Biomass consisting of waste from the food and feed industries as well as from wholesale markets accumulates in huge quantities during the production and sale of food and feed products. The organic substances have a high water content and a low lignocellulose content and are therefore ideal for fermentation. Microalgae produce up to 100 tonnes of dry matter per hectare per year (compared to 17 tonnes for sugar cane and 3.5 tonnes for wheat).

Nevertheless, the sustainable production of bioenergy from waste and algal biomass has so far not been sufficiently exploited. This is where the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart comes in. In cooperation with other companies and research institutes, the Stuttgart-based IGB operated a research facility where microalgae thrived in biogas production reactors. To this end, the German Federal Ministry of Education and Research (BMBF) launched the joint project "EtaMax - More biogas from low-lignocellulose waste and microalgae residues through combined bio-/hydrothermal gasification" within the "BioEnergy 2021" programme. As part of the 5-year project, which ended in early 2017, the scientists involved gained valuable experience and provided prospective approaches.

The project was coordinated by Dr.-Ing. Ursula Schließmann, head of the Department of Environmental Biotechnology and Bioprocess Engineering (UBT) at the Fraunhofer IGB and its aim was to convert organic matter, algae and wholesale market waste into biogas while producing as much energy as possible. Amongst other things, the researchers used a high-load fermentation process that was developed at the Fraunhofer IGB. The project mainly focused on regional production and use of regenerative methane made from biogas. “We have the perfect environment for this,” explains Schließmann, referring to the Fraunhofer Institute, which is seen as an ideal location for the project. Stuttgart Wholesale Market supplied fruit and vegetable waste for the EtaMax project. The purified biogas that was produced was also tested at a site near the IGB for its suitability as a fuel for powering CNG (compressed natural gas) vehicles.

”The composition of biowaste changes daily and requires a high degree of system flexibility,” says Dr. Schließmann, highlighting one of the major challenges involved in using organic waste to generate energy and fuel. “We not only had to deal with different types of fruit and vegetables, each with their own characteristics (such as acidity, seasonal composition), but also with a wide range of structural compounds.” For example, fibrous components (such as mango) were present alongside melon seeds and other hard seeds. “As part of the project, the Fraunhofer Institute for Process Engineering and Packaging IVV developed a shredder that efficiently shreds all these components in one plant, gently breaking down the proteins, carbohydrates and fats.” The substrate was stored in several containers. A management system specifically developed for the project was used to calculate the ratio in which the waste in the different containers had to be mixed to obtain the best possible results. Different parameters had to be taken into account. “The microorganisms that we used for fermentation require constant conditions,” says Dr. Schließmann, highlighting an important development step.

The reactors were used to efficiently convert fruit and vegetable waste into biogas in a two-stage process. This involved adapting a high-load fermentation process to the new type of biomass. The original process was developed at the Fraunhofer IGB in 1994 and used for fermenting sewage sludge and manure. Schließmann and her team were able to achieve degradation levels of up to 95%. The biogas yield was between 840 and 920 litres of biogas per kg of organic dry matter; the methane content was 55 to 60%. Schließmann emphasises the potential of this method: ”The system has a hydraulic residence time of just 17 days per stage and can be operated in a stable way even with changing fruit and vegetable waste. And all this is possible with a high degree of degradation and high biogas yield.”

Both carbon sources, i.e. organic waste and algal biomass, were combined with the objective of efficiently producing biogas. The team led by Schließmann developed a fully automated two-stage process in outdoor reactors for the material and energetic use of the algal ingredients. They then transferred this to a pilot scale. "We needed to find an algal strain that can cope with shifting light conditions. Too little or too much light limits algal growth and causes the alga to fade and die,” says Dr. med. Ulrike Schmid-Staiger, Group Manager Technical Microbiology at the Fraunhofer IGB.

”Our flat-plate airlift reactor ensures that the algal broth is optimally mixed. The light is evenly distributed across all cells, increasing the growth rate and algal cell concentration.” Depending on the cultivation conditions, the algae produce proteins, fats or carbohydrates that can be specifically extracted in a cascade matrix. Thus, a wide range of chemicals such as vitamins, omega-3 fatty acids and carotenoids can be supplied to the pharmaceutical, food and chemical industries. If the algae receive sufficient nutrient medium and carbon dioxide in addition to sunlight, their cells produce oil that can drive diesel engines.

The liquid fermentation residues from the biogas reactors were returned to the material cycle and used for cultivating the microalgae that would produce the diesel oil. It was possible to establish a mixed algae culture that was specially adapted to these liquid fermentation residues. Schließmann outlines the advantages: ”The nitrogen and phosphate components that are produced during fermentation are instantly eliminated and the material cycles are closed. This also helps reduce production costs for algal biomass.” The researchers also extracted from the biogas reactor the carbon dioxide that algae need in order to grow: the resulting biogas consists of about two-thirds methane and one third carbon dioxide. The major advantage of the process is obvious: everything is recycled.

Utilisation of all the methane produced

The resulting biomethane can be used as an alternative carbon source for producing useful industrial and pharmaceutical products. In addition, the gas can be fed directly into the natural gas grid, used as a heat source and as fuel for vehicles. The researchers tested a membrane system for purifying biomethane with which it was possible to remove carbon dioxide and other gas constituents such as hydrogen sulphide that were present in low concentrations. This system was used to produce batches with different methane concentrations for performance tests. CNG test vehicles were run on these fuel qualities. These studies are particularly interesting from an international point of view, since the quality of natural gas used for powering these vehicles is not the same the world over.

Further process development poses a challenge for the scientists. Methane in combination with oxygen can form flammable gas mixtures. In addition, substrate limitation, which is a result of methane’s poor water solubility, is a key pivotal point that needs be addressed. Bubble-free fumigation under adapted pressure and temperature settings as well as special membranes can be used to optimise the material utilisation of methane and to calculate the economic benefit.

The use of methane as an alternative substrate source for producing useful industrial and pharmaceutical products places considerably higher demands on the process than conventional glucose-based fermentation processes, for example. Methane and oxygen are gases that are poorly soluble in water. The researchers developed a membrane reactor that guarantees the supply of substrates and prevents substrate limitation from occurring. The flow of the reactor was simulated to optimise the design.

Further treatment of the fermentation residues and nutrients obtained offers opportunities for further optimisation. In contrast to pure algae cultures, waste fermentation results in large amounts of components that contain poorly degradable lignocellulosic components.

The Fraunhofer IGB researchers gained many insights from the project in areas ranging from individual process procedures to machine learning in plant development and control. "Automated process management is a big step towards achieving system stability, and thus transferring the system into industrial use," says Schmid-Staiger describing possible future prospects. However, a specific process must be established for each starting substrate. In the case of the EtaMax project, fermentation was geared towards Stuttgart Wholesale Market waste. The fermentation of other starting materials, e.g. from industrial waste or agriculture, which occur on a larger scale, could also be of interest for the Ministry of Agriculture and the Ministry of the Environment. Such cases also require substrate-specific process optimisation.

At present, the Fraunhofer IGB is working on technical processes for the reasonable utilisation of residues that accumulate during fermentation. Schließmann emphasises: “No project is set in stone. Project steps always need redesigning according to the substrate used and the objective. We already have a wealth of experience that we would like to bring in to other, flexible projects. “Projects can be designed according to specific requirements and in a future-oriented manner transferred from pilot scale to industrial scale – as sustainable, resource-saving and environmentally friendly processes for the material and energetic use of biomass.”