Dung: a solution to load-shedding

Cow dung could help us power South Africa. Many countries, such as Germany and the United States, have begun generating electricity from cow dung and food waste, through a process known as biogas production. In South Africa, a number of industries, including waste water treatment facilities and farms, have caught on to this technology, using it to generate heat and to power machines.

Biogas is produced in a digester, an oxygen-free space in which bacteria break down or digest organic material fed into the system. This process naturally produces biogas, which is mainly a mixture of methane and carbon dioxide. Biogas can be used for cooking, heating, lighting and powering generators and turbines to make electricity. The remaining liquid effluent can fertilise crops, as it is high in nitrogen, phosphorus and potassium.

There are many different digester sizes and models available, ranging from small, inexpensive ones made from recycled PVC plastic to enormous stainless steel industrial models that process thousands of litres of waste. Digesters can be found in backyards, farms, hotels and water treatment facilities; in short: anywhere with waste and a need for energy.

Until recently the world has relied heavily on electricity derived from fossil fuels such as coal, natural gas and oil. Once these fuels have been extracted from underground reservoirs, they are treated or cleaned at the coal mines or gas plants to remove impurities such as hydrogen sulphide, helium, carbon dioxide, hydrocarbons, moisture and metals. These fossil fuels are then transported to power plants, where they are burned in a boiler to make steam. This steam turns a big fan, called a turbine, to create electricity. This electricity, via high-voltage transmission lines, substations and distribution lines, eventually finds its way to your house.

But fossil fuels are a “dirty” energy source and they give off greenhouse gases when burned. Those gases are the major contributing factor to climate change.

Methane, the principal gas in biogas, is produced in a number of energy generation processes. However, unlike burning coal or using natural gas, biogas contains fewer particulates (very small particles of pollution) and produces a smaller quantity of greenhouse gases. Biogas production also does not have the “dirty” steps required for fossil fuel extraction, such as mining, cleaning and transporting.

This means that if we replace fossil fuels with biogas, we will lower greenhouse gas emissions.

But we know very little about the interaction of the bacteria inside the biogas digester. To use biogas as a sustainable fuel source, we need to understand and describe the bacteria population and growth dynamics inside the digester to produce biogas optimally.

We do know some of the process, though. Inside the digester, organic matter begins to decompose, because of the bacteria in the organic matter, and starts to produce methane. This process is called anaerobic fermentation because the decomposition takes place in an oxygen-free environment. It happens naturally in landfills, especially when the waste material gets wet and receives little sunlight. In a digester the methane can be collected and stored, but on a landfill it is released into the atmosphere and contributes to global warming.

Biogas production happens in four stages: hydrolysis (breaking complex molecules apart with enzymes that are produced by the bacteria), acidogenesis (when the resulting particles form simple molecules like fatty acids, amino acids and sugars), acetogenesis (simple compounds are then converted into organic acids such as acetic acid and butyric acid) and methanogenesis (finally methane and carbon dioxide result from the organic acids).

During each stage, different types of bacteria convert the organic matter in the cow dung or waste into biogas, which is mostly methane and carbon dioxide with a sprinkling of other gases like nitrogen and hydrogen sulphide. The solid matter that is left over after the fermentation process can be used as an organic soil fertiliser.

Any organic material can be used to make biogas, so it is an excellent way to dispose of agricultural waste, cow dung and sewage sludge.

A specific kind of bacteria, called methanogens, produce methane gas. A gene within these methanogens, known as methyl-coenzyme M reductase-subunit (mcrA), is exclusively found in these bacteria. Methanogens love organic material (including waste) because it has loads of calories and nutrition for them.

Our research is trying to find out how these methanogens (and their unique genes) interact with the chemicals inside the digester to discover the best way to produce methane gas.

We are finding more and more ways to use biogas as technologies develop. Compressed natural gas (CNG), for example, is biogas that has been compressed and can be used as a fuel for vehicles.

Currently we are testing a variety of feedstock including bran, maize and molasses for biogas production potential as well as optimising the conditions leading to maximum biogas production. We are also exploring the potential to use the effluent as fertiliser in local farms. The ultimate goal is to have biogas systems that will supply our university with clean energy.

Reitumetse Molaoa attends the University of the Free State.


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