Sewage sludge is an energy and nutrient rich resource, typically treated by anaerobic digestion and recycled to agriculture in the UK. Sludge provides soil with organic matter and essential nutrients which contribute to crop growth and offsets inorganic fertiliser use. However, this model has limitations:
• Sludge mineralisation releases CO2 and energy that could be recovered.
• The sludge land bank is constrained.
• Sludge contains emerging contaminants.
Pyrolysis has the potential to convert sludge into; energy-rich volatiles and char whose formation sequesters carbon and potentially facilitates nutrient recovery and manufacturing high-value products. A laboratory pyrolysis plant was designed and built to enable investigation into practical application of pyrolysis in a representative manner. Key process and energy findings were:
• Accumulation of carbon and soot-tar composites must be managed to avoid blockages.
• Energy-rich gas was formed (LHV = 15.6-19.7 MJ m-3), which comprised hydrocarbons, hydrogen, carbon oxides and other components.
• Conversion efficiency maximised at 800oC, with 59% conversion of sludge (HHV) to gas (LHV).
Sludge char retained one-third of sludge carbon by weight, including graphitic and heterocyclic structures which influences char surface chemistry. Sludge char is a Class-B candidate for soil carbon sequestration and analysis suggests it sequesters more than biosolids recycling. Char phosphate extraction with mineral acids is >90% effective, however co-extraction of iron and aluminium complicates phosphate valorisation. Char land application provides a circular-materials approach to phosphate recycling, assuming bespoke permits are obtained. Char contains heavy metals but is not expected to be ecotoxic because of their chemical form (e.g. sulphides). Solvent extraction is required to confirm organic contaminants do not pose a risk.
Several regulatory challenges (e.g. End-of-waste) have been identified that inhibit fuel-gas valorisation and char disposal or recycling. These may inhibit widespread adoption of pyrolysis and further development of novel processes that improve process reliability and product valorisation.