Feedwater Ltd manufacture and service water softeners throughout the UK
Softeners have two main modes, or cycles, of operation: service and regeneration. During the service cycle, the softener exchanges ‘hard’ cations for sodium, causing progressive depletion of exchangeable sodium to the point of exhaustion.
The softener is then ‘regenerated’ by saturating the resin bed with concentrated sodium chloride solution; sodium displaces the cations previously removed from the feed water.
The regeneration process actually involves a series of steps and typically requires about 1 to 3 hours to complete depending on softener make and model. The following information is a summary of the steps
a) During service, water flows downward through the bed, causing the resin spheres to become tightly packed. The first step of the regeneration cycle, backwash, loosens and expands the media by directing water flow upward through the resin bed.
Continued back washing cleans the resin of particles, including resin fragments or ‘fines’, trapped in the densely packed bed during the service cycle. In addition to cleaning, the loosened condition of the resin bed promotes uniform flow of brine and rinse water during later steps of the regeneration cycle.
b) Introduction of sodium chloride, or brining, occurs next. A brine tank containing water and sodium chloride cubes or pellets is located adjacent to the softener. The resulting solution is drawn from the brine tank by means of a Venturi or ‘eductor’ located in the control valve assembly usually at the top of the softener.
The volume of brine drawn and the duration of this step are individualized according to local circumstances, which include the volume and hardness of water which has passed through the softener during service, the extent to which sodium has been depleted from the resin, and the volume and configuration (height and diameter) of the resin bed.
c) Next, the softener is ‘slow rinsed’ to remove the brine solution present in the softener at the termination of brining. Slow rinsing is typically accomplished by stopping the inflow of brine and continuing water flow in the same direction and flow rate. This process ensures that the brine will be maximally utilized while removing any excess solution from the resin bed.
d) The final step in the regeneration process is the ‘fast rinse.’ Fast rinsing completes the removal of brine from the resin bed with a brisk flow of water being maintained in the same direction as during the service cycle. Fast rinsing shortens the time needed to completely remove excess brine from the resin and also serves to flush areas in which water flow is sluggish.
Applications and Advantages
The prevention of scale and deposit build-up in boilers, industrial washing processes and cooling towers have created a large demand for softened water.
Base-exchange (sodium ion exchange) systems are able to satisfy this demand economically and efficiently.
¢ Softened water has very low scaling tendencies because ion exchange reduces the hardness level of most waters to less than 2ppm
¢ Operation is simple and reliable; automatic and semi-automatic control valves are available at reasonable cost.
¢ Salt is inexpensive and easy and safe to handle
¢ No waste sludge is usually produced; waste disposal is not a problem.
¢ Within certain limits variation in water flow rate have little effect on water quality.
¢ Efficient operation can be obtained in units of almost any size; sodium ion exchange softeners are suitable for both large and small installations.
¢ Ion exchange resins have long life times (up to 10 years).
Base-exchange (sodium cycle) softeners efficiently reduce hardness levels in water. However the total solids content, alkalinity and silica are unaffected. If excessive silica or alkalinity concentrations are expected to lead to problems, for example, carbon dioxide in condensate from alkalinity, then alternative or additional processes should be considered.
Ion exchange resins are very efficient filters so do not function well on turbid waters. Operation with turbid waters causes resin bed fouling, short service runs and poor water quality.
Heavy metal contaminants such as iron and aluminium which are not removed during a normal regeneration will lead to resin fouling and poor quality water.
If iron or manganese is present in the water, the resin must be cleaned periodically.
Strong oxidising agents, such as chlorine, can attack and degrade the resin. Levels of chlorine greater than about 0.5ppm should be removed, for example, by activated carbon filtration, prior to the base-exchange softener.