The application of osmotically assisted reverse osmosis (OARO) processes for low-cost and sustainable zero-liquid discharge (ZLD) desalination
Keywords: Desalination
Membrane-based desalination
Zero-liquid discharge
Osmotically assisted reverse osmosis
Brine volume minimization

Freshwater scarcity is a globally increasing threat that is aggravated by population growth, economic development, the concomitant increase in consumption, and global climate change. Potable water is mostly extracted from natural fresh water sources, such as rivers, lakes, and aquifers, as these facilitate the cheapest and least energy intensive means of extraction. However, unsustainable extraction rates, which exceed the natural refill rate, inevitably lead to their depletion. Global warming further amplifies freshwater scarcity in arid regions, as these regions face higher evaporation rates and a growing reduction in the annual rainfall.
Desalination may potentially offer a long-term solution to our freshwater shortages, but it is expensive in terms of the resources required for its operation and this factor limits its accessibility, especially in low- and medium-income countries. Another concern is the environmental impact of desalination due to its high energy demand and the discharge of untreated brine, which may contain a wide range of pollutants. To combat both of these issues, the principal focus of this thesis is therefore on developing energy efficient brine volume minimisation (BVM) and zero-liquid discharge (ZLD) desalination processes.
Osmotically assisted reverse osmosis (OARO) is chosen as the sole focus of this study for its potential as a high-recovery, low-cost, energy efficient, and sustainable BVM and ZLD technology. The ability of OARO to draw water from concentrated brine (> 75 g/L) can be attributed to the combined effect of reducing the transmembrane osmotic pressure difference via the use of a draw solution and operating at high hydraulic feed pressures.
To first of all determine the technology’s potential for BVM, OARO has been incorporated into a standard ultrafiltration and reverse osmosis desalination plant to increase the process recovery. In total, six different OARO integrated flow processes, of which two are novel, are modelled numerically to determine their technical and economic feasibility in maximising the process recovery. At lower feed salinities, split feed OARO (SF-OARO) is the optimal OARO integrated flow process. Recoveries of up to 72% from a 35 g/L saline feed are possible when operating at the membrane burst pressure of 48.3 bar. Furthermore, the energy consumption of SF-OARO is approximately 4.00 kWh/m3, which is significantly lower than that in currently employed high recovery thermal processes, such as mechanical vapour compression (7-25 kWh/m^3). C-2, a cascading OARO (C-OARO) process, is a proposed novel OARO integrated flow process and is the most attractive at a higher feed salinity of 70 g/L. The maximum recovery of 44% is achieved at an average energy consumption of 6.37 kWh/m3. The presented numerical results demonstrate that the OARO integrated processes can competitively concentrate brine streams to concentrations of up to 125 g/L, whereas the maximum achievable brine salinity of a conventional single-stage RO process is generally limited to 75 g/L or less.
In order to improve the recovery of the standard OARO process, which utilises a non-responsive draw solute (i.e., NaCl), the incorporation of a thermo-responsive draw solute is investigated to see whether this novel combination can outperform 1) a forward osmosis (FO) process utilising the same draw solution, 2) the standard OARO process with non-responsive draw solutes, and 3) other high-recovery membrane processes. The findings from this parametric and technoeconomic analysis indicate that OARO mitigates several shortcomings of FO when utilising the same thermo-responsive draw solute, making it more energy efficient (energy savings of ≈10.8%), less susceptible to internal concentration polarisation, and more economical (cost savings of ≈15.5%). Furthermore, a key advantage of OARO is that it operates with lower draw solute concentrations, allowing for the draw solute to be regenerated at temperatures 12 °C lower than with FO operation. This means that low-grade waste heat (LGWH) is more easily utilised as an energy source. In comparison to other hydraulic pressure-driven ZLD technologies, such as low-salt-rejection RO (LSRRO), the electrical energy consumption of OARO can be significantly offset when powered by LGWH and using a thermo-responsive draw solute. The specific electrical energy consumption can be as low as 3.47 kWh/m3 when concentrating a saline stream from 0.6 M to 4 M using 4 membrane stages. At the same time, a 4-stage LSRRO system would consume approximately 48% more electricity to achieve the same concentration factor. Furthermore, OARO processes operating with a thermo-responsive draw solution can outperform those using non-responsive draw solutes. Brine concentrations of up to 245 g/kg can be achieved using technically feasible operating pressures and draw solute concentrations, while the number of required membrane stages is almost halved from 6 to 4.
The previously presented numerical results highlight the potential of OARO for low-cost and energy-efficient brine management. However, its performance can be significantly limited by membrane fouling. Hence, a comprehensive experimental study on OARO membrane fouling is also performed to explore the associated fouling mechanisms, and to evaluate the fouling reversibility in OARO via simple physical cleaning strategies. Firstly, the experimental results show that internal membrane fouling at the draw (permeate) side is insignificant. Flux behaviour in short-term operation is correlated to both the evolution of fouling and the change of internal concentration polarization. In long-term operation, membrane fouling constrains the OARO water flux to a singular, common upper limit, in terms of limiting flux, which is demonstrated to be independent of operating pressures and membrane properties. Generally, once the limiting flux is exceeded, the OARO process performance cannot be improved by higher pressure operation or by utilising more permeable and selective membranes. Instead, different cyclic cleaning strategies are shown to be more promising alternatives for improving performance. While both surface flushing and osmotic backwashing (OB) are found to be highly effective when using pure water, a full flux recovery cannot be achieved when a non-pure solution is used during OB; this is due to severe internal clogging during OB.
The presented numerical and experimental findings offer valuable insights into the technical and economic feasibility of OARO as a brine management technology during desalination. The presented findings also generate significant implications for OARO operation and fouling control.