Control of wastewater treatment plants has received more attention as a method to improve nutrient removal processes. Improved nutrient removal, results in an increased effluent quality, whilst minimising the energy consumption. Aeration control of oxidation ditches often focuses on adjusting the aeration intensity based on the nitrogen removal process, using ammonium measurements. The control systems do not incorporate biological phosphorus removal. Biological phosphorus removal, on the one hand, needs sufficient aeration and on the other hand, can deteriorate when subjected to excessive aeration. The complex relation to aeration, makes control of biological phosphorus removal difficult.

This research investigates the control of biological phosphorus removal in an oxidation ditch using simple sensors. Control systems often require expensive instrumentation. To limit the costs, cheap sensors using alternative monitoring variables for biological phosphorus removal are desired. Literature research was done to investigate the principles of biological phosphorus removal, possible alternative monitoring variables, current control systems and existing mathematical models and benchmarks that could contribute to a renewed control strategy. Some important variables found in literature were potassium, pH, ORP, ortho-P, nitrate, ammonium, conductivity, and DO. A measurement campaign was set up and executed to measure the trend of these variables throughout the oxidation ditch and to obtain more information on the state of the variables in the (un)aerated zones. A full-scale operating wastewater treatment plant in Hattem was made available to execute the measurement campaign.

The results obtained from the measuring campaign did not point towards alternative monitoring variables for biological phosphorus removal. Relationships between the variables and phosphorus found in literature, could not be directly derived from the measurement results. However, nitrate and ammonium measurements can give an insight into the biological phosphorus removal process. A change in ammonium and nitrate indicates an anaerobic, anoxic or anaerobic environment. Nitrate can in addition contribute to a control system by making sure that the zone before withdrawal from the oxidation ditch is anoxic, to inhibit P-release. However, it cannot on its own monitor the biological phosphorus removal fully as it cannot indicate the effect of the aeration intensity on biological phosphorus removal directly. The results from the measurements indicated a low biological phosphorus removal activity. From literature, it is known that several factors can influence the biological phosphorus removal, such as insufficient anaerobic zone and aeration settings.

To investigate the response of the WWTP to these factors, a BioWin model was constructed. This model simulates the performance of the treatment plant and investigates the impact of several adjustments, such as altered aeration and an extended anaerobic zone.

The simulation results showed that the DO setpoints of the aerators influenced the process in several ways. Increasing the aeration improved the results to a certain extent, after which the biological phosphorus removal deteriorated due to over-aeration. An extended anaerobic tank improved the biological phosphorus removal.
Unfortunately, a control system for biological phosphorus removal, using alternative monitoring variables, was not obtained. A benchmark of an oxidation ditch is suggested for further research and the implementation of biological phosphorus removal in a benchmark of an oxidation ditch was started.