As we transition from a fossil fuel-based world to a net-zero and bio-based economy, we must move towards more sustainable resource management. In Scotland, current options to dispose of sewage sludge (outside landfill) are very limited, with disposal to land increasingly considered unsustainable given the potential presence of recalcitrant chemical contaminants (organic/inorganic), microplastics, antimicrobial resistant (AMR) bacteria, etc., in sludge, which may in turn threaten the wider environment. However, this waste may be considered a resource with significant potential to be repurposed. In parallel, we also urgently need to close the terrestrial phosphorus (P) cycle, as it is well recognised that P is an essential resource for agriculture (and other industries), but there is now pressing concern over its future availability (as a finite, mined global terrestrial resource). One potential coupled solution to address both challenges is biochar. This project will explore the potential of granular biochar, generated from sewage sludge, to be used to remove excess P from wastewater (through adsorption) and in doing so create a ‘new’ solid material that could be used on land as a carbon (C) and P rich soil amendment.
Biochar is a pyrogenic carbon-rich material produced from waste biomass through pyrolysis under oxygen-limited conditions. The production of biochar from sewage sludge would transform a widespread waste bioresource into a ‘new’ material – which could then be utilised in novel ways. Producing biochar itself would also serve multiple aims, including solid waste repurposing, reducing carbon emissions (through creation of a stable form of solid C) and pollutant reduction (as biochar creation will (through thermal degradation) eliminate many organic pollutants present in sludge). Additionally, eutrophication driven by P-rich wastewater threatens freshwater worldwide. Wastewater treatment plants are required to remove P from their influents as there are strict limits for P release into lakes/rivers/seas. Hence, P ‘extraction’ from P-rich wastewater could both minimise eutrophication and recapture a valuable P resource that can then be re-used on land. Conversion of sludge to biochar, which is then (a) used to recover P from wastewater, and (b) then applied to land, is very much in line with the concept of a circular economy, with minimising pollution and with carbon neutrality.
However, multiple questions remain ‘if’ such a circular process were to be realised – not least regarding technical feasibility and the environmental safety of applying sewage sludge biochar to reduce wastewater P and then use such a material on land as a C rich P-laden soil amendment. Given rapid advances in biochar science and technology in recent years, together with a growing biochar market (i.e., its inclusion in the new EU Fertilising Product Regulations as a marketable product from July 2022), this PhD project has the potential to inform a step-change in sewage sludge handling and biochar production/use within the Scottish wastewater and agricultural sectors.
The student will address the following research objectives:
1. Characterisation and optimisation of sewage sludge (SS) biochar pyrolysis processes to meet multiple targets (e.g., optimise yield, minimise material/production costs, enhance P removal capacity).
2. Laboratory based batch and dynamic experiments with SS biochar (raw or thermochemically modified) to demonstrate P removal/recovery from model solutions and ‘real’ wastewaters.
3. Implementation of SS biochar in an existing pilot-scale P recovery unit to show a ‘proof of concept’ in a relevant environment; specifically at the Scottish Water Horizon (SWH) Wastewater Development Centre in Bo’ness.
4. Plant growth pot trials and other ecotoxicological studies to consider potential negative impacts of SS biochar if used in a water and soil environment.
5. Evaluate the cost effectiveness of SS biochar as a water treatment tool – assessment of its economic potential for scale-up and future use. Including life-cycle assessment (LCA) and energy/carbon budget.
The student will be based at the Environmental Research Institute, UHI North Highland, University of the Highlands and Islands in Thurso, Northern Scotland. The student will gain skills in environmental chemistry, engineering and material science and will benefit from access to a wide range of instrumental facilities available at the Environmental Research Institute.