Removal Performance And Membrane Fouling Mechanism Of Gravity-driven MBR Enhanced By Coagulation

With the rapid development of society recently,untreated discharge of decentralized wastewater seriously pollutes water environment that hinders the economic and ecological sustainable development.Decentralized sewage treatment system can effectively solve the sewage in the rural areas,and it is the key technology to improve the utilization rate of water resources.Gravity-driven membrane bioreactor(GDMBR)has great potential in decentralized wastewater treatment and reuse because of its low energy consumption and low maintenance.However,the level of stable permeability/flux is undesirable and limit its wide application.In the field of drinking water and wastewater treatment,coagulation is applied frequently,which is easy and low-cost to operate.Meanwhile,the addition of coagulation has the ability to control membrane fouling in membrane bioreactor(MBR).So far,whether coagulation-assisted process can improve the permeability and control membrane fouling of no or low shear GDMBRs need to be clarified.To this end,three lowpressure GDMBR were operated in parallel to reveal removal peroforemence and membrane fouling control of GDMBR enhanced by coagulation.Based on the dosing position,the coagulation can be divide into in-situ coagulation and pre-coagulation.Three GDMBR with in-situ coagulation and precoagulation or without coagulation were investigated.The results showed that in-situ coagulation could reduce the content of dissolved organic but increase the carbon sources in the sludge mixed liquor,and accelerate the growth rate of the microbial community of denitrification,but the constantly accumulating aluminum affected the growth of the microorganism for denitrification.The behavior that a small amount of aluminum flowed into the sludge mixture enhanced the removal of organic matter of effluent in GDMBR with pre-coagulation.At the same time,pre-coagulation reduced the organic compounds in the influent and delayed the stability of the denitrifying bacteria.Whether in-situ coagulation or pre-coagulation,the concentrations of residual aluminum and total phosphorus in the effluent meet the standard of emissions in China.Then,the permeate flux stability and fouling layer properties of in-situ coagulated GDMBR were discussed.It is found that in-situ coagulation significantly increased the permeability,however no stabilization of permeability occurred compared with none coagulated and pre-coagulated reactors.The addition of aluminum coagulation could transform soluble microbial product to Al floc and then deposited on the ultrafiltration membrane,so that the porosity of cake layer was increased.Moreover,coagulation prevented SMP and EPS attaching on the membrane surface,so as to alleviate the effect of these substances clustering on membrane and clogging the membrane pore.However,the long-range accumulation of aluminum would weaken the population of microorganisms on fouling layer.In fact,the ATP contents of the fouling layer in the in-situ coagulated reactor were nearly five times lower than the control.These phenomena demonstrate that the on-line addition of polyaluminium chloride limited the growth of biomass,and further likely impacted on the biological process within the bio-fouling layer.This is the potential reason for the unstable permeability in the in-situ coagulated GDMBR.In addition,the permeate flux stability and fouling layer properties of precoagulated GDMBR were studied.Conclusions can be drawn as follows: It is found that pre-coagulation significantly increased the permeability and stabilization of permeability occurred compared with none coagulated reactors.Due to the precoagulation and pre-settling processes,the organics in the feed water was reduced by 12%,it is the reason for the increase of stable membrane permeability in GDMBR.In addition,the residual Al in the feed water further impacted on the properties of the mixed liquor.The dissolved organics concentration in the mixed liquor were always lower than the control.Furthermore,the bio-fouling layer containing inorganic particle skeleton influence both of the structure and the biochemical properties of the fouling layer..It is shown that the residual Al in the feed could absorb the EPS in both the mixed liquor and the bio-fouling layer,which is the main reason for the improved stable level.In pre-coagulated GDMBR,the ATP contents of the fouling layer in the pre-coagulated reactor were still as high as the control reactor,which demonstrated that the biological activity kept a high level and a heterogeneous bio-fouling layer might develop on the membrane surface.This is the main reason for the stabilization of the permeability.To summarize,in-situ coagulation and pre-coagulation GDMBR would significantly alleviate GDMBR membrane fouling,increase membrane permeability and enhance the removal peroforemence.However,these GDMBRs should be used according to different conditions such as technical level,land price,etc.If the footprint is limited,the in-situ coagulation can be chosen,however,the membrane physical cleaning would be applied more frequently than the pre-coagulated GDMBR.The reason for the simple physical cleaning(more than every two months in the current research)is that the hydraulically reversible resistance account for 90% of the total filtration resistance,and the permeability/flux can be recovered easily.On the contrary,if the working area is sufficient,the pre-coagulation will be a good choice for the permeability enhancement due to less maintenance for a longer period compared with the in-situ coagulation case.