WATER
Determining when it is time to replace reverse osmosis membranes By Wes Byrne
O
n the surface, it might seem that knowing when to replace the membrane elements in a reverse osmosis (RO) system would be simple. In attempting to keep it that way, many companies will use one or more of the following guidelines for replacing their membrane: • Often it is after three years, which is when membrane manufacturers’ warranties end. • There is an increase in RO permeate conductivity, as related to its water quality requirement. • A designated reduction in the permeate/product flow rate occurs, as related to water demand. However, the reliance on simple limits may result in replacing the membrane prematurely. RO membrane elements have been known to last more than ten years, while still providing salt rejection and permeate flow rates similar to what they produced at startup. Performance deterioration caused by membrane age occurs so slowly, that age itself rarely plays much of a role in the life of the membrane. Rather, replacement is more often related to how well the RO system is maintained. If a strong oxidant like free chlorine contacts the RO membrane, it may dramatically reduce membrane life. It should be understood that any exposure to free chlorine will damage the RO membrane. The extent of damage will be related to the chlorine concentration and the amount of time it is in contact with the membrane. With continued exposure, RO salt rejection will decline and the permeate flow rate will increase. The extent of this is best gauged by normalizing the permeate flow rate for any changes in water temperature and operating pressures. If there has been an unacceptable decline in salt rejection that has occurred simultaneously with an increase in normalized permeate flow rate, it usually means that all of the membrane elements will need to be replaced. This includes those in a second pass if they received chlorinated permeate water from the first pass. Before assuming that all of the membrane elements need to be replaced, more insight can be gained by measuring the conductivity of the permeate water from each membrane vessel. Some rejection problems may be corrected by replacing only the membrane elements located in specific locations within the membrane vessel array, such as in the very lead end, when there is excessive fouling caused by suspended solids in the inlet water. Poor performance from the concentrate-end vessels could be a symptom of scale formation, or it might simply be related to warmer water temperature when using a low energy RO membrane. If the RO salt rejection declines, profile the membrane ves26 | June 2021
Excessive fouling is the most common cause of reduced membrane life.
sel permeate conductivities to isolate the location of the problem and where membrane elements may need replacement. Excessive fouling is the most common cause of reduced membrane life, which can be defined as allowing the RO feed-to-concentrate pressure drop to increase more than 15%, or the normalized permeate flow rate to decline more than 15%. If biological particles and/or silicate clay particles are allowed to collect and compact against the membrane surface, it will take longer for cleaning solutions to wet out and completely remove them. If larger particles are allowed to collect and plug off the flow channels through the membrane elements, the cleaning solution will not be able to access those fouled regions. If cleaning does not restore the original feed-to-concentrate pressure drop or normalized permeate flow rate, then more aggressive cleaning may be needed. This is more easily accomplished off-site. Membrane surface fouling will cause the normalized permeate flow rate to decline. Fouling is usually most severe in the lead-end membrane elements, due to their higher permeate flow production. As these elements lose permeation, downstream membrane elements are forced to produce more water and subsequently suffer from increased fouling. The inability to restore the normalized permeate flow rate with aggressive cleaning usually indicates that all of the (firstpass) membrane elements will need to be replaced. Large biological particles can be shed into the RO feed water from biofilm present in the piping and system components located after dechlorination. These particles can get caught Environmental Science & Engineering Magazine