Microalgae, carbon dioxide and their role in global warming
The briefest of summaries of global warming and greenhouse gases
Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities (methane – another important greenhouse gas is considered in the Bacteria, methane and biodegradable plastics page of this site). The latest measurement of carbon dioxide (at the Mauna Loa Observatory, Hawaii) shows carbon dioxide levels of 406.1 ppm (parts per million). These results show a significant increase in levels of atmospheric carbon dioxide over recent years. (See Figure to the right).
UN Climate Change Annual Report, 2017
Santos Climate Change Report, 2018
How carbon dioxide enters the atmosphere
CO2 emissions, 2018
Removing carbon dioxide from the atmosphere
- Increasing the efficiency of processes, such as fuel efficiency in vehicles
- Fuel switching from coal-fired electric plants to natural gas-powered facilities
- Use of wind power
- Use of solar power
- Use of nuclear power
- Reduction of deforestation
- Carbon capture, storage and utilization
Scrutinize CO2 removal methods: the viability and environmental risks of removing carbon dioxide from the air must be assessed if we are to achieve the Paris goals, 2018
Using microalgae to remove carbon dioxide from the atmosphere
Approximately 1.8 x 1014 kW of solar energy is intercepted annually by the Earth. About 60% of this amount or 1.08 x 1014 kW reaches the surface of the Earth. The rest is reflected back into space or absorbed by the atmosphere. The total annual solar radiation intercepted by the Earth is more than 7,500 times the world’s total annual primary energy consumption. This number is decreasing as human energy consumption increases.
The sun and the earth's climate, 2007
Advantages of using microalgae to absorb carbon dioxide
There are number of reasons why microalgae are an attractive option for absorbing carbon dioxide:
They grow very rapidly, doubling their numbers every few hours
They store energy as oils and carbohydrates which can be the basis of a wide range of valuable products
They can use carbon dioxide in the flue gases produced by power plants and anaerobic digestion facilities preventing release to the atmosphere
They do not compete with agriculture in that they can be grown in ponds (see later) situated on soils unsuitable for the growth of crops
The microalgal biomass produced can be used directly as human food, animal feed, feed for aquaculture, fertiliser, or can be extracted to produce biofuel, biodegradable plastics, lubricants, and a wide range of pharmaceuticals, cosmetics and nutrachemicals
After extraction, the residual biomass can be used as fuel for industrial boilers or as a feedstock for anaerobic digestion
Capturing CO2 from the air for accelerating growth of algae, 2017
Biocapture of CO2 by different microalgae-based technologies, 2018
Growing microalgae commercially
This is a large topic and will be dealt with in more detail at a later date. In summary, there are two main ways of growing microalgae on a large scale:
These are shallow artificial outdoor ponds in which sunlight is used as the source of energy. The pond is filled with water and nutrients and inoculated with a selected strain of microalgae. Often paddle wheels are used to circulate the growing microalgae around the pond.
A fraction of the water containing the microalgae is generally harvested each day and the microalgae concentrated by sedimentation, centrifugation or filtration. The resultant biomass can then either be used directly, or dried to a powder.
MicroBio Engineering Inc., 2018
These are closed systems of various designs, made from glass, plastic or metal, in the form of vertical tubes or bags, horizontal tubes, or panels. The photobioreactors may be used out of doors or within a building. They are more expensive to construct than open ponds but offer the advantage of greater control cover the growth conditions, by the use of artificial lighting and temperature control, and greater protection from contamination with unwanted micro-organisms such as rotifers that can use the microalgae as feed.
Dynamic modelling of microalgal production in photobioreactors, 2017
Commercially grown microalgae
The number of species of microalgae growing naturally in the world is unknown, but estimates range from 72,000 to several hundred thousand. To date, a very small number of microalgae have been grown commercially. Photographs of many of these are shown in the Microalgae page.
It is clear that there are many species of microalgae that have not yet been tested or cultivated that may have very useful characteristics and produce valuable products.
Valuable bioproducts obtained from microalgal biomass and their commercial applications: a review, 2018