Membrane distillation (MID) was used for treatment of the effluents generated during regeneration of an ion exchange system in the water treatment plant. The separation of these effluents was performed on the MD pilot plant equipped with the capillary modules made from polypropylene membranes. The precipitation of iron compounds was observed during the operation of MID installation. To assess the fouling phenomena one of the MID modules was subjected to autopsy after several weeks of process duration. A severe fouling of the membrane surface caused by the iron oxides was found. The morphology and composition of the precipitated layer were studied using the scanning electron microscopy (SEM) coupled with energy dispersion spectrometry (EDS). The Fourier transform infrared with diffuse reflectance spectroscopy (FTIR-DRS) was also employed for this analysis. The performance of fouled membranes was evaluated in a laboratory module assembled with the capillary membranes subjected to autopsy. It was found that the formed deposit did not significantly affected the permeate flux in the MID process. A possibility of the membrane regeneration by rinsing with HCl solutions was investigated. An increase of the permeate flux from 770 to 830 dm(3)/m(2)d was observed due to a partial dissolution of deposit from the membrane surface by hydrochloric acid. However, the dissolution all of the deposit (also including that precipitated into the pores) caused a partial wettability of the membranes, therefore the permeate flux was decreased from 830 to 650 dm(3)/m(2)d. The initial membrane permeability was restored for a short time, when the membranes were subjected to drying.