In 2020, Germany’s population collected over five million tonnes of biowaste in their organic waste bins. Most of this was composted, and some was fermented into biogas. Scientists in Baden-Württemberg think there's room for more. They want to transform biowaste into new products in a biorefinery. The idea is to produce biodegradable plant pots, mulch material, fertilisers, enzymes and biobased plastics in addition to biogas. The ‘Biowaste to Products’ (BW2Pro) project is being financed with 5.87 million euros in EU and state funding.
The pilot biorefinery is being set up on the site of the biowaste fermentation plant in the city of Backnang. Currently, 36,000 tonnes of biowaste are being fermented into biogas here each year and then combusted in the combined heat and power plant to generate heat and electricity. "Farmers in the region use the liquid digestate to fertilise their fields, while the solid digestate is composted," explains Dr. Lutz Bühle, Chief Technology Officer at Abfallwirtschaft Rems-Murr AöR. "Unfortunately, only 100 to 150 plants in Germany offer this opportunity, which leads to the recycling of a maximum of 25 percent of biowaste," says Dr. Hans Oechsner, head of the State Institute of Agricultural Engineering and Bioenergy at the University of Hohenheim. "Biowaste is exclusively composted in around 1,000 large composting plants," he adds. Yet it would theoretically be possible to go much further and use biowaste not only for energy and materials, but also as a secondary source of raw materials. This is exactly where BW2Pro comes in, and it has the potential to take Baden-Württemberg much closer to its goal of building a sustainable bioeconomy.
The biorefinery will process one metric tonne of biowaste per day. "On this scale, it’s easy for us to see where the difficulties arise and where we can still optimise," explains Oechsner. First, the waste is cleaned of stones, sand, foils, plastics and broken glass. This involves several steps of sieving and shredding. It is then thoroughly swirled in the biorefinery’s hydrocyclone, a process during which the biowaste, along with water, flows sideways into the hydrocyclone’s cylindrical section and circles downward. As it tapers, an upward vortex in the centre carries the lighter biowaste, while the heavy contaminants are pressed against the cyclone wall by centrifugal force and removed downward. "Biowaste is not homogeneous, so it is a challenge to treat it in a way that eliminates impurities for subsequent processes," says Dr. Claudia Maurer from the Institute of Sanitary Engineering, Water Quality and Waste Management at the University of Stuttgart. She is in charge of the treatment processes and coordinating the project together with Oechsner.
Once this step has been completed, the biowaste moves on to the next phase – thermal pressure hydrolysis. "Think of it as a robust pressure cooker," says Oechsner, who is testing the process with his team. The biowaste is heated to 160 degrees Celsius for about 30 minutes. A pressure of six bar is generated, compressing the cells. The cooking process is then suddenly stopped, causing the cells to relax abruptly and burst open. The nutrient-poor fibres can then be squeezed out by the nutrient-rich liquid from inside the cells, which contains carbohydrates, proteins and fats. Fibres and fluid now go their separate ways.
The fibres are dried and sorted. Novis GmbH presses fine, short fibres on site into small plant pots that are planted directly into soil and then disintegrate. Maurer and her team are carrying out detailed laboratory investigations to find out what happens to the pots in the soil. "And if there’s enough time, we’ll also look at plant pots made of fibres mixed with bio-resin or paste," says the agricultural scientist. The idea is that admixtures will allow the researchers to manufacture products other than plant pots.
Coarser fibres are being tested for their suitability as a mulch layer in nurseries. The issues are how thick the layer needs to be and whether plants can be sown through it. Mulching protects the soil from erosion and drying out and suppresses weeds. The fibres are broken down very slowly, so that the carbon they contain is not immediately released into the atmosphere as carbon dioxide, but is temporarily stored in the soil.
On the one hand, the nutrient liquid is fermented into biogas. This brings a higher yield than liquid manure and maize silage and a lower yield than waste fats from the slaughterhouse. The yield fluctuates because biowaste has different compositions over the course of the year. If there are a lot of shrub cuttings in autumn, it is lower; if there is a lot of food waste, it is higher. This is one of the challenges that Oechsner identifies: "It may be that certain biowaste is simply not suitable and is better composted." In the biogas process, all degradation steps normally run in parallel. With BW2Pro, the researchers rely on a two-phase system: carbohydrates, fats and proteins are converted into various acids by different microorganisms in several steps. These are temporarily stored and only metabolised into biogas by methane-forming microorganisms when needed. "In this two-phase process, we can work faster and more flexibly, produce biogas when it is needed and thus close gaps that arise with other renewable energies such as photovoltaics and wind," says the agricultural engineer.
Nutrients such as nitrogen, phosphorus or potassium remain in the liquid fermentation residue, because the biogas process mainly consumes carbon. The Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB wants to recover the nutrients from the digestate and produce fertiliser or raw materials. The digestate, which is now low in nutrients, is returned to the cycle. This is because liquid is needed to break down the biowaste during the thermal pressure hydrolysis process. It can also be used to irrigate fields.
Not all products are produced directly at the biorefinery. Some first need to be developed on a small scale in the laboratory. For example, two institutes at the University of Stuttgart are looking to use the nutrient-rich liquid to produce biobased plastics from polyhydroxyalkanoates (PHA). These biopolyesters are produced as reserve substances by certain microorganisms under deficiency conditions. The Institute of Interfacial Engineering and Plasma Technology is initially testing different feeding strategies, which change the composition and thus also the properties of the biopolymers. The Institute of Plastics Technology tests how the material can be processed and examines the properties. With additives, it is transformed into biobased plastics that can be processed on common plastics technology machines and thus also used industrially. The goal is to produce sample quantities for the plastics industry in larger bioreactors of 1,000 litres. Here, too, Maurer is investigating in the laboratory whether PHA are completely degraded in the soil and how long this takes.
Scientists at the Offenburg University of Applied Sciences want to produce the enzyme cellulase from liquid and fibres of plant biowaste using biotechnology. The enzyme will be used in the project to break down the cellulose of the plant cell wall into sugar. This would enable biogas and biobased plastics to be produced more quickly.
The exciting questions in the BW2Pro project are, on the one hand, whether the individual process steps work well and the interfaces function, and on the other, whether the system makes economic and environmental sense. The Institute of Energy Economics and Rational Energy Use at the University of Stuttgart is evaluating this by carrying out an economic analysis of the individual technologies and the process as a whole. Meanwhile, the ifeu - Institut für Energie- und Umweltforschung Heidelberg gGmbH will be carrying out environmental investigations. The two institutes will accompany the project throughout its entire duration so that any required optimisations can be made. An analysis is also being carried out of current recycling systems so that the different systems can be compared when the project comes to an end.
"If we can use the project to show that the system makes economic and environmental sense, the next step would be to set up a biorefinery on a workable scale within five years," says Oechsner. He believes this could spark off similar projects. "If the concept works, waste disposers will become producers, which has economic potential," says Bühle. It would have to be tested thoroughly, but after all, the technology they have developed started out as a prototype. "The Waste Framework Directive means that waste will soon be collected separately in other European countries as well," Maurer reports. These countries could then invest directly in a concept in which biowaste is used as a resource.
It goes without saying that any new systems of this kind need to be supported by the industry and society and the resulting products must be accepted. This can only succeed if information is provided. BIOPRO Baden-Württemberg is therefore supporting the project with a variety of communication measures.
BW2PRo is receiving 5.87 million euros in funding from the Baden-Württemberg Ministry for the Environment, Climate and the Energy Sector and the EU Commission as part of the ‘Bioeconomy Bio-Ab-Cycling’ ERDF funding programme. The project began in March 2022 and runs until March 2024. Currently, the scientists are characterising the biowaste, analysing what happens to it during thermal pressure hydrolysis, waiting for the building permit and preparing tenders; some of the required aggregates have already been ordered. They are finding creative solutions for increasing costs and delivery difficulties. The biorefinery is scheduled to go into operation in October 2022. "That will be exciting," says Oechsner, "and we'll see how we manage to create a wonderful cycle ."
Federal Statistics Office - Waste Balance 2020: https://www.destatis.de/DE/Themen/Gesellschaft-Umwelt/Umwelt/Abfallwirtschaft/Publikationen/Downloads-Abfallwirtschaft/abfallbilanz-pdf-5321001.pdf?__blob=publicationFile