Study On The Treatment Of Chlor-alkali Wastewater By Nanofiltration And Forward Osmosis

The treatment of the effluent from chlor-alkali industry has become a hot area ofresearch for its characteristics of serious pollution and difficulty to be treated. To date,methods such as calcium mirabilite, ion exchange, freezing denitration and bariumchloride have been employed to treat the chlor-alkali wastewater. The most popularmethod of freezing denitration suffers from the disadvantages of energy consumption,investment as well as operation cost. Therefore, it is of great importance to developnew method that has the properties of low energy consumption and greenenvironmental protection for the treatment of chlor-alkali wastewater.In considering of the problems facing with the currently adopted method to thetreatment of chlor-alkali wastewater, a new method combining the membraneprocesses of nanofiltration (NF) and forward osmosis (FO) was investigated in thisstudy for the treatment of chlor-alkali wastewater composed of18wt%NaCl and8wt%Na2SO4. NF was employed to treat the effluent for the separation of NaCl andNa2SO4presented in the chlor-alkali wastewater, while the FO was employed toconcentrate the permeate stream from the NF process. Treated wastewater containinglow content of Na2SO4and high content of NaCl could be reused for the dissolution ofsalt for the chlor-alkali industry. The effect of operation conditions on the separationperformance of NF, the performance of the diafiltration process using NF underdifferent operating conditions, the factors affecting the process of FO process, theperformance of FO process in concentrating the permeate stream of NF, theintegration of NF and FO were studied in this work. The main conclusions obtainedfrom the experimental results are as follows:(1) The sodium chloride and sodium sulfate presented in the chlor-alkaliwastewater could be effectively separated by NF through diafiltration. the sodiumchloride of chlor-alkali wastewater was not rejected by membrane NF1#adopted,while sodium sulfate was nearly completely rejected by the membrane. Operatingpressure, temperature and recovery ratio influenced the rejection of sodium sulfate.Relatively optimal conditions for membrane NF1#in treatment of chlor-alkali wastewater containing18wt%NaCl and8wt%Na2SO4through diafiltration were asfollows:2.0MPa,380L/h for the circulating concentrate and temperature of25~30℃.(2) In the treatment of chlor-alkali wastewater containing18wt%NaCl and8wt%Na2SO4through diafiltration, the theoretical NaCl content of the permeate ofstream (10L) from the continuous diafiltration process with membrane NF1#was11.38wt%, the experimental NaCl content of the permeate of stream (10L) frombatch diafiltration process with the addition of infiltration agent of10L water for fivetimes was10.56wt%, while the experimental NaCl content of the permeate of stream(10L) from batch diafiltration process with the addition of infiltration agent of10Lwater for one time was9.9wt%. The difference between the results of the batchdiafiltration processes was little.(3) The performance of the FO process was affected by the operation parametesincluding temperature, flow rate of feed solution, flow rate of draw solution as well asthe osmotic pressure difference between the feed and draw solutions. The optimaloperation conditions for the CTA-FO membrane used in this work were asfollows:25℃, flow rate of feed solution4.5cm/s, flow rate of the draw solution0.67cm/s. The flux of the CTA-FO membrane in concentrating the permeate stream of NFcould be predicted through the internal concentration polarization model.(4) The chlor-alkali wastewater could be treated for reuse through theintegration of three stages of NF and FO. The permeate streams from the three stagesof NF could be concentrate through FO process. The NaCl content of concentratedpermeate streams after mixing was about11wt%, while the Na2SO4content was lessthan0.5wt%, showing that the mixed stream could be used for the dissolution of salt.The treated feed solutions of the three stages of NF could be treated through freezingcrystallization after mixing, while the draw solution of the third FO process could betreated for reuse through NF process.