It is well known that anion resins are susceptible to fouling by the humic and fulvic acids sometimes found in surface waters.
These organic species, because of the relatively large molecular weights, become trapped within the resin matrix (to a greater or lesser degree depending upon the resin) and specific procedures have to be employed to cause recovery of the original ion exchange properties of the resin.
The symptoms of organic fouling include long rinse requirements, poor capacity and, in the case of strong base resins, higher silica leakage.
The most common forms of treatment involve the use of brine solution; the procedure is as follows :-
The resin should be treated at the end of the normal exhaustion cycle.
Three bed volumes of 10% w/v brine solution containing 2% w/v caustic soda should be prepared.
One bed volume should be introduced into the ion exchange unit at a flow rate not exceeding 2 BV’s per hour followed by a second bed volume -this second bed volume should be retained in the unit for as long as possible, but at least 4 hours. Some agitation, if possible, should be employed periodically throughout the retention period.
At the end of the retention period the last bed volume of brine should be passed through the resin at a rate of 1 BV per hour and the resin thoroughly rinsed with clean water until free from brine.
The resin should be subject to at least two complete regeneration cycles before being put back on line.
N.B. Brine at minimum 35°C should be employed or preferably as high as 60°C so as to produce a better organic elution effect.
This subject is covered to some extent in the section on iron fouling.
Occasionally the presence of iron is detected on the anion resin. This can arise from an iron/organic complex being present in the raw feed water.
In these cases, it is advisable to consider treatment of the anion resin with 6% hydrochloric acid immediately after the brine treatment. The procedure that should be followed is similar to that given for brining.
It is extremely important that all traces of hydrochloric acid are removed from the unit before introduction of the caustic soda regenerant.
It is important to ensure that the materials of construction are suitably resistant to hydrochloric acid.