| Membrane technology can realize resource utilization of digestate,achieving the goal of volume reduction and purification of digestate.The existing research mainly focuses on membrane pretreatment processes,membrane concentration processes combinations,and various membrane concentration processes have been proposed and applied for digestate.However,the extremely complex composition of digestate can cause serious membrane fouling resulting in difficulty in membrane cleaning and low flux recovery rate,but there is a lack of systematic research in this problem.In this paper,the purification efficiency and membrane fouling characteristics of polymeric and ceramic membranes with different pore sizes were investigated,and the coagulation process was used to regulate the digestate properties.The flux improvement mechanism of coagulation process combined with polymeric and ceramic membranes was investigated,and the membrane fouling structure characteristics were analyzed.Nanofiltration（NF）was used to concentrate the purified digestate from five purification processes,including polymeric membrane,ceramic membrane,coagulation,coagulation-polymeric membrane,and coagulation-ceramic membrane.The digestate concentration effects of different combination processes were compared,and the NF membrane fouling characteristics were identified.The main research conclusions of this paper are as follows:1,The polymeric membranes with pore diameters of 50.0 k Da,20.0 k Da,10.0 k Da,and 5.0 k Da have no significant differences in the permeate quality,but the membrane flux shows differences among them.The 20.0 k Da polymeric membrane showed a severe flux decrease due to particles blocking the membrane pores,and the 50.0 k Da polymeric membrane can always maintain the highest membrane flux.Microbial sequencing identified the main bacterial microorganisms in membrane fouling as Pseudomonas,Xanthomonas,and Burkholderiales.Organic fouling is the main type of membrane fouling during the digestate purification by polymeric membrane,and Na OH and Na Cl O have better cleaning effects on the polymeric membrane.It is recommended to select 50.0 k Da polymeric membrane for digestate purification,as it has high membrane flux and better flux recovery performance.2,There are significant differences in flux stability and permeate quality of ceramic membrane digestate purification with five pore diameters,and ceramic membrane with smaller pore diameters has stronger anti-fouling ability during digestate purifying.As the VRF reaches 5,the membrane flux of 50,200,500,800,and 1200 nm ceramic membranes decreases by 34.7%,8.8%,33.6%,40.5%,and 72.2%,respectively.The 200 nm ceramic membrane showed the strongest flux stability and the highest flux recovery efficiency after chemical cleaning.The 800 nm ceramic membrane caused the most serious membrane plugging and filter cake fouling by digestate particles.The 1200 nm ceramic membrane flux decreased the most,and its digestate purification effect was the worst.Polysaccharide and protein substances are the main components of organic membrane fouling in the digestate purification using ceramic membranes.The ceramic membrane fouling mechanism is the filter cake layers formation on the membrane surface,and the membrane plugging formation in the membrane pores by particles and colloidal substances.3,Based on the coagulation tests with different concentrations,the coagulant dosage of 0.9 g L-1PAC and 0.06 g L-1 PAM was proposed for digestate purification.When 0.9 g L-1 PAC was added,the minimum turbidity value of the supernatant was 97.1±7.7 NTU,and when the PAC exceeded this concentration,the turbidity value increased.When the dosage of PAM exceeds 0.06 g L-1,the floc sedimentation performance will not be further improved.Coagulation pretreatment can increase the flux of polymeric and ceramic membrane,and their membrane flux can be increased by 18.6%and 22.5%respectively.The removal of particles,colloids,and dissolved organic substances by coagulation can effectively enhance the membrane purification stability.The removal effect of coagulation process on dissolved organic matters in digestate is better than that of ceramic membrane and polymeric membrane.The SEM-EDS analysis indicated that coagulation can effectively alleviate the membrane fouling structures,and the main elements of pollutants on the membrane surface after coagulation were C,O,Ca,and P.Organic pollutants are the main type of membrane fouling.4,The five combined processes can effectively enhance the initial membrane flux of NF concentration,among which the coagulation-polymeric membrane-nanofiltration showed the highest membrane flux.In terms of the effect of pretreatment on flux enhancement ability:coagulation-polymeric membrane>coagulation-ceramic membrane>polymeric membrane>ceramic membrane>coagulation>raw digestate.When the VRF increases to 3 times,the difference of the flux enhancement effect of coagulation,ceramic membrane,and polymeric membrane pretreatment gradually disappeared.As VRF increase,the salt ion concentration in the NF concentrate continuously increases,and the osmotic pressure increase becomes the main factor for the membrane flux decrease.When the VRF reaches 4 times,the fluxes of the six NF concentration processes are all below 50 L m-2 h-1.Coagulation purification can alleviate the NF membrane fouling layer,but coagulation will cause the loss of dissolved organic substances in concentrate and bring potential risk of residual chemicals.The combination of coagulation with polymeric membrane or ceramic membrane can further alleviate NF membrane fouling.NF membrane fouling belongs to organic and inorganic composite membrane fouling,and its main components include silicon dioxide,calcium phosphate,colloidal substances. |
