Treatment of effluents from the regeneration of ion exchangers using the MD process

The membrane distillation (MD) process for the treatment of wastewater from ion-exchanger regeneration was proposed. The precipitation of salt deposits on the membrane surface was observed when such wastewater was directly used as a feed. A rapid decline of the permeate flux from 532 to 410 dm3/m2d was found. The problem of fouling was significantly diminished by the addition of Ca(OH)2 to the wastewater followed by filtration. The improvement of MD module performance was achieved after such pretreatment. The degree of water recovery equal to 50% was obtained without significant variations of the permeate flux. However, when the pretreated feed was subjected to twofold concentration, precipitation of the silicon compounds was observed. The deposit caused clogging of the inlets of capillary membranes and resulted in a gradual decline of the module efficiency. The two-fold concentrated feed was then treated by sedimentation and filtration, which permitted a further concentration of obtained retentate and enhanced the degree of water recovery to 75%.

Fig. 5. SEM image of the deposit formed on the membrane surface in the M 1 module during the concentration ofwastewater liB (pretreated wastewater II). Fig. 6. Results of SEM-EDS analysis of the deposit (presented in Fig. 4) formed during the concentrationof wastewater IIB. firmed that the application of two-stage wastewater treatment with the MD process achieved the degree of water recovery equal to 75%. After accomplishing the MD studies the membranes from M2 module were also subjected to autopsy. The SEM investigations revealed that the membranes remained clean; only small amounts of deposit were present on the membrane surface (Fig. 5). The results of SEM-EDS analysis of this deposit were presented in Fig.6. The deposit mainly contained the following elements: Ca, Si, Na, S and C1. The composition of The separation by the MD process of wastewater generated during the regeneration step of ion exchange resulted in the formation of deposits on the surface of the Accurel PP $6/2 membrane. As a result of this fouling phenomenon, the permeability of the membrane decreased; hence a rapid decline of module efficiency was observed. The composition of the deposit included C a S O 4 and silicon compounds. Severe membrane fouling excluded the possibility of separation of wastewater from ion-exchange regeneration without pretreatment by the MD process. The pretreatment of the above wastewater with lime precipitation, Ca(OH) 2, demonstrated that the majority of foulants was removed. Such pretreated feed was subjected to two-fold concentration in the MD process, and permeate flux decline was not observed. In the case of water recovery, a coefficient higher than 50% of the precipitation of silicon compounds was observed. This deposit clogged the inlets of the capillary membranes, and resulted in a rapid decline of the MD module's efficiency. Mechanical cleaning of the membrane inlets restored the initial efficiency of the module. The application of an additional treatment (sedimentation and filtration) for two-fold concentrated wastewater allowed further running of the MD process and achieved a degree of water recovery equal to 75%. Under these conditions, membrane fouling was limited, and therefore, the permeate flux was stable. The quality of the distillate obtained was maintained during the process and was found to be practically independent of the feed concentration, The distillate produced has electrical conductivity in the range of 1.5-2.5 #S/cm.