Intelligent air supply for wastewater treatment plants
The wastewater treatment plant at Rheda-Wiedenbrück is currently testing a brandnew control concept for AERZEN blower technology - the AERsmart control combination.
Modernisation work at the wastewater plant began in 2013. The citizens of the Rheda-Wiedenbrück region, as well as Germany’s largest pig slaughterhouse, are connected to this plant. A project objective was to supply the biological purification process more efficiently with air, by not only replacing old ventilation grids with new ones, but also by installing these 30 cm. deeper into the ground at the aeration basins. “In view of the surface area of the six basins, we could increase our processing volume by several hundred cubic metres, ” explains wastewater manager Hendrik Wulfhorst. Subsequently, however, 30 cm. of space gained resulted in an increase in the system pressure of 30 mbar, which had to be factored in accordingly in the design of the blower technology.
Before the modernisation of the plant, the biology had been run with a rather high excess of oxygen in the basins, mainly to cover safely any fluctuations in the entry values of the slaughterhouse. With the order to reduce operating costs and the associated CO2-emissions, a clear objective of the project was to link the aeration of the basins considerably more closely with the fluctuating waste water load and the resulting oxygen consumption. This implied, as a first step, a requirement for needs-oriented speed control of the four blower units made by AERZEN. The setpoint values are generated by the Programmable Logic Controller (PLC) from the data measured in the waste water - mainly in the form of ammonium and nitrate concentrations. In addition, there is an intelligent control system for the diaphragm regulating valves. They close slowly, when the required oxygen saturation in the water of the corresponding basin has been achieved. To avoid their closure leading to a higher pressure - and consequently resistance - in the pipe, in parallel the PLC reduces the target pressure. “Otherwise we would destroy energy by means of the diaphragm regulating valves, as within a constant pressure regulation the blowers have to work against the pressure loss caused by the diaphragm regulating valves,” explains Markus Haverkamp, project engineer from the supporting planning company aquaconsult. For the base load supply of the biology, providing alternately a circuit of ventilated and unventilated basins with three purification stages, the engineering office in Hanover came up a solution, with the planning and realisation involving, among others, a turbo blower made by AERZEN.
The type AT 150-0.85-GS achieves an intake volume flow of 4.800 cubic metres per hour with a motor nominal power of 143 kW at an intake pressure of 1 bar and a final pressure of up to 1.8 bar. For Cord Utermann, sales engineer at AERZEN, turbo blowers are classic examples of energy-optimised base load machines which should run permanently for 24 hours a day within nominal value parameters, as then they operate at the highest economic efficiency. “As with almost any turbo technology, energy efficiency falls as soon as the machines are driven into the partial-load range,” explains Utermann. As a consequence, concepts are to be developed for cleaning the varying dirt loads equally energy-efficiently during the day. For optimal energy efficiency in a wastewater treatment plant, the air requirement exceeding the base load has to be covered by displacement machines like positive displacement blowers and rotary lobe compressors. They are strong in high control ranges between 25% and 100%, and have good efficiency, even in part-load operation. Therefore, two AERZEN packaged units type Delta Hybrid (D 62 S) and one Delta Blower (GM 80 L) are also part of the compound system in the wastewater treatment plant in Rheda-Wiedenbrück. AERZEN has developed AERsmart in order to ensure that this quartet not only covers the necessary oxygen for the aeration tanks by a safe process, but also generates the required air volume in the most energy-efficient way in the compound system. According to Utermann,“ the high art of control engineering is to make transitions between the overlaying operating areas as fluent as possible and as energyefficient as possible for every load.” Since three different machines with diverging operational ranges and efficiencies are applied in the wastewater treatment plant in Rheda-Wiedenbrück, their operation must be co-ordinated in such a way “that the number of switching operations is kept as low as possible, as permanent switching-on and off increases wear,” according to Haverkamp. “Efficient air distribution among the aeration tanks and efficient machine selection are necessary for optimal overall efficiency,” he adds.
The demand for oxygen in the three clearance stages is the basis for the optimisation of the control system with the AERsmart control. The indices are processed by the central PLC of the plant and the resulting target pressure is transferred by Profibus to the blower control. AERsmart then takes care of the optimally combined operation of the four packaged units in relation to energy saving. “The turbo blower used here has, for example, the highest efficiency at a capacity of 83 per cent,” explains Utermann. If the air requirement is beyond this value, it can be more efficient to switch-off the base load machine completely and to cover the relatively low air requirement with both Delta Hybrid machines. “Our bacteria do not care where the oxygen comes from,” says Wulfhorst with a smile. But the manager of the wastewater treatment plant points out that the blower technology was projected in the run-up to the modernisation so that the performance of the turbo blower should be sufficient for the “normal” daily requirement. As an interim result, the wastewater treatment plant in Rheda-Wiedenbrück is saving about 30 per cent energy in the biology with the energy-optimised blowers, and a relatively simple process control which is more closely linked to the prevailing actual values. AERsmart provides a further five to eight per cent reduction in energy usage. The field test in the wastewatertreatment plant will prove how much the savings will be over a longer operating period. Rheda-Wiedenbrück is the first wastewater plant in Germany to test the AERsmart system under real conditions. “We can only see the complex connections of a wastewater treatment plant in the field. They cannot be displayed on a test bench. This is why intensive co-operation with our customers is so important. Only this can provide us with a close application reference for future-oriented developments,” summarises Utermann.