Effects Of SRT On Soluble Microbial Products And Membrane Fouling In MBR

Membrane bioreactor(MBR)exhibits obvious advantages in terms of the water reuse(zero emission),modification and upgrade of sewage treatment standard.However,membrane fouling hampers the application of MBR.Currently,membrane fouling mechanism is one of the research hotspots.The object of this work is to investigate the effect of sludge retention time(SRT)on MBR performance and the properties of dissolved microbial products(SMP)to reveal the main fouling mechanism.It was obtained that the higher SRT,the better sludge sedimentation.Specifically,MLSS concentration decreased gradually at low SRT(10,20d),remained stable at medium SRT(40d)and increased slowly at long SRT(90d).Meanwhile,the removal efficiency of COD enhanced gradually with the rise of SRT.As for NH4+-N,its removal efficiency was the best at the medium SRT(40d),which decreased at low or long SRT.The average concentration of SMP in the supernatant of MBR was SRT10d<SRT20d <SRT90d<SRT40d,in which protein was the major metabolic product,accounting for 39.14%~50.38%.The ratio of protein to polysaccharide was 1.4~1.9.The analysis of foulants on the membrane surface and membrane pores showed that the lower the SRT,the easier SMP accumulate in the membrane module.At the same time,protein contributed significantly to the formation of the cake layer fouling.On the contrary,humic acid and polysaccharide were more likely to cause irreversible fouling,and polysaccharides had the greatest contribution to the irreversible fouling.In spite of this,the degree of membrane fouling was opposite to the SMP concentration in MBR.The fouling rate also accelerated at low SRT and long SRT due to the relatively high content of polysaccharide and humic acid in the irreversible fouling.Moreover,the change of inorganic ion content in the membrane foulants was consistent with the SMP fouling trend,indicating that the interaction between SMP and ion enhanced the degree of irreversible fouling.At SRT of 40 d,MBR system exhibited better low fouling rate and better cleaning effect,and the content of inorganic ions,humic acid and polysaccharide were relatively low.Additionally,the proportion of irreversible fouling was low comparatively.According to SEM-EDX and FTIR analysis,microorganisms,particles,SMP(humic acids,proteins,polysaccharides and aromatic classes),and other inorganic materials such as silicate,phosphate and chlorides could form a cake layer on the membrane surface.The inorganic substances and a small amount of organic substances adsorbed and deposited in the membrane pores,resulting in the membrane pore blockage.This indicated that the fouling mechanism of the membrane was the cake-standard blockage.According to a constant flow blocking model and based on MBR running on-line monitoring of the transmembrane pressure,it was found that SRT had little effect on membrane fouling type,and cake,intermediate and standard blocking were the main mechanism of membrane fouling.The cake-intermediate and intermediate-standard blocking model could accurately describe and predict MBR membrane pollution.