Vacuum Membrane Distillation-Crystallization Process Of High Ammonium Salt Wasterwater

High ammonium salt wastewater is one of the most refractory wastewater in industry wastewater. In this paper, the batch mode of vacuum membrane distillation-crystallization process was carried out on the ammonium salt solutions. The results showed that high rejection rate of ammonium salt was achieved in vacuum membrane distillation process. The solutes of ammonium salt were recovered by coupling with crystallizer. The purpose of zero emission was achieved in theory. The detailed conclusions were as follows:Firstly, the effects of feed pH value on the concentrations of both free ammonia and permeate total nitrogen (TN) were investigated. When the feed pH value was below6, the concentrations of both free ammonia and permeate TN were kept at low level. Moreover,3wt.%NH4CI and (NH4)2SO4solutions were respectively treated by vacuum membrane distillation. The effects of operating conditions on permeate flux and permeate TN concentration were investigated. The results showed that the permeate flux reached up to48L·m-2·h-1and the ammonium salt rejection rate was above99.95%. The permeate TN concentration was below3mg/L, while feed pH value was3.Secondly, the empirical equation of heat transfer was fitted by experimental flux datas which were obtained at different feed temperatures and feed flow rates. A predictive model in term of permeate flux was built by combining physical parameters of ammonium salt solutions withthe empirical equation of mass transfer. The predicted values and experimental datas tended to agree. Furthermore, the concentration and temperature polarization coefficients (CPC and TPC) were estimated by the predictive model, and the effects of different operating conditions on the coefficients were investigated. Higher feed temperature leaded to greater CPC. Increasing feed flow rate promoted the decrease of TPC, but CPC was slightly increased. Higher TPC and lower CPC were obtained with the permeate pressure increasing. The influences of feed concentration on TPC and CPC were not obvious.Then, the ammonium salt solutions were concentrated by vacuum membrane distillation when feed pH value was3. The permeate flux decreased slowly at first. However, the permeate flux dropped rapidly at the end of concentration experiment. The feed supersaturation factor was closed to1. Moreover, in NH4C1system, the permeate TN concentration was kept in the range of3.2mg/L~4.7mg/L at first, butrapidly increased to 59.8mg/L at the end of concentration. Whereas, the permeate TN concentration was below3mg/L all the time with (NH4)2SO4system. The negative ions of ammonium salt have great influence on permeate TN concentration. The operating conditions also have effects on batch recovery. Lower permeate pressure resulted in earlier flux decline and lower batch recovery. Increasing feed temperature increased batch recovery, but leaded to flux decline early. Higher feed flow rate promoted the higher batch recovery and delayed the flux decline.The transfer resistances and supersaturation factor at membrane surface (SFm) were estimated by the experimental datas in concentration process. The membrane resistance and boundary layer resistance dominated at first, but the fouling layer resistance increased rapidly while the SFm closed to1. The corresponding feed concentration was taken as the critical point. Chosen the feed concentration below and beyond the critical point respectively, the behavior of permeate flux with operating time was investigated. The sharp decline of permeate flux indicated increase of fouling layer resistance, and the membrane fouling phenomenon with operating time were observed when the feed concentration was beyond the critical point. In addition, the permeate TN concentration also increased with operating time in NH4CI system. The deposition was found on both membrane surface and pore wall in NH4CI system from the SEM images of fouling membrane. However, large amount of (NH4)2SO4crystal deposits were observed on the membrane surface, but little fouling on the pore wall. The permeate flux recoveries were98.6%and53.6%respectively in NH4CI and (NH4)2SO4system after membrane cleaning. When the feed concentration was below the critical point, the permeate flux decreased slowly, and the permeate TN concentrations were below5mg/L.The crystallizer was carried out with the concentrated amminium salt solution. The results showed that the NH4CI crystals were precipitated from concentrated solution by cooling crystallization, whereas the (NH4)SO4crystal was obtained by adding the seed crystal. The optimized cooling method caused the larger NH4CI crystal size.Finally, the treatment of ammonium salt wastewater by coupling vacuum membrane distillation-crystallization was proposed. The membrane distillation was used to obtained the permeate water and desired concentration solutions and then crystals were precipitated by crystallizer. It was worth noting that the effluent with low temperature from crystallizer should be transported through the heat exchanger, in order to condense the permeate gas from membrane distillation process and increase the heat efficiency.