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Pyrolysis is the process whereby carbonaceous materials, such as biosolids, are heated between 400°C and 900°C in the absence of oxygen. Three main products are generated: a solid product called biochar, a py-liquid that consists of aqueous phase and non-aqueous phase liquid, and py-gas. The biochar holds value as a beneficial soil amendment and sequesters carbon. The py-liquid is potentially hazardous and needs to be dealt with (including potentially reducing it on-site via catalysis or thermal oxidation). Py-gas can be used on-site for energy recovery. Pyrolysis has gained recent interest due to concern over perand polyfluoroalkyl substances (PFAS) in biosolids. Although pyrolysis can remove PFAS from biosolids, it has been shown to produce PFAS that reside in py-liquid, and the fate in py-gas remains a knowledge gap. More research is needed to help close the PFAS and fluorine mass balance through pyrolysis influent and effluent products because pyrolysis alone does not destroy all PFAS. The moisture content of biosolids substantially affects the energy balance for pyrolysis. Utilities that already produce a dried biosolids product are in a better position to install pyrolysis. Pyrolysis has both defined benefits (solids reduction, PFAS removal from biosolids, and biochar production) as well as remaining questions (the fate of PFAS in py-gas and py-liquid, mass balance on nutrients, and py-liquid handling options) that will be answered through more pilot and full-scale demonstrations. Regulations and local policies (such as carbon sequestration credits) could affect pyrolysis implementation. Pyrolysis should be considered as an option in the biosolids stabilization toolbox with application being based on individual circumstances of a utility (e.g., energy, moisture content of biosolids, PFAS). Practitioner Points • Pyrolysis has known benefits but limited full-scale operational data. • Pyrolysis removes PFAS from biochar, but PFAS fate in gas phase is unknown. • Moisture content of influent feed solids affects energy balance of pyrolysis.• Policy on PFAS, carbon sequestration, or renewable energy could impact pyrolysis. [ABSTRACT FROM AUTHOR]

Sewage sludge conditioned by municipal solid waste incineration (MSWI) fly ash is introduced to condition sludge dewatering and to obtain high pH, high salinity, and high calcium ions. The dewaterability of sludge was dramatically improved after conditioning process with MSWI fly ash, resulting in 95% specific resistance to filtration (SRF) reduction. In addition, Yn index increased to 2.24 kg/m2·h from 0.71 kg/m2·h, three times higher than that of original sludge. Meanwhile, pH increased up to 12.54 as the MSWI fly ash dose increased; this was directly correlated with increase in salinity of the sludge. Box- Behnken design and response surface methodology were applied to determine the effects of major variables on moisture content and SRF. Findings show the increase of calcium dose and pH, as well as the suitable sodium salt favourable to sludge dewatering. Compared with nitrate ion, chloride ion has little effect on the dewatering of sludge water. As compared with nitrate salts, chlorine salts were insignificant on dewatering. [ABSTRACT FROM AUTHOR]