In order to achieve nitritation/anammox in sewage treatment

the feasibility of achieving partial nitritation using free nitrous acid (FNA) to treat activated sludge was studied in a system treating real wastewater. The results showed that the FNA inhibition on NOB was stronger than that on AOB since the decrease of nitrite oxidation rate was more than that of ammonium oxidation rate

after treating activated sludge by FNA. At the same time

FNA inhibition incresed with increasing FNA concentration at 0-0.75 mg HNO2-N·L-1. The nitrite accumulation rate (NAR) was only 1% after seeding activated sludge obtained from a wastwater treatment plant

indicating that nitrification was happened. When FNA treatment was used with a sludge retention time (SRT) of 15 days

nitritation was achieved since NAR increased to above 90%. The partial nitritation was achieved

and the ratio of of NO2--N/NH4+-N effluent was 1.24 on average when hydraulic retention time (HRT) decreased to 2.5 h. This effluent could meet the requirement of anammox reaction. Thus

partial nitritation could be achieved in sewage treatment system using the activated sludge treated by FNA when SRT and HRT were controlled.