Implementation of fungal-based desalination through capacitive deionization for urban water provision: a conceptual framework

The increasing demand for clean water in urban areas calls for innovative and sustainable water treatment solutions. Capacitive deionization (CDI), using fungal-based materials for desalination, offers potential benefits such as sustainability, low cost, and scalability, for urban water provision. However, few studies have explored the practical application of fungal-based CDI technology. This research assesses the feasibility of implementing fungal-based CDI technology in urban water provision systems, drawing on the key study from Chen et al.’s 2022 research, examining the preparation and performance of fungal-based CDI electrodes derived from Aspergillus niger. To create a reliable and up-to-date conceptual model, additional literature from indexed journals, focusing on CDI in desalination facilities from the past decade, is also reviewed. A conceptual framework was developed to demonstrate the potential integration of fungal-based CDI technology into urban water treatment systems, and taking into account factors such as capital and operational costs, scalability, and sustainability. The outcome of this study is a conceptual model that promotes further development of urban water provision through desalination, broadening the perspective on the application of emerging biotechnology, using fungal-based materials for water provision.


Introduction
Escalating freshwater demands in coastal regions necessitate efficient, cost-effective desalination technologies.Conventional methods like reverse osmosis [1] and thermal distillation [2] are not only energy-intensive and expensive but also less effective for low salt concentrations [3].Therefore, alternative solutions, such as using natural materials like fungi, have been explored [4][5][6][7][8].Fungi, specifically genetically engineered variants [9], promise a wealth of potential benefits, including high porosity, large surface area, and electrical conductivity, making them ideal for the electrosorption process [5,10].
Capacitive Deionization (CDI), a desalination technique that employs electric fields to remove salts [11,12], is another such method gaining significant attention due to its energy efficiency [13].CDI leverages electrodes with a large surface area to extract ions from water [14].Notably, Chen et al.'s 2022 research demonstrated the successful use of Aspergillus niger in CDI for water desalination [5,10].However, Chen et al.'s lab-scale study leaves a significant research gap for full-scale desalination plants.This paper aims to bridge this gap, advancing towards providing sustainable urban 1263 (2023) 012056 IOP Publishing doi:10.1088/1755-1315/1263/1/012056 2 water scarcity solutions.We introduce a conceptual framework for the application of fungal-based CDI in urban environments encompassing aspects like fungal cultivation, material processing, and system design, together with economic and environmental viability.By presenting this framework, we aspire to provide future researchers and stakeholders with a foundation to translate fungal CDI into practical use thus fostering sustainable approaches to urban water security.

Methods
Our study builds upon Chen et al.'s 2022 research on Aspergillus niger-derived electrodes for Capacitive Deionization (CDI) in desalination treatment, which will be divided into its preparation and performance of the electrode.The research is also supplemented with recent quality-assured literature focusing on desalination facilities employing CDI technology, with a restriction to works published in the past 10 years in indexed journals.This approach assures the currency and quality of information, enabling us to provide an up-to-date, reliable conceptual framework for practical application of fungalbased CDI in urban water treatment.

Capacitive deionization (CDI) for desalination
Capacitive deionization (CDI) is a method for desalinating water, where ions are removed from salt water through electrosorption to electrodes with opposite charges [11,12].The process uses electrodes, typically made from porous carbon-based materials, to store the ions in the form of electrical double layers (EDLs) in the pores of the electrodes [5].The ions are then desorbed, or removed when the electrode potential is reversed.This desorption process also simultaneously regenerates the electrodes for the next cycle of CDI [11][12], [14].
According to Porada et al., in order to obtain high efficiency in CDI, the electrodes must have a wide ion-accessible surface area for ion electrosorption, short routes for ion diffusion into the electrode pores [10], [15][16], and high electrical conductivity [3].This necessitates a porous carbon structure that is freestanding and devoid of binders and conductivity boosters.For CDI applications, a hierarchical porous structure with a mix of micropores and mesopores is appropriate because it enables effective ion transport and adsorption [5].Therefore, due its porous structure, fungal-based materials, as Chen et al. 2022 described, could provide a promising alternative to be used for the optimization of CDI.

Preparation of fungal-hypha activated carbon pad (FhACPad) electrode
Drawing on the methodology developed by Chen et al.'s 2022 research, the production of a Fungalhypha Activated Carbon Pad (FhACPad) electrode began with the fermentation of A. niger in a nutrient substrate sourced from autoclaved food waste (13.0 wt% solid content).Inoculated into a sterile food waste hydrolysate, the Aspergillus niger was cultured for 5 days at 28 °C in an air-bath shaker.Upon completion, the hyphal fibers were collected, washed, and formed into a sheet (~500 μm thickness) which was freeze-dried and cut into the desired dimensions.
The process of transforming the resultant hypha sheet into a FhACPad involved carbonization under an argon atmosphere, reaching a final temperature of 800 °C over a period of 2 hours.The electrode, which resulted in an 83.2% weight loss, was then treated with a 3.6 M KOH solution at 70 °C for 12 hours and subjected to a second heating process, again reaching 800 °C.This process only resulted in a minor 7.0% weight loss and allowed the maintenance of the electrode's dimensions, creating a stable, freestanding FhACPad ready for use.The fungal hypha sheet, originally 141.6 g/m², loses 83.2% weight upon carbonization, resulting in a 23.8 g/m² carbonized sheet (FhCPad), and a further 7.0% loss after activation, giving an experimental FhACPad mass of 0.00111 g for a 1 cm x 5 cm piece.Notably, this production process required no catalysts or adhesives.
For the purposes of comparison, conventional carbon electrodes were also fabricated using powdered activated carbon (PAC) combined with a glutaraldehyde-crosslinked polyvinyl alcohol binder.PAC electrodes were fabricated by affixing a 90 wt% powdered activated carbon and 10 wt% glutaraldehyde-crosslinked polyvinyl alcohol binder mixture to a graphite sheet.The binder was made by combining glutaraldehyde and polyvinyl alcohol at a 4.4 mol% ratio and incorporating activated carbon powder and deionized water for a final solid content of 30 wt%.This mixture was magnetically stirred overnight, cast onto a graphite sheet to a 100 μm thickness, air-dried, and then cross-linked in a vacuum oven at 80 °C for 12 hours.
For the scope of this study, it may be necessary to modify certain parameters such as the cultivation yield mass, the total volume of chemical solution, and the duration of production.Consideration should also be given to the cultivation facilities and their conversion into CDI, as scaling up production would necessitate the expansion of equipment to replicate the results at a full-scale plant level.

Performance, energy consumption, and cost analysis of FhACPad
The desalination performance of the Fungal-hypha Activated Carbon Pad (FhACPad) electrode, as investigated by Chen et al. 2022, demonstrated compelling results.The CDI experiments involving the FhACPad were conducted in a continuous flow mode, maintaining a flow rate of 1.0 mL/min and a cell voltage of 1.2 V during the adsorption period and -1.2 V during the desorption period.Utilizing a 10 mM NaCl feed solution, the FhACPad presented a significant desalination capacity at 35.6 ± 2.3 mg/g after 30 minutes of capacitive deionization (CDI), considerably higher than other reported carbon-based CDI electrodes.This was complemented by its efficient electrosorption of ions during CDI and lower energy consumption (124.8 kJ/mol) relative to traditional PAC electrodes (179.6 kJ/mol).The FhACPad also displayed the capacity to effectively remove common anions in saltwater, such as Cl -, NO3 -, and SO4 2-.Interestingly, in a multi-solute system, the FhACPad demonstrated increased removal of SO4 2-due to its higher valence that facilitated faster transport across the ion exchange membrane.
Regarding long-term stability, the FhACPad maintained consistent desalination capacities (~32.0 mg/g) throughout 40 cycles of multi-solute solution treatment, with less than a 10% variation in total salt removal.The decrease in CDI current efficiency over time (78.4% at the 2nd cycle to 73.1% at the 40th cycle, with efficiency average of 76.4%) was likely due to fouling and deterioration of the ion exchange membrane.Still, the rate of decrease slowed over the cycles suggesting a trend towards stabilization.The FhACPad's efficient desalination performance and high desalination capacity can be attributed to its three-dimensional carbon network, high conductivity, and low ion diffusion resistance.Notably, these properties, along with its low energy consumption and superior long-term stability, potentially make the FhACPad a cost-effective alternative to conventional electrodes for desalination applications.

Results and Discussions
The conceptual framework in this study will be divided into several parts: input, system design, supporting facility and output.This framework function as a schematic for the future works that require fungal-based CDI construction for desalination, that will be described each of its parts for its significance.

Figure 2. General framework for fungal-based CDI desalination
The framework describes the general idea of fungal-based CDI desalination.The input is divided into two parts: fungal material cultivation for CDI and feed-water of desalination that suits CDI treatment.The system design depicts two essential components: performance of fungal-CDI and operation and maintenance (O&M).While the supporting facility design is composed by infrastructure which could serve the fee-water intake system for the input.Infrastructure would also provide the system design where CDI desalination takes place.As for the output, the total treated water produced is the expected value that represents the overall CDI system for desalination, accompanied by the brine disposal that will be supported by the infrastructure, to be managed as waste.
Every part of the framework above are integral to each other, e.g.where the increase or decrease in one area might determine how the other part will perform.Therefore, understanding thoroughly every aspect of the conceptual framework described will be essential to optimize the desalination treatment.The explanation of each is as follows:

Fungal cultivation and conversion to CDI
The preparation of FhACPad communicated in the literature review earlier, is essential to ensure the quality of the fungal-based CDI to be exactly as Chen et al. 2022 conducted.However, in order to scale up the treatment, adjustment of the cultivation and its CDI conversion properties must be calculated, and this could only be achieved by understanding the characteristics of each.
Fungal materials, while cost-effective for CDI, can be susceptible to environmental stressors, such as high temperature and salinity, which along with pH changes can reduce efficiency and stability over time [17][18].Their cultivation, processing, and characterization, face challenges due to the lack of standard protocols, varying growth rates, morphology, contamination risks, and concerns over quality control [8,19].The scalability and automation of fungal material production, as well as their regulatory and social acceptance, present additional obstacles [19][20][21].Overcoming these limitations may involve robust genetic engineering, surface modifications, or use of naturally resistant fungal strains, in concert with optimization of DI operating conditions.The development of standardized protocols and automation could enhance scalability and at the same time, reduce costs.Education efforts could foster social and regulatory acceptance.Optimizing fungal material preparation for CDI, including pore size, surface area, and electrical conductivity, could enhance salt removal efficiency.By addressing these issues, the feasibility and effectiveness of fungal-based CDI for water desalination can be enhanced.

Feedwater of desalination
Capacitive Deionization (CDI) is a particularly effective method for desalinating feed-water with low salinity.This is because CDI operates at a voltage lower than 1.2 V to prevent Faradaic reaction [22][23], and its energy consumption is directly related to the total ions removed [24].As a result, CDI is highly energy efficient when dealing with a salt rejection of less than 25% and high water recovery [25], making it especially suitable for treating brackish water with salinity levels less than 10 g/L.However, the quality of the feedwater can influence the effectiveness of CDI.For example, feedwater containing silica, organic matter, and pathogens may necessitate additional treatment steps to ensure the optimal quality of the desalinated water [26].
Pretreatment of feedwater and periodic cleaning are necessary to maximize desalination efficiency and ensure the longevity of the fungal electrodes.CDI systems typically require cartridge filtration as pretreatment [27], and may need additional measures to remove organic matter.The conventional pretreatment can achieve a water recovery of approximately 99% and requires low electricity consumption [28].The maintenance costs for cartridge filtration are relatively low, ranging from 0.015-0.021USD/m 3 [29], and this further contributes to the cost-effectiveness of fungal-based CDI.

Performance of fungal-based CDI
The effectiveness of a CDI desalination facility hinges on the materials used, such as electrodes [30], and the operating conditions, including flow rate, applied current, and pressure.To assess performance, a standard evaluation procedure should be established, such as the use of the total cycle time [31].This involves defining the feed salinity and required salinity removal ratio, and then measuring three key indicators related to cost-effectiveness: specifically energy consumption, water recovery ratio, and clean water productivity [26,32].
Chen et al.'s 2022 research demonstrated the Fungal-hypha Activated Carbon Pad (FhACPad) as an effective CDI desalination electrode.The FhACPad showed a desalination capacity of 35.6 mg/g over 30 minutes, with a 76.4% salt removal efficiency.In the experiment, 10 mM NaCl feedwater flowed at 1.0 mL/min for 30 minutes, totaling 30 mL.The FhACPad reduced the salt concentration from 10 millimoles to approximately 2.46 millimoles, leaving 24.6% of the original salt concentration, which represents the clean water produced.
Scaling up this research to a full-scale plant requires adjustments.The FhACPad electrode mass must increase proportionally to the feedwater volume to maintain the same salt removal efficiency.The feedwater flow rate needs adjustment to maintain sufficient contact time for effective desalination.Finally, energy consumption, recorded at 124.8 kJ/mol for the FhACPad, must be managed, as the scale of the system increases, so too will the energy requirements, and this will need to be factored into the design and operation of the full-scale plant.

Operation and maintenance (O&M)
Operating a fungal-based Capacitive Deionization (CDI) desalination plant involves various costs, that include capital expenses for equipment and buildings, operational costs for energy and chemicals, and maintenance costs, particularly for the fungal electrodes.The total cost also includes the operations of different parts of the plant, such as feedwater intake, pretreatment, the main desalination unit, posttreatment, and brine disposal.The energy efficiency of the CDI process, which is directly tied to the salinity of the feedwater and circuit resistance, plays a significant role in the operational costs [26].
The kinetic efficiency of the CDI process which impacts the productivity of clean water [33], depends on factors like the average salt adsorption rate.The costs associated with the manufacturing and scalability of the fungal-based electrodes are key to the total costs of CDI systems, which possibly could account for between 27 to 73% of the total cost [26].Prolonging the lifetime of CDI systems to more than 2 years is crucial for cost-effectiveness [33].Balancing capital and energy costs, considering other design parameters such as dimension and weight, can help minimize the total cost for producing fresh water [34].Improved engineering design, including efficient desalination units and effective brine management, is key to reducing overall desalination costs [26].

Total treated water produced
The total volume of water treated using fungal-based capacitive deionization (CDI) is largely contingent on the system's efficiency and operational parameters.In general, the greater the surface area of the fungal material and the better its electrical conductivity, the more salt can be removed per unit of water, thereby enhancing the overall treatment capacity [5,32].Moreover, optimizing the system's operating conditions, such as the voltage applied to the CDI cells and the flow rate of the feed water, can significantly influence the total volume of water treated [26].However, several factors can limit the total amount of water treated.The lifetime and durability of the fungal materials used in CDI can play a crucial role [34].Given their susceptibility to environmental stressors [17,18], there may be a reduction in performance over time, which in turn affects the total volume of water that can be treated.Furthermore, the scalability of the CDI system itself poses a challenge.While smaller systems are excellent for proof-of-concept and testing, scaling up to industrial levels necessitates careful optimization of the system design and operational parameters [26,34].Overcoming these limitations will enhance the total volume of treated water, contributing significantly to water desalination efforts.

Brine disposal
The management of brine, a by-product of the desalination process, is a crucial aspect of the overall cost and environmental impact of Capacitive Deionization (CDI) systems [35].The quantity of brine produced depends on the water recovery ratio, with lower ratios resulting in a greater volume of concentrate brine.The quality and quantity of this brine determine the optimal approach to its management and potential utilization.The salinity of brine from CDI is typically 7-8 g/L [26].The disposal of brine can pose significant environmental and economic challenges.Surface water discharge, the most common disposal practice, is often limited to coastal desalination plants and can affect local water chemistry and aquatic life [36,37].
Alternative disposal methods include combined sewer disposal, which costs about 1.26 USD/m 3 [38], and various utilization approaches such as mixing with raw water for irrigation [39] or harvesting salinity gradient energy [40].However, these methods may require additional treatment, especially if the feedwater is groundwater, as the concentrate brine can contain high concentrations of harmful gases.For CDI systems, particularly those using fungal-based electrodes, the management of brine is an important consideration in the overall operation and maintenance costs.By optimizing brine management strategies, the cost-effectiveness and environmental impact of fungal-based CDI systems can be improved [26].

Infrastructure
Infrastructure is the foundation on which every component of the fungal-based Capacitive Deionization (CDI) system rests.At the input stage, the feedwater intake system is an essential part of the infrastructure, responsible for the collection and transportation of water to the desalination unit.The water undergoes preliminary processing, or pretreatment, before being subjected to the CDI process.This pretreatment, facilitated by the infrastructure, may involve steps to remove impurities like silica, organic matter, and pathogens [26,28].The infrastructure's design also ensures necessary measures for handling the variability in fungal materials, offering standardized protocols for their cultivation, processing, and characterization.These protocols help to mitigate the issues of susceptibility to environmental stressors, quality control, and scalability of fungal-based CDI, thereby maintaining the efficiency of the overall system.
Further, the infrastructure takes care of the CDI desalination process, housing the main desalination unit and subsequent post-treatment facilities.The performance of the fungal-based CDI system, evaluated based on specific energy consumption, water recovery ratio, and clean water productivity [32], can be significantly influenced by the supportive infrastructure.This infrastructure ensures the operational stability of the CDI process, taking into account factors such as the applied current, pressure, and flow rate.In terms of output, infrastructure ensures effective management of both the produced treated water and the disposal of the resultant brine.By optimizing the water recovery ratio, the infrastructure helps to control the volume of brine produced, thus minimizing potential environmental impact and aiding in the system's cost-effectiveness.In essence, a well-designed and well-implemented infrastructure can significantly contribute to overcoming the challenges associated with fungal-based CDI, thus making it a viable option for large-scale water desalination projects [26].

Conclusion
In addressing urban water scarcity, this research proposes a novel, sustainable solution using a fungalbased Capacitive Deionization (CDI) system.The Fungal-hypha Activated Carbon Pad (FhACPad) electrode demonstrated impressive desalination performance, with significant desalination capacity and efficient electrosorption of ions, while affording less energy consumption than traditional methods.The work introduces a conceptual framework for implementing this technology, detailing elements such as fungal cultivation, material processing, system design, and economic and environmental considerations.The outlined technical diagrams serve as blueprints for future application and development of the system, with specific attention to elements like feedwater quality, fungal cultivation, desalination performance, operation and maintenance costs, and brine disposal.Addressing these elements is crucial to realizing the full potential of fungal-based CDI as a feasible and effective water desalination solution.The success of this solution, including the capacity for scaleup, will depend on overcoming challenges such as fungal material vulnerability, quality control, and infrastructure requirements.