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Algae, and particularly microalgae are not only suitable for environmentally friendly production of many commodities, but also for the use of waste streams. They grow excellent on carbon dioxide from flue gases, residual water of agro-industrial companies and even diluted digestate from manure. In return they produce valuable raw materials. Algae recycle nutrients that thus remain available in the nutrient cycle, instead of being wasted and pollute the water. Algae are not only promising as waste converters and recyclers. The algal cell contains many useful substances and microalgae are cultivated increasingly for the production of valuable raw materials. For example, it is possible to produce oil, proteins, starch and pigments (e.g., beta-carotene). Applications of these materials are numerous, ranging from biodiesel and bioplastics to colorants and hamburgers.

Microalgae, also called phytoplankton by biologists, are very small plant-like organisms between 1-50 micrometres in diameter without roots or leaves. Together with the seaweeds (macroalgae or large aquatic plants), microalgae are part of the so-called aquatic biomass. Microalgae are very common (hundreds of thousands species exist) and occur both in freshwater and seawater where they form the basis for most food chains. Most species contain chlorophyll, use sunlight as an energy source and convert carbon dioxide (CO2) into biomass. In this process of photosynthesis the algae produce oxygen (O2). On a global scale microalgae produce more than 75% of the oxygen required for animals and humans.

Normally microalgae are not visible by the naked eye, but if the water is eutrophic, massive algal blooms occur, changing the water in a green, brown, blue, or orange liquid mass. Only a few tens of thousands, out of a total of 200,000 to 800,000 different species, have been described in literature. With so many unknown algae species an almost inexhaustible source of possibilities exists.

The genetic analysis and ranking of all types of microalgae is still in progress and there is not yet a complete and consistent classification. At the moment taxonomists have distinguished the following main groups:

  • Green algae. With 7500 species they form one of the largest groups of algae. These algae contain chlorophyll (like in plants) and a large amount of proteins. In addition, under stress conditions they produce starch and oil that are stored inside the cell. Green algae exist as unicellular or multicellular species. Chlorella is a well-known single-celled species, which also is grown commercially.
  • Red algae are a group of 5000 mostly multicellular marine species, living in the tidal zone of the sea.
  • Diatoms. With more than 100.000 species, this group of unicellular algae produces most of the biomass on earth. They are an indispensable food source for the zooplankton in freshwater and seawater. These algae have a particularly attractive skeleton of silica that fits together like two halves of a sphere. Diatoms produce mainly oil that is stored in the cell. By varying the amount of oil they can regulate their buoyancy.
  • Brown algae. Virtually all 1500 to 2000 species are multicellular algae existing almost exclusively in the sea. Brown algae like kelp are often found on the beach, and are therefore regarded by many people as the traditional seaweed.
  • Gold algae. This group of 1000 species of beautifully colored unicellular algae exists mainly in fresh water, but also a number of marine species are known. They possess flagella that are used for displacement.


Biogas is produced out of biomass through anaerobic digestion. The process consists of four steps, each of which carried out by different groups of bacteria:

1. Hydrolysis: large organic polymers such as carbohydrates, fats and proteins are broken down into smaller constituents, (e.g. simple sugars, amino acids, fatty acids and water);

2. Acidogenesis: further breakdown of remaining components, done by the acidogenic bacteria which convert the material into short-chained fatty acids, alcohols, CO2, hydrogen and ammonia;

3. Acetogenesis: organic acids are formed, the basis for the eventual methanogenesis. The bacteria responsible for this phase, the acetogens are highly sensitive to temperature fluctuations. The methanogenesis itself also slowly start.

4. Methanogenesis: Ninety per cent of the total amount of methane is formed during this phase. Also CO2 is released and, in small proportions, also water, H2S and N2. The content of methane in biogas typically varies between 50% and 60%.