This　study　aimed　to　investigate　the　influence　of　biomass　properties　and　high　mixed　liquor　suspended　solids　(MLSS)　concentrations　on　membrane　permeability　in　a　pilot-scale　hollow　fibre　membrane　bioreactor　treating　domestic　wastewater.　Auxiliary　molasses　solution　was　added　to　maintain　system　operation　at　constant　food-to-microorganisms　ratio　(F/M　=　0.13).　Various　physicochemical　and　biological　biomass　parameters　were　measured　throughout　the　trial,　comprising　pre-thickening,　thickening　and　post-thickening　periods　with　reference　to　the　sludge　concentration　and　with　aerobic　biotreatment　continuing　throughout.　Correlations　between　dynamic　changes　in　biomass　characteristics　and　membrane　permeability　decline　as　well　as　permeability　recovery　were　further　assessed　by　statistical　analyses.　Results　showed　the　MLSS　concentration　to　exert　the　greatest　influence　on　sustainable　membrane　permeability,　with　a　weaker　correlation　with　particle　size　distribution.　The　strong　dependence　of　absolute　recovered　permeability　on　wet　accumulated　solids　(WACS)　concentration,　or　clogging　propensity,　revealed　clogging　to　deleteriously　affect　membrane　permeability　decline　and　recovery　(from　mechanical　declogging　and　chemical　cleaning),　with　WACS　levels　increasing　with　increasing　MLSS.　Evidence　from　the　study　indicated　clogging　may　permanently　reduce　membrane　permeability　post　declogging　and　chemical　cleaning,　corroborating　previously　reported　findings.