5. Achieving the Zero-Liquid-Discharge Target Using the Integrated Membrane System for Seawater Desalination

Membrane desalination technology has emerged in recent years as the most viable solution to water shortage. However, despite the enormous improvement in membrane desalination technology, some critical developments are still necessary in order to accomplish possible improvements in the process efficiency (to increase recovery), operational stability (to reduce fouling and scaling problems), environmental impact (to reduce brine disposal), water quality (to remove harmful substances) and costs. In particular, cost- effective and environmentally-sensitive concentrate management is today recognized as a significant obstacle to extensive implementation of desalination technologies. As a result of the significant impact of desalination plants on the environment, the requirements for concentrate management are brine disposal minimization and zero liquid discharge (ZLD), both being the demanding targets for several applications. Conventional pressure-driven membranes such as MF, NF and RO were integrated with the innovative units of membrane contactors such as Membrane Distillation/Crystallization (MD/MC). The integration of different membrane units represents an interesting way for achieving the ZLD goal due to the possibility of overcoming the limits of the single units, thus improving the performance of the overall operation.

The present research study focuses on the evaluation of the integrated membrane system which merges membrane contactor technology with conventional pressure-driven membrane operations for seawater desalination. Sensitivity studies were performed for several configurations of the integrated system to obtain the most sensitive parameter in the total water cost and optimal design of the system.

The results revealed that the pressure-driven membrane operations were very sensitive to the feed concentration and the cost of electricity consumption. On the other hand, MD processes were not sensitive to the variation of the feed concentration or the electricity costs. The most sensitive parameter in the total water cost of the MD plant was the cost of steam which contributed to values as high as high as 11.4 % in case of MD without a heat recovery system. The best tolerance to the variation of these parameters was obtained when using the integrated membrane system of pressure-driven membranes and MC processes.