Microalgae as a Unit Operation in Biorefineries: Nutrient Recycling and Biological CO₂ Utilization

Fraunhofer IGB develops sustainable, resource-efficient and ecologically friendly production processes for the production of valuable products and the energetic use of microalgae combining the use of flue gas from combustion processes or offgas from biotechnological fermentations like bioethanol as the carbon source (combined heat and power stations with biogas or natural gas).

Our intention first is to recover valuable products from microalgae followed by digestion of the residual biomass to biogas. Carbon dioxide is recycled to the algal cultivation process. For a positive net energy balance the use of waste gas as the carbon source is a basic requirement for photoautotrophic microalgae biomass production.

Additionally, recycling of nitrogen and phosphorous from anaerobic digestion effluents is possible.

Wastewater streams from biogas plants used for sewage sludge digestion at high loading rates – so-called high-load digestion – are characterized by high concentrations of ammonium and phosphate, reaching up to 1,300 mg NH₄ per liter and 200 mg phosphate per liter, respectively. Typically, in these wastewater streams, ammonium is converted to nitrogen or precipitated together with phosphate in energy-intensive process steps. Reuse is therefore not possible. In various projects, Fraunhofer IGB has demonstrated that wastewater streams with high N and P content can be used for algae production to close nutrient cycles and replace synthetic media for algae cultivation.

This represents a further step towards the production of algae biomass for energetic utilization (oil, biogas) in combination with the sustainable recycling of water and nutrients which also considerably reduces costs and energy demand.

In the project ”More biogas from low-lignocellulose waste and microalgae residues through a combined bio/hydrothermal gasification, EtaMax) it was possible to successfully use filtrate water from two different municipal biogas plants as a culture medium during initial tests with Phaeodactylum tricornutum, an alga containing the omega-3-fatty acid EPA (eicosapentaenoic acid). Depending on the origin of the filtrate water, it was only necessary to add phosphate to achieve an optimal N-to-P ratio for continuous biomass production in the flat panel airlift photobioreactors. The biomass productivities achieved with filtrate water were even higher than those generated with the synthetic medium. 

In the RoKKa project, the removal of nutrients from filtrate water using microalgae was combined with the utilization of CO₂ from the sewage treatment plant's digester gas.

For every kilogram of microalgae biomass produced, the amount of carbon equivalent to approximately two kilograms of CO₂ is sequestered. Microalgae technology is therefore ideally suited as a unit operation for biological CO₂ sequestration and the production of co-products in biorefineries, linking the disposal or treatment of a waste stream with the value creation generated by the resulting products.

Microalgae module for CO₂ utilization at wastewater treatment plants

In the RoKKa project, a microalgae processing module was able to convert nutrient-rich process streams from the wastewater treatment plant, as well as CO₂ from the plant’s digester gas, into biomass and valuable storage materials. To supply nutrients to the microalgae, magnesium ammonium phosphate produced in the ePhos module was added to the filtrate water – which is rich in ammonium nitrogen – to compensate for the low concentration of phosphate in the filtrate water and achieve an optimal nitrogen-to-phosphorus ratio. The microalgae strain used, Phaeodactylum tricornutum, produced plant-stimulating polysaccharides, known as beta-glucans. These can help plants defend against fungal infections such as powdery mildew and may partially replace chemical pesticides in the future, for example in viticulture.

Microalgae module for utilizing CO₂ from fermentation processes

The goal of the SmartBioH₂-BW project was to integrate a biorefinery into an existing industrial environment at the Evonik Operations GmbH site in Rheinfelden. Using two interconnected biotechnological processes (purple bacteria and microalgae), the aim was to produce biohydrogen and other bio-based products from industrial wastewater and waste streams. Here, a microalgae plant was coupled to the purple bacteria module, which produces carbon dioxide (CO₂) as a byproduct. The algae bind it into their biomass, producing additional hydrogen and starch in the process.

An algae module was also integrated into the “H₂Wood – Black Forest” project to utilize CO₂ from bacterial fermentation. Using the microalga Chlorella sorokiniana, storage compounds such as starch and carotenoids like lutein can be synthesized under the influence of light as additional co-products that can be utilized by various industrial sectors.