Softening Plants
WAT-softening plants use the ion exchange process to generate softened water. The sodium salts of a sulfonic acid -SO3Na are the functional groups, which are bonded to a cross-linked organic matrix.
Cations, especially the hardness former calcium and manganese are exchanged against sodium ions. Anions are not exchanged.
Matrix - (SO3Na)2 + Ca2+ = Matrix - (SO3)2Ca + 2 Na+
Therefore the total salt concentration and the pH-value do not change. After exhaustion of the exchanger capacity an automatically operated regeneration is triggered. The exchange operation is reversed by rinsing the exchange resin with a sodium chloride solution.
The countercurrent regeneration can be triggered time-, volume- or quality-dependent. For industrial plants and constant raw water qualities usually a quantity-dependent triggering of regeneration is chosen. A process titrometer (Testomat) can be installed to monitor the soft water quality. If varying water qualities are typical, a quality-dependent control with a Testomat is recommended.
A high-quality monospheric ion exchange resin is used for softening plants. This type of resin allows an almost complete loading of the resin before a hardness breakthrough occurs, which means that an optimal use of regenerant is achieved. Compared to conventional softening plants the quantity of rinsing water is reduced about 30%.
For regular water qualities the quantity of wastewater is about 1% of the generated soft water quantity.
The inner shell of the composite-pressure vessels is made of polyethylene (PE) which fulfils the chemical and mechanical standards and the high standards of hygiene. The vessels are designed for operating temperatures up to 50 °C and operating pressures up to 10 bar and are resistant in compliance with the german pure water law.
They are also designed to pass a 0-to-10.5 bar pressure cycle test of 250 000 cycles without failure.
To suit the particular needs of each installation different microprocessor controller are available for controlling the service and regeneration sequences. The operation and control of all functions can be regulated by a programmable microprocessor controller which is equipped with a folio-keyboard and a basic programming memory (EEPROM). The service and regeneration sequences are regulated, up to a nominal piping diameter of DN65, by regeneration control valves which are manufactured from high quality red brass. The valves can be nickel-plated if necessary. Plants with a nominal piping diameter of DN80 are regulated by 2/2-way pneumatically operated diaphragm valves or butterfly valves (both Gemü). The use of regeneration control valves for regeration is limited. The valves are mounted on noncorrosive coated steel frames.
Areas of Application
Drinking water supply with hard water
Drinking water with a high alkaline earth concentration of 2.5 mol/m3 (7°dH) or more tend to deposit boiler scale in supply lines, warm water generators and sanitary systems. Therefore a softening plant and a blending device for the drinking water supply of bigger buildings is recommended, so that the alkaline earth concentration can be reduced to 1.5 mol/m3.
Feed water for steam generation
For the operation of steam generators the feed water has to be free of alkaline earth ions to avoid deposition. If an economic thickening in the circulating water is not possible, because of a high alkalinity of the raw water, an additional preceding decarbonization is necessary.
Feed water for hot water circulating systems
To avoid deposition of boiler scale in heating systems, the feed water has to be treated dependent on the capacity of the heating system and the tendency of deposition of boiler scale in compliance with guideline VDI 2035. Water softening is a cost-effective way of water treatment.
Note: Raw water with high salt concentration should be desalinated with a reverse osmosis plant.
Feed water for cooling water systems and airwashers
The decisive cost factor of the operation of recirculating cooling water systems and airwashers is the consumption (wasteful consumption) of water and wastewater. Use of softened water allows an operation with a high thickening of the water before deposition occurs. If a blending device is installed, an optimum between deposition tendency and corrosiveness can be achieved.
Raw water with a high alkalinity should, for economic reasons, rather be decarbonized or desalinated.
Softening as a preliminary stage of reverse osmosis
Reverse osmosis is a cost-effective desalination process where a thickening of the feed water (salt concentration increases) occurs.
By first removing hardness formers with a softening plant a deposition of calcium carbonate on the membrane surface can be prevented which allows a higher thickening of the water. This on the other hand means a higher permeate recovery and less costs for wastewater disposal.
Process- and Rinsing water
Softened water often fulfils the requirements for
process water or rinsing water used for surface treatment of metals such as metal cleaning and phosphatizing.
