Section 4.6.4 briefly describes the removal of iron and manganese from municipal water supplies using ion exchange processes. The paragraph states several limitations on the use of ion exchange:
However, Section 4.4.2 describes in detail the use of cation exchange processes for the removal of hardness from water. It should be noted that is the dissolved solids content or the sodium concentration of the water are issues, then ion exchange should probably not be used.
Section 4.4.2.1 summarizes pre-treatment requirements for waters to be passed to the exchange unit. Section 4.6 is referenced, with the additional stipulation that turbidity counts higher than 5 NTU should not be passed to the unit.
A resin with an exchange capacity less than or equal to 20,000 grains/cu ft should be used when regeneration is accomplished with a brine solution exchanging 0.3 lbs brine / kg of hardness removed. The vessel itself may either be upflow or downflow with automatic regeneration, unless manual regeneration is approved by the "reviewing authority." As usual, all automated equipment must provide manual override controls.
The exchange basin itself must be at least 3' deep. The flow within the reactor should kept lower than 7 gal/min/sq ft of bed. Backwash must be provided at a rate of 6 to 8 gal/min/sq ft of bed.
Sections 4.4.2.6 through .19 elaborate on control mechanisms and design criteria for the unit, as well as chemical storage and waste disposal. Section 4.4.2.10, however, makes an interesting stipulation: ion exchange bypass pipes must be installed so that hard and soft waters can be blended to obtain a desired hardness. This allows a water works to save costs on an efficient ion exchanger by not having to run it full time, and to save on the amount of water that is actually treated. This is important to remember while designing a unit, because not necessarily all water from the preceeding unit will be subject to ion exchange. Therefore, flow considerations may be lower in the ion exchange unit than in the rest of the plant.