Removal of nitrogen from waste water systems by biological nitrification and denitrification in an activated sludge system is considered. One of the major methods proposed for controlling excess algal production in receiving waters, the process consists of aerating activated sludge mixed liquor solids for detention times sufficient for nitrifying bacteria to oxidize ammonia to nitrate. The mixed liquor then goes to an anaerobic denitrification zone where the nitrate is biologically converted to nitrogen gas. Pilot plant studies preceding the current studies are discussed prior to presentation of the work on pure culture kinetic studies and development of ion selective cathodes for identification of nitrogen fractions. For the conditions used, the rate of denitrification by Pseudomonas denitrificans followed the Michaelis pattern, not being inhibited by high concentrations of CH3COONa as the substrate. Although the rates denitrification at the various KNO3 concentrations did not follow the Michaelis pattern, inhibiting effects of nitrate were not observed at KNO3 concentrations as high as 8.0 mM/l. The use of ion selective cathodes for detecting ions such as the nitrogens was investigated. It appears that the selectivity of ion-selective cathodes can be increased greatly by the addition of time dependent retarding voltages. (Author)