Aerobic　operation　of　an　immersed　hollow　fibre　membrane　bioreactor,　treating　municipal　wastewater　supplemented　with　molasses　solution,　has　been　studied　across　mixed　liquor　suspended　solids　(MLSS)　concentrations　between　8　and　32　g　L-1,　the　higher　concentrations　being　normally　associated　with　thickening　operations.　Only　a　marginal　loss　in　membrane　permeability　was　noted　between　8　and　18　g　L-1　when　operation　was　conducted　without　clogging.　The　sustainable　operational　flux　attainable　above　18　g　L-1　was　highly　dependent　upon　both　the　MLSS　concentration　and　the　state　of　the　membrane.　A　temperature-corrected　flux　of　28　L　m(-2)　h(-1)　(LMH)　was　sustained　for　18　h　at　an　MLSS　of　8　g　L-1　using　membranes　close　to　initial　their　virgin-state　permeability.　This　value　decreased　to　around　14　LMH　at　20　g　L-1　and　5　LMH　at　32　g　L-1　MLSS　for　an　aged　membrane　whose　permeability　had　been　recovered　following　clogging.　Below　the　threshold　flux　operation　without　significant　clogging　was　possible,　such　that　the　membrane　permeability　could　be　recovered　with　a　chemically　enhanced　backflush　(CEB).　Above　this　flux　clogging　took　place　at　a　rate　of　around　7-14　g　solids　per　m(2)　membrane　per　m(3)　permeate　volume　passed　irrespective　of　the　MLSS　concentration.　The　permeability　of　the　unclogged　membrane　was　depressed　and　could　not　be　recovered　using　a　standard　CEB,　indicative　of　irrecoverable　pore　clogging.　The　outcomes　corroborated　previously　reported　observations　concerning　the　deleterious　long-term　impacts　of　clogging,　and　confirmed　the　critical　importance　of　operation　at　a　sustainable　flux　value.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.