A research team at the ZHAW Institute of Chemistry & Biotechnology, based in Wädenswil in the Swiss canton of Zurich, has succeeded in comprehensively recovering liquid organic industrial waste, according to a statement. This waste, which is a valuable energy source due to its high carbon content, is generated in facilities such as paper mills, dairies, and slaughterhouses, as well as in fruit juice production. Experts from Austria, including researchers from the University of Natural Resources and Life Sciences, Vienna (BOKU) and engineering company AAT Abwasser- und Abfalltechnik GmbH in Wolfurt, Vorarlberg, also participated in the project.
The biorefinery project known as VARESI involves a three-stage process for energy recovery. It was tested in practice at the ZHAW campus in Wädenswil using dewatered paper sludge. According to the statement, the first stage involves fermentation, which produces methane that can be used, for example, in heating systems or combined heat and power plants to generate electricity and heat simultaneously.
In the second stage, the carbon in the fermentation residues is converted into biochar under pressure and heat through hydrothermal carbonization (HTC). The biochar can be used as a climate-neutral fuel. The liquid fraction of the digestate becomes process water for the third stage, which has been further enriched with carbon through the HTC process. This water undergoes a second round of fermentation, producing additional methane. Carbon that cannot be broken down by microorganisms is filtered out and carbonized, resulting in purified water that can be used in industrial applications.
To demonstrate the benefits of this process, the research team calculated how much energy the TELA GmbH paper mill in Niederbipp in the canton of Bern would generate through this innovative method of liquid waste recycling. According to their findings, around 34,000 megawatt-hours of thermal energy could be produced annually from the approximately 20,000 metric tons of dry matter in the mill’s paper and bio-sludge. Compared with the conventional recycling process involving dewatering and incineration, the yield equates to an additional 12,600 megawatt-hours, corresponding to 7 per cent of the paper mill’s total energy demand in 2023. By substituting natural gas, annual CO2 emissions could be reduced by about 13 per cent, according to the statement.
In their end project report, the researchers suggest that energy yield could be further increased if the HTC process were tailored even more precisely to the specific substrate. Project leader Hajo Nägele describes the VARESI concept as a key technology. He explains that it opens the door to economically constructing and operating decentralized biorefineries for locally generated wet biomass. The novel process concept could also support the decarbonization of sectors such as wastewater treatment and agriculture, he adds, and can be efficiently combined with nutrient recovery, including nitrogen and phosphorus. ce/mm