Alkaline scaling in the membrane distillation process

Demineralisation of tap water, produced from surface water (lake), by a membrane distillation process has been investigated. The temperature caused the decomposition of bicarbonates presented in the feeding water and the formation of alkaline scaling on the membrane surface. In the present work the influence of various operating parameters such as the feed temperature and the hydrodynamics conditions on the formation of the deposit layer, mainly calcium carbonate, was studied. The deposits were analysed by scanning electron microscopy (SEM) coupled with energy dispersion spectrometry (EDS). For the low flow rate a deposit layer was more compact than that formed for higher flow rates. The morphology of the formed deposit has a significant influence on the permeate flux decline, and a higher flux was found for the less compact deposit layer. The alkaline scaling was retarded for the feed temperature below 353 K and the flow rates above 0.6 m/s.

The obtained results demonstrate that the direct application of tap water as a feed for the MD process results in a rapid decline of the permeate flux due to the deposition of a significant amount of CaCO3 onto the membrane surface (scaling). The high feed temperature accelerated the ! decomposition of HCO3 ions. Therefore, taking into consideration the intensity of alkaline scaling formation, the feed temperature should be below 353 K in the MD process. The hydrodynamic conditions occurring in the module have a considerable influence on the module efficiency. For the used module design the flow rate of the feed should be above 0.6 m/s. This magnitude of the flow rate is also reducing the scale level.