Towards the implementation of circular economy in the water softening industry: A technical, economic and environmental analysis

To reduce the environmental impact of the industrial sectors, circular strategies should be implemented to purify the effluents and recover raw materials. In this context, a novel integrated methodological approach is proposed to identify the most suitable strategy to improve the sustainability of the water softening industry via the treatment and recycling of the produced wastewater. Different concentration technologies and energy supply systems are compared to find economically feasible and environmentally friendly treatment systems. The investigated chains present the same pre-treatment step (nanofiltration and crystallization) and different concentration technologies: Multi-Effect Distillation (MED), Membrane Distillation (MD) and the coupling of Reverse Osmosis and Membrane Distillation (RO-MD). In the case of electricity supplied by the grid, the MED and the RO-MD chain are economically competitive with the state of the art (Levelized Brine Cost (LBC) between 4 and 6$/m3, lower than the regenerant solution cost, equal to 8$/m3). Moreover, the specific CO2 emissions due to the energy required by the treatment processes (10.8 kgCO2/m3regenerant for the MED chain and 16.7kgCO2/m3regenerant for the RO-MD chain) are lower than those produced by the current system (19.7kgCO2/m3regenerant). Varying the feed flow rate, the MED-chain is more feasible at larger plant sizes for its lower energy demand, while the chain including RO-MD shows lower costs at smaller plant sizes for its lower investment costs. When a photovoltaic-battery system is coupled, both the MED-chain and RO-MD-chain show a CO2 emission reduction of more than 75% with respect to the state of the art. Furthermore, their LBC values are very competitive, especially if the plant is located in a region with high solar potential.