Abstract
A sustainable circular economy involves designing and promoting products with the least environmental impact. This research presents an experimental performance investigation of direct contact membrane distillation with feed approaching supersaturation salinity, which can be useful for the sustainable management of reverse osmosis reject water. Traditionally, reject water from the reverse osmosis systems is discharged in the sea or in the source water body. The reinjection of high salinity reject water into the sea has the potential to put the local sea environment at risk. This paper presents a design of a solar membrane distillation system that can achieve close to zero liquid discharge. The theoretical and experimental analysis on the performance of the lab scale close to zero liquid discharge system that produces supersaturated brine is studied. The lab-based experiments were conducted at boundary conditions, which were close to the real-world conditions where feed water temperatures ranged between 40 °C and 85 °C and the permeate water temperatures ranged between 5 °C and 20 °C. The feed water was supplied at salinity between 70,000 ppm to 110,000 ppm, similar to reject from reverse osmosis. The experimental results show that the maximum flux of 17.03 kg/m2·h was achieved at a feed temperature of 80 °C, a feed salinity of 10,000 ppm, a permeate temperature of 5 °C and at constant feed and a permeate flow rate of 4 L/min. Whereas for the same conditions, the theoretical mass flux was 18.23 kg/m2·h. Crystal formation was observed in the feed tank as the feed water volume reduced and the salinity increased, reaching close to 308,000 ppm TDS. At this condition, the mass flux approached close to zero due to crystallisation on the membrane surface. This study provides advice on the practical limitations for the use of membrane distillation to achieve close to zero liquid discharge.
Highlights
There remain major concerns over the environmental impacts of reverse osmosis reject water
The heat transfer in the feed boundary layer (Qf ), which is the convection heat due to In DCMD configuration, viscous flow through the membrane is neglected as the feed the temperature difference from the bulk feed temperature and the feedwith temperature at bf and permeate streams interact with the membrane under atmospheric pressure, the the surface of the membrane
In DCMD configuration, viscous flow through the membrane is neglected as the feed and permeate streams interact with the membrane under atmospheric pressure, with the total pressure kept constant at about 1 atm [29]
Summary
There remain major concerns over the environmental impacts of reverse osmosis reject water. These methods require a high and considerable amount of energy to process [5]. Membrane distillation (MD) process can provide nearly 100% salt and non-volatile. Membrane distillation (MD) process can provide nearly 100% salt and non-volatile solute rejections; besides, it can operate at high salinities near saturation. MD can be considered as the optimum solution for both the water and the management of brine from conventional desalination [14,15,16]. Water vapor from the feed side travels travels through through the the pores pores in inthe thehydrophobic hydrophobic membrane membrane due due to tothe thesaturation saturationpressure pressure difference difference across generated by by the the temperature temperaturegradient.
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