Microalgae contain oils, or 'lipids', that can be converted into biodiesel. The idea of using microalgae to produce fuel is not new, but has received recent renewed attention in the search for sustainable energy. Biodiesel is typically produced from plant oils, but there are widely-voiced concerns about the sustainability of this practice. Biodiesel produced from microalgae is being investigated as an alternative to using conventional crops, such as rapeseed: microalgae typically produce more oil, consume less space and could be grown on land unsuitable for agriculture. However, many technical and environmental issues, such as land use and fertiliser input still need to be researched and large-scale commercial production has still not been attained.
Using microalgae as a source of biofuels could mean that enormous cultures of algae are grown for commercial production, which would require large quantities of fertilisers. While microalgae are estimated to be capable of producing 10-20 times more biodiesel than rapeseed, they need 55 to 111 times more nitrogen fertiliser: 8-16 tonnes per hectare per year. Such quantities of nitrogen and phosphorus could damage the environment. Additionally, it could limit the economic viability of using microalgae. Nitrogen and phosphorus found in algal waste, after the oils have been extracted, must therefore be recycled. The research suggests that 'anaerobic digestion' could accomplish this goal.
Anaerobic digestion of the algal waste produces carbon dioxide, methane and ammonia. Left-over nitrogen and phosphorus compounds can be reused as fertiliser to the algal process. Using the methane as an energy source can further enhance energy recovery from the process.
In the laboratory study, the researchers highlighted some key issues to be addressed in microalgal production:
- Sodium (in salt) can inhibit the anaerobic digestion process when using marine algae, although researchers suggest suitable bacteria (anaerobic digesters) can adapt.
- Digestion of the algae can be enhanced and the methane yield increased by physical or chemical pre-treatment to break down cell walls and make the organic matter in the cells more accessible.
- The nitrogen content of certain algae can be high, resulting in greater levels of ammonia which can also inhibit the digestion process. One strategy to overcome this problem uses a 'codigestion' process, whereby other organic waste, which is higher in carbon and lower in nitrogen, is added to the algal waste. For example, paper waste, food waste or sewage sludge can be added to the process. However, this can raise questions concerning 'downcycling', which is generally discouraged.
- Another strategy would be to choose species of microalgae that naturally have a higher carbon to nitrogen ratio.
The results suggest that if the lipid content of the microalgae is less than 40 per cent, more overall energy would be recovered if just methane is produced directly from the algae, without first extracting the lipids. This method of methane production would need to be fully assessed against other methods, such as biomethane production from waste, to understand its viability. However, if the goal is to produce biodiesel, the key implication is that the species of microalgae used for cultivation should be chosen carefully: the algae should have greater than 40 per cent lipid content. Additional energy recovery through methane is then possible through anaerobic digestion of the algal waste after oil extraction.