*3.2.3 Benefits of microalgal WWT*

Molinuevo-Salces and co-workers [76] pointed out the benefits of microalgalbased WWT systems to include:


Starch-based textile de-sizing wastewater (TDW) was treated with the microalgae, *Scenedesmus sp*. to remove organic carbon with lab-scale reactors, which achieved 92.4% color removal, reduction in chemical oxygen demand (COD) by 89.5%, carbohydrates by 97.4% and organic acids by 94.7% [22]. Phasey and co-workers [23] averred that cultivation of microalgae using municipal and agricultural wastewater in high rate algal ponds (HRAP) partitions nutrients into microalgal biomass, which can be recovered and reused.

### *3.2.4 Microalgal WWT challenges*

In spite of all the advantages, some challenges have to be surmounted before the microalgal WWT protocol can be applied. The challenges include (1) land requirement, (2) effect of wastewater characteristics, (3) environmental and operational condition influence and (4) biomass harvesting and valorization [14]. However, limitations such as algae biomass separation from water, process efficiency in cold climates and limited ability of the algae biomass to reduce micropollutant content in wastewater discourages full-scale use [77].

#### **3.3 Microbial fuel cells for wastewater treatment**

In order to build a sustainable platform for the future society needs to substantially reduce its reliance on fossil fuels. This reduction can then minimize the global scale of pollution. As has been discussed in this chapter, these two global challenges could be concurrently addressed through the application of wastewater treatment technologies which reduce pollution and provide the starting blocks for biofuels. In recent years, a paradigm shift has occurred where wastewater, which can also be referred to as waste matter, is being used by industries generate electricity. In particular, studies have illustrated that a number of biological processing methods can

#### *Emerging Trends in Wastewater Treatment Technologies: The Current Perspective DOI: http://dx.doi.org/10.5772/intechopen.93898*

be used to produce bioenergy or bio-chemicals while treating industrial wastewater. Specifically, brewery wastewater treatment has been highlighted for the application of microbial fuel cells (MFCs) [78]. One such instance of this method is using MFCs to simultaneously treat wastewater and produce bioenergy which is most referred to as bioelectricity. Production of these bio-products happens from simply converting the organic and chemical energy contained in wastewater to electrical energy. To further explore these possibilities, this section first describes an MFC, second it discusses applications of MFCs in wastewater treatment, and thirdly it reviews the different techniques and operations that use MFCs to treat wastewater while concurrently producing electricity. In addition, it also describes other applications and bioenergy products of this technique, its advantages and disadvantages, further promising applications of the MFC technology in wastewater treatment. An MFC is a device that converts organic matter to electricity using microorganisms as the biocatalyst. Typical MFCs have three major components: electrodes, separator, and electrogens. All MFCs contain two electrodes, which, depending on the design, can either be separated into one or two chambers. These chambers operate as completely mixed reactors. As illustrated in **Figure 7** below, each electrode is placed on each side of the membrane, which can either be a proton exchange membrane (PEM) or a cation exchange membrane (CEM). The anode faces the chamber that contains the liquid phase, and the cathode faces the chamber that only contains air [79].

Aforementioned literature proposed the use of carbon, graphite, and metalbased materials as electrodes. For example, materials made from carbon cloth, carbon paper, carbon felt [80], graphite granules, carbon mesh [81], platinum, platinum black and activated carbon with single or tubular or multi-electrode configurations are suitable as electrodes [82]. These electrodes should have properties which render them biocompatible and stable In addition high electrical conductivity, and large surface area is recommended [83, 84]. The cathode can be exposed to air or other additional electron acceptors like permanganate, chromium hexacyanoferrate and azo dye, etc. [85]. The separator is either a cation exchange membrane [86] or a salt bridge [87] which is used to keep the chamber. The potential difference generated between the two chambers drives the electrons to move through the circuit while microbial degradation of wastewater acts as the substrate to generate bioelectricity [88]. MFCs were first considered to be used to treat wastewater as early as 1991 [89]. Municipal wastewater contains a multitude of organic compounds that can fuel MFCs. The amount of power generated by MFCs in the wastewater treatment process can potentially halve the electricity demand in a conventional treatment process which consumes a significant amount of electric power

#### **Figure 7.**

*Schematic diagram and pictures of a typical double-chamber microbial fuel cell (MFC), sourced from Logan et., 2006 [78].*

for aerating the activated sludge. MFCs yield 50–90% less solids to be disposed of than conventional activated sludge treatment methods. Anaerobic digesters, are sometimes integrated with aerobic sequencing batch reactors to overcome the challenges of sludge disposal [90]. Furthermore, organic molecules such as acetate, propionate and butyrate can be thoroughly broken down to CO2 and H2O. A hybrid MFC incorporating both electrophiles and anodophiles are especially suitable for wastewater treatment because more organics can be biodegraded by a variety of organics. MFCs using certain microbes display a special ability to remove sulphide as necessary in wastewater treatment [91]. MFCs can enhance the growth of bio electrochemically active microbes during wastewater treatment, thus enabling operational stabilities. Continuous flow, single-compartment MFCs and membrane-less MFCs are favored for wastewater treatment amidst concerns in scale-up of other technologies [92–94]. Sanitary waste, food processing wastewater, swine wastewater and corn stover are all favorable biomass sources for MFCs because they are rich in organic matters [95–97]. Up to 80% of the Chemical Oxygen Demand (COD) can be reduced in some cases [96, 98] and a columbic efficiency as high as 80% was obtained by Kim et al. [99].

MFC technologies are a promising yet novel strategy in wastewater treatment, as the treatment process itself becomes a method to capture energy in the form of electricity or hydrogen gas, rather than being a net consumer of electrical energy. In the early 1990s Kim and colleagues illustrated that bacteria could be used in a biofuel cell as an indicator of the lactate concentration in water [80], which in turn supports electricity generation [81]. Although the power generation was low, it was not apparent whether the technology would have much impact on reducing wastewater strength. In 2004 this changed, and the link between electricity generation with MFCs and wastewater treatment was clearly forged when it was proven that domestic wastewater could be treated to practical levels while simultaneously producing electricity [82]. The amount of electricity produced in this study, while low (26 mW/m2), was considerably higher than previously obtained with other wastewater types. Research conducted prior to 2004 had shown that organic and inorganic matter in marine sediments could be used in a novel type MFC design [83], making it apparent that a wide variety of substrates, materials and system architectures could be used to generate electricity from organic content with bacterial biomass. Still, power levels in all these applications were relatively low. The final development that raised the current interest in MFCs was peaked when power densities of two orders of magnitude greater was produced in an MFC with the addition of glucose [84]. This application had no need for exogenous chemical mediators or catalyst thus ensuring this operation was purely biological.

Following these demonstrations, the competition was on to advance a rather practical approach to MFC applications. The first objective being the development of a scalable approach and design of the MFC for various wastewater treatment types [78]. While the energy that could be harnessed from the wastewater may not be enough to power a typical city, it has been reported that a substantial amount of energy can be used to power the WWTPs. As can be observed in the few studies discussed above on MFC technology, the per capital basis of the energy is not particularly substantial and impressive. Also, it can be noted that the most significant energy savings associated with the use of MFC for wastewater treatment, besides generation of electricity and removal of high strengths pollutants form these recalcitrant substrates, is savings in expenses for aeration and solids handling in typical WWTPs. The main operating costs for wastewater treatment are, aeration, sludge treatment and pumping. It has been argued that aeration alone can account for half of the operational costs at a typical WWTP [85]. Reducing this cost can also ensure that WWTPs become net producers of energy if MFCs are integrated with other treatment technologies.

*Emerging Trends in Wastewater Treatment Technologies: The Current Perspective DOI: http://dx.doi.org/10.5772/intechopen.93898*

#### *3.3.1 Applications of microbial fuel cells in wastewater treatment*

Applications of MFCs in wastewater treatment include a variety of advantages like long-term sustainability, use of renewable resources, degradation of organic and inorganic waste, bio-hydrogen production, and removal of compounds like nitrates, etc. [86]. The electrochemical active microbial community requires an in-depth understanding of its solution chemistry to engage in full-scale implementation and exploitation of MFC technology for electricity generation. [9]. Under ideal laboratory conditions, these systems have produced power densities of 2 to 20 mW/m<sup>2</sup> [87]. However, the amount of biomass-based energy produced by microbial processes is very low. It has yet to reach to its full potential to work in pilot scale units. It has also been noted that the success of specific MFC applications in wastewater treatment will depend on the concentrations and biodegradability of the organic matter in the effluent, the wastewater temperature, and the absence of toxic chemicals [78]. One of the first applications could be the development of a pilot-scale reactor at industrial locations where a high quality and reliable influent is available. Food processing wastewaters and digester effluents are good candidates. Moreover, decreased sludge production could substantially decrease the payback time. In the long term, dilute substrates, such as domestic sewage, could be treated with MFCs, thus decreasing society's need to invest substantial amounts of energy in their treatment. A varied array of alternative applications could also emerge, ranging from biosensor development and sustained energy generation from the seafloor, to bio-batteries operating with various biodegradable fuels. While full scale, and highly effective MFCs are not yet within our reach, the technology holds considerable promise, and major hurdles will undoubtedly be overcome by engineers and scientist in the near future [88]. The growing pressure on our environment, and the call for renewable energy sources will further stimulate development of this technology, to full scale plant operation. As part of the aforementioned applications of MFC in wastewater treatment, potential for application of this technology it as a typical sensor for pollutant strength analysis for in situ process monitoring and control [89]. The proportional correction between the columbic efficacy of MFCs and the strength of the wastewater can propose MFCs to be potential biological oxygen demand (BOD) sensors [80]. An accurate method to measure the BOD value of a liquid is to calculate its Columbic yield. A number of works, namely [80, 90] showed a strong linear relationship between the Columbic yield and the strength of wastewater in BOD concentration range. MFC-type BOD sensors are advantageous because they have excellent operational stability, and good reproducibility and accuracy. An MFC-type BOD sensor constructed with the microbes can be kept operational for over five years without extra maintenance [80]. These biological sensors promise a longer service life than ordinary versions of BOD sensors reported in literature.

#### *3.3.2 Promising techniques of MFCs in wastewater treatment and electricity valorisation*

Waste biomass is a cheap and relatively abundant source of electrons for microbes capable of producing electrical current outside the cell [85]. Rapidly developing microbial electrochemical technologies, such as microbial fuel cells, are part of a diverse platform of future substantial energy and chemical production technologies. In this section, we discuss the key advances that will enable the use of exo-electrogenic micro-organisms to generate biofuels, hydrogen gas, methane, and other valuable inorganic and organic chemicals. Moreover, this section will scrutinize the crucial challenges for implementing these systems and compare them to similar renewable energy technologies. Although commercial development is already underway in several different applications, ranging from wastewater treatment to industrial chemical production, further studies are still required regarding efficiency, scalability, system lifetimes and reliability of MFCs in the field of wastewater treatment and bioenergy production [85].

Power generation using domestic wastewater in the flat plate system was developed and found to be capable of continuously generating electricity from the organic matter in the wastewater while undergoing treatment [82]. Following an acclimation period of approximately 1-month, constant power generation from wastewater was obtained with the Flat Plat Microbial Fuel Cell (FPMFC) over a period of five months. For wastewater containing 2463 mg COD/L, an average power density of 560 mW/m2 was obtained with a hydraulic retention time (HRT) of 2.0 h (0.22 mL/ min flow rate; 164 mg/L log mean COD) and an air flow rate of 2 mL/min with a 470 *ohms'* resistor. Under these operating conditions, the COD removal rate was 1.2 mg/L min (58% COD removal), and the maximum power density was achieved at a flow rate of 0.22 mL/min. This power density was about 10% higher than that obtained under typical operating conditions with a 470 *ohms'* resistor.

Continuous wastewater treatment and electricity generation using a Single Chamber Microbial Fuel Cell (SCMFC) was successfully piloted with feasible results [82, 91]. It was found that the system could generate 26 mW/m<sup>2</sup> at the maximum power density while reducing 80% of the COD. In a specially designed, smaller batch system by Liu et al. [92] showed that up to 28 mW/m2 of power could be generated with domestic wastewater. It was further demonstrated that by removing the proton exchange membrane (PEM), they could generate a maximum of 146 mW/m2 of power. In these systems, the anode was separated from the PEM/ cathode or plain cathode in a large chamber, but the anode chamber was not mixed except by the flow of liquid into the system. In other MFCs, the anode chamber was often mixed in [93–95] . In hydrogen fuel cells, the electrodes are usually combined into a single strip separated by a PEM. This is necessary to keep the two electrodes near to enhance proton conduction between the two electrodes. However, PEMs such as nafion are permeable to oxygen, resulting in the transfer of small amounts of oxygen from the cathode chamber to the anode chamber.

Domestic wastewater treatment was examined under two different temperature gradients, (23 ± 3°C and 30 ± 1°C) and flow modes (fed-batch and continuous) using a single-chamber air–cathode microbial fuel cells (MFCs) in view of the effect of operating parameters which affect the production of electricity [94]. Temperature was an important parameter which influenced efficiency and power generation. The highest power density of 422 mW/m<sup>2</sup> (12.8 W/m3 ) was achieved under continuous flow and mesophilic conditions, at an organic loading rate of 54 g COD/L-d with reduction of COD by 25.8%. Energy recovery was found to depend significantly on the operational conditions (flow mode, temperature, organic loading rate, and Hydraulic Retention Time (HRT)) as well as the reactor architecture. The results demonstrate that the main advantages of using temperature gradients, in series MFC configurations for domestic wastewater treatment are power savings, low solids production, and higher treatment efficiencies.

A study on MFCs used to produce electricity from different compounds sources, including acetate, lactate, and glucose has proven its ability in high efficiencies and versatility in applications for wastewater treatment [96]. Clearly, the possibility to produce electricity in a MFC from domestic wastewater, while at the same time accomplishing biological wastewater treatment (reduction of COD) was emphasized. Tests were conducted using SCMFC containing eight graphite electrodes (anodes) and a single air cathode. The system was operated under continuous flow conditions with primary clarifier effluent obtained from a local wastewater

#### *Emerging Trends in Wastewater Treatment Technologies: The Current Perspective DOI: http://dx.doi.org/10.5772/intechopen.93898*

treatment plant. The prototype SCMFC reactor generated electrical power (*maximum of 26 mW/ m*<sup>2</sup> ) while reducing the COD by about 80%. The power output was proportional to the hydraulic retention time over a range of 3 to 33 h, and to the influent wastewater strength over a range of 50–220 mg/L for COD. Current generation was controlled primarily by the efficiency of the cathode. Optimal cathode performance was obtained by allowing passive air flow rather than forced air flow (4.5–5.5 L/min). The Columbic efficiency of the system, based on COD reduction and current generation, was <12%, indicating that a substantial fraction of the organic matter was not accessible to the microorganisms thus limiting the current generation. Bioreactors based on power generation in MFCs may represent a completely new approach to wastewater treatment. If power generation in these systems can be increased, MFC technology may provide a new method to offset wastewater treatment plant operating costs, whilst making advanced wastewater treatment more affordable for both developing and industrialized nations.

The development of electric power from MFCs was initially investigated for its potential contribution to applications in space research [97]. It was discussed that one of the determining factors in MFC technology was the use of applied microbial cultures, which are responsible for converting electric energy from the chemical bonds in the substrates. In the last decade, despite the intensive development there is a knowledge gap regarding electricity production from microbes and the screening for electricity production. The fast screening method was based on microbial iron (III) – reduction, and do not require any MFC infrastructures. The method is suitable for the evaluation of numerous microbe species or strains simultaneously; and in this way there is possibility to extend the range of potential MFC biocatalysts and be able to predict the electricity generation from the chosen cultures. The knowledge which was generated from this study concerning the growth – iron (III) – reduction, substrate utilization, adhering and biofilm forming properties, extracellular conductive proteins and redox mediator production measurements is essential for the utilization of *G.toluenoxydans* and *S. xiamenensis* species for the different types of MFC applications (wastewater treatment and/or energy production). This information is vital for further strain-improvement and to create an efficient MFC design for electricity production. *S.xiamenensis* DSMZ 22215 species can catalyze *maltose* or *maltodextrine* efficiently. This ability makes the microbes available to be useful in MFC systems for the treatment of starch-based wastewaters *(e.g. Brewery wastewater, starch wastewater and the pulp and paper industry).*

Simultaneous wastewater treatment for biological electricity generation, through the membrane electrode assembly air-cathode MFC in starch processing wastewater (SPW) as substrate, was proven in this study [82]. Over the entire experimentation time, it was perceived that the optimum voltage output of 490.8 mV and power density of 293.4 mW/m<sup>2</sup> was ascertained with a current density of 893.3 mA/m2 . An internal resistance of 120 ohms was also recorded within the third cycle of experiments. Removal efficiencies for COD and <sup>+</sup> *NH N* <sup>4</sup> − increased with time, with a maximum of 98.0% and 90.6%, respectively. This was higher than most reported works on MFC operations. High values of nitrate removal might have been a result of both biological and physiochemical processes. Columbic Efficiency (CE) was not high (maximum 8.0%) and was mainly caused by other electron acceptors in the SPW, and oxygen diffusion during long operation periods. SEM revealed the presence of biofilm on the anode, in which short rodshaped bacillus might have been the dominating bacteria responsible for MFC operation. This study demonstrated the feasibility of using MFC technology to generate electricity and simultaneously treat SPW with high removals of COD and <sup>+</sup> *NH N* <sup>4</sup> − , thus providing an attractive alternative to reduce the cost of wastewater treatment whilst generating electricity from a renewable resource.
