An integrated approach to enhance the desalination process: coupling reverse osmosis process with microbial desalination cells in the UAE

The growing need for better sources of fresh water has led to water desalination become a dominant technology in the water industry, especially in arid countries like the UAE. Across the globe, Reverse Osmosis (RO) has become the key method used to desalinate seawater. Due to the high energy requirements of RO desalination, the need to reduce the energy load has become a pertinent area of research. Microbial Desalination Cells (MDCs) are an emergent technology that show great promise being integrated into the RO desalination process. Studies have shown that a signi ﬁ cant portion of the energy utilized in RO desalination could be eliminated by using MDCs as a pretreatment process. In this study, the integration of various MDC types into the pre-treatment process for Reverse Osmosis Desalination were compared and explored. Existing MDC integration setups were brie ﬂ y explained. Research was split into possible con ﬁ gurations for the integration. This includes optimization of key parameters such as anodic inoculum, feed inlet ratios and accompanying pretreatment processes. The limitations and challenges faced in the integration were investigated and the required future studies aligned with subject was deliberated.


INTRODUCTION
Fresh water scarcity is becoming a global issue due to the available fresh water in the world being rapidly depleted.
Fresh water makes up only 2.5% of the total water resources in the world, and less than 1% of it is accessible. This has led a large focus on alternative methods of obtaining fresh water, such as desalination of seawater or wastewater treatment, across the world (Qasim et al. ). In dry and arid countries such as the UAE and other GCC countries, desalination is the most-utilized method of obtaining drinking water with over 80% of drinking water generated exclusively from the desalination process. The demand for expansion of desalination facilities has steadily been increasing worldwide, with a 7-8% annual growth rate in the UAE. The However, the MDCs would place a greater emphasis on electrodialysis compared to the MFCs. This provides a method for diluting saline water or treating wastewater by removing organic matter. MDCs have been shown to possess tremendous potential incorporated into the desalination processes, either as a pre-treatment to the existing processes or the standalone units if scaled up (Saeed et al. ).
Further details about the history of the MDCs and their configurations, the integrating of different technologies with MDCs, and a summary of RO desalination are discussed in the next sections. In addition, a thorough study was conducted on the proposed MDC-RO integration setup, the possible conditions and configurations of MDCs that could be used in the RO integration. The limitations and opportunities for the future research were also investigated.

MICROBIAL DESALINATION CELLS
MDCs are a variant of MFCs that are constructed in a similar fashion to the MFCs. They rely on the electric potential generated by the exoelectrogenic bacteria in the anodic chamber in order to facilitate ion-transport through ionmembranes (Das ). MDCs typically consist of three chambers; an inoculated anodic chamber, a desalination chamber, and then a cathodic chamber. An Anion Exchange Membrane (AEM) separates the anodic chamber from the middle desalination chamber, and a Cathode Exchange Membrane (CEM) separates the middle chamber from the cathodic chamber. An external wire connects the anodic and cathodic chambers. Saline water is fed into the middle desalination chamber. Organic material is fed into the anodic chamber in the form of wastewater. This is then utilized by the bacteria as a substrate, resulting in the biofilm formation on the anode electrode. Electrons are released in this process, which then travel towards the cathode through the external wire. This leads to the development of an electric potential inside the MDC.
Due to this electric potential difference, anions and cations in the desalination chamber move towards the anode through the AEM, and towards the cathode through the CEM, respectively. This results in the water feed being desalinated in the desalination chamber (Saeed et al.

; Das ).
In order to optimize the application of the MDCs, its compartments and configurations have gone through several modifications over the last two decades. The first configuration was the air-cathode MDC, which used oxygen from air as the terminal electron acceptor. This was an effective configuration due to the high reduction capacity of oxygen and the high affordability and obtainability of air. However, it had a lot of shortcomings that required the expensive modifications, such as the use of platinum catalysts. It also required a large amount of maintenance

DEVELOPMENTS IN INTEGRATION OF MDC TECHNOLOGY WITH OTHER PROCESSES
Over the past decade, research has been conducted into integrating MDC technology with different processes to enhance their efficiencies. The attempts at integration have been collated in Table 1.
All the research shows that an integration of the two technologies would allow for a massive overall reduction in energy consumption, as well as an eco-friendlier method to obtain potable water.
The goal of this study was to find the optimal configuration of MDC that would integrate easily into the RO desalination process. This optimal configuration should provide high water-recovery and have minimal (or negligible) drawbacks. This study will do a comparative review of the integration of different MDC configurations into pretreatment for RO desalination. In addition, explicit attention will be given to three forms of MDCs; Stacked MDCs, Osmotic MDCs and Upflow MDCs.

MDC CONFIGURATION COMPARISON
Since the MDC is the core aspect of the proposed setup, a comparison needs to be done into the different MDC  Figure 1.

Osmotic MDCs (oMDCs)
The concept of the oMDC was put forth by Zhang (2011).
The proposed setup involves the traditional AEM in the MDC replaced with an FO membrane. This configuration would significantly improve the desalination capability, but with a reduction in the electricity-producing capability of the MDC. It also allows for higher saltwater dilution and better removal of organic matter from the wastewater streams. A schematic diagram of an oMDC is shown in A very high desalination capacity was achieved with this configuration, and was theorized to be a good addition into the pretreatment section of RO, and if scaled up enough, could possibly work as a replacement for it.

Upflow stacked MDCs (USMDCs)
The concept of USMDCs was first put forth in (Wang et  The schematic diagram of this is shown in Figure 4. The common RO configurations include a pretreatment stage followed by the main RO unit. The main configuration is a vessel with two chambers separated by a semipermeable membrane with a pressure device (usually a     Along with the MDC unit, pretreatment processes such as flocculation and microfiltration can be included in the setup. These can occur before or after the MDC process, as needed, to ensure that the feed stream is ready for the RO block. After the completion of the pretreatment process, water is fed into the traditional RO chamber where the remaining salinity is removed. This configuration allows for desalination while consuming much less energy than of a standalone RO system as the MDC unit consumes no energy and reduces the number of RO passes. After this step, the desalinated water can be disinfected and neutralized to ensure that it is safe for further use.

ALTERNATIVE CASE STUDY
An ideal subject for the MDC integration would be the

PROPOSED SETUP CONFIGURATIONS
For such a potential integration process, there are several key parameters to consider, including the inoculum used in the anodic chamber, the ratio of wastewater to seawater introduced to the MDC, and the pretreatment processes that could accompany the MDC.

Inoculum
The

Comparative look at different MDC setups
Due to the proposed setup being accommodable for different types of MDCs, it is important that a comparative numerical analysis of the different types of MDCs is done. where the salinity level was a preset value. The results from controlled salinity were then extrapolated to match the salinity of seawater and, hence the efficiency of seawater desalination was approximated. In this UMDC setup, actual seawater was used as an influent, and the results showed a 20% decrease in desalination rate as compared to the synthetic seawater. It was concluded that if the UMDCs were integrated into the pre-treatment process for the RO desalination, 30% of the initial total dissolved solids (TDS) could be removed. This integration resulted in a reduction of 2.9 kWh/m 3 in consumed energy in RO and saved 22% of energy compared to the solo RO desalination system. A downside of the UMDCs application is that the desalination rate tends to be inversely proportional to the power output. At high-current operations, high desalination levels are attainable, with a trade-off being a drastic drop in power output. However, when the power output is not critical and the focus is on the desalination process, the operation at the maximum current configuration is not disputable.

The use of USMDCs
Wang et al. () proposed a novel integrated USMDC setup consisting of a fusion of SMDCs and UMDCs. The study found that the integrated USMDC would have an average desalination rate of almost 25% greater than that of the SMDC or UMDC setup. The desalination rate in this study remained at a stable average of 91.6% for 120 days. It also had twice the CTE of the UMDC, with barely any change in the pH of the system. Currently, due to the benefits provided by this combination, it would be a pertinent technology for further future researches. The downside of such setup would be a relatively expensive to construct, and due to the novelty of the idea, there is insufficient data to confirm whether this is the optimal configuration to be utilized in the RO pre-treatment integration.

Overall MDC choice
The Comparative data of all setups with regards to the different properties has been tabulated in Table 3.

LIMITATIONS
The limitations of the integrated setups are the same problems generally faced when using the MDCs.

Biofouling of the membranes
The membrane is undeniably one of the most crucial aspects This reduced pH can lead to the death of the inoculum present in the chamber, since many inoculums require very specific pH conditions. It also increases the ohmic resistance, leading to lower current generation (Noori et al. ).
Biofouling on the cathodic side is generally a less- Biofouling is altered by multiple factors such as temperature, tidal forces, humidity and sunlight which essentially affect the growth of microorganisms in seawater (Maddah & Chogle ).
In general, FO membranes tend to foul much more easily than the IEMs. This leads to greater maintenance requirements on the oMDCs when compared to three-chambered MDCs and SMDCs. The benefit of the oMDC is that that FO membranes are more cost effective than the IEMs.
Biofouling also requires extensive and frequent cleaning operations. These cleaning processes also heavily impact on water production since the desalination process needs to be ceased in order to conduct the cleaning process. This is a into the FO membranes as they were found to improve output water quality and increase membrane selectivity.
One benefit of using the oMDCs is that the FO membranes are generally far more economically viable as compared to AEMs. This greatly reduces the maintenance expenses. (Koók et al. ) noted that the biofouling of membranes was an eventuality that cannot be avoided. Hence the best option would be to conduct further research in membrane technology to create longer-lasting membranes.
The operating time for the MDC would vary depending on the situation. It was generally observed that increasing the operating time led to an increase in biofouling rate (Miskan et al. ). Hence it would also be pertinent to find a method to increase the lifetime of the membrane without changing the membrane itself. An example would perhaps be by using different ratios of liquids in the chambers.
Mitigation methods for biofouling of cathodes include regenerating and refabricating cathodes that have fouled,

OPPORTUNITIES AND FURTHER RESEARCH
There are many areas of research that could significantly benefit the integration of MDCs into RO desalination.
The first major area of research would be membrane technology. Since biofouling is a major concern in MDCs, development of membranes that are more resistant to the fouling would be very useful. This extends to both the FO and IE membranes. Membranes with higher biofouling resistance would not need to be replaced as frequently and would require less maintenance time. A pertinent area of research would be better methods to control the pH inside an MDC. As it stands, there aren't many methods that exist that consistently help control pH.
It would be extremely beneficial to design a control system that can accurately measure and control pH. A possible solution could be a system that injects trace amounts of acid or base, as required, in order to keep the pH stable. The results revealed the possibility of using activated carbon from biomass waste (such as coconut shells) as electrode material.

CONCLUSIONS
With the advent of a large freshwater scarcity across the world, desalinated water is quickly becoming one of the major ways to obtain clean water for domestic use.
The most commonly used process to desalinate water is the RO process. This method is hampered by extremely large energy requirements. MDCs are a technology that show immense potential to reduce energy and economic loads on the RO desalination process. MDCs have the potential to be integrated into the RO pre-treatment process due to their high-water recovery and dilution capacity.
Countries like the United Arab Emirates, which depend heavily on desalinated water for domestic use, could benefit heavily from integrating MDCs into the RO desalination process. There still needs to be a significant amount of research done in order to combat the many drawbacks and limitations involved in the MDC integration, such as the biofouling of the membranes and the scaling up of the integration. Research should also be done in order to look at ways to best optimize the integration, such as by improving the efficiency of the MDCs, or better integrating other pretreatment processes into the process. However, once resolved, MDCs could open the path for a significantly more sustainable process to obtain potable water, both in terms of energy and resources utilized.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.