During the cheese process NaCl and CaCl₂ are often added. Some of these salts will be contained in the whey and are in some cases unwanted, if for example the whey is used in baby food production, where a low salt content is aimed at.

Different processes have been developed and shall in short be mentioned here:

• Membrane filtration
• Ion exchange
• Electrodialysis

MEMBRANE FILTRATION
In the nanofiltration process ≈50% of the salts is removed (depending on the perme-ability of the lactose chosen in the membrane) together with water in the nanofiltration plant.

Nanofiltration is an RO process with relative open membrane structure allowing in particularly small monovalent ions, such as Na⁺ and Cl^- to pass through the membrane.

The process uses pressure lower (20-30 bar) than that applied in a normal RO plant to ensure that as much of the salts and as little as possible of the lactose passes through the membrane. Typical figures for molecule penetration for different membranes are:

		Case 1	Case 2
Lactose penetration into the permeate:	%	1	4
NaCl penetration into the permeate:	%	50	73

The simultaneous removal of water and monovalent ions without losing major organic components like lactose and proteins, makes the nanofiltration an important tool for a combined concentration and demineralization process.

ION-EXCHANGE
To visualize this process, two lots of resins with different characteristics should be used, see Fig. 142. The resins usually consist of polystyrene and divinyl benzene in form of small spheres with a diameter of 0.4-0.8 mm having a large number of attached bonds on their surface which can absorb (reversibly) one specific type of ions. The ions are divided into cations (H⁺, Na⁺, K⁺, Ca⁺⁺, etc.) and anions (OH^-, Cl^-, SO4^--, etc.). When the whey is passed through the first lot of resins, the cation-exchanger, all the cations are replaced by H⁺, whereas all the anions pass through the cation-exchanger unaffected. During the subsequent passage of the decationized whey through the second lot of resins in the anion-exchanger, all the anions are replaced by OH^-. The transfer of whey through the resin beds continues, until the resins are saturated with cations and anions. This point is controlled by means of a conductivity meter, after which the resin beds are purged of whey, washed with water and regenerated by means of acid and alkali solutions. These solutions should be sufficiently concentrated to remove the absorbed cations and anions and replace them by H⁺ and OH^- bringing them back to their previous state. The treatment with acid and alkaline results at the same time in a sufficient cleaning. After the regeneration the resins are washed with clean water   preferably condensate from an evaporator - and are then ready for another batch. A typical cycle would be two hours' production and four hours' regeneration. Whey treated in an ion-exchange plant will be demineralized 90-98%. The processing/holding time is 20 min., and as the processing temperature is < 10ºC, no bacteriological growth is experienced.

Fig. 142  Flow-sheet. Ion-exchange

ELECTRODIALYSIS
In an electro-dialysis plant a number of ionselective membranes, made from approxi-mately the same material as the resins, is applied in form of cells. Each cell   consisting of one cationic and one anionic membrane - is arranged in a stack analogous to a plate heat exchanger. The spacing between each membrane is about 1 mm. Between each of the above mentioned cells flows a diluent salt solution.

The entire stack is placed between a series of DC electrodes providing an electric field. Under influence of the electric field, the cations will migrate in the direction of the electric current - i.e. towards the -pole - and pass through the cation membrane into the flushing salt solution which is recycled during operation. The anions will migrate in the opposite direction of the electric current - i.e. towards the +pole - and pass through the anion membrane into the flushing salt solution. Like the flushing salt solution the whey is recycled during operation via a batch tank with a conductivity meter attached. To obtain sufficient conductivity of the whey to be treated a preconcentration to 20-28% solids, as well as a processing temperature of 30-45ºC, is necessary. The recirculation time depends on the required demineralization level and may reach up to 6 hours, if 90% demineralizing is aimed at. Naturally, this results in bacteria growth. To avoid this, the temperature may be lowered, which, however, will result in an increased plant size, and thus higher investment, or formaldehyde may be added. As the salt content decreases in the whey the conductivity decreases, and it is necessary to increase the applied voltage to compensate for this. To keep the salt content in the flushing water at an optimum level it is diluted as the process continues. Excess salt solution is passed to the drain. The cleaning of the membranes is done by passing acid and alkaline through the plant, an operation lasting 2 to 6 hours depending on the plant manufacturers.

Spray drying of demineralized whey is done as for normal sweet whey as discussed on page 259. However, due to the lower salt content, the drying becomes easier.