Heat recovery with AERZEN blowers
As a part of an extensive renovation project, the city of Filderstadt modernised the Bombach wastewater treatment plant, located in the district of Bonlanden. Now, Delta Hybrid rotary lobe compressors made by AERZEN provide optimal oxygen supply, and, thanks to heat recovery measures, the energy balance has improved further.
Although nowadays process control engineering is getting more and more sophisticated, the biological processes at a wastewater treatment plant cannot easily be transferred to other plants, as “the microorganisms are very sensitive to changes,” explains Sven Gayring, deputy works manager in Filderstadt-Bonlanden. And, while process control engineering is becoming more and more important, simultaneously the demands for efficiency are increasing.
The technology at the Bombach wastewater treatment plant was no longer up-to-date. During ist fundamental modernisation, a particular focus was on biology, which accounts for the lion’s share of energy costs, mainly due to the blowers maintaining the oxygen supply for the microorganisms. For this, the city of Filderstadt now uses four packaged units type Delta Hybrid (D 24 S, 1005 m³/h, 30 kW) made by AERZEN. The packaged unit technology combines the advantages of blower technology and compressor technology, and is not only energy saving but also one hundred percent process safe, as the packaged units provide absolutely oil and absorption material-free air. This means that the packaged units work at optimal levels of energy efficiency both in cases of low air requirement and under higher loads.
To be able to control the volume flow for the basins precisely, all four Delta Hybrid machines are speed-controlled in a frequency range of between 21 and 50 hertz. With a motor power rate of 30 kW each, the units have been spatially combined in one building, but their blower capacity, with a maximum discharge pressure of 1.6 bar (abs.), is split: One packaged unit takes over the basic supply of biology 1, while the other three supply the newly-built aeration basin. Having installed the four rotary lobe compressors in a single room, the wastewater treatment plant has the option to use the radiant heat from the units in a quite simple way. An extraction system installed in the room collects the heated air and distributes it to other rooms in the technology area. There is even greater potential for heat recovery from the blower air itself. This results from the increase in air temperature during compression. To make use of this potential for better energy efficiency, a pipe bundle heat exchanger has been installed in the main pipe. Water, withdrawing a temperature difference of up to 17°C from the warm air, flows through this heat exchanger. The potential, with a flow capacity of 20 cubic metres per minute each, is for an estimated thermal output of 14 kW. This capacity is primarily used for the generation of hot water and for heating the newly built operations building. “Heat recovery in wastewater treatment plants is becoming a more and more important subject,” says works manager Martin May. “It is important to utilise all sources of energy as effectively as possible.”
In the new biology 2, the microorganisms quite specifically decompose nitrate in anaerobic environments. If a sufficient volume of oxygen is available, their preference is to consume ammonium compounds. To be able to control these two degradation processes in one basin, the air supply has been divided into zones. The oxygen content can be controlled precisely according to the prevailing nitrate and ammonium concentrations via the speed of the Delta Hybrid machines made by AERZEN. With a cascaded operation of speed variable blowers and compressors, wastewater companies can optimally adjust the necessary oxygen contents for microbiological cleaning to achieve maximum energy efficiency. Moreover, with comparatively simple technical solutions, the heat originating from the blower air can be used effectively. With this approach, wastewater treatment plants can become self-sufficient in meeting demand for warm water - there is no longer any need for them to use fossil fuels.