How to Generate Electricity from Dirty Water Using Microbes - Printable Version

+- (
+-- Forum: General (
+--- Forum: Renewable Energy (
+--- Thread: How to Generate Electricity from Dirty Water Using Microbes (/thread-30539.html)

How to Generate Electricity from Dirty Water Using Microbes - FarmTech - 02-19-2023

Do you know that any dirty water (waste water)  containing micro organisms can be used to generate electricity? The dirty water can be manure slurry, waste water from agro processing, urine, brewery wastewater, kitchen wastewater etc. This can be achieved using a microbial fuel cell (MFC). So how did scientists achieve this feat?

One of the simplest setup consist of a 2 chamber container. The chambers are separated by a membrane which can be a costly material like Nafion or a cheap one like earthenware. Two electrodes are placed in the chambers and connected to a load (such as LED lights, sensors etc) using copper wires. One of the electrodes is called the anode (negative) and the other a cathode (positive). The anode chamber is filled with wastewater and sealed to prevent oxygen from entering while the cathode chamber is open to air.

[Image: mfc-2-chambers.jpg]

The anode can be made from graphite, carbon brush (best), carbon cloth, carbon veil, stainless steel mesh etc. The cathode can be made from activated carbon (AC), modified activated carbon, AC mixed with polytetrafluoroethylene (PTFE) and pasted on a stainless steel mesh, carbon black, carbon black modified with aerogel, carbon nanotubes (CNT), carbon nanofibers (CNF) graphene etc.

Don't let these names scare you though. Some of them are quite simple to obtain. For example, you can use the lead in pencil as graphite. You can make activated carbon by treating charcoal with chemicals like lemon juice, bleach, potassium hydroxide, calcium chloride, phosphoric acid etc.

What happens in a MFC is that microbes attached to the anode surface will eat organic matters and produce:

1. Carbon dioxide (CO2)
2. hydrogen ions H+ and
3. electrons.

The electrons will flow externally from anode to the cathode as follows: from the microbes to the anode; anode to copper wire; copper wire to the load and load to the cathode. This electron flow generate electricity that powers the load.

Internally, the hydrogen ions will flow to the cathode chamber through the separating membrane where it will combine with electrons and oxygen to form water.

One disadvantage of microbial fuel cell (MFC) is their low power output. A MFC with a 1 square meter anode surface area will produce only about 0.4 miliwatt depending on the type and components used. You need about 1 watt (1000 miliwatt) to light up a high power LED light. So a MFC needed to light up this LED will need to have an anode surface area of about 2500 square meter (Roughly one-third of a football field!). But there is hope due to new scientific advances that have been made after years of research. By using specially made electrodes and more effective configurations, power output have been  boosted (up to 2 watts per square meter!). By connecting the output of many MFC in series or parallel, MFCs have been successfully used to power LED lights and even charge mobile phones.

Pic of 1w Led
Pic of series parallel

Soil-based microbial fuel cells
In this type of MFC, the anode is buried few centimeters in the soil while the cathode rest on top of the soil (or it is slightly buried), and is exposed to air. The soil must be wet and rich in organic matters like dead leaves, manure etc.

[Image: SoilMFC.png]
A Soil Microbial Fuel Cell

Plant microbial fuel cells
This one is similar to soil-based microbial fuel cells. Just that plants are grown in the soil. The plant roots will secrete nutrients that will feed soil microbes. These microbes will release electrons that will produce electricity. As a result of this feeding of microbes, plant MFC will generate more electricity than soil-based MFC.

[Image: plant-mfc.jpg]
A Plant Microbial Fuel Cell

Apart from generating electricity, MFC can be used to remove excess nutrients from wastewater and reduced its biochemical oxygen demand (BOD) before releasing it into rivers and streams. Excess nutrient can cause algae bloom in rivers, streams and oceans. This is a situation where algae, a type of water plant, will grow fast and cover the water surface. Then they will die and decay. As they decay, they use up water oxygen and cause fish and other water creatures to die in masses.

Biochemical Oxygen Demand (BOD) is the amount of oxygen required by microorganism to breakdown organic matters. If you release wastewater into water bodies without first treating them, microorganisms will use more oxygen to breakdown the organic matters. This will kill or stress water creatures.

MFCs have also been used to remove pollutants like antibiotics, metal ions etc from wastewaters.

There are 3 other variants of MFC, namely microbial electrolysis cell (MEC), microbial desalination cell (MDC) and microbial electrosynthesis.

1. Microbial Electrolysis Cell (MEC): In this cell, microorganisms and an external power supply are used to produce hydrogen gas. Part of the electricity comes from microbes and the rest comes from an external power supply like battery. Thus less energy input is required.

[Image: m-electrolysis.jpg]
Microbial Electrolysis Cell

2. Microbial Desalination Cell (MDC): Here, microbes are used to turn salty water into fresh, drinkable water. According to wikipedia, only 0.3% of the earth's water supply is usable for human consumption, while over 99% is sequestered by oceans, glaciers, brackish waters, and biomass. With MDC, these unusable water can be made fit for human use. 
[Image: desalination.jpg]

A Microbial Desalination Cell

There are other ways to desalinate salty water such as reverse osmosis, nanofiltration etc. But MDC is preferable because of its lower costs, energy and environmental impacts.

3. Microbial Electrosynthesis: This is where microbes are feed carbon dioxide (CO2) and electricity to produce chemical products like acetic acid, ethanol, butanol, butyric acid, hexanoic acid and hexanol. This can be achieved while treating wastewater. This is important because CO2 is the major cause of global warming.  Unfortunately, this technology is still at an infancy stage.

[Image: electrosynthesis.jpg]
Image Source:

A typical MES setup consists of two chambers, namely anodic chamber and cathodic chamber; separated by a proton exchange membrane (PEM) that allows the protons to migrate from the anodic to the cathodic chamber. At the anode, water molecules split into protons, electrons, and gaseous oxygen. The oxygen escapes the anodic chamber, protons are transferred to the cathodic chamber through the PEM, and the electrons are drawn to the cathode through an external circuit. In the cathodic chamber, the electrons and protons or energy carriers such as such as hydrogen and CO2 are combined by biocatalysts (eg acetogen microbes) to produce primarily volatile fatty acids (VFAs) like formate, acetate, butyrate, etc. An external power supply is needed to achieve this.

The world have gone a long way technologically. But it seems like Africa is left behind mainly due to lack of research fundings.