Deep　understanding　of　pH　variation　during　biological　nitrogen　removal　is　critical　for　ensuring　the　effective　operation　of　nitrification/denitrification　or　nitritation/denitritation　processes　on　a　real-time　basis.　Two　tank-type　glass　vessels,　with　a　6　L　working　volume　for　each,　were　used　as　sequencing　batch　reactors　(SBRs)　to　investigate　variation　of　pH　in　biological　nitrogen　removal　process　(nitritation,　nitratation,　denitratation　and　denitritation).　Same　amount　of　ammonium　nitrogen　(NH4+-N)　and　nitrite　nitrogen　were　used　as　initial　nitrogen　substances　for　oxidation　operation　mode,　whereas　nitrate　(NO3--N)　and　nitrite　nitrogen　(NO2--N)　were　used　for　reduction　mode.　Results　showed　that,　pH　decreased　during　nitritation　and　increased　during　denitritation.　On　pH　profile,　＇ammonium　valley＇　indicated　the　end　of　nitritation,　instead　of　nitrification　and　no　characteristic　bending　point　appeared　at　the　end　of　nitrification.　There　was　no　evident　break　point　appeared　on　pH　profile　when　denitratation　finished.　Moreover,　the　pH　profile　during　denitritation　is　more　convex　than　that　during　denitrification.　Alkalinity　consumption　was　carried　out　mainly　in　nitritation　process　and　alkalinity　was　produced　only　in　denitritation　process.　Same　amount　of　total　alkalinity　will　be　produced　during　reduction　of　same　amount　of　nitrate　and　nitrite,　but　the　components　of　alkalinity　are　different.　©　2011　IEEE.