Hierarchically porous biochar preparation and simultaneous nutrient recovery from sewage sludge via three steps of alkali-activated pyrolysis, water leaching and acid leaching

•Hierarchically porous biochar was derived from sludge via KHCO3-activated pyrolysis.•Porous biochar prepared at 900 °C has a high specific surface area of 1476.0 m2/g.•Water-soluble potassium salts were formed during KHCO3-activated sludge pyrolysis.•Porous biochar showed a high adsorption rate of 14.4 mg/(g∙min).•Water leaching solution rich in nutrient elements could be reused for plant growth.