Integrated membrane process for the treatment of desulfurization wastewater

In the desulfurization process for purification of coking gas, a huge amount of wastewater is generated. In order to recover the usable substances such as suspended sulfur (SS) and ammonium salts, e.g., (NH₄) ₂S₂O₃ and NH₄SCN, in the wastewater and also to avoid severe environmental problems in the case of improper disposal, an integrated membrane process mainly consisting of ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) was proposed to treat the wastewater. In the UF process, a ceramic membrane was employed to remove SS and an efficiency of 99.9% and a steady flux of about 500·L·m ^-2·h^-1 were achieved. In the NF step, (NH ₄)₂S₂O₃ was separated from NH ₄SCN with a retention ratio of 95.0%, and finally 83.0% of (NH ₄)₂S₂O₃ was recovered in the retentate whereas 99.2% of NH₄SCN was recovered in the permeate via dialysis with deionized water. In the RO process, NH₄SCN can be recovered with an efficiency of 99.0% through a four-pass filtration process. Meanwhile, the RO permeate can be reused in the salt diafiltration process. The results show that the proposed integrated membrane process is technically feasible and economically efficient for the treatment of desulfurization wastewater generated in the coking industry.