The high water hardness levels found in some water pose practical problems for both domestic and industrial customers. These range from scaling in boilers, water heaters and hot water systems, to long-term scaling in water mains, needing to use more soap and detergent when washing, having problems when cooking vegetables through to requiring larger than usual demineralization facilities.
Softening
Softening water through catalytic lime softening leads to an improvement in water comfort for the customer.
Chemical Process
The sand grains act as catalysts or crystal nuclei that are coated with the calcium carbonate precipitate which is obtained thanks to the localized and rapid increase in the pH of the water.
The grains grow and form carbonate pellets 1 to 2 mm in diameter. These pellets are then extracted on a regular basis from the base of the reactor once they have reached the optimum size.
The reactor is fed with sand after each extraction in order to ensure that the quality of the sand remains constant. The water is injected at the base of the reactor through a diffusion floor which ensures homogenous flow apportionment
The aim of catalytic lime softening is to soften the water through calcium carbonate precipitation.
This will also lead to a reduction in alkalinity. Either lime or sodium hydroxide can be used.
The precipitation reaction occurs when it comes into contact with a bed of sand fluidized by a rising water current in a reactor. The bed expands by maintaining a high fluidization speed (80 to 100 m/h).
Physical-chemical process (ion exchange)
Softening technology is based on ion exchange resin systems. It focuses on removing the hardness from water due to calcium and magnesium ions.
Softeners do not reduce the total amount of impurities in water, they simply exchange the types of ions from calcium and magnesium into sodium.
Softeners are often used as pre-treatment to reverse osmosis to prevent the build up of hardness scale and flow rate depending on the amount of hardness in the feed water.
A cation resin in the sodium form is used and operates with sodium chloride for regeneration.
When all the exchange sites on the resin have given up their sodium ions, the resin is said to be "exhausted" and can remove no more hardness ions.
It then requires regeneration where a strong sodium chloride solution is passed over the resin, displacing the hardness ions and putting it back into the sodium form.
Softeners can be regenerated co-current or counter-current.
The resin may be used again to soften the water.