Valorising activated sludge biomass waste via low-temperature processing towards a circular economy.

Most of the sewage sludge in Scotland is marketed for agricultural use, land reclamation and incineration. However, regulations on sludge usage are set to become stricter which will reduce the volume of sludge recycled and used in agriculture and land reclamation. This is a worldwide problem and with the prospect of more sludge being produced and wasted because of new regulations, there has been a recent surge in valorisation projects to find alternative uses for sludge in today’s modern society. Furthermore, sludge production causes approximately 40% emissions, therefore, there is new research to reducing the emissions and find a sustainable technique to dealing with sewage sludge. Torrefaction is a process used to improve biomass properties, the products from which have been used, as fuels or reducing agents. Due to the milder processing conditions used in torrefaction, compared with those used in the manufacture of biochars, with temperatures between 200-300°C and atmospheric pressure used, the final character of torrified carbons is aimed at calorific value rather than textural development. A handful of recent publications have looked at the application of torrefaction to process sewage sludge, but these have all involved a dewatering step, which consumes additional energy and produces associated emissions.