Sugar Industry Wastewater Treatment Through Photosynthetic Microbial Desalination Cells: A Sustainable Approach ^†

Abstract

The expansion of the sugar industry has resulted in large amounts of untreated effluent, necessitating the development of energy-efficient treatment technologies, like microbial desalination cells (MDCs). However, the high cost and potential toxicity of chemical cathode catalysts limit MDC performance, making biocathodes a promising alternative. This study investigates the efficiency of a Scenedesmus obliquus-inoculated photosynthetic microbial desalination cell (PMDC) in the cathode chamber to treat sugar industry effluent, desalinate water, and generate electricity. The performance of the PMDC is compared to that of traditional MDCs. The results showed that the PMDC achieved 21.6% desalination, 73.8% anode COD removal, and a maximum power density of 6.8 mW/m², outperforming MDC by 6.43%, 18.5%, and 112.5%, respectively. These results demonstrate that the PMDC offers an effective, energy-efficient alternative to MDCs with added benefits of nutrient removal and algal biomass production at the cathode, making it a promising solution for water and wastewater treatment.

1. Introduction

Sugarcane production is experiencing rapid growth in response to the escalating demand for sugar and bioethanol [1]. This has led to the generation of large quantities of effluent, as with every ton of cane processed, the industry requires 1500–2000 dm³ of water [2] and generates 1000 dm³ of wastewater [3]. Moreover, sugar industry effluents are categorized as having high chemical oxygen demand (COD), biological oxygen demand (BOD), total dissolved solids (TDSs) [2], and ammonium [3], with small traces of metals and nutrients [4]. Managing industrial wastewater has become a serious challenge worldwide [5], prompting the exploration of energy-saving technologies like microbial desalination cells (MDCs). They are a bio-electrochemical system and serve as an extension to modified microbial fuel cell (MFC) technology [6], which facilitates desalination along with wastewater treatment and energy generation, thereby reducing the reliance on electricity for desalination purposes [7,8]. Despite the multifaceted capabilities of MDCs, their effectiveness is hindered by cathodic reduction kinetics. The use of cathode catalysts in conventional MDCs has drawbacks in terms of cost and toxicity [9]. Alternatively, an algae-assisted cathode chamber can produce oxygenic photosynthesis, eliminating the need for expensive and toxic catalysts or mechanical aeration [10].

The foundational research by Kokabian and Gude [11] advanced the exploration of sustainable water desalination and energy generation technologies by incorporating algae as a viable and eco-friendly biocathode in photosynthetic MDCs (PMDCs). Subsequent studies have since focused on employing algal biocathodes in PMDCs and compared their performance with traditional MDCs in treating various types of wastewater, for instance, palm oil mill effluent [12], oil refinery wastewater [13], petroleum wastewater [14], and dairy wastewater [15,16]. However, the treatment of sugar industry wastewater remains unexplored as an anolyte. This research is crucial for sucrose-based industries such as sugar production, beverage manufacturing, and breweries. Moreover, previous studies have independently investigated PMDCs for industrial and domestic wastewater treatment; there is a notable lack of research on their combined treatment. Such integration can alleviate pressure on freshwater resources by enabling the use of desalinated saline water in industrial processes, including those in the sugar industry. Furthermore, the PMDC remains relatively unexplored for treating domestic wastewater in the cathode chamber with a biocatalyst, Scenedesmus obliquus.

The study aims to address these gaps through the following objectives: (i) to compare the performance of MDCs and PMDCs in terms of sugar industry wastewater treatment, energy generation, and desalination, and (ii) to treat domestic wastewater by employing Scenedesmus obliquus as an algal catalyst in the cathode chamber of PMDCs. These objectives aim to contribute to the advancement of sustainable wastewater treatment and energy generation technologies.

2. Materials and Methods

2.1. Reactor Configuration

Two lab-scaled three-chambered rectangular MDCs named MDC and PMDC were constructed using acrylic sheets with a total volume of 1:0.5:1 for the anode, desalination, and cathode separated by ion-exchange membranes. Graphite rods and carbon fiber brushes were used as cathode and anode electrodes. The cathode chamber of the PMDC had an electric motor for intermittent mixing, while the MDC had an aerator. Illumination at the cathode chamber of the PMDC was provided using an LED light of 18 watts and a digital timer to ensure 12:12 h of light intensity. A copper wire was linked to each electrode and extended outside the MDC setups to develop an electrical circuit for electron transport at a fixed resistance of 1000 Ω. Details of the reactor configuration are mentioned in

Table 1. Details of reactor design and configuration.

Materials	Specifications
Reactor’s working volume (A/D/C)	900:450:900 mL (1:0.5:1)
Surface Area of Graphite Rods	32.28 cm2
Surface Area of Carbon Fiber Brushes	183.13 cm2
Surface Area of Ion Exchange Membranes	128 cm2
Light Intensity for PMDC	150 μmol/m/S
Mixing for PMDC	145 rpm
Aeration for MDC	100 mL/min

* A/D/C = (anode/desalination/cathode).

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2.2. Reactor Inoculation

Synthetic sugar industry wastewater (per L) was synthesized using 2000 mg C₁₂H₂₂O₁₁, 135 mg (NH₄)₂SO₄, and 15.06 mg KH₂PO₄ according to modified recipes from the literature [17,18]; sodium bicarbonate was used for pH adjustment. This synthetic sugar wastewater had a COD of 2000 mg/L, NH₄-N concentration of 29.4 mg/L, PO₄³⁻ concentration of 7.88 mg/L, and electrical conductivity (EC) of 5.16 mS/cm and was taken into the anode chambers of both the MDC and PMDC. For the PMDC, synthetic domestic wastewater contained (per L) 188 mg C₆H₁₂O₆, 134 mg NH₄Cl, 60 mg KH₂PO₄, 200 mg NaHCO₃, 111 mg NaSO₄, 250 mg NaCl with 1.25 mL trace element solution [19], and 100 mL of a 0.1M phosphate-buffered solution adjusted to pH 7. This domestic wastewater had a COD of 200 mg/L, NH₄-N concentration of 39.2 mg/L, and PO₄³⁻ concentration of 23.2 mg/L with an EC of 12.32 mS/cm. Meanwhile, for the MDC, the catholyte consisted of 0.1 M phosphate-buffered solution (pH 7) having an EC of 12.13 mS/cm. Synthetic saline water was mimicked by adding 35 g/L NaCl to distilled water, having an EC of 56.0 mS/cm. An anaerobic mixed sludge was introduced into the anode chambers in both reactors in a 1:2 sludge-to-wastewater ratio. Meanwhile, 1 g/L of Scenedesmus obliquus was used in the PMDC cathode with synthetic domestic wastewater as the biocatholyte.

2.3. Reactor Operation

The setup was initially operated in an open circuit for two weeks, as shown in Figure 1, to promote biofilm formation on the electrode surface. During the first week, fluctuations were observed, indicating the dynamic process of biofilm development and the stabilization of metabolic activity. As the operation progressed into the second week, the voltage trend for both systems began to stabilize. By the end of the second week, both systems achieved near-stable voltage, confirming the formation of a mature biofilm and preparing the reactors for closed-circuit operation. Following this, the circuit was closed by a 1000 Ω resistor connecting the anode and cathode. The reactors were operated in batch mode with 4 days of hydraulic retention time (HRT) using synthetic solutions in the anode, desalination, and cathode. All experiments were conducted at a consistent room temperature of 30 ± 2 °C.

2.4. Reactor Analysis

The voltage data were recorded by a data acquisition system through a PC. The current and power output were calculated using Ohm’s law. The polarization curves were developed by varying external resistance from 4 to 100,000 Ω using a resistance box. Microalgae concentration was determined at 680 nm by a UV–vis spectrophotometer. Dissolved oxygen (DO), pH, and electrical conductivity (EC) were recorded using an oxygen meter (WTW Multi 9310, Germany) and a multimeter (inoLab pH/Cond 720, Germany), respectively. The American Public Health Association (APHA) standard methods [20] were followed to measure COD, NH₄⁺, and PO₄³⁻, while desalination was measured in terms of conductivity (EC) reduction [14,21].

3. Results and Discussion

3.1. Wastewater Treatment at Anode and Cathode

The effectiveness of the PMDC in anode sugar wastewater treatment was assessed and compared with the MDC control in terms of COD removal, as shown in Figure 2a. During a four-day batch operation, the MDC achieved an average anode COD removal of 62.3%, while the PMDC achieved 18.5% higher COD removal than the MDC. The PMDC demonstrated superior COD removal by utilizing algal oxygen and wastewater as electron acceptors, fostering a highly oxidizing environment that facilitated rapid electron consumption [15]. Compared to the MDC, the PMDC achieved greater efficiency in treating sugar wastewater at the anode. Additionally, it provided the added advantages of domestic wastewater treatment with nutrient (N and P) removal and algal biomass production in the cathode chamber. At the cathode, the PMDC achieved an average COD removal of 80.7%, PO₄³⁻-P removal of 22.2%, and NH₄⁺-N removal of 30.2%, as depicted in Figure 2b. The enhanced photosynthetic activity in the microalgal cells was responsible for the higher NH₄⁺-N removal. Nevertheless, this correlation with algal growth deviated from a linear pattern. The observed inconsistency could potentially be explained by the migration of NH₄⁺-N from the cathode to the desalination chamber across the cation exchange membrane [22]. Additionally, the concentration losses in the catholyte could have been caused by the volatilization and oxidation of ammonia [23]. In addition, the higher phosphate concentration in this study probably led to lower phosphate removal (38–43%), as the phosphorus removal mechanism involves microalgal absorption and precipitation at high pH [24]. As the pH value was below 8, it is anticipated that phosphate removal was primarily facilitated through microalgae assimilation [25].

3.2. Power Generation

In the four-day batch cycle, the MDC exhibited an average voltage of 216.4 mV, while the PMDC had a 15.1% higher average voltage than the MDC, as shown in Figure 3a. However, when evaluating the system’s efficiency in converting the substrate into electrical energy, the coulombic efficiency (CE) of this study was found to be relatively lower than in the literature, with the PMDC achieving a CE of 0.41%. In contrast, it was 0.38% for the MDC, as depicted in Figure 3b. The lower coulombic efficiencies of this study with higher COD removal suggest that the electrogenic bacteria were not dominant within the MDCs. This could be due to using a mixed microbial culture sourced from anaerobic sludge, where methanogens possibly held a dominant presence. This dominance of methanogens could hinder substrate conversion into electrical energy, possibly due to competition between methanogens and exoelectrogens. This leads to electron loss, potentially affecting electricity production and lowering CE [26].

Additionally, the PMDC demonstrated a maximum PD of 6.8 mW/m², which was approximately 52.9% higher than the maximum PD of the MDC, as shown in Figure 4. This enhanced performance is linked to the efficient oxidation–reduction reaction (ORR) facilitated by in situ oxygen production from algae, reducing the internal resistance of the PMDC by 62.74% relative to the MDC, which was 1768.3 Ω. The MDC’s cathode, aerated externally, exhibited lower efficiency due to slower oxygen diffusion and the potential formation of intermediate byproducts, which hindered the ORR.

3.3. Desalination and Conductivity

In the MDC and PMDC, the desalination efficiency was evaluated in terms of electrical conductivity (EC). The PMDC achieved an average desalination efficiency of 21.58%, making it 6.88% more efficient than the MDC in desalination, as shown in Figure 5a. The higher voltage generated by the PMDC facilitated improved ion migration across membranes, accelerating the desalination process. This efficiency is reflected in the movement of ions across membranes during the desalination process, which is further supported by observed changes in EC and pH levels at the anode and cathode. Starting from an initial EC of 4.17 mS/cm at the anode, the PMDC showed a 7% higher increase in anodic EC compared to the MDC, which reached an EC of 6.29 mS/cm at the end of the batch cycle, as presented in Figure 5b, and a corresponding pH reduction during this duration was evident; the PMDC exhibited a pH decrease from 7.0 to 6.59. In contrast, the MDC showed a decrease from 7.0 to 6.39. This indicates that the PMDC had approximately 3.13% less of a pH decrease than the MDC in this duration. The pH reductions are attributed to bacterial substrate consumption, accumulation of hydrogen, and/or ion migration from the desalination chamber. At the same time, EC variations were observed in cathode chambers. The PMDC exhibited an increase in the average EC of 13.71 mS/cm from 12.9 mS/cm at the cathode, while the MDC had an average EC rise of 13.44 mS/cm. This indicated that the PMDC had a 2.00% increase in EC compared to the MDC, as shown in Figure 5b. Additionally, the PMDC showed a pH increase from 7.04 to 7.53, while the MDC exhibited an increase from 7.04 to 7.31. This indicates that the PMDC had a 3.00% higher pH increase compared to the MDC. This pH increase could be due to algal photosynthesis and ion migrations.

3.4. Dissolved Oxygen Levels at the Cathode

Dissolved oxygen (DO) at the cathode is a key and controlling factor of MDC performance, significantly influencing its operational efficiency and environmental impact. Figure 6 represents the DO concentrations of the MDC and PMDC cathode chamber. Due to the continuous daily oxygen supply and the absence of a passive oxygen source, like algae in the PMDC cathode, the MDC demonstrated nearly constant DO, averaging at 6.01 mg/L in 4 days of batch operation with little changes throughout the experiments, contrasting with the higher DO observed in the PMDC, which was 8.03 mg/L under light conditions and 4.79 mg/L under dark conditions on average. However, the DO concentrations in the PMDC cathode chamber were higher until day 2 of the 4-day batch mode, after which they began to decline. This decrease is attributed to the reduced algal growth, which peaked during the first 2 days and remained static from day 3. Beyond the 4th day, the algal growth had started to decrease, as shown in Figure 6.

In the four-day batch cycle, the average algal growth was 1821 mg/L, with a growth rate of 205.25 mg/L/d. Also, in this study, as aeration in the MDC control was provided without using any catalyst at the cathode electrodes (graphite rods), it might have led to hydrogen peroxide (H₂O₂) formation as an intermediate. This may have limited the oxygen reduction reaction (ORR) at the cathode, as hydrogen peroxide can be produced through a two-electron pathway instead of the more efficient four-electron pathway. Although hydrogen peroxide can be further reduced to water, its formation indicates a less efficient oxygen production process [27]. In contrast, algal biocathodes serve as an excellent source of oxygen, outperforming conventional aeration due to their unique capability of in situ oxygen production through algal photosynthesis, bypassing the need for intermediate production steps and ensuring a more reliable and efficient oxygen supply.

4. Conclusions

This study demonstrated that photosynthetic microbial desalination cells (PMDCs) are a highly effective and sustainable solution for treating sugar industry wastewater, generating electricity, and desalinating water. By utilizing Scenedesmus obliquus in the cathode chamber, the PMDC outperformed conventional microbial desalination cells (MDCs) in multiple aspects, including higher COD removal, better desalination efficiency, and greater power output. Additionally, the PMDC successfully treated domestic wastewater while producing valuable algal biomass. This research highlights the potential of PMDCs as a more efficient, eco-friendly alternative to traditional MDCs, with promising applications in industrial wastewater management and resource recovery.