Recovery of water and chemicals from ion exchange regeneration effluents (1995)

Ion exchange is a cyclic process involving loading the ion exchange resin with ionic species from the raw water or process stream during the service stage of the cycle, followed by desorption of these species during the regeneration stage of the cycle. Regeneration is generally carried out using a concentrated solution containing the ion that is exchanged (usually as an acid, base or salt). The exchange is stoichiometric, however more than stoichiometric amounts are required to achieve complete regeneration. For example, for an ion exchange process based on countercurrent regeneration, one equivalent of salts will be removed with the addition of at least 1,2 to 2,0 equivalents of extra salts to the system derived from the regeneration chemical. Hence, the salinity discharge to the environment increases at a greater rate than the rate of salinity increase in the raw water.

Disposal of the resulting regeneration effluent can incur high waste disposal costs and it may lead to pollution of the environment. The build-up of salts in an aqueous environment eventually causes the water to be unfit for use unless subjected to expensive treatment. The excess salts would then still have to be discharged to the environment. The recovery and reuse of regeneration chemicals will reduce the discharge of these chemicals into the environment and will result in the recovery of resources that may have an economic benefit.

AIMS
The primary aim of this project was to investigate methods of reducing the amounts of chemicals that enter the aquatic environment due to the discharge of ion exchange resin regeneration effluents. This could be achieved by recovering and recycling of regeneration chemicals. Alternatively, the dissolved salts could be transformed into stable immobile (insoluble) compounds. A secondary aim was to reduce the volume of the regeneration effluents.

TECHNIQUES
A suitable recovery and recycling process for ion exchange regeneration effluents must include methods of :

• isolating the fraction of regeneration effluent that is to be processed,
• purifying and concentrating the chemical components that are to be recovered,
• converting the chemical components into a usable form, and
• using the recovered chemicals, either as regenerant make-up or for another application.

The form of the recovered chemicals need not be the same as that of the original regeneration chemicals, for example, sulphuric acid could be used to produce gypsum.
The following are possible methods of recovering water and chemicals from ion exchange regeneration effluents and reducing the use of regeneration chemicals :

1. Use of pressure-driven membrane processes (reverse osmosis, nanofiltration, ultrafiltration, and cross-flow microfiltration) for pretreating and purifying regeneration effluents,
2. Salt-splitting by electrodialysis/electrolysis,
3. Salt-splitting by bipolar electrodialysis,
4. Recovery of sodium hydroxide using membrane filtration and electrolysis,
5. Acid recovery by diffusion dialysis,
6. Combined ion exchange - precipitation processes,
7. Segregation and recycling of the regeneration effluent, and
8. Use of alternative regeneration chemicals.