利用海水汲取液的沼液正渗透浓缩技术

Concentration of biogas slurry with forward osmosis technology

为实现沼液浓缩以提高其应用价值,该试验以海水作为汲取液,采用正渗透技术对山东某海滨猪场的沼液进行浓缩。试验结果表明,正渗透浓缩倍数为2时的膜通量最高可达到9.5 L/(m2·h);沼液浓缩后的总含盐量、总钾、总磷、总氮、化学需氧量的回收率均可以达到96.7%以上,在保证较高膜通量的条件下沼液体积最高可减小为原液的1/4;对数据拟合分析的结果表明,在沼液浓缩倍数为3时,正渗透膜具有较高的浓缩效率,可达到3.9 L2/(m2·h·g)。综上,采用正渗透技术对沼液进行浓缩的方式是可行的,这不仅缓解了沼液不能及时消纳所造成的资源浪费,而且还可以提高沼液作为肥料的应用价值。

Biogas slurry is the residue after the anaerobic fermentation of excrements from breeding industries. It is rich in nitrogen, phosphorus, potassium, and other nutrients. Biogas slurry is already known to be a valuable resource as a liquid fertilizer that increases crop yields and improves soil quality. However, due to the limited farmland areas around the breeding industries and the inconvenience of separating biogas slurry in application and transportation, research on the concentration of biogas slurry has become increasingly important. Forward osmosis（FO） is an isolation technology based on the natural phenomenon of osmosis with the transportation of water across a semi-permeable membrane. The driving force of water transport is the osmotic pressure difference between the two sides of the membrane, which is contrary to the ordinary pressure-driven membrane processes, such as ultrafiltration and reverse osmosis. Related research on the FO technology has grown significantly, but its application to the concentration of biogas slurry has not yet been reported. This paper reported on the application of the FO technology to the concentration of the biogas slurry, so as to improve its practical value, at a seashore pig farm in Shandong Province, using seawater as the draw solution. Considering that the driving force of FO membrane technology is the concentration difference, variations in the FO membrane flux were determined at different draw solution fluxes（30, 40, 50, 60 and 70 L/h）, so as to investigate the effect of draw solution flux on the membrane flux. Furthermore, we studied the influence of concentration multiples（2, 2.5, 3, 3.5, 4） on the FO membrane flux. During all the experiments, biogas slurry concentration time, total dissolved salt（TDS）, total potassium, total phosphorus, total nitrogen, and COD were also determined in the concentrated biogas slurry and their recovery ratios were calculated. Then the relationship between the TDS in the concentrated biogas slurry and the FO membrane flux was investigated. Based on the experimental results, we concluded that the FO membrane flux reached as high as 9.5 L/（m^2·h） with a concentration multiple of two; the recovery ratios of TDS, total potassium, total phosphorus, total nitrogen, and COD in the biogas slurry after the concentration process were all above 96.7%, while the volume of biogas slurry was reduced to 1/4 of the original at most, under the condition of ensuring a higher membrane flux. The characteristics of the seawater used as the draw solution changed little in this research. Furthermore, the results showed that the FO membrane worked with high efficiency with a biogas slurry concentration multiple of three, and the membrane efficiency was as high as 3.9 L2/（m^2·h·g）. In summary, it is feasible and effective to concentrate biogas slurry with FO technology, which not only alleviates the wastage of nutrients in the biogas slurry, but also enhances the application value of biogas slurry as a liquid fertilizer.