Sustainable zero liquid discharge desalination

The main purpose of this study is to develop a sustainable zero liquid discharge desalination (SZLDD) system. A mathematical model was developed to evaluate the experimental values of the membrane water mass flux, heat transfer coefficients, the membrane/liquid interface temperatures, the temperature polarization coefficient and the evaporation efficiency. This model was solved numerically using MATLAB® software, and its results were used to predict the actual performance of the membrane unit.

Firstly, the membrane distillation coefficient was evaluated from the computer model data and was subsequently used to estimate water fluxes. Experimental tests were performed using 0.0572 m² of PTFE membrane manufactured by Membrane Solution (85% porosity, 45 µm thickness, 0.22 µm nominal pore size). Feed solutions are aqueous NaCI solutions with 1000-200,000 mg/L (0.1-20 %) in concentration and its temperature ranges at 40-80°C, and feed flow rate was 2 l/min. The temperature and flow rate of permeate water were fixed at 20°C and 3 l/min, respectively. The experimental observation showed that the vapour mass flux through the membrane pores increased with feed temperature, but decreased with feed concentration. It was found that the predicted mass fluxes agreed reasonably with the experimental data, except at a high feed concentration. The temperature polarisation coefficients increased with concentration and decreased with increasing temperature. The membrane heat transfer rates and the permeate flux have been discussed in this thesis.

Secondly, the interest of using solar powered membrane distillation systems for desalination is growing worldwide due to the membrane distillation (MD) attractive features. At later stage, this study experimentally investigated the utilization of direct contact membrane distillation (DCMD) coupled to a salinity-gradient solar pond (SGSP) for sustainable freshwater production and reduction of brine footprint on the environment. A mathematical model for heat and mass flux in the DCMD module and thermal model for SGSP were developed and coupled to evaluate the feasibility of freshwater production. The experiment results on RMIT University SGSP coupled with DCMD are presented. The feed stream of 1.3% salinity was heated up by the SGSP and circulated through DCMD module then discharged in an evaporation pond. Also, a thermal energy system was used to recover heat from the outlet brine stream of DCMD and use it as preheating for inlet feed water stream. Results were compared and showed that if the flow is laminar, the connecting DCMD module to the SGSP could induce a marked concentration and temperature polarisation phenomenon that reduces fluxes. Therefore turbulence has to be created in the feed stream to reduce polarisation. Also, to reduce the environmental footprint, the brine is recirculated after passing through the heat exchanger.

Thirdly, DCMD unit was connected directly to SGSP to achieve zero liquid discharge desalination. The system contains a hydrophobic microporous PTFE membrane module and a plastic pipe circulating all over the pond water surface to be used as cooling system. The pipe also functions as a wave suppression system where it is floating over the top of the pond water surface. The system was sourced by the hot and high concentrated saline water that is extracted from non-convective zone as a feed solution, then, the brine discharges at the lower convective zone of the solar pond. Therefore, if the saturated brine is used to produce salts, there will not be any brine left over which may lead to zero liquid discharge desalination. . The system was modelled theoretically and solved by Matlab simulation program. It was found that the system has the ability to deliver 52 l/day of fresh water for 1m2 of membrane coupled with SGSP, consuming almost 11 kW/m2 of thermal energy. Also, the transmembrane coefficient of the used PTFE membrane was proved to be 0.001 kg/ m2/Pa/ hour.

Finally, an economic assessment was conducted to evaluate the feasibility of SZLDD to be used as sole source of fresh water of a community of 100 homes at MENA region. It was found that a 300 m2 of membrane need to be coupled with a 70000 m2 of SGSP which occur a unit cost of AU$ 5.4 for 1 m3 of fresh water. This cost can be reduced to $1.98/m3 by using a new available MD system which is a significant improvement.