Softening is an ion exchange process used to remove ions from the water that contribute to alkaline scale formation, primarily calcium and magnesium. As water containing these ions passes through the softening resin bed, the ions are exchanged with sodium ions residing on the surface of the resin.

When no more sodium ions are left for this exchange process, the resin must be regenerated. Regeneration is accomplished with a brine solution of sufficient strength to cause an exchange between the sodium ions in the brine and the calcium and magnesium ions on the resin. Once regeneration is complete, the resin is ready to begin the softening cycle again.

All types of Ion Exchange Technology, co-current, counter-current and packed bed (upcore), can be used for water softening. The most economical way to produce the required effluent quality determines the choice of the system. The definition of the "most economical way" is not the same for all companies. Some consider the capital cost, others the operating cost of the system, as predominant. Consequently, a given set of requirements not always leads to the same choice of system. Generally, there is a compromise between the regenerant consumption and the unit size for all systems.

Most ion exchange softeners are co-current, meaning that the service flow and the regenerant flow will pass the IX unit in the same direction (downflow). It is the simplest, and the most reliable, process to operate. The hardness content in the bottom layers of the resin bed will still be considerable after the regeneration has been completed. This causes the reaction to partially reverse in the bottom layers of the bed and hardness leakage is the result. The required softened water quality will define the minimum regeneration level for a co-current softener (varies with the feed water composition). The production of very high quality softened water requires an extremely high regeneration level. The salt consumption per unit of softened water becomes prohibitive.

Counter-current regeneration is applied when the soft water quality requirements exceeds those that can reasonably be achieved with co-current regeneration. In counter-current units, the regenerant flow moves in the opposite direction as the service flow. The resin layers that are contacted last by the service water are those that have seen a large excess of regenerant and contain very little hardness ions. The partial reversal of the reaction, causing the leakage in a co-current unit, does no longer occur. Theoretically the leakage of a counter-current unit would be zero. Practically, leakage of 0.1 to 0.2% is achieved, almost independent of the regeneration level. During the upflow regeneration, the resin bed must not be disturbed. An extra (inactive) layer of resin must be added at the top of the unit. During brine injection, an extra flow comes from the top of the unit to keep the resin bed down. A counter-current unit always contains more resin then its co-current counterpart. It also must be kept in mind that saving salt by the use of low regeneration levels increases the resin volume (and the size) of the unit.

More recently, "Packed Bed" (Upcore) technology has entered the picture. Here, the ion exchange columns are almost completely filled with active resin. The 80-100% freeboard in the regular IX units, used to backwash and expand the resin bed for removal of suspended solids, is no longer present and the suspended solids are controlled by other means. Packed bed units also employ counter-current regeneration. The leakage levels are low but do not reach those obtainable by true counter-current units. The brine injection step of the regeneration cycle must be performed at high linear flow rate because the resin bed has to be lifted and kept compacted against the top collector of the unit. This usually necessitates higher regeneration levels and lower efficiency than those that can be used for a true counter-current unit.

The differences between the performance of the three softening technologies, operating under the same conditions regarding flow and influent water composition, is presented below. The best choice of the system to be used may be influenced by the composition of the influent water. For this reason, two calculations are given, the second set for water with much higher hardness and TDS.

General comparison of Systems

As is shown on the table, the hardness of the feed water is very important for the equipment sizes. For feed water with low hardness, the equipment size is determined by the system flow and the run length follows from it. At higher influent hardness, the amount of chemical work becomes the determining factor. For a given performance, packed bed units are generally the smallest and the most efficient. Regular removal of suspended solids is necessary for all systems in order to maintain proper plug flow complication. The removal from packed bed units is the most difficult to handle in this respect.

Influent Hardness: 85 ppm as CaCO3  TDS: 139 ppm as CaCO3  System flow: 50 USgpm (net)  Time between regenerations: 12 hours. Two units, one on line

	UNIT	CO-CURRENT	COUNTER-CURRENT	PACKED BED
Regeneration water (average)	USgpm	1.0	1.2	0.6
Effluent hardness	ppm CaCO3	1.8	0.2	0.4
Regeneration level 100% NaCl	Lbs/ft3	4.0	4.0	6.0
NaCl (100%) consumption	Lbs/1000USgal	1.21	1.21	1.20
Resin per unit	ft3	10.8	12.6	7.3
Vessel diameter	ft	2.0	2.0	1.5
Vessel straight side	ft	6.5	7.5	5.1

Influent Hardness: 250 ppm as CaCO3  TDS: 304 ppm as CaCO3  System flow: 50 USgpm (net)  Time between regenerations: 12 hours. Two units, one on line

	UNIT	CO-CURRENT	COUNTER-CURRENT	PACKED BED
Regeneration water (average)	USgpm	2.4	3.6	1.6
Effluent hardness	ppm CaCO3	3.5	0.2	0.4
Regeneration level 100% NaCl	Lbs/ft3	6.0	4.0	6.0
NaCl (100%) consumption	Lbs/1000USgal	4.14	3.60	3.40
Resin per unit	ft3	24.8	37.4	20.8
Vessel diameter	ft	2.5	3.0	2.0
Vessel straight side	ft	9.0	10.0	7.6

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