Membrane Maintenance

The membrane maintenance degradation depends on several factors, such as temperature and temperature variations, pH and pH variations (as can occur with membrane cleanings) and the presence of oxidative substances such as Chlorine. These factors can influence alone or in combination the decay of the membranes while certain ions can act as catalysts. For example iron deposits are known to enhance the effect of chlorine in the degradation of the membrane.
The membrane maintenance can include measures to avoid temperature fluctuations and the minimization of the impact chemicals cleanings by reduced frequency or less extreme pH regimes. Also the use of additives like anti-scalents can be implemented to keep the membrane surfaces clean of scale and iron deposits. If under normal operation super saturation is created membrane maintenance should include a flush of all the water present in the feed channel at a production stop. The water used for flushing preferably should be permeating, since that is the most stable water available.

Anti scalents

Anti scalents are used in reversed osmoses and nano filtration. Both membrane processes lead by definition to concentration of the ions present in the feed water. Salts can be formed from cations (i.e. positively charged ions, such as Calcium, Barium and Strontium) combined with anions ( i.e. negatively charged ions such as Carbonate, Sulphate and Phosphate). This is a process well understood and can be predicted if the pH and temperature of the feed water is known. The selection programs of the membrane manufactures indicate the super saturation level of most common salts. By using anti scalents these super saturation levels can be exceeded without having actual scale formation on the membrane. Anti scalents keep these salts in solution only for a limited time, which is dependent of type of anti scalent used, temperature and type of salts involved. This is why a flushing of the supersaturated water at the feed channel is needed at a production stop as normal membrane maintenance operation.

Repairing damaged membranes

Separately from a decreased production, membranes are also replaced if the permeates quality starts to decrease. This can be measured by an increased average salt passage (ASP), which should be normalized to a reference temperature. The normalization is needed since warmer water opens the membranes and allows more ions to pass the membrane. If under warmer conditions the requested quality cannot be met, membrane maintenance may include a temporary coating. Also if new membranes are not available on short term a life time increasing coating can be considered. This coating can be provided by chemical suppliers and can be applied in a very short cleaning procedure. Figure 1 shows the reduced salt passage from an average 6 % before to less than the required 5 % after applying 4 Memcoat 01 as membrane maintenance at a plant in Belgium.