Hybrid Seawater Desalination Technology Based on Reverse Osmosis and Membrane Distillation Methods

The article presents the results of a computational and analytical study of hybrid reverse osmosis–membrane distillation (RO–MD) technologies for desalination of Caspian Sea water, suggesting the production of an additional amount of desalinated water by the MD method from RO concentrates heated to 50–80°C by the waste heat of boiler-fuel combustion products. Two options for solving the problem of the CaCO₃ and CaSO₄ scaling of membranes have been studied: with preliminary nanofiltration (NF) or sodium cationization (Na) of seawater, as an alternative to the use of an antiscalant (AS) or acid. The negative environmental effect of most plants (eutrophication of water bodies) and their low efficiency at high concentrations of desalinated water are taken into account. The Langelier saturation index (СаСО₃) and the degree of concentrate saturation (СаSO₄) were used as criteria for membrane scaling. The NF and RO processes were studied using the computer program ROSA, and the MD and Na processes were studied by computer simulation of the corresponding design models. It has been found that at a 70% permeate yield at the NF and RO stages, the possibility of calcium scale deposition on the RO and MD membranes is excluded, but it can occur on NF membranes, thereby requiring the use of antiscalants. At the same time, the additional production of permeate at the MD stage from RO concentrates reaches 40% of the amount of permeate at the RO stage and the total power consumption according to the scheme is 1.88 kWh/m³. Reducing the calcium hardness of sea water to 50 µeq/dm³ by Na cationization makes it possible to refuse both the use of AS and sulfuric acid acidification with additional production of MD permeate of 27% relative to the RO permeate. The power consumption rises to 2.5 kW h/m³. To employ the known advantages of NF without the use of AS, a hybrid Na–NF–RO–MD scheme is proposed. It has been established that at 80% yields of NF and RO permeates, it is sufficient to reduce the hardness of sea water from 16 to 5.5 meq/dm³ to prevent CaSO₄ scaling at all stages of treatment and to exclude CaCO₃ scaling by acidifying the softened water.