Turning sewage into electricity and farming nutrients
A new method converts sludge waste solids into multiple valuable products
Today’s wastewater treatment processes produce large quantities of ‘biosolids’. This waste material, which contains high levels of persistent micropollutants, is typically incinerated, sent to landfills, or spread on farmland. But alongside the harmful pollution, biosolids also contain energy-storing organic carbon, as well as nitrogen and phosphorous, which are useful as fertiliser. UK startup Onunda is therefore looking at this waste stream differently, seeing it as a source of valuable resources.
Leveraging a combination of hydrothermal and thermochemical conversion technologies, the company is perfecting a novel approach to sludge waste treatment. The process destroys PFAS and microplastics, while recovering value in the form of clean energy and nutrients.
The technology can be integrated into existing wastewater treatment infrastructure to supplement and enhance existing capacity, or it can alternatively be delivered as a standalone solution. In the first stage of the process, which involves no solvents, the water in the feedstock performs ‘work’ on the organic solids and contaminants, ‘hydrolysing’ them into harmless aqueous – or ‘watery’ – constituents. This watery slurry is then sent to the second process stage where it is dewatered and dried to a target moisture content. In the final stage, the dried ‘hydrochar’ is gasified into a high calorific value syngas and a biochar residue.
"The company is perfecting a novel approach to sludge waste treatment. The process destroys PFAS and microplastics, while recovering value in the form of clean energy and nutrients."
The syngas is cleaned and a portion of it is used to generate electricity that powers the plant. Any remaining electricity is sent to the power grid or used by the adjoining wastewater treatment plant in cases where the technology is integrated into an existing facility. The biochar, meanwhile, is used to create a pollutant-free agricultural fertiliser. A third product of the process – a nutrient-rich liquor – can be fed to an anaerobic digester to produce biogas or used as a liquid fertiliser in hydroponic applications.
Not only can Onunda’s technology be used as an additional treatment to create value from biosolids, but the company’s plants can also provide additional capacity for the treatment of primary sludge. As a result, wastewater treatment plants can direct excess sludge to an Onunda unit, rather than discharging untreated sewage into waterways.
Did you know?
There are increasing concerns that PFAS in biosolids could contaminate the food supply. In January, the US EPA released its draft risk assessment for two forever chemicals, PFOA and PFOS. The document highlighted that these chemicals could cause human health risks exceeding the agency’s acceptable thresholds when present in land-applied sludge at levels of one part per billion. The assessment also found that there were risks associated with PFOA and PFOS regardless of whether sludge is applied to the land, incinerated, or landfilled. Onunda’s technology destroys PFAS while also recovering value from sludge and biosolids. It further provides additional capacity to avoid sewage discharge, another challenging issue for the sector.
