Prepared by: Purolite International
FACTORS TO BE CONSIDERED
a) Reliability
Both RO and IX are well established reliable technologies with a good track of performance world wide. Future technical developments are not expected to have a major influence on plant and process costs.
b) Feed Water Pre-treatment Required
Both processes require pre-treatment to remove suspended solids to a low level to avoid fouling. However IX is more tolerant of suspended solids and RO requires additional pre-treatment by micro-filtration. Membranes are also subject to scaling by hardness present in the feed water and require either a softening plant as part of the feed water pre-treatment or the addition of anti-scaling chemicals.
c) Quality of Treated Water
IX can produce demineralised water with a conductivity of less than 0.5uS/cm from a simple SAC-SBA combination and less than 0.1μS/cm with the addition of a mixed bed SAC/SBA unit.
Even the best performing RO plants cannot meet the treated water quality of a simple IX plant and a subsequent IX unit is required to achieve boiler feed water quality. (SAC is strong acid cation resin, SBA is strong base anion resin).
d) Flexibility
Ion exchange plants tend to be more flexible than RO, for example in terms of performance over a wider range of temperature variations and the ability to recover from high suspended solids in the feed.
e) Fouling by Organics
Both RO membranes and IX resins can be fouled by organics present in the feed water. IX resins are much more easily cleaned than RO membranes without long plant shut down and use cheap cleaning chemicals; salt and sodium hydroxide.
However RO has a place in producing demineralised water and when used in combination with IX can produce the highest quality boiler feed water. The role of RO is in treating high TDS waters and in order to establish guidelines for selection of RO versus IX it is necessary to carry out detailed cost comparisons.
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