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.