Waste water has to be cleaned carefully prior to discharge into waters. In the sewage plant Berchtesgaden the oxygen required for this was generated by three turbo compressors in the past. “Although the turbo units were adjustable between 56 and 100 per cent, there were considerable problems concerning the regulation“, reminds Georg Lenz, Production Manager of the sewage plant. “The fact is that the turbos did not work only extremely inefficient; even more oxygen was discharged into the clarifiers than required for the supply of the bacteria. This excess was troublesome concerning sewage, as an overcharge with oxygen influences the sewage process negatively. More precisely: The use of turbo compressors was difficult for the generation of oxygen in our sewage plant”, explains Lenz.
Meanwhile, the three turbo compressors installed in 1991 have been replaced by speed-controlled Aerzen units. First of all, an Aerzen Delta Blower replaced a turbo and was operated together with the other two turbo compressors – in case of low demand, Aerzen unit ran alone. The decisive step to today’s solution followed 2007 and 2009 when the two remaining turbos were replaced by two speed-controlled Aerzen rotary lobe compressors of the newly developed, oil-free compressing series Delta Hybrid.
The station is operated with a maximum pressure of 0,55 bar. „The way we operate the compressors is related to the particular demand and to highest possible energy efficiency and not under the aspect of wellbalanced use. The expenditure for the acquisition of a blower is only a small one in relation to the longtime costs for energy.” emphasizes Production Manager Lenz and shows the advantages of the new Aerzen concept: The power of the three speed-controlled rotary lobe compressors were selected depending on demand with optimal gradation so that the whole spectrum of individual requirements can be covered in a very energy-efficient way. In spite of the actual power range from 50 up to 100 per cent, the units controlled via frequency converter always work very efficiently. Moreover, the new station works in every requirement situation with highest-possible energy efficiency.
The oxygen station is run depending on demand by a superordinated control in the central control system of the sewage plant and a programmable control (SPS) as interface between processor and the blowers. In the three Aerzen units only the input signals are converted into certain speeds via frequency converters. In case of failure of a machine the central control system activates the redundancy unit via an emergency program. Sensors determine the oxygen content and the oxygen requirement in the clarifiers. The oxygen content is adapted in the basins. Production Manager Lenz: “Therefore, we require a very wide power spectrum to comply with the required ideal set value.
The Aerzen units offer their power spectrum from 19 up to 80 m³/min whereat we can operate every frequency converter between 25 and 50 Hz depending on demand. We can operate each unit individually or – if required – also in combination with a second unit.”
