ZHAW Team Converts Liquid Bio-Waste into Energy

(CONNECT) A research team at the Wädenswil-based According to a press release, the ZHAW Institute of Chemistry & Biotechnology has succeeded in achieving the comprehensive recovery of liquid organic industrial waste. Such waste, which is high in energy value due to its high carbon content, is generated, for example, in paper mills, dairies, slaughterhouses, or during the production of fruit juices.

The biorefinery project, known as VARESI, involves a three-stage process for energy production. It was tested in a practical setting at the ZHAW campus in Wädenswil using dewatered paper sludge. In the first step, fermentation produces methane, which can be used in heating systems or combined heat and power plants to generate both electricity and heat.

In the second step, the carbon in the fermentation residues is converted into biochar under pressure and heat (hydrothermal carbonisation, HTC). It can be used as a climate-neutral fuel. The liquid components of the fermentation residues provide process water for Step 3, which has been further enriched with carbon through the HTC process. This is then used to produce additional methane through further fermentation. Microbiologically unusable carbon is filtered and carbonised. What remains is purified water that can be used in industry.

To demonstrate the benefits of this process, the research team calculated how much energy the TELA GmbH paper mill in Niederbipp, Bern, would generate through this innovative method of recycling its liquid waste. According to this, approximately 34,000 megawatt-hours of thermal energy could be produced annually from the roughly 20,000 metric tons of dry matter in their paper and bio-sludge. That is 12,600 megawatt-hours more than with the conventional treatment process involving dewatering and incineration. The additional energy output is equivalent to 7 percent of the paper mill's total energy demand in 2023. Replacing natural gas could reduce annual CO2 emissions by about 13 percent. 

In their final 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 in the future. Project Manager Hajo Nägele considers the VARESI concept to be a key technology. It opens up the possibility of the cost-effective construction and operation of decentralised biorefineries for locally generated wet biomass. This innovative process concept could also support decarbonisation in sectors such as wastewater treatment and agriculture, and can be efficiently combined with the recovery of nutrients such as nitrogen and phosphorus. ce/mm