In the aeration tank, the turbo provides ventilation

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Courtesy of Aerzener Maschinenfabrik GmbH

Robust in operation: at Emsbüren, the AERZEN turbo saves quite a lot of energy

At Emsbüren wastewater treatment plant, Wasserverband Lingener Land invested in new blower technology for the aeration tank. In comparison with the old assembly, the turbo blower made by AERZEN saves between 100 and 200 kilowatt hours every day. Modernisation of wastewater treatment plants in order to reduce their energy consumption benefits the entire population.

Wasserverband Lingener Land has four wastewater treatment plants in the towns of Lengerich, Freren, Spelle and Emsbüren. About 35,000 citizens in nearly 12,000 households are connected. With a capacity of 16,000 population equivalents, Emsbüren wastewater treatment plant is the biggest, and in the course of its modernisation, it was equipped with a turbo blower made by AERZEN. The compact unit supplies the aeration tank cyclically with oxygen for the oxidation of ammonium to nitrate. This had been preceded, in 2015, by equipping the basin, which has a depth of six metres, with modern fine-bubble diffusers made in Austria. According to the estimation of Hermann Schräer, a local skilled worker, the energy savings at the wastewater treatment plant, which amount to around 30 per cent in overall operation, are mainly due to the new aeration concept.

Modernisation with great effectiveness

“In the purification process, we are now working so productively that we could take one of our two aeration tanks out of operation,” says Schräer. And this increase in effectiveness directly improves resource efficiency- for example, only one submersible mixer has to be used, instead of two. “The agitator only has a power of 3 kW, but that is three kilowatts saved over a long operating period,” adds Schräer. After all, the agitators are working around the clock, as in Emsbüren the biology is not aerated in different zones of a basin but intermittently in one basin. This has a diameter of 24 metres and a capacity of 2,500 cubic metres. The Aerzen Turbo blower TB 50-0.8S, with its electrical motor power of 42 kW and a maximum speed of 42,000 rpm, supplies a volume flow of up to 2,000 cubic metres per hour. The turbo blower installed beside the aeration basin in a compact building has to reach in its performance class a differential pressure of up to 800 millibar. This value is sufficient as the maximum back pressure at the ground of the biology, with a water depth of six metres, is 600 millimetres. As the air supply is provided only a few metres away from the basin, the efficiency increases once again. Shorter pipings reduce friction losses, thus resulting in a lower flow resistance in the system.

Energetically optimised turbo

The cyclic reduction of the nitrogen bound in ammonium and nitrate compounds makes it necessary that aerated and nonaerated phases alternate as far as time is concerned. At present, the daily operation comprises nine aeration cycles. The capacity of the turbo blower is controlled within the redox curve via the current actual value of the oxygen saturation in the water. “For the nitrification phase, an oxygen concentration of 2 mg/l has proved to be successful. If this value has been achieved the PLC reduces the capacity of the turbo,” explains Schräer. If the wastewater plant blew in more air and increased the oxygen concentration to about 3 mg/l, on the one hand this would mean wasted money, and on the other hand the time required for the anaerobic nitrate removal would increase. At Emsbüren, they operate on the basis that a wastewater treatment plant will usually observe a COD value of 70 mg/l, but their average value is 40 mg/l. Therefore, Schräer assesses the available technology as “a very good solution, particularly for small wastewater treatment plants. ”During an aeration cycle, first of all the turbo blower starts operating for a few minutes at 100 per cent capacity, to set the wastewater in the basin in motion. For the remaining time, the plant runs energetically optimised at about 60 per cent of the maximum capacity. At present, the time span of the aerobic and anaerobic phases is fixed at approximately two hours. During the night, and with less inflow, longer periods apply and the considerably lower air requirement is covered by a small positive displacement blower. At Wasserverband Lingener Land (WVLL), the turbo blower is the heart of the biology “and is running very well,” says the skilled worker. In comparison with the replaced technology (an older turbo blower), the operation is extremely safe and energy efficient. The Aerzen Turbo type TB 50-0.8S starts with a power of 42 kW and then reduces to 23 kW. As improved energy efficiency always involves the correct design for the necessary air requirement, the blower capacity has been designed exactly for this basin. The old blower, manufactured in 2001, had been dimensioned generously and had a connection capacity of more than 70 kW - too much for the aeration system of 2,500 cubic metres and its basin with a depth of six metres and a diameter of 24 metres. While magnetic bearings involve an extremely complex regulation and monitoring of the function, the Aerzen Turbos work with a simple and effective air foil bearing. A specialist in blower and compressor technology uses as a maintenancefree friction partner a 2-pot coating. One of these materials is polytetrafluorethylene. PTFE is a thermoplastic, which, due to its very low coefficient of friction, is used as a non-stick coating.

Conclusion

The wastewater treatment plants of WVLL Emsbüren show the energy saving advantages offered by turbo blowers even in relatively small biological basins. Moreover, the robust construction of the turbo series TB makes it possible to end continuous operation and to operate the turbo cyclically instead. Thus, this procedure forms the basis for a simple and effective modernisation of small municipal wastewater treatment plants in the countryside. At Wasserverband Lingener Land they are already planning their next projects.

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