Effects of membrane pore size on nutrient enrichment and membrane fouling behavior in nanofiltration of ultrafiltration permeate from biogas slurry concentration

Abstract: Ultrafiltration (UF) can effectively concentrate the biogas slurry, but a large number of nutrients are still remained to be further permeated using nanofiltration (NF) and precision membranes. The pore size of NF membranes can directly dominate the retention characteristics and flux of the membrane, thus affecting the concentration performance. In response to the lack of research on the effect of NF membrane pore size on nutrient enrichment and membrane fouling behavior in the concentration process of biogas slurry, this study aims to determine nutrient retention and the characterization of membrane fouling. The research object was taken as the chicken manure in the biogas slurry of UF, while the NF membranes were set with the membrane pore sizes of 2.0, 1.0, 0.5 nm for the concentration. Results showed that the retention rates of chemical oxygen demand (COD) by NF membranes of 2.0, 1.0, 0.5 nm were 68%, 77%, and 81%, respectively. The retention rate of COD was higher, as the membrane pore size decreased. By contrast, the retention rates of total nitrogen (TN) by NF membranes of 2.0, 1.0, 0.5 nm were 29%, 30% and 35%, respectively. The retention rate of TN was higher, as the membrane pore size decreased. The retention percentages of total phosphorus (TP) by NF membranes of 2.0, 1.0, 0.5 nm were 68%, 71%, and 73%, respectively. The retention of TP was higher with the decrease in membrane pore size. The retention rates of total potassium (TK) by NF membranes of 2.0, 1.0, 0.5 nm were 19%, 21% and 23%, respectively, where the retention rates of TK were higher, as the membrane pore size decreased. Once the biogas slurry was concentrated to 1/3 of the original volume, there was no difference in the running time between 2.0 and 1.0 nm membrane (1082-1204 min), but the running time of 2.0 and 1.0 nm was significantly less than that of 0.5 nm one (3 200 min, P<0.05), indicating that the smaller pore size NF membrane was less efficiency in the concentration. Energy Dispersive Spectrometer (EDS) revealed that the contents of the organic element (C, O, and N) on the surface contamination layer of 2.0, 1.0, 0.5 nm membranes were 92.9%, 92.0%, and 93.3%, respectively, indicating the dominated organic contamination. The inorganic contamination of 1.0 nm membranes was more severe than those of 2.0 and 0.5 nm membranes, resulting in a looser contamination layer on the membrane surface. There was a slow decrease in the flux and higher concentration efficiency. Compared with the 1.0 nm membrane, the smaller pore size of the 0.5 nm membrane and the larger initial flux of the 2.0 nm membrane both led to the formation of an organic-inorganic dense contamination layer on the membrane surface, resulting in a rapid reduction in the flux. The Fourier Transform Infrared Spectrometer (FTIR) showed that the organic pollutants on the surface of 2.0, 1.0, 0.5 nm membranes were mainly macromolecular organic substances, such as protein and humic acid, whereas, the organic pollutants on the surface of 0.5 nm membranes were also contained the polysaccharides. X-ray diffraction (XRD) demonstrated the high overall diffraction intensity of the NF membrane, indicating the relatively stable and pure state of crystal pollutants on the membrane surface. The crystal pollutants were identified by SiO2, FeS2, and Ca2Fe2O5, leading to the organic-inorganic dense contamination on the membrane surface. The smallest contact angle was achieved in the 0.5 nm membrane. The FITR was also combined to reveal the adsorption of the more hydrophilic polysaccharides on the membrane surface. The membrane fouling pore blocking occurred during NF concentration, as a combination of complete, standard, and intermediate pore blocking. There was no influence of membrane pore size on the cake layer formation. The resistance of the membrane originated mainly from the filter cake layer, where the resistance of the filter cake layer accounted for the largest proportion (81.9%-95.6%). The 0.5 nm membrane with the smallest pore size was easy to plug the pore for the formation of a dense filter cake layer, leading to a rapid decrease in flux. As such, the NF membrane with a pore size of 1.0 nm was highly recommended to integrate with the UF in the two-stage concentration of biogas slurry for stable water flux and effective nutrient retention.