Wastewater　treatment　generates　considerable　amounts　of　sludge.　Reducing　sludge　volume　by　dewatering　is　an　important　part　of　many　wastewater　treatment　processes.　We　investigated　the　effects　of　a　combination　of　sodium　dodecyl　sulfate　(SDS)　and　polydimethyldiallylammonium　chloride　(PDMDAAC)　on　anaerobically　digested　sludge　dewaterability.　Capillary　suction　time　(CST)　and　specific　resistance　to　filtration　(SRF)　were　measured　as　indicators　of　sludge　dewaterability.　Extracellular　polymeric　substances　(EPS)　content,　particle　size,　zeta　potential,　and　bound　water　in　sludge　floc　were　also　measured.　We　found　that　CST　increased　from　348.5　to　530.5　s　as　the　SDS　dosage　increased.　However,　with　a　dose　0.03　g　g(-1)　DS　PDMDAAC　only,　SRF　and　CST　were　3.78　x　10(12)　m　kg(-1)　and　79.2　s;　with　a　combined　dose　of　0.025　g　g(-1)　DS　SDS　and　0.03　g　g(-1)　DS　PDMDAAC,　SRF　and　CST　decreased　to　3.21　x　10(12)　m　kg(-1)　and　68.7　s.　These　results　indicate　that　above　a　certain　dose,　SDS　reduces　sludge　dewaterability　and　that　an　optimum　dose　of　SDS　in　sludge　pre-treatment　would　improve　subsequent　PDMDAAC　conditioning.　Sludge　conditioned　with　a　combination　of　SDS　and　PDMDAAC　increases　dewaterability　more　than　conditioned　with　SDS　only　or　PDMDAAC　only.　Freeing　EPS-bound　water　with　SDS　followed　by　re-flocculation　using　PDMDAAC　gave　improvement　in　sludge　dewaterability.