Applying　low　frequency　ultrasound　could　effectively　pretreat　waste　activated　sludge　(WAS)　undergoing　anaerobic　digestion　(AD),　because　the　ultrasound　can　break　up　the　flocs　and　cell　walls.　In　this　study,　we　applied　ultrasound　to　three　types　of　biological　nitrogen　activated,sludge　in　an　ice　bath.　Power　densities　ranged　from　0　to　1.30　W　mL(-1)　and　reaction　times　ranged　from　0　to　30　min.　The,　three　types　of　sludge　included:　short-cut　nitrification　denitrification　sludge　(SCND-SBR),　complete　nitrification　denitrification　sludge　(CND-MUCT),　and　anaerobic　ammonium　oxidation　sludge　(AMX-SHARON).　Experiments　showed　that　reductions　in　average　particle　size　were　significantly　decreased,　due　to　breakage,　at　an　ultrasonic　duration　of　10　min　with　different　energy　densities.　More　soluble　chemical　oxygen　demand　(SCOD)　was　detected　when　the　ultrasonic　density　was　higher　and　the　duration　was　shorter,　compared　to　when　the　density　was　lower　and　the　duration　was　longer.　Results　suggest　that　ultrasonic　density　has　a　greater　impact　on　the　SCOD　disintegration　degree　(DDcoD)　as　compared　to　operation　time.　Soluble　proteins　(SP)　and　NHz-N　were　released　during　the　ultrasound;　however,　this　process　does　not　inhibit　the　subsequent　methanogenic　activity　during　the　AD　process.　The　ultrasonic　density　of　1.04　W　mL(-1)　was　the　most　economic　and　effective,　with　the　highest　efficiency　in　reducing　particle　size　and　releasing　SCOD.　Therefore,　applying　ultrasound　can　accelerate　the　hydrolysis　rate,　a　rate-limiting　factor　during　AD.　This　may　ultimately　reduce　treatment　construction　and　operational　costs.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.