Nitrogen　removal　via　nitrite　(the　nitrite　pathway)　is　beneficial　for　carbon-limited　biological　wastewater　treatment　plants.　However,　partial　nitrification　to　nitrite　has　proven　difficult　in　continuous　processes　treating　domestic　wastewater.　The　nitrite　pathway　is　achieved　in　this　study　in　a　pilot-scale　continuous　pre-denitrification　plant　(V　=　300　L)　treating　domestic　wastewater　by　controlling　the　dissolved　oxygen　(DO)　concentration　at　0.4-0.7　mg/L.　It　is　demonstrated　that　the　nitrite　pathway　could　be　repeatedly　and　reliably　achieved,　with　over　95%　of　the　oxidized　nitrogen　compounds　at　the　end　of　the　aerobic　zone　being　nitrite.　The　nitrite　pathway　improved　the　total　nitrogen　(TN)　removal　by　about　20%　in　comparison　to　the　nitrate　pathway,　and　also　reduced　aeration　costs　by　24%.　FISH　analysis　showed　that　the　nitrite　oxidizing　bacteria　(NOB)　population　gradually　reduced　at　low　Do　levels,　and　reached　negligible　levels　when　stable　nitrite　pathway　was　established.　It　is　hypothesized　that　NOB　was　washed　out　due　to　its　relatively　lower　affinity　with　oxygen.　A　lag　phase　was　observed　in　the　establishment　of　the　nitrite　pathway.　Several　sludge　ages　were　required　for　the　onset　of　the　nitrite　pathway　after　the　application　of　low　DO　levels.　However,　nitrite　accumulation　increased　rapidly　after　that.　A　similar　lag　phase　was　observed　for　the　upset　of　the.　nitrite　pathway　when　a　DO　concentration　of　2-3　mg/L　was　applied.　The　nitrite　pathway　negatively　impacted　on　the　sludge　settleability.　A　strong　correlation　between　the　sludge　volume　index　and　the　degree　of　nitrite　accumulation　was　observed.　(C)　2008　Published　by　Elsevier　Ltd.