Sewage sludge (or biosolids) produced during wastewater treatment contain significant amounts of phosphorus. Pyrolysis treatment can produce a safe material while sequestering carbon
however, it underutilises the phosphorus in the resulting biochar, leaving it partially inaccessible to plants. Here the effect of doping calcium-rich sewage sludge (14% calcium) with potassium acetate, combustion wood ash and its water-extract on phosphorus availability, and carbon and nitrogen release during pyrolysis are investigated at 500 and 700°C. The aim was to produce an efficient circular economy material with superior P release properties. Potassium acetate addition increased the amount of water-extractable phosphorus in sewage sludge biochar produced at 500°C by three-fold (to 3.6% of the total phosphorus content). As hypothesised, potassium doping was less effective in increasing phosphorus availability than reported previously due to the high calcium content in sewage sludge that locked up phosphorus. In a wheat pot trial, doped biochar still resulted in 28% higher stem biomass and 29% grain phosphorus uptake compared to biochar without potassium acetate and the water-extractable P content in thermally treated sewage sludge correlated with phosphorus uptake and plant biomass. Replacement of potassium acetate with combustion wood ash did not improve phosphorus availability in biochar. However, extracting wood ash with water first and using this extract for sewage sludge doping added 2.5% potassium and doubled the water-extractable phosphorus content in the resulting 500°C-biochar. This proof-of-concept highlights suitability of waste-derived wood ash extract as dopant to boost the potassium content and phosphorus availability in sewage sludge biochar. This innovative circular economy process produces enhanced sewage sludge biochar fertilisers from two waste materials with significant economic and environmental benefits, including diversion of waste from landfill and emission reductions.