Introduction

In an industrial production estimation, including electricity generation, costing has always been the major aspect to be considered for industrializing any technology. Taking into account the existing renewable energy technology, wind energy prices depend on test discount rates, wind speeds, installation costs and the capital repayment periods. Capital costs including land acquisition, cost of plant, grid connection and initial financing costs have to be considered as one of the components for electricity generation costs. The other costs except components (grid connection, electrical installation and foundation) varied based on the country of installations and the turbine size, where they cost around 24% of total turbine costs for UK and Germany and < 20% in Denmark and Spain (Dincer 2011). In 2010, 1350 €/kW is the average cost of an installed wind turbine in Spain with an average connection cost of 140 €/kW (Schallenberg-Rodriguez 2013). Besides, the operating costs such as local authority rates, labor costs, materials costs (wind turbines: $700–3000/kW based on rated power), maintenance costs, insurance, and rent have to be included as well while defining the energy prices. The battery replacement for every five years along with the operation and maintenance costs will sum up to around 5% of the capital costs (Dincer 2011). The maintenance costs are different depending on the type of machine used. For older generation machine, the maintenance costs are around 3% of the turbines capital cost, while 1.5 – 2% of capital cost is required for newer machine (Bolinger 2012). In Spain, the average of operation and maintenance costs has reduced from 40 $/MWh (year 1982)–10 $/MWh (year 2010). The last component, fuel costs, are zero for renewable energy like wind, wave, geothermal and solar installations (Milborrow 2012). Renewable energies are investigated to secure the source of energy supplies and reduce the pollution caused by burning fossil fuels. However, they are mostly applied in certain countries as the climate or topography varies in each country and are difficult to transport or store the energy generated (Elshobary et al. 2021).

Nowadays, microbial fuel cell (MFC) technology is attracting much attention of the society due to the potential in producing electricity and treating wastewater simultaneously. It is because the high energy consumption of 20 to 45 kWh/population equivalent-year was required for wastewater treatment and significant amount of greenhouse gases emissions were observed (Bolognesi et al. 2021). A life cycle assessment conducted by Zawartka et al. (2020) has indicated that highest greenhouse gases were analyzed in the central wastewater plant compared to the sewerage system, septic tanks and household wastewater treatment plant. Furthermore, the sensitivity analysis performed had showed that a sensitivity of ± 10.3% in greenhouse gases emission was caused by a ± 30% change of energy consumption in central wastewater treatment plant, indicating the significant of greenhouse gases emission related to energy consumption (Zawartka et al. 2020). Hence, a better alternative in treating wastewater from industrial and household is required to lower the environmental impact. The MFC technology is introduced as it reduces the environmental footprint, secures electricity supplies and achieves effective wastewater treatment at the same time. The performance of MFC is influenced by several parameters such as materials of system, architecture, microorganism community and substrate source as well as the operating conditions of the system.

The MFC anode oxidizes organic matter to form electrons, which are conducted through an external loop to a receptor in the cathode reaction chamber (such as O2 or ferricyanide) and combined with the excess protons that pass through the proton exchange membrane to form water (Zhou et al. 2012). Microalgae have received more attention because it has the potential to be an efficient electron acceptor during photosynthetic reactions within a cathodic chamber or as electron donors at the anodic end in the MFC system (Cui et al. 2014). Using microalgae as the microbial in the cathode of MFC, the nutrients from wastewater and the CO2 produced by anode microbial metabolism can be removed effectively (Kusmayadi et al. 2020). Moreover, microalgae biofuels are considered as the third-generation biofuels (Maity et al. 2014). Mixotrophic culture is a feasible strategy to reduce the cost of microalgae cultivation (Wang et al. 2014). In order to optimize the nutrient composition for microalgae cultivation, the extra addition of nutrients such as carbon, nitrogen and phosphate is required while using wastewater as a single feedstock. Wu et al. (2013a, b) have constructed a tubular photobioreactor with a cathode MFC which is able to produce oxygen from Chlorella vulgaris. The relationship between the voltage of microalgae MFC and dissolved oxygen was explored in the work of Wu et al. (2013a, b). Results showed that the biomass concentration has a significant positive relationship with the voltage output (Wu et al. 2013b). It is reported that using 5–14% (v/v) CO2 to domesticate microalgae led to a microalgae productivity of 1247 ± 52 mg/L and the microalgae cannot grow further without the CO2 supplies in the case of using only acetic acid as sole carbon source (Cui et al. 2014). He et al. (2014) have observed that 92.1% of COD removal efficiency can be obtained with a maximum power generation efficiency of 2572.8 mW/m3 and 14.1% of coulombic efficiency (CE) by using immobilized Chlorella sp. in microalgae MFC. They also concluded that Chlorella vulgaris was a common electron acceptor in MFC cathode which can also hold the CO2 at the same time (He et al. 2014). The deposition type MFC with Chlorella vulgaris as the biocathode has an electricity production capacity of 21 mW/m2 (Wang et al. 2010). A maximum power generation efficiency of 21.4 mW/m2, which is about 2.8 times higher than non-biological cathode, was obtained (Wu et al. 2013b). In the work of Zhou et al. (2012), the suspension culture of Chlorella vulgaris is used as cathode of the MFC. They found that the electricity production efficiency was about 88% higher than immobilized microalgae with 2.5 W/m3 of maximum current density and 9.4% of CE (Zhou et al. 2012). Wang et al. (2010) have developed a new MFC system (microbial carbon capture cells, MCCs) to provide the cathode microalgae growth with CO2 in the exhaust gas while the generated electricity was 5.6 W/m3 which can reach the exhaust gas in the cathode (Wang et al. 2010).

Reports available in the literature showed that sulfate-reduction bacteria was used to convert sulfate to hydrogen sulfide (Lee et al. 2014). The sulfate-reduction bacteria are chemoheterotrophic in nature and utilize organic matters as carbon source to produce electricity (Bratkova et al. 2019). In the process of oxidizing organic matter, the sulfate reduced to hydrogen sulfide and simultaneous electron transfer takes place. The electrons move from the external circuit to the anode to generate electric current (Bratkova et al. 2019). At this stage, the organic matter and sulfate are removed from wastewater. Rabaey et al. (2006) have reported that the MFC technology converts the sulfides into elemental sulfur and removes the organic matter from wastewater (Rabaey et al. 2006). Cai and Zheng (2013) have studied the simultaneous treatment of sulfide (60 mg/L) and nitrate (10.5 mg/L) in a dual-tank MFC. Maximum current density of 138.31 mA/m2 and a stable current density of 12–19 mA/m2 were obtained, while the final products were nitrogen and sulfate (Cai and Zheng 2013). Liu et al. (2015) have reported that the development of baffle type MFC with initial sulfate concentration of 60 mg/L and chemical oxygen demand (COD) of 800 mg/L which operated for 24 h hydraulic retention time. Total sulfate and COD removal efficiency of 70% and 55% were obtained, respectively (Liu et al. 2015).

Immobilization technique refers to the entrapment or anchoring of biocatalysts inside an inert carrier to make it more stable and reach the target function. For the biocatalyst, immobilization provides several advantages of biodegradation efficiency improvement such as multiple usages of remediation agents, shear forces against in a bioreactor, harsh environmental tolerance and storage period increasing in the environmental biotechnology application (Bayat et al. 2015). Immobilization techniques include absorption, binding on a surface, cross-linking, entrapment and encapsulation (Kumar et al. 2016). Our previous study has developed the immobilization technique to entrap the microalga Chlorella sp. to form a microalgae/polymeric matrix granule within the cross-linking solution. Peak removal efficiencies for NH4+-N, COD and color were 80.2%, 70.8% and 77.9%, respectively; 10% of fatty acid methyl esters achieved in a bioreactor consists of the matrix granule at hydraulic retention time of 48 h with continuously fed textile wastewater for 440 h (Wu et al. 2020).

The aim of the present study is to develop an integration dual-chamber MFC which uses the immobilized anaerobic bacteria in the anodic chamber and immobilized microalgae in the cathodic chamber to increase electricity generation efficiency. Higher electricity generation efficiency is desired for MFC in order to replace the current/existing technology like generating power using non-renewable source (fossil fuels and natural gas) and ensure the continuous supply of electricity. The application of wastewater in MFC as microalgae culture medium will aid in reducing material cost of MFC where microalgae replace expensive catalysts in improving the performance of MFC and the results obtained in this work help society to identify the potential microalgae strain in maximizing the electricity production.

Materials and method

Materials

The following were used: urea, sucrose, yeast extract, NaH2PO4⋅2H2O, potassium sulfate (K2HPO4), peptone, ammonium nitrate (NH4NO3), potassium nitrate (KNO3), sodium nitrate (NaNO3), ammonium chloride (NH4Cl), ammonium sulfate ((NH4)2SO4), magnesium sulfate hydrate (MgSO4⋅ H2O, Na2SO4), sodium citrate, ammonium chloride (NH4Cl), ferrous ammonium sulfate hexahydrate (Fe(NH4)2(SO4)2), calcium chloride (CaC12), potassium ferricyanide (K3Fe(CN)6), Wolf’s vitamin solution, Wolf’s mineral solution, phosphate buffer solution, sodium alginate (C6H9NaO7), deionized water (DI water) and distilled water. All chemicals used were of analytical grades and purchased from Katayama Chemical Co., Ltd. (Taiwan).

Microorganism and culture medium

Cathodic microalgae and culture medium

Four species of microalgae (Chlorella sp. G29-5, G31-8, W23-21, and W25-381) used in study were collected from Taiwan and screened using freshwater medium (urea 0.03 g/L, sucrose 1 g/L, yeast extract 0.5 g/L, NaH2PO4⋅2H2O 0.005 g/L, K2HPO4 0.05 g/L). The prepared freshwater medium was dispensed in a spiral test tube, and the operation value was kept at 10 mL, representing that the volume of freshwater medium used to screen the microalgae was maintained at 10 mL. The microalgae were picked up by the inoculating loop, and the spiral tube with the microalgae cell was placed on the sieve algae spiral turntable. A swine wastewater with the characteristics of COD 6850 mg/L, NH4+-N 1.0 g/L and pH 7.5 was collected from a swine farm in central Taiwan. The environmental parameters of swine manure concentration (5X and 10X dilution ratio), pH (4, 6, 8 and 10) and extra nitrogen sources (urea, yeast extract, peptone, NH4NO3, KNO3, NaNO3, NH4Cl and (NH4)2SO4) with concentration of 0.5, 1.0 and 2.0 g/L for microalgae cultivation were optimized at 30℃ and 30 rpm for 7 days. The light intensity was about 3000 lx, and the photoperiod for the cultivation was 24 h. The variation of OD680 (optical density of culture at wavelength of 680 nm) was measured using UV–visible Spectrophotometer (S1205, UNICO, USA), and the pH of culture was measured using pH meter (MP 220, Mettler Toledo, USA) for initial and final (after 7-day cultivation). The methods used to measure OD680 and pH were stated under the section “Analytical procedures.”

Anodic anaerobic inoculum and culture medium

The anaerobic bacteria used in the experiment were screened from the waste sludge, which were collected from domestic sewage at Feng Chia University (sewage sludge), a wastewater treatment plant at food fructose manufactory (fructose sludge), and the Wenshan landfill in central Taiwan (landfill sludge). The anaerobic inoculum was domesticated in a glass vial. The domestication was done by adding 20 mL of wastewater, 20 mL of sulfate-reducing bacteria medium (sodium lactate 3 g/L, MgSO4. H2O 2 g/L, Na2SO4 1.15 g/L, sodium citrate 5 g/L, NH4Cl 1 g/L, K2HPO4 0.5 g/L, Fe(NH4)2(SO4)2 1 g/L, Wolf’s vitamin solution 1 mL, and Wolf’s mineral solution 1 mL into 1 L of distilled water) (Lee et al. 2014) and 60 mL of phosphate buffer solution. Domestication means to choose and breed the wild species artificially in the man-made niches for obtaining the cultivated variants that meet the desired requirement (Steensels et al. 2019). Air was then removed from the headspace via argon sparging for 5 min. The operation volume was 75 mL at pH 7. The batch experiments were conducted at 35 ℃ and placed in an incubator shaker at 100 rpm.

Immobilization technology for anodic inoculum and cathodic microalgae

The sodium alginate solution of 20 g/L was sterilized at 110 °C for 10 min and was mixed with the anodic anaerobic bacteria and cathodic microalgae biomass solution, respectively. Then, the mixture of solution (biomass and sodium alginate) was put into a 50-mL syringe and pressed into CaC12 solution of 40 g/L to form the immobilized anaerobic bacteria and microalgae cells with diameter of 4 mm, respectively. The immobilized cells were washed with DI water and kept at room temperature for 1 d before seeding in the MFC chambers.

MFC structure

After optimizing the environmental parameters for the cathodic microalgae, fed-batch model operation was conducted in two-chamber MFCs to investigate the simultaneous wastewater bioremediation and bioelectricity generation. Anodic and cathodic chambers with working volumes of 75 mL were divided by cation exchange membrane (AMI-7000, Membranes International, Ringwood, NJ). Carbon cloth (5 × 3 cm, 30 cm2 projected area; W1S1010, Ce Tech Co Ltd, Taiwan) was used as the electrode, and an external resistance (R) of 100 Ω was used to connect the electrodes. The MFCs were incubated at 35 °C without mixing. The anode chamber was seeded by the enriched sewage sludge, fructose sludge and landfill sludge and kept anaerobically by flushing it with N2 before inoculation and during the feeding. The anodic growth medium conductivity was 12–13 μS/cm depending on the pH. The 50 mM K3Fe(CN)6 cathode electrolyte in 100 mM phosphate buffer (adjusted to the initial pH to 7.0) was used in the first experiment to investigate the suspended and immobilized anaerobic inoculum electricity generation performances.

Furthermore, raw swine wastewater (COD 6850 mg/L, NH4+-N 1.0 g/L and pH 7.5) was applied as the cathode electrolyte. The fresh swine wastewater was used to replace 10% of working volume in the MFC every two days, and the liquid samples were collected to detect the water quality during the cultivation for 21 days. The cathode solution conductivity was 24–28 μS/cm depending on the pH.

Electrochemical measurements and calculations

Electrochemical measurements were collected by a data acquisition system (Jiehan 5000, Taiwan) at 3 min intervals. The current (I) was calculated from the recorded cell voltage (V) and connected external resistance (R) via Ohm's law. Then, the power density was calculated via the equation of power density (W/m3) = I2 (A) ∙ R (Ω)/V (m3). Here, R is the external resistances (100 Ω) and V is the anolyte chamber volume (75 mL).

Performance analysis was performed at the end of each experiment by measuring cell voltage and anode potential after 30 min of stabilization with external resistances (100 Ω) at the open circuit mode. Power density and polarization curves were drawn from performance analyses results. Internal resistances were further estimated from the slopes of polarization curves according to polarization curve (Ma et al. 2016).

Cyclic voltammetry (CV) was applied to draw the polarization and power curves by changing the external resistance from 1 M Ω to 5 Ω (RBOX-408, Resistance decade box, Lutron Electronic Enterprise Co Ltd, Taiwan) and recording the voltage every 5 min. Internal resistances were further estimated from the polarization curve slopes according to the polarization curve (Ma et al. 2016). The measured cell voltage and the consumed COD of swine manure concentration were applied to calculate the coulombic efficiency (CE) over the periods according to Eq. 1 (Luo et al. 2009):

$$\mathrm{C}\mathrm{E}=\frac{8{\int }_{0}^{{t}_{\mathrm{b}}}I\mathrm{d}t}{\mathrm{F}{\mathrm{V}}_{\mathrm{a}\mathrm{n}}\Delta \mathrm{C}\mathrm{O}\mathrm{D}}$$
(1)

where tb is the measurement period (d), F is Faraday's constant (96,500 C/mol × e), Van is working volume of anode chamber (L), and \(\Delta\) COD is consumed COD concentration (g/L).

Analytical procedures

The microalgae concentration was measured using a UV–Visible spectrophotometer (S1205, UNICO, USA) at 680 nm (A680) wavelength. The sample optical density (OD) was measured at this wavelength. The samples were diluted, assuming that the absorbances of samples are greater than 0.8, where absorbance is the amount of light absorbed by the sample. Standard method analytical procedures were used to determine the COD concentration and pH (American Public Health Association 1995).

Results and discussion

Optimization of the cultivation parameter for cathodic microalgae

Swine manure concentration

Swine manure with different concentrations and conditions (sterilization/un-sterilization) were studied to optimize the cultivation parameters for cathodic microalgae (Chlorella sp. G29-5, G31-8, W23-21, and W25-381). These parameters were studied as they influence the growth rate of microalgae biomass due to the amount of light penetration and conditions of the system. Figure 1 shows that the OD of all types of microalgae strains cultured using 5X dilution of swine manure ranged from 0.783 to 0.898 (sterilization) and 0.982 to 1.348 (un-sterilization), while OD obtained using 10X dilution of swine manure ranged from 0.784 to 0.900 (sterilization) and 0.856 to 1.06 (un-sterilization). The growth of all strains in both 5X and 10X was found to be comparable within the span of 70 h, indicating that the nutrients in higher dilution rate were sufficient for the optimum growth of biomass as well. Since swine manure contained high fertilization content, additional nutrient from the medium through less dilution will not necessarily improve the biomass productivity (Meng et al. 2018). It was also apparent that the lower dilution rate 5X has produced a medium solution which is slightly cloudier than the higher dilution 10X, but the light penetration into the system was not affected significantly as the microalgae showed similar optical density readings for both dilution rates. Besides diluting the swine manure, Leite et al. have claimed that the piggery wastewater used for microalgae cultivation can be mixed with diluted municipal wastewater in reducing the cost of microalgae cultivation and increasing the nutrient concentration at the same time (Leite et al. 2019). Based on the results, 5X dilution was chosen as the optimum dilution for next study as a slightly higher value range for un-sterilized condition was obtained using the mentioned dilution rate.

Fig. 1
figure 1

Microalgae (a Chlorella sp. G29-5, b Chlorella sp. G31-8, c Chlorella sp. W23-21, d Chlorella sp. W25-381) growth performance and pH value variations

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pH of the culture

Figure 2 displays the graph of OD versus microalgae at different pH values, which indicates that the microalgae did not grow at pH 4 and pH 10. However, they grew very well at pH 6 and pH 8. Different species of microalgae can tolerate a different range of pH values and some species stunt due to over acidic or alkaline condition. For Chlorella sp., the suitable range of pH was found to be pH 6–7 (Sakarika and Kornaros 2016; Qiu et al. 2017). Mediums which are too acidic tend to show very poor biomass growth, as the acids have the potential to disrupt the cell wall, thereby killing the microalgae rather than promoting the growth. As the pH increases from 8 to 10, the flocculation of cells will occur and results in the aggregation of the microalgae cells which prevents them from multiplying effectively (Sakarika and Kornaros 2016). For the pH values higher than 10, it possibly leads to cell lysis and reduces the productivity of biomass. It was reported that the increased pH value of wastewater has increased the concentration of uncharged species, causing the ammonia to be diffused into more acidic intracellular matrix (Li et al. 2020). The results obtained are in agreement with the suitable range of pH, showing the highest optical density at pH 6 and good optical density at pH 8. The pH value of various systems used under stable condition for microalgae cultivation was reported at around pH 8, supporting the result obtained for this study (Zhou et al. 2019). Therefore, it is not necessary to adjust the pH value to achieve the optimal growth conditions (the initial pH value of the swine manure is about pH 7.5) of microalgae in swine manure culture medium. The optimum condition of 5X diluted swine manure at its initial pH value was selected for the study of nitrogen sources for microalgae cultivation.

Fig. 2
figure 2

Microalgae growth performance at various pH values

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Nitrogen source

In our previous study, extra K2HPO4 4 mg/L and urea 1 g/L were added into the medium while cultivating the microalgae, Chlorella sp. G23 using textile wastewater as feedstock at pH 10 with aeration (carbon dioxide sparging). The highest total fatty acid methyl ester content of 20 ± 4% was accumulated with the addition of extra nutrients (Wu et al. 2017). It is well known that nitrogen is the primary nutrient source for the growth of microalgae. Therefore, the type and concentration of nitrogen source have major influence on microalgae growth. In this study, 8 types of nitrogen sources with different concentrations have been used for the growth of microalgae in 5X diluted swine manure. Figure 3 illustrates that the microalgae, Chlorella sp. G29-5, G31-8, W23-21, which used 2.0 g/L of yeast extract as nitrogen source, have showed higher growth rate. Besides, specific type of microalgae species has higher growth rate corresponding to certain type of nitrogen sources at various concentrations, where low concentrations (0.5 g/L) of peptone and yeast extract promoted growth activity of Chlorella sp. W25-381, and high growth activity of Chlorella sp. G31-8 was gained using 2.0 g/L of NH4NO3. Higher biomass concentration was successfully obtained using organic nitrogen sources such as yeast extract and peptone. Yeast extract was an exceptional nitrogen source that was able to give the highest concentration of microalgae biomass production, due to the composition of yeast extract as it contains nitrogen and various compounds such as amino acids, peptides and carbohydrates. This contributes to additional nutrient content like organic carbon from the carbohydrates, making the cultivation into a mixotrophic condition and improving biomass generation (Kim et al. 2016). Nitrogen sources involving ammonium did not show very high productivity of microalgae as excessive ammonium can lead to inhibitory effects on cell growth from the lack of ATP formation in the chloroplast, which subsequently leads to the inhibition of photosynthesis (Ramanna et al. 2014). Similar results were obtained in the work of Wu et al., where better biomass productivity was gained using potassium nitrate as nitrogen source compared to ammonium chloride and ammonium nitrate (Wu et al. 2013a). Among all types of nitrogen sources, this study showed that the suitable nitrogen source in obtaining the highest yield of microalgae biomass is yeast extract.

Fig. 3
figure 3

Microalgae growth performance at various nitrogen sources at concentrations of 0.5, 1.0 and 2.0 g/L

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Suspended/Immobilized anaerobic inoculum in the anode and K 3 [Fe(CN) 6 ] solution in the cathode

After 1 month of cultivation using both suspended and immobilized anaerobic inoculum (domestic sludge, fructose sludge and landfill sludge) in anodic chamber and the K3[Fe(CN)6] solution in the cathodic chamber, data of the MFC such as peak voltage, internal resistance, power density, current density and COD removal efficiency are given in Table 1. These data are significant to be examined to ensure the functionality of system in producing electricity and treating the sludge. A system with high value of the data for all aspects are desirable, except for internal resistance as high internal resistance restricted the delivery of current and terminal voltage to the load. It was observed that most of the data obtained for immobilized inoculum are higher compared to the suspended inoculum used in the system. In the case of using suspended inoculum in anode, highest peak voltage and coulombic efficiency (CE) were obtained using sewage sludge and the highest COD removal, current and power density were gained using landfill sludge. As for fructose sludge, data obtained for all aspects were lowest except CE and internal resistance.

Table 1 Electricity generation efficiency from suspended and immobilized anaerobic inoculum in anode and K3[Fe(CN)6] solution in cathode
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For immobilization method, the immobilized anaerobic bacteria were used to instead of the suspended anaerobic bacteria in the cathode of MFC. The experimental results obtained from the MFC following the voltage variation cycle law of three type of sludge after 1 month of incubation showed that the data values obtained vary from data obtained using suspended inoculum. Using sewage sludge at the anode gained the highest current density; fructose sludge to have the highest COD removal, power, and current density, while using landfill sludge to get highest peak voltage and CE. In Table 1, the sewage sludge is reported to have lowest CE and peak voltage, while lowest current density and COD removal were found using landfill sludge. CE as the parameter indicating the charge efficiency of the transaction of electrons in fuel cells are utmost important. The total CE between suspended and immobilized anaerobic inoculum was found to be increased by 1.7%, 8.7% and 10% for sewage sludge, fructose sludge and landfill sludge, respectively. The data obtained using immobilization techniques showed positive results in enhancing the MFC with all types of sludges at anode, especially for landfill sludge. This study was conducted as the existing literatures are either focusing on suspended or immobilized inoculum, where the comparison study between two methods is rarely conducted. With anaerobic inoculum from different sludge, the results showed that the immobilization methods are more effective in energy generation as it was observed that the electricity produced for immobilized inoculum is higher compared to suspended inoculum. Through this study, immobilization method has also demonstrated to be advantageous in the applications involving microorganisms (Senko et al. 2019) and it has been selected for the following studies.

Combination of immobilized anodic anaerobic bacteria and immobilized cathodic microalgae

The voltage performances of the immobilized anodic anaerobic bacteria and four cathodic immobilized Chlorella sp. strains (G29-5, G31-8, W23-21 and W25-381) were analyzed. It was observed that MFCs were unstable at the beginning of cultivation period as the biofilms did not grow on the surface of electrodes (data shown). After 14–16 days of cultivation, the voltage performance became stable and the highest maximum voltage reached is 155.0 mV from Chlorella sp. G29-5 as anodic MFC with the lowest ones from Chlorella sp. W25-381 at 115.2 mV. After 21 days of cultivation, the open circuit voltage of these anodic Chlorella sp. MFCs was increased, up to values ranging from 340.2 to 485.5 mV. The ascending values of Chlorella sp. for both maximum voltage and open circuit voltage are in the sequence as stated, Chlorella sp. W25-381 < W23-21 < G31-8 < G29-5. Figure 4 depicts the polarization and power densities of the four Chlorella sp. strains after 21 days of cultivation. The results showed that the maximum power density for these four MFCs was 505.6 mW/m2, which was carried out from the anodic Chlorella sp. G29-5 at the current density of 0.222 mA/cm2 and a voltage of 245.3 mV. This value is higher than other anodic MFCs Chlorella sp. Therefore, Chlorella sp. G29-5 was the suitable microalgae strains to be utilized in the cathodic of MFCs as the highest voltage performance and power density were observed. The application of swine manure as a substrate for electricity production in this work has shown to produce good power density. Past literature has showed the potential of swine manure as substrate for power generation either in common type or up-flow MFC, showing the agreement with the results obtained in this study (Min et al. 2005; Ma et al. 2016). Good power density allows higher amount of power to be delivered based on its volume, which show the potentiality of swine manure in producing electricity. There are a lot of compounds in waste resources which can aid in the production of electricity, showing that waste resources are good organic source for MFCs. Various researches reported that Chlorella sp. is a good electron acceptor in a dual-chamber MFC (Gonzalez del Campo et al. 2014; Gouveia et al. 2014; Commault et al. 2017). With further optimization, the cultivation of algae using waste resources will contribute to the simultaneous production of clean energy along with wastewater pollutants removal.

Fig. 4
figure 4

Polarization and power densities of the four Chlorella sp. strains after 21-day cultivation

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Figure 5 shows the cyclic voltammetry (CV) curves of the four Chlorella sp. strains after 21 days of cultivation. In the forward scan results, the anodic peaks of the anodic chamber and cathodic Chlorella sp. chamber are observed at various cell voltage and current density, as followed: (1) Strain G29-5: 330.0 mV (82.69 mA) and 580.4 mV (57.06 mA),( 2) Strain G31-8: 335.5 mV (72.09 mA) and 520.3 mV (72.25 mA), (3) Strain W23-21: 190.0 mV (65.99 mA) and 583.5 mV (57.81 mA), (4) Strain W25-381: 190.2 mV (58.79 mA) and 780.6 mV (48.53 mA). All the microalgae strains showed quite similar cycles of current density, with only a minor difference observed due to the different structure and composition of each individual strain. Moreover, the reverse scan results showed that there was no significant anodic peak found. In the cathodic chambers, only Chlorella sp. W23-21 has no significant cathodic peak. The Chlorella sp. G29-5, strain G31-8 and strain W25-381 were observed to have the cathodic peaks of 40.15 mV (– 12.26 mA), 260.3 mV (–4.53 mA) and – 19.91 mV (– 10.24 mA), respectively. The forward scan results in both anodic and cathodic chambers reveal that the anodic anaerobic microorganisms oxidize the organic matters and reduction reaction occurred by cathodic microalgae metabolism efficiently simultaneously. Moreover, the reverse scan results show this reaction is irreversible.

Fig. 5
figure 5

Cyclic voltammetry (CV) curves of the four Chlorella sp. strains after 21-day cultivation

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Figure 6 shows the COD and NH4+-N removal efficiencies of the four Chlorella sp. strains after 21 days of cultivation. The cathodic chambers have higher NH4+-N and COD removal efficiencies as compared to the anodic chamber. In the anodic chamber, the immobilized anodic anaerobic bacteria and immobilized cathodic Chlorella sp. G29-5 have shown highest COD removal efficiency that is 43.6%, while the lowest COD removal efficiency was obtained using cathodic Chlorella sp. G31-8 MFC that is 37.2% as shown in Fig. 6a. Besides, the highest NH4+-N removal efficiency of 47.1% was observed using Chlorella sp. G31-8, while the lowest, 41.7% was obtained at cathodic Chlorella sp. W23-21 MFC. However, the cathodic Chlorella sp. G29-5 MFC has the highest peak values for NH4+-N removal efficiency among all microalgae strains, which is equivalent to 77.7%. It was reported that NH4+-N removal efficiency up to 95.9% was achieved using the immobilized microalgal-based photoautotrophic MFC, where Chlorella sp. was utilized in the MFC of the reported work (Wang et al. 2019). The peak COD removal efficiency in the cathodic chamber is 76.4%, which occurred in the Chlorella sp. G31-8 chamber, and the lowest peak COD removal efficiency is 56.4%, observed in Chlorella sp. W25-381 chamber from Fig. 6b. From the results, a significant difference can be observed in the COD and NH4+-N removal efficiencies between the strains, showing up to 15% variation for COD in the anodic chamber and up to 20% variation in the cathodic chamber. The microalgae strains showed good pollutant removal and bioremediation capabilities, and higher wastewater treatment ability was achieved with immobilized anodic anaerobic bacteria and immobilized cathodic Chlorella sp. G29-5. Several reports stated that the COD removal efficiency was affected by the reactor design, biomass quantity, C. vulgaris growth rate, wastewater composition and hydraulic retention time (Cuellar-Bermudez et al. 2017; Wollmann et al. 2019). Although the Chlorella sp. G31-8 has the highest COD removal efficiency at cathodic chamber, Chlorella sp. G29-5 was selected as the most suitable microalgae strain as it has the highest COD and NH4+-N removal efficiencies at anodic and cathodic chambers, respectively, as well as comparable COD and NH4+-N removal efficiencies at cathodic and anodic chambers, respectively, with the highest values.

Fig. 6
figure 6

COD and NH4+-N removal efficiencies in a anode and b cathode of the four Chlorella sp. strains after 21-day cultivation

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A microalgae-based MFC system produces a syntrophic interaction between bacteria and microalgae. This system has a negligible net energy consumption. The microalgae-based MFC mechanism is comprised of biodegradable substrate oxidation and electron generation at the anode and carbon dioxide (CO2) evolution at the cathode. Mekuto et al. (2020) mentioned that the organic matter undergoes glycolysis following by a citric acid cycle (CAC) that consumes acetate (in the form of acetyl-CoA) and water and subsequently reduces the nicotinamide adenine dinucleotide (NAD+) into NADH. Lastly, the CO2 was produced as a waste by-product. The NADH generated by the citric acid cycle is fed into the oxidative phosphorylation (electron transport) pathway (Mekuto et al. 2020). The net result of these two closely linked pathways is nutrient oxidation that produces usable chemical energy in the form of ATP. Shewanella oneidensis and Geobacter sulfurreducens are the most common microorganisms in MFC systems due to their high electrochemical activities. Saratale et al. (2017) reported that the cathodic microalgal oxygen production depends mainly on the electron transformation from water into nicotinamide adenine dinucleotide phosphate (NADP+) through the photosystem I (PSI), photosystem II (PSII) and cytochrome b6f complex, by small plastoquinone and plastocyanin mobile molecules in the photosynthesis reaction (Saratale et al. 2017).

As compared to other renewable electricity generation technologies, microalgae-based MFC integrates several functions while generating electricity. Microalgae-based MFC utilizes bacteria and microalgae to produce electricity through photosynthesis and metabolic reaction (Kusmayadi et al. 2020) while treating the wastewater with high nutrient loading (act as essential elements for microalgae growth) during the electricity production. The application of wastewater as microalgae culture medium has reduced the fresh water supply required for this technology, and the alternative of expensive catalysts in enhancing cathode’s performance by circulating water containing dissolved oxygen is not required as well in microalgae-based MFC (Cui et al. 2014). Pumping cost of circulating water continuously is saved as oxygen is produced as by-product in microalgae cultivation and acts as the terminal electron acceptor in the fuel cell. Additional advantage is that carbon fixation is performed by microalgae as CO2 is one of the nutrients required for microalgae growth. The renewable electricity generation technologies like wind, sun and tides energy have a major disadvantage that microalgae-based MFC overcame; the electricity production is affected by climate or location of country. This factor has limited the development of such technologies in some countries as well as the share of renewable energy in the global energy consumption, where the usage of renewable energy only takes up to 16.70% in the total energy consumption worldwide (Islam et al. 2014). Furthermore, the microalgae biomass produced in the microalgae-based MFC acts as biofuel feedstock as the microalgae contain high lipid and carbohydrate contents. Various types of valuable components within microalgae biomass such as astaxanthins, phycobilins and proteins (Yen et al. 2013) can be extracted and applied in pharmaceutical, nutraceutical and animal feed industries, reducing the cost required by this technology. This technology is able to lighten the issue of energy insufficient and environmental pollution due to over-disposing of wastewater into waterbodies and high CO2 emission from human activities. It is also reported that less sludges are formed using MFC compared to the conventional aerobic activated sludge treatment (Kusmayadi et al. 2020).

Cost estimations

In this work, a capital cost estimation was calculated for a 1000-L MFC system according to the local Taiwan’s market, with the price quoted from the respective manufacturers. The analysis was conducted for large scale as it provides information for the potential usage of this MFC system in industrial scale. As shown in Table 2, the material cost of this MFC system required around $956.40 for carrying out the electricity generation. The materials utilized in this system were economical as a potential system to be industrialized. It is reported that the material properties were not influence significantly to the power output of MFC, as similar results on anodic current densities, power outputs and charge transfer resistances were obtained using both high-performing and low-cost anode materials (Stoll et al. 2016). As compared to the previous studies, the price of anode in this study was much lower compared to a system with anode made up from a graphite brush with different materials. The price of the anodes is $450, $150 and $1925, respectively, for titanium core, hard carbon felt and carbon foam material, while it only costs $7.5 for anode in this study (Stoll et al. 2016). Besides, the MFC in this study did not require catalysts, reducing a significant cost compared to the common type of MFC using catalysts in enhancing its performance. The cost spent on catalysts is high if expensive catalysts such as platinum are used, costing around $90- $130/g. Microalgae utilized in this system has similar function as catalyst that enhance the performance of MFC, and the advantage is to generate extra income (around $26–34/kg biomass) (Shen et al. 2009) to cover the capital and processing cost. The estimated capital cost was lower compared to a stack system of MFC, where the total amount is $10,430 for equivalent volume of system with this study (Zhuang et al. 2012). Although the capital cost of equivalent working volume in the current study is lower than other literatures, the efficiency of power density in the current system requires improvement. It is reported that the capital cost per power generated for this system is $ 490.46/mW, comparatively higher than other literatures studies, which ranges from 2.74 to $48/mW (Zhuang et al. 2012; Stoll et al. 2016). The consideration of the extra income gained from the microalgae biomass for further process was not performed in the analysis, which required further investigation and studies to validate the economic potential of the current system. Other expenses including labor cost, tax and shipping costs were excluded as the prices were subjected to the location. In terms of whole system, there is no doubt that microalgae-based MFC is potential as a green approach to treat wastewater, produces microalgae biomass and generates electricity.

Table 2  Material cost estimation of the laboratory size, 75 mL, and 1000-L MFC system
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Conclusions

The utilization of microalgae-based MFC has showed significant COD and NH4+-N removal efficiencies at cathode chamber, and power density of the MFC varied with the type of microalgae strain used. The optimum results obtained for highest nutrient removal and electricity generation are to use Chlorella sp. G29-5 at cathodic chamber and fructose sludge in anodic chamber. Besides, the immobilization technology enhances the energy generation efficiency compared to suspended inoculum; higher maximum voltage, current density, power density, coulombic efficiency and COD removal efficiency with lower internal resistance were obtained. The combined immobilized anaerobic bacteria and cathodic Chlorella sp. G29-5 in a dual-chamber MFCs have simultaneously treated swine manure and generate electricity in the cathode chamber. Microalgae-based MFC technology can utilize the effluents from photofermentation, anaerobic hydrogen or methane fermentation systems. The microalgae biomass is the potential feedstock for these systems to produce biohydrogen and biomethane or sell as an extra income to cover the cost of MFC. This integrated system was found to be useful in electricity generation and wastewater treatment. A biorefinery system is required to increase the economic feasibility of bio-electrochemical systems.