Composting of waste algae: A review

• The review highlights that: Composting has been successfully used at pilot scale to manage waste algae removed from eutrophied water environments.
• Algae are characterised by a relatively low carbon to nitrogen (C/N) ratio, which can result in nitrogen loss as NH3 and even N2O.
• Algae are characterised by high moisture content and low porosity, which together make aeration challenging.
• Algae are of high salinity, metals and toxin content, which can affect composting progress and application of the product as a fertiliser.
• That these challenges, peculiar to algae, can be overcome by co-composting the algae with other materials.

Although composting has been successfully used at pilot scale to manage waste algae removed from eutrophied water environments and the compost product applied as a fertiliser, clear guidelines are not available for full scale algae composting. The review reports on the application of composting to stabilize waste algae, which to date has mainly been macro-algae, and identifies the peculiarities of algae as a composting feedstock, these being: relatively low carbon to nitrogen (C/N) ratio, which can result in nitrogen loss as NH3 and even N2O; high moisture content and low porosity, which together make aeration challenging; potentially high salinity, which can have adverse consequence for composting; and potentially have high metals and toxin content, which can affect application of the product as a fertiliser. To overcome the challenges that these peculiarities impose co-compost materials can be employed.