Exploiting online in-situ ammonium, nitrate and phosphate sensors in full-scale wastewater plant operation.

In-situ nutrient sensors are now a proven technology. Having ion membranes eliminates the need for ultrafiltration, and consequently the sensors can be located at suitable places in any of the reactors. This gives the potential for new control structures for the control of nitrification, denitrification, and phosphorus removal. In the paper some examples of such controllers are demonstrated as used in a full-scale wastewater treatment plant. A successful control implementation scheme at full-scale plants includes three steps: monitoring, experimenting and controlling. The benefit of implementing process control based on nutrient sensors is real: by implementing precipitation dosage control a savings of 41% compared to flow proportional dosage can be reached, while the savings compared to constant dosage is 73%. An increase in nitrate recirculation shows significant improvement in the nitrogen removal ability at very low cost. Reliable nutrient sensors are not the only prerequisite for a successful control system. The design of actuators, such as drives, compressors and valves, is often overlooked. Furthermore, the lower level controllers have to work properly before the more advanced controllers can work adequately. A collection of practical experience regarding such issues is given in this paper.