Experimental　studies　were　carried　out　on　a　bench-scale　nitrogen　removal　system　with　a　predenitrification　configuration　to　gain　insights　into　the　spatial　and　temporal　variations　of　DO,　pH　and　ORP　in　such　systems.　It　is　demonstrated　that　these　signals　correlate　strongly　with　the　operational　states　of　the　system,　and　could　therefore　be　used　as　system　performance　indicators.　The　DO　concentration　in　the　first　aerobic　zone,　when　receiving　constant　aeration,　and　the　net　pH　change　between　the　last　and　first　aerobic　zones　display　strong　correlations　with　the　influent　ammonia　concentration　for　the　domestic　wastewater　used　in　this　study.　The　pH　profile　along　the　aerobic　zones　gives　good　indication　on　the　extent　of　nitrification.　The　experimental　results　also　showed　a　good　correlation　between　ORP　values　in　the　last　aerobic　zone　and　effluent　ammonia　and　nitrate　concentrations,　provided　that　DO　in　this　zone　is　controlled　at　a　constant　level.　These　results　suggest　that　the　DO,　pH　and　ORP　sensors　could　potentially　be　used　as　alternatives　to　the　on-line　nutrient　sensors　for　the　control　of　continuous　systems.　An　idea　of　using　a　fuzzy　inference　system　to　make　an　integrated　use　of　these　signals　for　on-line　aeration　control　is　presented　and　demonstrated　on　the　bench-scale　system　with　promising　results.　The　use　of　these　sensors　has　to　date　only　been　demonstrated　in　intermittent　systems,　such　as　sequencing　batch　reactor　systems.