reactors. The excellent performance of these reactors relies on granules of excellent bio-activity and high settling ability as well as enhanced mass transfer rate which is induced by the strong hydraulic condition. In order to shorten granulation period and prevent granule disruption and wash-out, the effects of hydrodynamic shear force on the granulation and granule disruption with quantitative methods are studied systematically in this paper. Experimental results show that: (1) Apparent shear force is defined to denote the hydrodynamic shear force in upflow reactors and a mathematical formula is developed. (2) Three granulations under the conditions of"low organic loading without N2 injection","low organic loading with N2 injection"and"high organic loading without N2 injection"respectively with granule fragments as nuclei in an Internal Circulation Anaerobic reactor are completed successfully. The granulation with granule fragments as nuclei consists of three periods including High-speed Period, Decelerating Period and Stabilization Period. (3) High hydrodynamic shear force induced by hydraulic loading which reach up to 0.7m3/(m2·h) and high sludge organic loading like 2.6 kgCOD/(kgVSS·d) can enhance the granulation. If reactors are operated under the condition of"high organic loading with N2 injection", process to reach the stable performance and granulation can be enhanced. (4) In granulation, high hydrodynamic shear force which is from 1.5 to 2.0Pa, lead to smooth and dense granules with high settling ability and high strength as well as high secretion of ECP. Collided Granules are observed in the three granulations. (5) Increase of hydrodynamic shear force with successive or stepwise modes can evoke granule disruption. For the successive increase mode, sludge wash-out is main cause of operation failure of reactors. The negative effects of stepwise increase mode are relatively less serious and the increase mode is more reliable to prevent granule disruption. If the increasing rate of hydrodynamic shear force and the adaptation time for granule is appropriate, the negative effects of the increasing hydrodynamic shear force can be eliminated and it is probable to obtain better granules under the high hydrodynamic shear force condition.