Filamentous　bulking　sludge　due　to　excessive　growth　of　filamentous　bacteria　is　a　serious　operational　problem　in　activated　sludge　plants.　The　addition　of　chemicals　is　one　of　widespread　ways　to　control　filamentous　bulking.　In　this　study,　filamentous　bulking　in　a　continuous　activated　sludge　system　was　found　to　be　mainly　caused　by　Eikelboom　Type　021N　filamentous　bacteria　likely　due　to　low　substrate　concentration　gradients.　These　Type　021N　bacteria　were　found　to　be　resistant　to　chlorination,　maintaining　cell　integrity　at　a　dosage　of　up　to　80　mg　Cl/gSS.　An　alternative　biocidal　agent,　cetyltrimethyl　ammonium　bromide　(CTAB),　exhibited　a　much　stronger　biocidal　effect　on　these　filaments,　which　significantly　improved　sludge　settleability.　Type　021N　with　filamentous　index　of　5　was　selectively　killed,　but　floc-formers　recovery　their　activity　after　CTAB　termination.　The　study　implied　that　CTAB　might　have　more　penetration　capacity　to　cell　wall　of　chlorine-resistant　Type　021N　bacteria.　We　therefore　suggest　the　penetration　property　of　filament　cell　wall　should　be　considered　or　tested　before　the　selection　of　biocide　type　in　practice.　Crown　Copyright　(C)　2012　Published　by　Elsevier　Ltd.　All　rights　reserved.