Study On Sludge Yield And Biofilm Periodicity In Saturated Packed Biofilm Reactor

Saturated packed biofilm reactor is a new wastewater treatment process with a very high packing rate（70～90%）,which is based on moving bed biofilm reactor（MBBR） process.The process has been applied and proved as low investment,small land occupation and high pollutant removal rate.However,there are few farther studies on this process.At present,most of the studies stay on the effluent effect,and there is no report on the sludge yield as well as the periodic change of pollutant removal and biofilm.In this paper,the saturated packed biofilm reactor process is taken as the research object.Firstly,three new methods for high-efficiency determination of water and sludge quality index are established.Then,a mathematical empirical model was established to study the effect of flushing parameters,such as flushing frequency,flushing filling rate and flushing time on sludge yield,and the optimal operation conditions of the process were obtained.Finally,the removal efficiency of pollutants,material balance and the change of biofilm in the operation cycle were studied,and the periodicity rule of the process was summarized.The main conclusions are as follows:（1）Based on the principle of light scattering,a method for the determination of SS in water samples was established by UV/Vis spectroscopy technology.A method for the determination of Total Phosphorus（TP） in sludge and a method for the determination of observed yield coefficient(Y_obs) in activated sludge were established by headspace gas chromatography.The three methods have good reproducibility and high accuracy,and their relative standard deviations（RSD）are within 2.63%,2.60% and 5.46%respectively,which provides a powerful detection method for this study.（2）The sludge yield of the saturated packed biofilm reactor was affected by flushing frequency,flushing filling rate and flushing time.An empirical model of sludge yield is established.The linear coefficient between the predicted value and the actual value of the model is R²=0.9581.Combined with the response surface model and the empirical model of sludge yield,the optimal flushing conditions of the system are 1.25 d per time,60% filling rate and 0.5 h.The average concentrations of COD,BOD₅,NH₃-N,TN,TP and SS are 25.67,4.78,1.86,12.45,0.38 and 7.51 mg/L,respectively,which can meet the class A standard of discharge standard for pollutants of urban sewage treatment plant（GB 18918-2002）,the sludge output is between 231.1-287.0 g/d,and the target of high pollutant removal rate and low sludge output can be met at the same time.（3）The removal rate of pollutants in the saturated packed biofilm reactor changes periodically.As for the removal of COD,BOD₅ and SS,their removal rates increased firstly and then decreased,while the removal rate of TP increased continuously and the removal rate of NH₃-N increased first and then remained.Through the analysis of material balance,TOC,TN and TP on effluent accounted for 5.96%,5.03% and 21.96%of the influent respectively,which indicated that the removal effect of TOC and TP were better than TN.（4）In the saturated packed biofilm reactor,the biofilm showed periodic change in the normal treatment mode.In per cycle,the biomass showed S type growth,the soluble extracellular polymers（S-EPS） gradually increased,the loose bound extracellular polymers（LB-EPS）,the tightly bound extracellular polymer（TB-EPS） and the extracellular polymer polysaccharide/protein component（PS/PN） firstly increased and then decreased.Y_obs firstly slowly increased and then fast decreased.Scanning electron microscopy（SEM） showed that the biofilm was mainly composed of bacteria with smooth surface in the early stage,and the number of cocci with mucus on the surface increased significantly in the middle stage,but there was no significant change in the number and morphology of bacteria in the later stage.（5）The packed biofilm of the saturated packed biofilm reactor showed periodic changes in the flushing mode.In per cycle,the biomass and Y_obs first increased rapidly and then decreased slowly,LB-EPS and TB-EPS first decreased and then remained,and PS/PN gradually increased.SEM micrograph showed that the loose structure of the packing biofilm was removed first,and then a small part of the material closely combined with the packing was removed.