Concentrate circularity: A comparative techno-economic analysis of membrane distillation and conventional inland concentrate management technologies

Abstract

Inland desalination with reverse osmosis (RO) offers a promising alternative to increase potable water resources. However, large volumes of concentrated brine are generated as a byproduct of the process. Concentrate disposal for inland regions displaces valuable water resources and can make up to 33 % of the total cost of desalination. Conventional disposal systems are limited by location, hydrogeology, climate, and policies, and few place value on resource recovery, thus limiting the implementation of desalination facilities in water-stressed regions. Membrane distillation (MD) is an alternative technology that can minimize disposal volume while maximizing water recovery for beneficial reuse. MD uses thermal energy gradients to desalinate the concentrate stream achieving near zero-liquid discharge. Furthermore, several MD configurations include heat recovery, making MD an energy-efficient alternative. A techno-economic assessment (TEA) of air-gap MD (AGMD) for RO concentrate management was performed and compared to three conventional concentrate management systems: evaporation ponds, deep-well injection (DWI), and concentration-crystallization. TEA results indicate that compared to DWI at 1.09 $/m3, evaporation ponds at 1.47 $/m3, and concentrators at 6.2 $/m3, respectively, AGMD is a competitive concentrate management technology producing water at 0.90 $/m3 when operating conditions are optimized and low-grade heat is available. When operating at high salinity (>70 g/L) the selection of operating conditions, specifically module length and circulating flowrate, is critical. Results of this study support the economic viability of AGMD in contrast to current industry standards and highlight the importance of resource recovery to promote a circular water-energy economy for regions relying on reuse and desalination.