Fast responding biosensor for on-line determination of nitrate/nitrite in activated sludge

A microscale biosensor for nitrate/nitrite was used for in-situ monitoring in an activated sludge plant. The biosensor is based on diffusion of nitrate/nitrite through a tip membrane into a dense mass of bacteria converting the ions into nitrous oxide with subsequent electrochemical detection. The biosensor signal was in very close agreement with the nitrate/nitrite concentration in the plant as analysed by conventional laboratory techniques. The concentration of the only interfering substance, nitrous oxide, was so low that it did not have any effect. The response time of the applied biosensors was less than 30 s, and the detection limit was less than 50 μg N l−1. Detachment of the ion-permeable membrane in the tip of these first-generation biosensors limited the period with continuous monitoring to 5 days.