Effect Of Tubificidae’s Predation On MBR Excess Sludge Characteristics

Activated sludge treatment of wastewater results in the generation of aconsiderable amount of activated sludge that has to be wasted. An ideal way to solve theproblem is to minimize excess sludge production by micro-fauna’s predation.Ourprevious studies have shown that static sequencing batch worm reactor （SSBWR） had agood performance in sludge reduction, and the presence of Tubificidae was observedthroughout the operational period after its inoculation.In this study, comparisonexperiments were carried out to investigate the effect on sludge reduction, sludgesettleability, sludge dewaterability, sludge viscosity, surface charge and microbial activityinduced by Tubificidae’s predation.A serials of parameters,including fractal dimension,lengths of filamentous/floc area（LFA）, roundness, form factor, three-dimensional aspectratio were also extensively examined to gain a comprehensive insight into the effect oftubificidae’s predation on sludge morphology.During the100-day running period, the specific sludge reduction rate by thepredation of Tubifex tubifex was0.15±0.02mg （mg·d）^-1, demonstrating that Tubificidaehad a good performance in sludge reduction. SVI decreased by28.9%, indicating thatthe settleability of sludge was greatly improved by the presence of worm. The compactstructure of sludge floc and the decrease of extracellular polymeric substances （EPS）were the important factors contributing to the better settleability. The normalizedcapillary suction time （NCST） and the specific resistance to filtration （SRF） of sludge inSSBWR was2.45times and1.16times larger than that of initial excess sludge,respectively, showing a badly deterioration of sludge dewaterability, which couldmainly result from a decrease in floc size. Microbial activity was also influenced by theworm’s predation. Specific oxygen utilization rates of heterotrophic bacteria,ammonia-oxidizing bacteria and nitrite-oxidizing bacteria decreased by7.09%,7.84%and8.29%, respectively. In addition, results of the experiments also showd that surfacecharge of sludge floc increased by13.8%, while sludge viscosity decreased by14.49%,which may result from the decrease in EPS amounts.The related coefficients rpvalues were calculated by the software SPSS （StatisticalProgram for Social Sciences） to estimate the strength of the correlations.The resultsshowed that EPS （rp=-0.835） was correlated inversely to Zeta potential significantly,butEPS had a positive （rp=0.64） correlation with sludge viscosity. Sludge particles sizedistribution （PSD） had strong effect on SVI （rp=0.939） and NCST （rp=-0.896）.Furthermore, SMP is a predominant factor affecting sludge dewaterability.In order to characterize quantitatively microbial aggregates and protrudingfilaments, a novel image analysis procedure was proposed in Matlab8.0environment. Roundness increased by35.3%while fractal dimension, LFA decreased by12.8%,18.2%, respectively. This indicated that sludge floc became stronger and more regularafter tubificidae’s predation. However, worms had no impacts on the boundaryroughness of sludge floc.Finally, MBR+SSBWR was established to study the influence of SSBWR sludgemembrane fouling process. The results showed that COD, NH₃-N removal efficiencycan reach93%,100%, respectively. Sludge recirculation can effectively mitigatemembrane fouling.