Concentration of biogas slurry with forward osmosis technology

Abstract: 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/(m2·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/(m2·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.