Skeleton　builders　are　essential　for　achieving　deep　sludge　dewatering.　In　this　study,　a　novel　spent　coffee　ground　(SCG)-based　skeleton　builder　was　developed　to　attain　deep　sludge　dewatering　by　combined　conditioning　with　FeCl3,　and　possible　mechanisms　were　examined.　Through　different　surface　analysis　techniques,　it　was　demonstrated　that　at　a　pyrolysis　temperature　of　300　?,　the　spent　coffee　ground　biochar　(SCGB-300)　has　an　intact　pore　structure,　a　rigid　carbon　skeleton,　and　large　oxygen-containing　functional　groups,　making　it　the　best　skeleton　builder　for　sludge　dewatering.　When　combined　with　FeCl3　for　conditioning,　the　structure　of　SCGB-300　remained　intact　under　high　pressure　and　played　important　role.　The　rich　porous　structure　facilitated　water　drainage.　During　the　sludge　conditioning,　small　amount　of　positive　charge　on　the　surface　of　SCGB-300　further　increased　the　zeta　potential　of　sludge　through　charge　neutralization.　At　the　same　time,　the　adsorption　of　SCGB-300　removed　viscous　hydrophilic　substances　and　further　improved　the　dewatering　performance.　At　an　optimum　dosage　of　6%　(dry　solid,　DS)　FeCl3　and　30%　SCGB-300　(DS),　the　moisture　content　of　sludge　was　reduced　from　85.47%　to　63.35%,　and　the　dewatering　rate　was　increased　from　46.08%　to　70.03%.　Therefore,　SCGB　is　a　promising　skeleton　builder　for　sludge　conditioning　and　deep　dewatering.