Operation, control and assessment of a full-scale membrane distillation unit for treating desalination brine in the context of greenhouse production

Membrane distillation (MD) is a thermal desalination technology with a niche application in the concentration of brines. This paper aims to assess the operation, control, and performance of the largest full-scale vacuum-assisted air-gap membrane distillation (V-AGMD) system reported to date in the literature, using biomass heat as thermal source and real brine from a reverse osmosis unit with salinity 63.7 g/L^-1 as feed. The levelized cost of water of a large-scale V-AGMD plant has been estimated using experimental results. The MD unit comprises up to ten multi-envelope air-gap modules, and is installed in the facilities of the AgroConnect project at IFAPA, an agricultural research centre close to the University of Almería (Spain). Results show that cooling limitations in the V-AGMD operation cancel out the beneficial effect of vacuum assistance on vapor diffusion, especially when the feed temperature and the feed flow rate are increased, hindering the condensation, and hence reducing the permeate productivity even down to 40% in comparison with conventional AGMD operation. This work demonstrates that full-scale V-AGMD can take part in a future optimized sustainable water network to supply water for intensive agricultural activity in the south-east of Spain, although a sufficient cooling supply is mandatory to avoid operational issues.