Combined membrane and thermal desalination processes for the treatment of ion exchange resins spent brine

The disposal of industrial wastewater effluents represents a critical environmental issue. This work focuses on the treatment of the spent brine produced by the regeneration of ion exchange resins employed for water softening. For the first time, a comprehensive techno-economic assessment and an analysis of the energy requirements of the treatment chain are carried out, via the simulation of ad hoc implemented models. The chain is composed of nanofiltration, double-stage crystallization and multi-effect distillation. The valuable product is the brine produced by the multi-effect distillation, which can be re-used for the regeneration. Therefore, the treatment chain's economic feasibility is evaluated via the Levelized Brine Cost, which includes the terms of cost and revenue of every unit in the chain. Varying the nanofiltration recovery, the treatment system always turns out to be economically competitive, since the Levelized Brine Cost is lower than the current cost of the fresh regenerant solution (8 $/m3). In particular, the lowest value of 4.9 $/m3 is found for a nanofiltration recovery of 25%. Moreover, the cost of the reactant used in the crystallization and the revenues of Mg(OH)2 and Ca(OH)2 play a prominent role in all scenarios. Regarding the energy demand, the thermal energy required by the evaporator is the main contribution and covers more than 30% of the operating costs (excluding the cost of the crystallization reactant, which is balanced by the hydroxides revenues). Therefore, the costs can be significantly reduced when waste heat is available in the industrial site. Overall, the treatment chain is economically feasible and allows reducing the industrial environmental impact by recycling waste streams and waste heat.