Renewable Electricity and Hydrogen Production via Decentralized Wastewater Treatment Systems

Abstract

Urban wastewater could be converted into energy if microbial electrochemical technologies (METs) like microbial dual-chamber electrolysis cells (MDEC) or microbial fuel cells (MFC) are applied as a treatment method. Mathematical modelling of MFC and MDEC for wastewater treatment and energy recovery has been developed in this study. The Radaue method has been used to solve ordinary differential equations (ODEs), and the model outputs were successfully validated with previous experimental and modelling data. A case study in Montreal, Canada, has also been considered for testing the application of METs on an urban scale with a total daily wastewater flow of 75,000 L/day. The results show that from 1 m³ of wastewater, MDEC and MFC can generate 0.077 kg H₂ and 0.033 kWh, respectively.

1. Introduction

In urban areas, wastewater treatment facilities and water supply systems are major energy consumers [1]. Undoubtedly, by depleting fossil fuel resources, the cost of energy rises, and the carbon footprint of wastewater treatment plants (WTPs) should be considered when mitigating climate change. Often, the energy costs in WTPs are high and put a strain on municipal budgets [2]. Among available options to replace fossil fuels, green hydrogen and green electricity (from wind turbines and solar panels) are good solutions for sustainable development [3,4]. Hydrogen could be produced from a variety of technologies [5], including thermochemical (pyrolysis, gasification, combustion, and liquefaction), electrochemical (such as water electrolysis), and biological processes (biophotolysis, dark fermentation, and photo fermentation) [6]. Not only is the energy content of hydrogen 2.4 times higher than methane and almost 2.8 times higher than gasoline [7,8], but burning hydrogen does not have any detrimental environmental impacts, such as producing greenhouse gas emissions [9]. Hydrogen could be used in different sectors, such as space or water heating and electricity production, through technologies such as fuel cells (FC) and combustion engines [10].

Wastewater and its byproducts, such as residuals, contain kinetic, thermal, and chemical energy with values of about 10 to 14 kJ/g COD based on recent studies [3,4]. Microbial electrochemical technology (MET), for example, MFC for bioelectricity generation and microbial electrolysis cells (MECs) for biohydrogen production, are emerging green technologies due to their ability to produce bioenergy and recover value-added products such as methane and treated water [11,12] (see Figure 1). They can provide part of each area’s electricity, heating and lighting demands. Therefore, such technologies can play a crucial role in having a Positive Energy District in future cities.

The valuable resource used in MET for biological treatment and energy recovery could be urban wastewater containing biodegradable compounds, including carbohydrates and proteins [13,14]. The main motive for pursuing this technology is the potential for complementing the existing costly wastewater treatment systems with a technology that can be self-sustainable or even has a net positive energy output while pollutants are removed. MFC and MEC are new bioreactor types that typically comprise two chambers separated by a membrane. In an anode chamber, microorganisms oxidize organic substrates and release electrons and protons during an anaerobic chemical reaction [15]. Then in MFC, electrons are transferred through an external circuit, and protons migrate through the solution across the membrane to the cathode chamber, where they bond with oxygen and electrons to form water and generate electricity [2]. In MECs, in the cathode chamber, the generated electrons transfer to the cathode side through an external circuit to combine with protons to generate hydrogen in the absence of oxygen [16]. MECs were introduced in 2005 as a promising and well-investigated type of MFC, except that it involves a sealed cathode and external voltage to generate biohydrogen [17,18]. Since the external voltage needed for hydrogen production via MECs is much less than the water electrolyzer, this approach is more efficient.

In recent years, a few modelling and experimental approaches have been proposed for both MFC and MEC. Pinto et al. [19] took a two-microbial population MFC model to compare different operating modes and reactor configurations. The results indicated that the electrical load could control the ratio between the anodophilic and methanogenic populations, and the co-existence of the two populations reduced reactor performance. They concluded that if reactors were connected in series, they would certainly improve treatment efficiency. The authors improved their previous study by applying ODEs to calculate real-time process control and described biomass growth and retention in the anode chamber [20]. They found that the highest generated electricity of an MFC was achieved by changing the operating conditions, which made the proposed model suitable as a convenient tool for offline process optimization. Likewise, researchers currently demonstrate a few studies in pilot-scale MECs to reach the goal of the industrialization of METs to decrease the cost of urban wastewater treatment and energy recovery from it. The first pilot-scale MEC located in England for domestic wastewater treatment was published by Heidrich et al. [21]. The researchers considered a reactor consisting of six modules with a volume of 120 L fed by local wastewater and monitored collecting gas through the system over three months. They measured chemical oxygen demand (COD) (0.14 kg COD/m³ d) and its related cost of energy (2.3 kJ/g COD), which proved that the less-recovered energy from their system (70%) did not have any effects on generating hydrogen due to the high performance of the technology resulting from its exemplary configuration. Alavijeh et al. [22] proposed a model for both MECs and MFCs. At first, they modeled an MFC based on experimental results and various simulated variables of liquid bulk and biofilm. Secondly, they extracted the MEC model from the MFC by changing the boundary conditions. As an innovation, they introduced biofilm local potential modelling for MEC simulation with simple linear boundary conditions. Therefore, the performance of the MEC was identified according to the variations in microbial distributions, methane, and hydrogen production at different applied voltages. In addition, they simulated the polarization characteristics reached from experimental data. All of these evaluations specified that the proposed model was successfully able to predict both MFC and MEC performance.

In this research, mathematical modelling of MDEC and MFC is adopted from the works of Gonzalez et al. [5] and R.P. Pinto et al. [20]. Two scenarios were defined to compare the power generation rates between only using MFC (first scenario) and the integration of both MDEC and FC (second scenario) as a wastewater treatment method and energy recovery pathway at urban scales. The mentioned scenarios were evaluated for a real case with two non-domestic blocks located at 4000 Saint Patrick Street, Montreal, Canada (see Figure 2). Based on the data provided by [23], in Montreal, the average water consumption, which is equal to wastewater generation, is about 300 L/day per capita. In non-residential buildings, water consumption is usually less than in residential buildings. For an assumed generation of 200 L per person and 375 building users, the total daily generation is 75,000 L/day of wastewater. Moreover, the potential of using the generated hydrogen and power through METs as a transportation fuel has been investigated.

2. Materials and Methods

2.1. Mathematical and Numerical Method

In this research, the mathematical modelling of the proposed MET is based on competition between microbial populations, including anodophilic and methanogenic, for acetate consumption, which is considered the primary carbon source in the effluent of the anode chamber. This study presumes that the soluble amount of acetate in wastewater is about 550 mg/L. Anodophilic microorganisms degrade acetate and release electrons and protons plus CO₂, but the products of acetate consumption via methanogenic microorganisms are methane and CO₂, so the desired microorganism is anodophilic because it releases the necessary stuff to form hydrogen and generate electricity. Moreover, in respect of the charge-transferring mechanism from a carbon source and anodophilic microorganisms to the anode, the charge is assumed to involve intracellular mediators, which exist in the reduced ($M_{r }$) and oxidized ($M_{o }$) forms. The mass balance equations describe the behaviour of substrate consumption, microbial growth, and competition. Thus, a fast numerical solution is needed to solve the differential equations relating to biomass growth and retention in the anodic compartment. Some essential indicators, including pH, temperature, and pressure, are constant during the chemical processes in MDEC and MFC. In METs, where the substrate gradient in the biofilm is neglected, the uniform distribution of microbial populations and ideal mixing is assumed in the anode part. In continuing as a first step, the model was established for a 1-L reactor volume and was then scaled up for urban areas, using the mentioned data in the case study part.

2.2. Equations for Mathematical Modeling of MDEC and MFC

The following equations can describe the conceptual model. Furthermore, it can be seen that in some parts, the equations are common between MDEC and MFC, especially in the anode chamber.

2.2.1. Chemical Reactions at Anode and Cathode Chambers (MDEC)

Anode chamber

$${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{M}}_{\mathrm{o} }\to 4{\mathrm{M}}_{\mathrm{r} }+2{\mathrm{CO}}_2$$

(1)

$$4{\mathrm{M}}_{\mathrm{r}} \to 4{\mathrm{M}}_{\mathrm{o}}+8{\mathrm{e}}^{-}+8{\mathrm{H}}^{+}$$

(2)

$${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{O}}_2 \to { \mathrm{CH}}_4+{ \mathrm{CO}}_2$$

(3)

Cathode chamber

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-} \to 2{\mathrm{OH}}^{-}+{ \mathrm{H}}_2$$

(4)

2.2.2. Chemical Reactions at Anode and Cathode Chambers (MFC)

Anode chamber

$${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{M}}_{\mathrm{o} }\to 4{\mathrm{M}}_{\mathrm{r} }+2{\mathrm{CO}}_2$$

(5)

$${\mathrm{M}}_{\mathrm{r}} \to { \mathrm{M}}_{\mathrm{o}}+{ \mathrm{e}}^{-}+{ \mathrm{H}}^{+}$$

(6)

$${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{O}}_2 \to 4{\mathrm{CH}}_4+{ \mathrm{CO}}_2$$

(7)

Cathode chamber

$${\mathrm{O}}_2+4{\mathrm{e}}^{-}+4{\mathrm{H}}^{+}\to 2{\mathrm{H}}_2\mathrm{O}$$

(8)

2.2.3. Anodic Compartment Material Balances

The equations below refer to the equilibrium of acetate, anodophilic and methanogenic microorganisms for continuous flow reactors (MDEC and MFC), where D is the dilution rate equal to the flow rate divided by the reactor volume (V_r), which is assumed to be equal to 1 in this study. S,$X_a$, and $X_m$ are the concentration of acetate (mg S/L), anodophilic microorganisms (mg $X_a$/L) and methanogenic microorganisms (mg $X_m$/L), respectively (mg X/L).${\alpha }_a$ (0.54) and ${\alpha }_m$ (0.48) are the constant values of dimensionless biofilm retention.

$$\frac{dS}{dt}=D\left(S_0-S\right)-q_a{X}_a-q_m{X}_m$$

(9)

$$\frac{d{X}_a}{dt}={\mu }_a{X}_a-K_{d,a}{X}_a-{\alpha }_{a}D{X}_a$$

(10)

$$\frac{d{X}_m}{dt}={\mu }_m{X}_m-K_{d,m}{X}_m-{\alpha }_{m}D{X}_m$$

(11)

2.2.4. Hydrogen Production Rate in MDEC

• MDEC

$$Q_{H_2}=Y_{H_2}\frac{I_{MEC}}{mF}\frac{RT}{P}$$

(12)

$Y_{H_2}$ is the hydrogen yield, and the value for that is 0.9$; m$ is the electrons per mole of the mediator.

2.2.5. Methane Production Rate at MFC

• MFC

$$Q_{C{H}_4}=Y_{C{H}_4}{q}_m{X}_m{V}_r$$

(13)

where $Y_{C{H}_4}$ is the methane yield, and the value for that is 0.3 (mL ${\mathrm{CH}}_4/\mathrm{mg} \mathrm{S}$).

2.2.6. Intracellular Material Balances

Intracellular mediators exist in the reduced ($M_r)$ and oxidized ($M_o)$ forms and since a constant pool of the mediator per anodophilic microorganism is assumed, the equation below can be applied in modelling steps. The 0.05 (mg M/mg X) is the total mediator fraction in this work.

$$M_{total=}{M}_r+M_o$$

(14)

• MDEC

$$\frac{d{M}_o}{dt}=\frac{\gamma }{V_r{X}_a}\frac{I_{MEC}}{mF}-Y_M{q}_a$$

(15)

where $Y_M$ is the yield rate for the oxidized mediator, and is equal to 3.3 (mg M/mg A).

• MFC

$$\frac{d{M}_o}{dt}=-Y{q}_a+\gamma \frac{I_{MFC}}{mF}\frac{1}{V_r{X}_a}$$

(16)

2.2.7. Microbial Kinetic Equations

In the equations below, a stands for anodophilic and m stands for methanogenic microorganisms. So, ${\mu }_a$,${\mu }_m$,$q_a$,$q_m$ are belong to the growth rate and consumption rate of mentioned microorganisms respectively.

$${\mu }_a={\mu }_{max,a}\frac{S}{K_{s,a}+S}\frac{M_o}{K_{ME}+M_o}$$

(17)

$${\mu }_m={\mu }_{max,m}\frac{S}{K_{s,m}+S}$$

(18)

$$q_a=q_{max,a}\frac{S}{K_{s,a}+S}\frac{M_o}{K_{ME}+M_o}$$

(19)

$$q_m=q_{max,a}\frac{S}{K_{s,m}+S}$$

(20)

2.2.8. Electrochemical Equations

As MDEC require an applied voltage and MFC provide power, the equations in this part are totally different. Therefore, they are divided into two separate sections.

• MDEC

The required voltage for MDEC is computed through the equation below.$E_{CEF}$ is the value of electrode potential.

$$E_{app}=E_{CEF}-I_{MFC}{R}_{int}-{\eta }_{conc}-{\eta }_{act}$$

(21)

Further, concentration losses and activation losses equations are as follows:

$${\eta }_{conc}=\frac{RT}{mF}\mathit{ln}\left(\frac{M_T}{M_r}\right)$$

(22)

$${\eta }_{act}=\frac{RT}{\beta mF}{\mathit{sinh}}^{-1}\left(\frac{I_{MEC}}{S_A{i}_0}\right)$$

(23)

The current in MDEC can be determined through the equation below.

$$I_{MEC}=\frac{E_{CEF}+E_{app}}{R_{int}}-\frac{RT\left[\mathit{ln}\left(\frac{M_T}{M_r}\right)+\frac{1}{\beta }{\mathit{sinh}}^{-1}\left(\frac{I_{MEC}}{S_A{i}_0}\right)\right]}{R_{int}}$$

(24)

By applying the equation below, the internal resistance can be calculated.

$$R_{int}=R_{Min}+\left(R_{Max}-R_{Min}\right)e^{-k_R{X}_a}$$

(25)

• MFC

By subtracting ohmic, concentration, and activation losses from the value of the electrode potential, the generated voltage from MFC can be assessed.$E_{thermo}$ is known as the maximum voltage of MFC.

$$I_{MFC}{R}_{ext}=E_{thermo}-I_{MFC}{R}_{int}-{\eta }_{conc}-{\eta }_{act }$$

(26)

The concentration and activation losses can be calculated using the equations below.

$${\eta }_{conc}=\frac{RT}{mF}\mathit{ln}\left(\frac{M_T}{M_r}\right)$$

(27)

$${\eta }_{act}=\frac{I_{MFC}}{S_A{i}_{0.ref}}\frac{RT}{mF}\left(\frac{M_r}{M_o}\right)$$

(28)

The MFC current equation is shown below

$$I_{MFC}=\frac{\left(E_{OCV}-{\eta }_{conc}\right)}{\left(R_{Ext}+R_{Int}\right)}\frac{M_r}{M_r+\epsilon }$$

(29)

The relationship between internal resistance and minimum and maximum observed resistance can be seen in the equation below.

$$R_{int}=R_{Min}+\left(R_{Max}-R_{Min}\right)e^{-k_R{X}_a}$$

(30)

The open-circuit voltage is estimated as follows:

$$E_{ocv}=E_{Min}+\left(E_{Max}-E_{Min}\right)e^{\frac{-1}{k_R{X}_a}}$$

(31)

2.2.9. Design Parameters and Variables

The parameters and numbers needed to solve the above equations are listed in

Table 1. The operation factors for the thermodynamic modelling of the decentralized wastewater systems.

Factors and Description	Value and Units	Factors and Description	Value and Units
$E_{app}$ (Applied potential)	0.6 (V)	$R_{max} ($Maximum internal resistance)	2000 (Ω)
$E_{CEF}$ (Counter electromotive force for the MEC)	−0.35 (V)	$R_{min} ($Minimum internal resistance)	25 (Ω)
$E_{Min}$ (Minimum EOCV)	0.01 (V)	Y (mediator yield)	22.75 (mg M/mg S)
$E_{max}$ (Maximum EOCV)	0.61–0.66 (V)	$q_{max,a} ($Maximum anodophilic reaction rate)	13.14 (mg S/mg X d)
$K_{ME}$ (Mediator half-rate constant)	0.01 (mg M/L)	$q_{max,m} ($The maximum methanogenic reaction rate)	14.12 (mg S/mg X d)
$K_{d,a}$ (Decay rate of anodophilic microorganisms)	0.04 (1/d)	${\mu }_{max,a} ($The maximum specific growth rate for anodophilics)	1.97 (1/d)
$K_{d,m}$ (Decay rate of methanogenic microorganisms)	0.01 (1/d)	${\mu }_{max,m} ($The maximum specific growth rate for methanogens)	0.3 (1/d)
$K_{s,a}$ (Half-rate constant of anodophilics)	20 (mg S/L)	β (Oxidation transfer coefficient or reduction)	0.5
$K_{s,m}$ (Half-rate constant ofMethanogens)	80 (mg S/L)	γ (Mediator molar mass)	663,400 (mg M/mole M)
$K_R$ (Constant, which determines the slope of the curve in the equation)	0.024 (L/mg X)	M (Electrons per mole of mediator)	2 (mol e−/mol H2)

.

3. Results and Discussion

3.1. Model Validation

In this section, MDEC and MFC are validated by the works of Gonzalez et al. [5] and R.P. Pinto et al. [20]. In this step, it is vital to validate the model and ensure that it is reliable and accurate. To evaluate the MFC model accuracy, the acetate consumption rate and methane production rate were compared with the results from R.P. Pinto et al. [20] (see Figure 3 and Figure 4). Figure 3 demonstrates that the concentration of acetate at the beginning of the process is at its highest value, and during this time, it will be consumed by microorganisms and goes to zero values over time (the batch system assumed). Figure 4 shows that the average methane outputs from our model could be considered to be about 0.67 (mL CH₄/L of wastewater (L_w), day), and it closely follows the reference measured.

In addition, the validation of the rate of hydrogen production in MDEC with Gonzalez et al. [5] is depicted in Figure 5. The hydrogen production rate reaches its highest value in the first five days, and it remains steady for the rest of the process due to the continuous flow conditions. Therefore, the validation figures in this part indicate that the outputs of MDEC and MFC models agree well with the previous modelling carried out by other researchers. Moreover, the mean squared error formula has been used to compare the simulation results with previous works as follows:

$$MSE=\frac{1}{n}{\displaystyle \sum }_{i=1}^n{\left(\frac{y_i^{Ref}-y_i^{sim}}{\overline{y}}\right)}^2$$

(32)

where n is defined as the number of data points, $y_i^{exp}$ is the experimental value (reference), and $y_i^{sim}$ is presented as a simulation value. Thus, the MSE for acetate consumption, methane, and hydrogen production are 0.7%, 0.09%, and 0.1%, respectively.

3.2. Cycle Description

In this work, an advanced wastewater treatment system was suggested to assess the application of simulation codes for wastewater treatment and energy recovery on an urban scale. Based on Figure 6, two scenarios are defined to compare the application of MFC and MDEC as wastewater treatment methods in city areas. In the first steps of both scenarios, the wastewater would be collected and pretreated and then injected into the METs to start the treatment process. Then, in the first scenario, the power output of the MFC can be used in two different applications, such as charging electric cars or directly used to cover a proportion of the electricity demands in the generation location. In the second scenario, hydrogen, as the main product of the MDEC can be considered as a transportation fuel or feed for FC to generate power. It should be noted that the applied voltage can be covered by renewable sources of green electricity, such as solar panels and wind turbines, to run the MDEC. In addition, treated water from both scenarios could be reused for various applications such as toilet flushing, irrigation, and carwashes in the same location. Therefore, as mentioned in Figure 2, a real case study is applied to examine the proposed systems in Figure 6.

3.3. Results of the Case Study

In applying the mathematical modelling of MDEC and MFC, 0.077 kg H₂ and 0.033 kWh energy can be obtained from using these technologies to treat 1 m³ of wastewater. Therefore, along with the first scenario, the generated electricity from the total collected wastewater in the mentioned location using MFC is about 2.5 kWh. Then, by applying the second scenario, the generated hydrogen via MDEC is nearly 5.7 kg. The total energy required to run the MDEC in this location is nearly 129 kWh, which could be supplied from renewable energy sources such as wind turbines or solar power. As shown in Figure 6, the hydrogen generated from the MDEC is used for power generation via fuel cells (FC). The FC feed is pure hydrogen, so by integrating the MDEC and FC, power can be generated from the wastewater treatment process. It should be noted that the maximum efficiency of FC is about 60%. Therefore, in this part, a comparison between the direct power generation from MFC and the indirect power generation from MDEC-FC has been conducted (see

Table 2. Energy generation via MFC and MDEC-FC by treating 1 m³ of domestic wastewater.

Type of METs	Hydrogen Production (kg)	Energy Content (kWh)	Energy Consumption (kWh)	Energy Generation via FC (kWh)	Net Energy Generation (kWh)
MFC	-	0.033	-	-	0.033
MDEC-FC	0.077	2.58	1.72	1.55	−0.17

).

Table 2 shows that the generated energy from the MDEC-FC system is negative because of the low efficiency of FC. It can be concluded that by using the generated energy from MDEC-FC for the same system, 0.17 kWh more is needed to treat 1 m³ of wastewater. It is, therefore, necessary to investigate the efficiency of other pathways, such as combustion engines, for power generation using the generated hydrogen from the wastewater treatment process. Both MDEC and MFC generate low amounts of energy, but the benefit of these technologies is in treating the wastewater with low power consumption at the same location of generation.

3.4. The Application of METs in the Transportation Sector (Green Cars)

Applying alternative, sustainable, and renewable fuels, such as green electricity and hydrogen, as the driving force for automobiles has become prominent worldwide in recent years. Both fuel cell electric vehicles (FCEVs) and electric vehicles (EVs) have the potential to replace older-generation petroleum-fuelled cars and have no CO₂ emissions. One difference between these two kinds of cars is that FCEVs can fill up with hydrogen faster than charging EVs with electrical power [24]. Also, EVs are limited by the ranges that they can travel because of the capacity of their batteries. According to data in the literature, it is assumed that for driving 100 km by FCEVs, 0.9 kg H_2, and in EVs, 15 kWh of electrical energy is needed [25,26]. As mentioned in Figure 6, the generated hydrogen and electricity from METs can be used in FCEVs and EVs, respectively.

Table 3. The application of METs outputs as fuel for green cars.

Type of Green Car	The Required Energy for Hydrogen by Green Cars to Drive 100 km	Selected METs	Energy Consumption by METs	Total Wastewater Requirement
EVs	15 kWh	MFC	-	455 m3
FCEVs	0.9 kg	MDEC	27 kWh	13 m3

reveals the information and comparisons regarding the application of METs to provide the required fuels for green cars, and it shows how much wastewater should be treated via METs to drive 100 km by green cars.

4. Conclusions

MDEC and MFC are decentralized approaches that can treat wastewater locally without being sent to central wastewater treatment plants through piping systems. In addition, the treated wastewater could be reused for different purposes, such as toilet flushing, car washing, and irrigation. This paper shows that H₂ can be produced from MDEC, and electricity can be generated from MFC. Both technologies have the advantage of low power consumption for treating wastewater with a small energy generation contribution. The application of METs in the transportation sector has been analyzed as a contribution to sustainable energy production from waste compounds. However, attention must be paid when interpreting the results of this study. The microbial electrochemical technologies discussed here are expensive, and conducting accurate benefit-cost ratios and Life Cycle Assessment studies is an open area of research.