Optimization of forward osmosis process for concentration of biogas slurry

Abstract: Biogas slurry is one of the products of anaerobic fermentation of livestock wastewater, which contains substantial quantities of organic matter, nitrogen, phosphorus, potassium and other nutrients. For environmental protection and resources recycling, the biogas slurry could be used as a fertilizer. However, the biogas slurry could not be completely used for nearby farmland and it is difficult to transport such large volume of biogas slurry. Therefore, concentration becomes important for economic transportation and practical application of the biogas slurry as a fertilizer. In recent years, forward osmosis (FO) concentration technology has attracted growing interest. However, there were few researches concerning concentration of the biogas slurry with FO technology. To facilitate the extensive application of FO process for concentration of biogas slurry, further and comprehensive works should be carried out. In this study, a forward osmosis process was built for concentration of the biogas slurry and the performance of this process under different conditions was investigated. The results indicated that the membrane orientation should be set as FO model when the active layer was facing the feed solution. When the support layer was facing the feed solution (pressure retarded osmosis, PRO model), although the initial water flux was higher, the water flux declined quickly. Under the FO model, the hydraulic shear caused by the cross flow could prevent the deposition of the solutes of the biogas slurry over the active layer of the membrane, thereby alleviating the membrane fouling. By comparing the performance of the FO process under different cross flow velocities of 10.2, 15.4, 20.5, 25.6 and 30.7 cm/s, the cross flow velocity of 20.5 cm/s was considered to be preferred for this process. When the cross flow velocity was increased to be 25.6 and 30.7 cm/s, the water flux decline quickly, which was possibly due to the severer internal concentration polarization and membrane fouling caused by the higher initial water flux. To select the ideal draw solution, four inorganic draw solutions as NaCl, MgCl2, Na2SO4 and MgSO4 solutions with the same concentration were tested for this process. Due to the highest osmotic pressure of MgCl2, the highest water flux was obtained when MgCl2 was used as the draw solute. To obtain the optimal concentration of MgCl2 draw solution, the performance of the FO process with MgCl2 solution concentrations of 1.0, 1.5, 2.0, 2.5 and 3.0 mol/L was further evaluated. The results indicated that the increase of MgCl2 solution concentration could heighten the water flux. However, when the concentration was increased to be 2.5 and 3 mol/L, the enhancement of the water flux was not significant. Therefore, comprehensively considering the water flux and the cost of draw solution, the MgCl2 solution concentration of 2 mol/L was advised to be used for this FO process. Under the above optimal conditions, the volume of the biogas slurry was reduced to be 1/5 of the original volume by the FO process. Soluble organic substances, total phosphorus, total nitrogen, ammonia nitrogen and total potassium concentrations in the concentrated biogas slurry significantly increased. The rejection efficiencies of the above substances were 82.2%, 96.4%, 83.8%, 83.6% and 80.2%, respectively. The above results indicated the excellent performance of the FO process.