On-line　process　control　may　cause　substantial　changes　to　the　microbial　community　in　a　biological　wastewater　treatment　system.　Recent　studies　have　shown　such　effects　can　be　exploited　in　control　system　design　to　achieve　an　optimised　microbial　community.　Excellent　progress　has　been　made　on　the　elimination　of　nitrite-oxidising　bacteria　(NOB)　in　biological　nitrogen　removal　wastewater　treatment　systems　using　on-line　aeration　control,　enabling　nitrogen　removal　via　the　nitrite　pathway.　Control　methods　for　eliminating　NOB　are　now　available　for　both　continuous　systems　and　sequencing　batch　reactors,　and　have　been　demonstrated　with　both　domestic　and　various　types　of　industrial　wastewaters.　The　elimination　or　reduced　growth　of　glycogen　accumulating　organisms　(GAOs),　a　competitor　of　polyphosphate　accumulating　organisms　(PAOs),　in　enhanced　biological　phosphorus　removal　(EBPR)　systems　via　pH　and　carbon　source　control　has　been　conceptually　demonstrated　through　the　use　of　enriched　cultures.　However,　these　strategies　are　not　yet　ready　for　the　control　of　practical　EBPR　processes.　Sludge　population　optimisation　also　involves　selecting　the　most　desirable　organism　or　a　consortium　of　organisms　to　perform　a　required　function.　This　is　particularly　important　for　nitrification,　one　of　the　most　important　and　delicate　steps　in　modern　wastewater　treatment　plants.　Results　from　both　experimental　and　simulation　studies　suggest　that　reactor　operation　could　have　a　major　impact　on　the　nitrifier　community　structure,　which　should　be　further　investigated　in　future　studies.　©　2008　Springer　Science+Business　Media　B.V.