The　global　energy　crisis　is　approaching　due　to　rapid　population　growth　and　overexploitation　of　fossil　fuels.　Therefore,　the　development　and　use　of　new　and　renewable　energy　sources　is　already　in　the　extreme　urgency.　This　work　developed　a　novel　technology　to　efficiently　produce　renewable　liquid　bioenergy　from　discarded　wastes,　by　effectively　transforming　sewage　sludge　into　high-value　medium　chain　fatty　acids　(MCFA).　The　maximum　MCFA　yield　in　the　anaerobic　sludge　fermentation　was　revealed　to　be　10.6　times　of　control　when　utilizing　sewage　sludge　with　1.78　mg-N/L　free　nitrous　acid　(FNA)　pretreatment.　The　carbon　flow　from　sewage　sludge　into　MCFA　in　the　fermentation　system　was　significantly　enhanced　with　appropriate　levels　(0.71–1.78　mg-N/L)　of　FNA　pretreatment.　Compared　to　FNA　pretreatment,　however,　its　direct　addition　severely　inhibited　total　products　(i.e.,　carboxylates　and　complex　alcohols)　generation　because　of　the　toxicity　on　live　cells　(decreasing　to　8.3　%–13.9　%)　in　sludge.　Kinetic　models　(one-substrate　and　two-substrate)　were　utilized　to　investigate　the　mechanism　of　MCFA　promotion　by　FNA　pretreatment　on　anaerobic　sludge　fermentation,　in　which　linear　relationship　analysis　between　FNA-derived　organic　release　and　the　fitted　parameters　were　also　performed.　The　results　indicated　that　the　conversion　of　refractory　materials　into　rapidly　bioavailable　substrates　for　MCFA　production　contributed　to　increasing　MCFA　production　rate　and　potential.　Moreover,　the　relative　abundances　of　functional　microorganisms　related　to　hydrolysis-acidification　and　chain　elongation　process　increased　under　FNA　pretreatment,　further　favoring　the　MCFA　production.　This　study　provides　a　novel　and　effective　technology　of　sludge　energy　recovery　that　can　achieve　the　next-generation　sustainable　sewage　sludge　management.　©　2022　Elsevier　B.V.