| To alleviate membrane fouling during membrane enrichment of digestion biogas slurry, pretreatments of coagulation and electrocoagulation prior to nanofiltration (NF) system were studied in this paper, respectively. Key operating parameters of coagulation and those of electrocoagulation were optimized, and relationship between independent operating parameters of coagulation/electrocoagulation and turbidity removal rates was determined. Effects of pretreatments on alleviating NF membrane fouling and on nutrients variations during the membrane enrichment of digestion biogas slurry were also evaluated. Moreover, the running cost of coagulation/electrocoagulation and characteristics of foulants on NF membrane surface were analyzed. The main results of this study were drawn as follows:(1) Polyaluminium chloride (PAC), polymerized ferric sulfate (PFS) polymerization ferric chloride (PFC), basic aluminium chloride (BAC) were selected as coagulants for coagulation, turbidity removal rate generally increased with the increase dosage of coagulants in the range of 1-6 g/L, and variation trend of zeta potential were different due to different coagulants. Response surface methodology (RSM) was used to optimize the key operating parameters. Quadratic response surface model was obtained between turbidity removal rate and the three main independent parameters (PFS dosage, PAM dosage and mixing time). The results showed that linear terms of PFS dosage and PAM dosage, quadratic term of PFS dosage, interaction terms of PFS dosage and mixing time had a significant effect on turbidity removal rate. The optimum conditions for achieving 100% turbidity removal were 3.56 g/L,50.5 mg/L and 4 min for PFS dosage, PAM dosage and mixing time, respectively.(2) The effects of a combined process of coagulation-NF membrane on alleviating membrane flux reduction rate and nutrients variations were investigated. During the process of membrane enrichment without coagulation, membrane flux reduced 58.3% from 42.9 L/(h·m2) to 17.9 L/(h·m2) at volume reduction factor(VRF) of 1 to 10. However, the reduction rate of membrane flux was 20% and 25.8% after coagulation, respectively. At VRF of 7, the running time of NF process could be prolonged to 32 h or 44 h with coagulation pretreatment, running time of NF was 20 h for enriching untreated biogas digestion slurry. Moreover, after addition of 250 mg/L PFS and 5 mg/L PAM, concentrations of NH4+-N, TN, and COD in concentrate at VRF of 7 were 268±9.5 mg/L,1754.2±42 mg/L,1038.5±9.5 mg/L, respectively, the running cost of the coagulation was 3.2 yuan RMB/m3.(3) RSM was used to optimize the operating parameters of electrocoagulation process of biogas digestion slurry, and to describe interactive effects of the three main independent parameters (current density, reaction time and A/V ratio) on turbidity removal rates and electric energy consumption. The results showed that turbidity removal was significantly affected by linear terms of current density and A/V, quadratic terms of current density and reaction time, interaction terms of current density and reaction time, reaction time and A/V; linear terms of current density, reaction time and A/V, quadratic terms of current density, interaction terms of current density and reaction time had an effect on electric energy consumption. The optimal conditions obtained from the compromise of the two desirable responses were as follows:current density of 35.7 A/m2, reaction time of 24 min and A/V of 20.7 m2/m3. Under the optimal conditions, membrane flux increased 22.2% in the process of membrane enrichment, and the running cost of electrocoagulation was 1.96 yuan/m3.(4) To illustrate the fouling mechanism of NF membrane, composition of foulants and deposit morphology on NF membrane surface were determined by Scanning Electron Microscopy (SEM)-X-ray Energy Dispersive Spectrometry (EDS) and Fourier Transform Infrared Spectrometry (FTIR). Comparad with electrocoagulation, coagulation can remove aromatic compounds except for the organohalogen compounds. |
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