High recovery rate solar driven reverse osmosis and membrane distillation plants for brackish groundwater desalination in Egypt

Abstract

This thesis explores the feasibility of extracting and desalinating brackish groundwater in Egypt using solar driven high recovery rate reverse osmosis and membrane distillation desalination plants to help establishing decentralised agricultural communities.

Groundwater properties and potential for sustainable development from seven main hydro-geological systems in Egypt were investigated. It was found that approximately 55% of Egypt’s area has access to brackish groundwater, 47% of which has access to aquifers with moderate to high potential for development.

The feasibility of high recovery rate photovoltaic driven reverse osmosis desalination plants was investigated. Using commercial simulation tools, it was found that the plant can operate at recovery rates of 75 to 90% with unit water costs of 0.7 to 1.65 USD/m3 with the typical brackish groundwater composition and depths found in Egypt. Moreover, it was shown that such plants are cost competitive with similar plants driven by diesel generators if the subsidies on diesel are removed.

The feasibility of replacing standard photovoltaic modules with photovoltaic/thermal collectors to reduce the energy consumption of the reverse osmosis plant by heating the water was explored. The annual performance of the photovoltaic/thermal collectors was analysed using TRNSYS. It was concluded that for such application, photovoltaic/thermal collectors have no economic advantage.

The feasibility of using hybrid reverse osmosis/membrane distillation plants to increase the recovery rate was also investigated. A mathematical model was built using MATLAB to simulate the performance of a commercial full scale spiral wound permeate gap membrane distillation module. The model gave good agreement with experimental results available in the literature. A TRNSYS model was built to analyse the annual performance of the solar driven membrane distillation plant. It was found that the evaporation losses from the cooling tower greatly limited the recovery rate where no more than 10% enhancement was feasible. Such small enhancement in the recovery rate resulted in a 1.9 to 3.6 fold increase in the unit water costs. It was concluded that higher recovery rates are possible with high recovery rate membrane distillation modules with low cooling requirements; and that solar driven hybrid plants can be economically feasible if: a source of waste heat from a renewable energy source is available to drive the membrane distillation process; the specific heat consumption of the process is reduced by 4 folds; and the module costs drop by at least 3.5 folds.