Research project to revolutionise liquid waste management

Aarhus University

AN Engineer at Aarhus University (AU), Denmark has received a US$1.65m grant to develop technology that could revolutionise liquid waste management.

The Kr11.2m (US$1.65m) ERC Starting Grant, awarded by the European Research Council, will support the REBOOT project, expected to deliver more sustainable liquid waste management and to enable recovery of valuable resources, including almost 100% phosphorus.

Phosphorus is ranked amongst the top 20 most critical raw materials by the EU. The resource is scarce, and it is predicted that reserves will only meet demand for a further 50–100 years. Depletion of the resource will have dire consequences for humanity.

Europe, which does not have its own phosphorus reserves, primarily imports it from North Africa where it is obtained from mines as phosphate rock. Phosphoric fertiliser is critical for maintaining the yields normal in modern agriculture. The mining, refining, and transport of phosphate rock for fertiliser produces 3.1 kg of CO₂ per kilogramme of phosphoric fertiliser. According to project head Patrick Biller, a chemical engineer and Assistant Professor in the Department of Engineering at AU, the EU imports 2.6m t/y of phosphorus.

REBOOT aims to develop a single, cohesive process for the extraction of valuable materials from liquid waste.

The project will use state-of-the-art technology known as continuous hydrothermal liquefaction (HTL) for recovery. HTL is a process that uses high temperature and pressure to convert biomass into bio-crude, which is similar to fossil crude and can be refined into the same products, such as chemicals, aviation and shipping fuels, and bitumen. According to Biller, the process has high feedstock versatility, it does not require dry feedstock, and in a continuous reactor it only takes 15 minutes.

Manure can contain large amounts of antibiotics and sewage sludge can also contain hazardous substances, such as microplastics, oestrogens, pathogens, and pharmaceutical products, including antibiotics. These substances make it difficult to recycle the liquid wastes directly. Beneficially, the high temperatures and pressures to be used in the HTL reactor will destroy environmentally hazardous substances in the liquid waste so the phosphorus recovered will be clean, safe, and plant-available.

Biller expects to extract 95–99% of the phosphorus from the liquid waste, and 65% of the carbon as biocrude. Other end-products from the process are expected to be fresh water, hydrogen, and CO₂. The recovered phosphorus will be used to produce fertiliser, and the biocrude will be upgraded using the hydrogen through hydrogenation. The fresh water could be discharged in a water body or used for processes such as irrigation and the carbon dioxide could be captured to make carbon negative fuels, or if it is pure enough it could be sold to drinks companies for use in carbonated drinks.

Challenging aspects of the research will include removing the inorganic fraction from a continuous system under high pressure and temperature. Additionally, the researchers will have to develop a method enabling them to manage the process water. As the biomass slurries only contain about 20% dry matter, a lot of water will be produced by the process and it will contain a lot of valuable carbon and nitrogen. Conventional water treatments will be unsuitable for the process water due to the high chemical oxygen demand (COD) value, which indicates the dirtiness of the water. In REBOOT, researchers are to attempt to develop microbial electrolysis cells that will enable conversion of organics into hydrogen.

In addition to agriculture the research could also have significance to problems associated with the lack of sewage treatment in many developing countries, such as the spread of disease and other health effects.

The project will use a pilot reactor from a previous study, which will be modified for REBOOT.

The project is to start in January 2020, when Biller will assemble a research team. The current research grant covers a period of five years. The project will run at pilot scale at the Department of Engineering’s Centre for Biorefining Technologies at Foulum, which is home to one of the world’s largest HTL reactors.

Patrick Biller said: “I'm very grateful to have been awarded this grant, which makes it possible to develop this exciting new technology that will enable us to recover valuable phosphorus from waste otherwise difficult to manage.”