Second Pass RO Systems

[WaterEng]

It is interesting to see that a series of 2nd pass RO systems are losing rejection in less than 18 months of operation. The systems are nearly identical in both pretreatment and design with the only commonality being interpass NAOH injection to increase the carbonate levels in aid of higher 2nd pass rejection.

Has anyone else seen this trend? Is the caustic a problem or could it be another case of chloramine kinetics?

Thanks for the input.

 

[GarySCWSVI]

It may very well be the NaOH feed that is the cause. I may also very well be Chlorine/Chloramine. I suggest you have one of the modules postmortemed to determine the cause. Chlorine/Chloramine can degrade the membrane. NaOH could cause membrane scaling and cause a pH in excess of the membrane specifications cause damage. There could also be any number of other potential problem causers.