纳滤膜孔径对沼液超滤透过液养分富集与膜污染行为的影响

超滤（ultrafiltration,UF）能有效浓缩沼液,但透过液中仍含有大量养分,可进一步采用纳滤（nanofiltration,NF）等精密膜浓缩利用。NF膜的孔径会直接影响膜的截留特性和通量,从而影响浓缩性能。为了探究NF膜孔径对沼液浓缩过程养分富集效果和膜污染行为的影响,该研究以鸡粪沼液的UF透过液为研究对象,分别采用800 D、500 D、100 D的NF膜（平均膜孔径分别为2.0、1.0、0.5 nm）进行浓缩,重点分析养分截留效果和膜污染特征。结果表明：不同孔径的NF膜均能高效截留化学需氧量（chemical oxygen demand,COD）和总磷（total phosphorus,TP）,截留率可达68%以上,但对总氮（total nitrogen,TN）和总钾（total potassium,TK）的截留较低,仅为19%～35%。随着膜孔径降低,NF对COD、TN、TP、TK的截留效果略有提高,但整体差异不明显。不同孔径NF膜在沼液浓缩过程均出现了明显的水通量降低。与1.0 nm的NF膜相比,0.5 nm膜较小的孔径和2.0 nm膜较大的初始通量均会导致膜表面有机-无机致密污染层的形成,从而造成水通量快速降低;而1.0 nm膜表面形成的以无机晶体为主的污染层较为疏松,通量下降较为缓慢。综合养分截留效果和水通量变化规律,确定孔径为1.0 nm的NF膜更适用于浓缩沼液的UF透过液,研究结果可为推动沼液膜浓缩的发展与工程应用提供理论与技术支撑。

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.