Enhanced hydrolysis/acidogenesis and potential mechanism in thermal-alkali-biofilm synergistic pretreatment of high-solid and low-organic-content sludge

•Biofilm assistance improved the performance of thermal-alkali pretreatment.•TAB was feasible to treat high-solid and low-organic-content sludge.•TAB primarily facilitated solubilization and hydrolysis via stimulated proteolysis.•TAB eliminated autochthonous microorganisms in the waste activated sludge.•TAB enriched specialized fermentative microorganisms and relevant functional genes.