The　biotic　factors　(bacterial　abundance,　enzyme　activity)　and　abiotic　factors　(sludge　disintegration,　organics　solubilization)　in　sludge　thermophilic　fermentation　at　various　solids　concentrations　were　investigated　to　clarify　the　effects　on　hydrolysis　and　volatile　fatty　acids　(VFAs)　production.　The　results　show　that　with　the　solids　concentration　increasing　from　volatile　suspended　solids　(VSS)　5-40　g/L,　the　abiotic　effects　of　particles　disintegration　played　a　leading　role　in　hydrolysis,　resulting　in　the　decrease　of　hydrolysis　rate　constant　(k(H))　(from　0.1181　to　0.0623)　and　solids　degradation　ratio　(from　55.19%　to　36.63%),　while　the　biotic　effects　dominated　above　VSS20,　reinforcing　bacterial　abundance　and　hydrolase　activity.　The　maximal　soluble　chemical　oxygen　demand　(SCOD)　production　yield　positively　correlated　with　VSS,　and　the　value　for　VSS40　(436.52　mg/g　VSSfed)　increased　by　50%　over　VSS5　(298.21　mg/g　VSSfed).　Additionally,　the　solubilization　and　conversion　of　carbohydrates　were　respectively　enhanced　by　2.6-folds　and　13.4-folds　at　higher　VSS,　while　for　proteins　they　almost　decreased　by　50%.　The　VFAs　production　was　comprehensively　affected　by　the　carbon　substrate,　ammonium　and　acid-forming　enzyme,　with　the　highest　accumulation　yield　(232.17　mg　CODVFA/g　VSSfed)　for　VSS30;　meanwhile,　a　higher　percentage　of　acetate　and　n-butyrate　but　lower　of　propionate　were　achieved　with　VSS　increasing.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.