The use of wastewater for energy generation by biodigestion and generation of biogas is a well established process in use at numerous waste-water treatment plants. A new approach, based on microbial fuel cells, which offers a scalable alternative with much potential, is in the development stages. The technique also has an application to acid mine water drainage treatment.
Waste treatment is a problem for all human settlements from small villages to large cities. The basic processes developed for waste-water treatment (activated sludge, trickling filters and lagoons) were developed over a century ago, and have changed little with respect to the fundamental approach of oxidising organic matter to remove the organic load on receiving water bodies. The traditional method of aeration produces a water which, although clear of solid matter, is rich in nutrients and has devastating effects when discharged into sea or freshwater. Waste-water treatment remains an economic burden to industries and the public.
Organic matter in waste-water has energy value, particularly industrial and agricultural waste-waters which have high concentrations of organic matter. It is estimated that domestic waste-water contains 9,3 times more energy than the treatment consumes . Biodigestion of waste-water sludge has been a step forward, producing biogas which can be used in internal combustion gas engines to produce electricity, but is an involved process that requires large digesters with a low efficiency factor and a low thermal efficiency gas engine. The most successful and widely used biological technology for wastewater treatment is the activated sludge process. In the process, pumping and aeration are the predominant energy consuming, for example, 21% of the total treatment energy demand is consumed by pumping and 30 to 55% consumed by aeration.
Due to the high cost of operation and high energy demand, alternative approaches to treatment of waste-water are being developed. A substantially different approach, based on electricity production directly from organic matter in waste-water is being pursued. This approach is based on the anaerobic oxidation of organic matter in a microbial fuel cell. The fuel cell offers the possibility of direct conversion of the energy in organic matter into electricity with the potential of a much simplified process and higher conversion efficiency. Power output at this stage of development is low, but is expected to step up with further development.
Microbial fuel cells
A microbial fuel cell (MFC) is a bio-electrochemical device that harnesses the power of respiring microbes to convert organic matter in waste-water directly into electrical energy. At its core, the MFC is a fuel cell, which transforms chemical energy into electricity using oxidation-reduction reactions. The key difference is that MFCs rely on living biocatalysts to facilitate the movement of electrons throughout their systems instead of the traditional chemically catalysed oxidation of a substrate at the anode and reduction at the cathode. In this field the term substrate is used to describe a substance on which the microorganism acts to produce a chemical reaction, in this case organic matter contained in waste-water, usually in dissolved form.
The process behind MFCs is cellular respiration. Nature has been taking organic matter substrates and converting them into energy for billions of years. Cellular respiration is a collection of metabolic reactions that cells use to convert nutrients into adenosine triphosphate (ATP), which fuels cellular activity. The overall reaction can be considered an exothermic redox reaction, and it was with this in mind that an early twentieth century botany professor at the University of Durham, MC Potter, first came up with the idea of using microbes to produce electricity in 1911 .
In order for a fuel cell to work a complete circuit is needed. In the case of the MFC the cathode and an anode are separated by a cation selective membrane and linked together with an external conductor through the load. When an organic “fuel” is fed into the anode chamber, the bacteria oxidise and reduce the organic matter to generate the life sustaining ATP that fuels their cellular machinery. Protons, electrons, and carbon dioxide are produced as byproducts, with the anode serving as the electron acceptor in the bacteria’s electron transport chain.
The electrons pass from the anode to the cathode through the external load connection. At the same time protons pass freely into the cathode chamber through the proton exchange membrane separating the two chambers. Finally an oxygen present at the cathode recombines with hydrogen and the electrons from the cathode to produce water, completing the reaction.
The use of biological organisms responsible for catalysing electrochemical reactions, gives these systems a level of complexity that is perhaps above that of already complex electrochemical systems (e.g. batteries, fuel cells and supercapacitors). The main differences of MFCs to the conventional low temperature fuel cells (direct methanol fuel cell or proton exchange membrane fuel cell) are :
A MFC consists of an anode and a cathode separated by a cation specific membrane. Microbes at the anode oxidise the organic substrate, generating protons which pass through the membrane to the cathode, and electrons which pass through the anode to an external circuit to generate a current. The problem is collecting the electrons released by bacteria as they respire. This leads to two types of MFCs: mediator and mediatorless. The mediatorless MFC is the most promising and is the main version used in developments. There are two basic versions of the MFC, the two cell and the single cell.
Two cell MFC
This is illustrated in Fig. 1 . The cell consists of two compartments, containing the anode and cathode, separated by a permeable membrane. The anode cell contains the substrate (wastewater or organic material) and the anode, which is coated with a surface film of microorganisms. The cathode cell contains the cathode and the electrolyte. Substrate is fed to the anode cell and oxygen to the cathode cell. The anode cell is maintained in and anaerobic state i.e. is kept free of oxygen.
Single compartment MFC
The single compartment uses an external air cathode which is separated from the inside of the cell by the membrane (Fig. 2). The air cathode version gives a higher power density than the two chamber version. In practice the MFCs are coupled together in stacks to provide the required voltage.
The anode forms a containment area for the micro-organism, which forms a biofilm on the surface of the anode. The activity of the cell is dependant upon the amount of bacteria that is contained in the anode, and thus on the surface area of the anode. The selection of the proper electrode material is crucial for the performance of MFCs in terms of bacterial adhesion, electron transfer and electrochemical efficiency. There have been many approached to scaling up the power production using different carbon-based materials such as carbon paper, carbon felt, carbon fibre as well as carbon nanotube-based composites. To implement the MFC technology in practice, the cost of materials must be reduced and power densities must be maximised.
The output power of MFCs is greatly constrained by the surface area of electrodes. The ohmic losses are directly proportional to the resistance of the electrode. The easiest way to decrease the resistance is to increase the effective surface area while keeping the volume the same, hence enhancing the efficiency of the anode. The anode is typically constructed of a material with a very high surface area, typical types being carbon nanostructures. The anode material significantly impacts the biofilm formation and the electron transfer between the micro-organism and the electron acceptor.
Cathodes reduce oxygen to produce water and cathode materials are chosen to have catalytic properties for oxygen reduction. Platinised carbon electrodes are commonly employed as oxygen-reducing cathodes in MFCs. However, high cost and catalyst poisoning prevent the practical applications of Pt-based cathodes. The high-cost Pt is often replaced by other nonprecious electrodes like Mn2O3, and Fe2O3.
The single chanber version uses an air cathode, which allows air form the outside to enter the chamber and react with the protons travelling from the anode. Air cathode cells have been found to have higher power density then immersed cathodes. A typical air MFC cathode has three layers: a diffusion layer , a conducting support material and a catalyst.
MFCs and waste-water treatment
All types of waste-water containing organic matter can be treated by this process, including domestic waste-water, brewery effluent, and much else. Several plants are in operation and have shown good results. Use of MFCs for waste-water requires a design which allows the waste-water to flow through the cell over the anode surface. Various configurations have been adopted for this purpose, including the tubular MFC where the cathode is placed on the outside of the tube and the anode occupies the full internal space. Waste-water flows through the anode from one end to the other.
The mechanism of oxidation and reduction in the MFC is not clearly understood, and various reactions have been proposed to explain the process. An example using acetate as the substrate follows:
Anode: CH3COOH+2H2O →2CO2 +8e– + 8H+ (1)
Cathode: 2O2 +8e– + 8H+ → 4H2O (2)
Overall: CH3COOH+2O2 →2CO2 +2H2O + Electricity (3)
A disadvantage of the system is relatively low energy production. However as the purpose of the plant is water purification, any electricity produced is a bonus. Developments focused on improving power output are showing results. Cell voltage and current density vary depending on cell type, microorganism used, and substrate. Power output may be expressed in several ways:
Values published in the literature vary widely and there seems to be some confusion about how to express power density, and it seems that in some cases the geometric surface area of the anode is used, and in others the active surface area. Examples of published values from different sources are shown in Table 1.
|Substrate||Source||Current density||Area power density||Vol. power density|
|Domestic wastewater||||–||–||1,7 W/m3|
|Domestic wastewater||||–||–||3,7 W/m3|
|Domestics wastewater||||0,06 ma/cm2||–||–|
|Domestic wastewater||||–||26 mW/m2||–|
|Sewerage sludge||||–||6000 mW/m2||–|
|Anaerobic digestor effluent||||–||–||58 W/m3|
|Brewery wastewater||||0,18 ma/cm2||–||–|
|Brewery wastewater||||–||–||830 mW/m3|
|Abattoir wastewater||||0,130 ma/cm2||–||–|
|Glucose||||1540 mW/m2||51 W/m3|
Other applications of the wastewater based MFC principle include electrolysis, and methane production.
Producing hydrogen gas is possible at very high yields by electrohydrogenesis, in reactors that have various names, usually referred to as microbial electrolysis cells (MECs) . The MEC is based on modifying a microbial fuel cell (MFC) in two ways: adding a small voltage (>0,2 V) to that produced by bacteria at the anode; and by using an oxygen free cathode. The addition of the voltage makes it possible to produce pure hydrogen gas at the cathode. This MEC system is operated as a completely anaerobic reactor. The voltage needed to be added can be produced using power from an MFC. The protons and electrons produced by the bacteria are recombined at the cathode as hydrogen gas, a process called the hydrogen evolution reaction (HER). This is shown in Fig. 4.
The voltage required to produce hydrogen from dissolved organics is of the order of 0,4 V , and the bacteria produce between about 0,2 to 0,3 V. Thus, only 0,2 V or more needs to be added to make hydrogen gas in the MEC. This voltage is much less than that needed for water electrolysis, which is about 1,8 V. It takes a lot of energy to split water, but “splitting” up organic matter by the bacteria is a thermodynamically favourable reaction when oxygen is used at the cathode. In the MEC process, no oxygen is present and the reaction is not spontaneous for hydrogen production unless a small boost of voltage is added to that produced by the bacteria. Thus, the MEC process is more of an “organic matter electrolysis” procedure (versus water electrolysis) .
The simplified reaction is:
Anode: C2H4O2 + 2 H2O → 2 CO2 + 8 e– + 8 H+ (4)
Cathode: 8 H+ + 8 e– → 4 H2 (5)
This is a process whereby carbon dioxide is converted to methane using electric current and a microorganism catalyst. The process is usually intended for CO2 capture or conversion and is usually used to convert surplus energy from renewable sources in to a storable energy carrier. The process can be combined with the microbial fuel cell to convert the CO2 generated by the fuel cell to methane. Fig. 4 illustrates the process.
The simplified reaction is:
CO2 + 8H+ + 8e– →CH4 + 2H2O (6)
The examples given above are mostly at laboratory or pilot stage, but one company has produced a market ready model. Cambrian Innovation’s EcoVolt  uses a MFC in tandem with a secondary set of electrodes to convert carbon rich waste water streams into methane gas by the electromethanogenisis process. The methane can be routed back to the water treatment plant to provide heat and energy or converted by power plant included in the unit. The system is claimed to produce 35 kW of electricity while processing up to 400 kl of wastewater/day. Several units are in operation at breweries and wineries . An experiment has shown that neat urine can be used in an MFC to power a cell phone, which puts a new slant on emergency power systems .
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