Study On Forward Osmosis Process For Waste Activated Sludge Thickening And Digestion

With the rapid growth of the urbanization process,the amount of wastewater is correspondingly increasing in China.In the meanwhile,large quantities of waste activated sludge(WAS)will be produced in the process of wastewater treatment in wastewater treatment plants(WWTPs),and the cost of WAS treatment and disposal is equal to that of wastewater treatment.Therefore,the WAS treatment and disposal have attracted more and more attentions.Currently,common sludge treatment processes include sludge thickening and digestion.However,the traditional sludge thickening and digestion processes have some problems such as low treatment efficiency,easy to cause secondary pollution and complicated operation.The forward osmosis(FO)technology has received extensive attentions in recent years owing to its excellent effluent quality,low pollution trend and low energy consumption.In view of the problems existing in the traditional sludge treatment processes and the advantages of FO technology,we tried to apply FO technology for the thickening and digestion of WAS.However,current researches focused on the application of FO technology for thickening(FST)and digestion(FSTD)of WAS are limited,and the mechanisms of FO membrane fouling under high sludge concentration are still unclear.Therefore,this study is focused on the effect of FO technology for sludge thickening and digesting,and the effluent quality,FO membrane operating performance and FO membrane fouling behavior were also investigated.In addition,according to formula derivation and experimental verification,a mathematical model of sludge concentration variation was established for predicting the variation of sludge concentration and sludge digestion rate.The main contents and results are listed as follows:(1)The feasibility of FO technology for sludge thickening and digesting was investigated.The results indicated that the FST and the FO technology combined with the MF technology for deep thickening sludge(MF-FO)can successfully concentrate the WAS to more than 30 and 50 g/L,respectively.In addition,in the FSTD process,the sludge digestion rate exceeded 40% when the sludge concentration of WAS reached 30 g/L.(2)The water quality of FO membrane effluent in FST,MF-FO and FSTD was investigated.The results showed that the effluent quality of the FO membrane met with the standards of "Urban Wastewater Reclamation-Urban Miscellaneous Water Quality"(GB/T18920-2002)during all processes of the sludge thickening and digestion due to the high retention performance of the FO membrane.(3)The performance and the fouling behavior of the FO membrane in the process of sludge thickening and digestion were investigated.It was found that the FO membrane flux decreased significantly due to the increasing sludge concentration during the sludge thickening and digestion process.Moreover,the sludge concentration had a critical influence on the FO membrane fouling.The deposited fouling layer on the FO membrane were mainly the gel layer when the sludge concentration was lower than 30 g/L,while a thick mud cake layer would gradually form on the surface of the FO membrane once the sludge concentration exceeded 30g/L.(4)The factors affecting the variation of sludge concentration in the process of FST,MF-FO and FSTD were investigated.The results indicated that HRT had a significant effect on the variation of sludge concentration in the processes of sludge thickening and digestion.A mathematical model of sludge concentration variation was established by formula derivation and experimental verification,and it can successfully predict the variations of sludge concentration and digestion rate under different HRT conditions.