In the last years especially in bigger size plants there is a tendency to use advanced method of control e.g. fuzzy systems or artificial neuronal networks. The use of these systems requires a lot of information from process based on sensor measurements. The initial costs as well as costs for maintenance can be high. Furthermore often people at site are not familiar with all kind of equipment (especially sensors for the various nitrogen components). Therefore sometimes sensors after some months of operation don´t deliver data at all or don´t deliver reliable data can be used for process control.
Einstein already said once: “Everything should be made as simple as possible, but not simpler“. So a control system, which uses only small number of process data supplied by stable, rugged and if possible more or less maintenance-free sensors, can be an option even for bigger size plants. Beside DO- and NH4-Nconcentration the air flow rate into each aerated tank respectively zone is used more often, because rugged and precise sensors are available on the market and the delay in the control loop due to time for dissolving the oxygen in the wastewater is reduced. Even the task of air distribution to various tanks can be solved easily and more stable when the air flow rate is used as an additional parameter.
There are control strategies using air flow rates on the market available and used for some years already even in multi-line plants with upstream denitrification. They use PI- or PID-controller with disturbance estimation and feedforward control because load changes in the influent of the sewage treatment plant cause a dynamic system.
But not only signal quality and availability of process parameter as well as the aeration controller are responsible for a good control. Even the actuators in the control loop must act precise and reproducible. Otherwise the control result based on very precise calculation by best controller can be poor, poor in adjusting quality of control valves and/ or poor regarding energy efficiency. Only if all components in the system are aligned (full system integration), best process functionality and an economical success can be achieved.
Technical basics are described in the German standard papers of the German Wastewater Association DWAM264, DWA-M229-1 and DWA-A 268. Especially the new DWA-M 229-1 contains many helpful information regarding suitability of the various types of control valves for aeration air, regarding control performance as well as pressure drop (= operational costs respectively energy efficiency). In the past these facts were not considered at all in any evaluation of projects. The main two tasks of the controller are: 1st the load-depending air supply to an aerated zone (to compensate uneven distribution of water to different tanks) or tank and 2nd the air distribution between several tanks (to compensate different pressure situations due to different water level, differences in head loss of aerators etc.). These are typically operational problems in common header systems. If these tasks cannot be solved quite well, over- and under-aeration take place, energy consumption of the plant is higher than necessary and effluent quality can be worse than possible with a good working system.
A short introduction of a control system, consisting of an air flow meter, especially for aeration air developed and optimized valves as well as an improved PID-controller shall be provided. The controller specials are: a self-learning P-action component, a double-stage I-action component and a D-action component based on experience, but flexible. There are some further correction possibilities e.g. water temperature and salinity (which effect solubility of oxygen in the water), stroke-based and air flow range-based amplification/ damping to adjust the control characteristics and improve control performance.
The common system of the “most open valve” (MOV) is supplemented by the “system of the most important valve” (MIV) to improve control performance when header pressure shall be adjusted based on actual load situation, without any loss of process safety, to safe energy.
Additional calculation of actually required DO-SET values based on effluent quality in reference to the NH4N concentration can be applied. This can lead to a further saving of power consumption.
This PID-controller is unique world-wide. Various data from real plants in different countries will be presented.