Using waste nutrients to cultivate microalgae – Swansea team publishes new article in Bioresource Technology

The paper explains the ways in which algae can be cultivated using excess nutrients, the optimum two-step approach used to maximise productivity, and how this biomass is optimally suited for use as an animal feed ingredient.
Implementation of circular approaches in industry, by minimising waste and optimising reuse of resources, is of critical environmental importance. Microalgal cultures are particularly adept at waste remediation and are also incredibly versatile in how they can be processed and applied. This article explains in more detail how the ALG-AD team have demonstrated the ability of microalgae to remediate organic waste at an industrial scale.

Anaerobic Digestion (AD) is the main approach used in the EU and the UK to treat organic waste. There are more than 17,000 AD plants in operation in the EU, who primarily process organic wastes to produce electricity from biogas. Digestate is a side-stream of AD, with nearly 10 million tons of excess digestate produced. Digestate is a type of sludge, high in nitrates and phosphates, which is typically spread on land as a fertiliser – however with strict limits on how much, and when, this can be used. This is to prevent the nutrient run off, leading to pollution in waterways, which can have devastating effects on the local ecosystem.
The ability of microalgae to solve environmental issues, such as wastewater treatment or eutrophication has been demonstrated, but only at a relatively small scale. Microalgae can grow and produce biomass using different culture systems, growth strategies and in different geographical conditions. There are three different growth modes: autotrophy, heterotrophy and mixotrophy. Autotrophic cultures are the most common, and these organisms use light as an energy source, CO2 as a carbon source and nutrients (mainly nitrogen (N) and phosphorous (P)) to grow. Heterotrophic cultures grow in darkness and with a regular carbon source. Mixotrophic cultivation is the growth mode where microalgae simultaneously use inorganic CO2 and organic carbon sources in the presence of light, to increase the biomass over a short period.

 

The ALG-AD team have successfully demonstrated a combination of two different growth modes (autotrophic-mixotrophic growth) at industrial scale, at our UK pilot facility at Langage AD, Plymouth.

As well as the team developing approaches to optimise microalgal productivity using digestate as a cultivation medium, the cultivated biomass has also undergone extensive analysis to understand the characteristics of the microalgae, and how it compares to commercially grown equivalents.
Data shows that the algae grown on digestate are higher in protein than the commercially cultivated equivalent, which is particularly interesting when the final aim is to use the biomass as feed for animals. The algae also shows interesting increases in carotenoids, which are molecules known for their health boosting properties. These results reinforce the exciting potential for quality microalgae biomass produced under mixotrophic conditions to be used for animal feed purposes. There are also other high value metabolites that can be extracted from microalgae, such as pigments, omega-3 oils and amino acids, which can be used in a range of industrial applications.

Currently, microalgal biotechnology is in its youth, but is increasing in global prevalence, with applications in industrial fields ranging from pharmaceuticals and cosmetics to animal feed (e.g. aquaculture) and biostimulants.
Our work on ALG-AD has proven that microalgae can be used at scale to remediate excess nutrients, and produce a quality, sustainably cultivated biomass with multiple commercial applications. Legislation at present is unclear and difficult to navigate, however with establishment of a suitable legal framework, this technology has the potential to remediate thousands of tonnes of digestate, and therefore use the nitrates and phosphates within that digestate, without the risks of pollution linked to storage or returning this to land.
This new circular economy industrial approach could help to alleviate eutrophication risks, produce large quantities of microalgae biomass for animal feed production, and create thousands of new sustainable jobs.


The team is always keen to speak to anyone interested in exploring algal cultivation approaches, so for more information please get in touch with Dr Claudio Fuentes Grunewald 

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