A biological municipal wastewater treatment plant with a considerably fluctuating discharge quantity can only perform its function with utmost energy efficiency and reliability if the process air consumption of the aeration basin can be adjusted continuously and is fully automated in order to deal with widely fluctuating load quantities. The example of the wastewater treatment plant shows how this is achieved with the use of a speed controlled Aerzen turbo blower.
The sewage treatment plant in Bremervörde was designed to have an overall capacity of 30,000 EGW (population equivalents) and with approx. 29,000 EGW it is now nearly at full capacity. At present, up to 3,000 m³ waste water are fed into the sewage treatment plant every day. However, approx. 1,200 up to approx. 1,500 m³ can be generated by one single production, the quantity fluctuates particularly due to the rest from work on weekends. The amount of wastewater generated by the resident population in the catchment area in the course of the working week with approx. 1,500 m³/day is nearly constant.
View into the blower station of wastewater treatment plant Bremervörde. At the front the AT-turbo blower, behind it two positive displacement blowers Delta Blower, with which process air for the aeration basin (small photo) is generated.
It is obvious that a wastewater treatment system with a wastewater spectrum of 50 up to 100 per cent can only be run with optimal energy utilization when the selected process air generators adjust fully automatically to all the requirements. Sewage work operator Heiko Müller says: “Therefore in November 2010 we realized a new concept for the generation of process air. Currently, one new speed-controlled Aerzen turbo blower as active base load plant covers, by itself, our process air requirements across the entire spectrum.
When so required, we can switch in addition two older model fully automatic positive displacement blowers for generation of the peak load. With this well-tailored distribution of the base load onto an accordingly dimensioned turbo blower and the peak load onto up to two positive displacement blowers we are now operating our process air station particularly energy efficient. The subject ‘energy costs’ is for us extraordinarily important, because for the process air generation we have to spend about 75 per cent of the total energy costs of our wastewater treatment plant.”
Larger control range
Already in the more distant past the process air for the aeration basin was generated by means of positive displacement blowers made by Aerzener Maschinenfabrik. These old units were replaced in 1999/2000 by two Aerzen blowers series Delta Blower. One unit covered the requirement, the second one served as redundancy. Both units were already speed controlled, so that they could be adjusted to the fluctuating requirement in a range of between approx. 35 and 50 Hz. However, in their lower performance range the blowers could not cover anymore the even lower load requirement.
Therefore, supplementary units had to be found for Bremervörde’s process air generation requirements. These had to be speed controlled on the one hand and to have a wide performance spectrum on the other. Additionally, if possible they should be able to run 100 % in speed-controlled load-run operation across the entire requirements spectrum.
In November 2010 a new Aerzen turbo blower type AT 100 – 0.6 (driving power 75 kilowatts) was installed. Since it was commissioned the unit has run 10,700 operating hours (as of February 2012) and consequently ran almost entirely without no-load operation periods around the clock in volume controlled load operation – which proves its optimal design. This unit, with a capacity spectrum of 36 up to 80 m³/min, now serves as base load generator for process air supply of the wastewater treatment plant in Bremervörde.
The two Aerzen positive displacement blowers of the old supply concept can be connected automatically as stand-by units and as peak load generators. As per information from Heiko Müller about 75 per cent of the electrical energy used in the Bremervörde wastewater treatment plant is used for generating process air. Therefore for the new concept the particularly economical handling of electrical energy was an absolute precondition.
Heiko Müller, sewage work operator at Bremervörde "We are convinced that with the present combination with one turbo blower for the base load and two positive displacement blowers for peak load and stand-by, we have realized an ideal and very recommendable concept."
Before making the decision to purchase they had determined that under consideration of the available depth of the basin a discharge pressure of 0.4 bar is sufficient for optimal aeration. Onto this value theimpeller of the blower was customized for the highest possible energy efficiency. Higher pressures up to max. 1.0 bar are possible by using corresponding impellers.
All three units, the new Aerzen ATturbo blower as well as the two approx. 12 year old Aerzen positive displacement blowers series Delta Blower, were installed optimally in a separate building. The supply air enters from the outside. A filter mat prevents dust from penetrating into the unit. Warm exhaust air supported by a fan escapes the room. All the units supply via a busbar in the station directly into the aeration basin. A probe in the basin continuously measures the oxygen content and controls the delivery volume of the process air via the speed of the units. At the end of the treatment process the treated water is discharged into the adjacent river Oste with a purity degree of approx. 95 per cent. “With this concept we meet our requirements optimally and with highest possible energy efficiency. The turbo blower works with its broad performance spectrum as ideal base load unit in spite of our widely fluctuating input quantities between 1,500 and 3,000 m³/day. Depending on the requirements, the Aerzen positive displacement blowers can be connected fully automatically as peak load units”, says Heiko Müller and continues: “With our new concept for generation of process air as per our experience we found an optimal solution with highest possible energy efficiency. Our expenditure on maintenance amounts to just a few minutes a year for changing the filter mats.
And the specialists from Aerzen guarantee an optimally adapted overall concept. We are convinced that with the present combination with one turbo blower for the base load and two positive displacement blowers for peak load and stand-by, we realized an ideal and very commendable concept. With this we can supply our biologically working wastewater treatment plant and make it safe to operate, reliable and especially energy efficient with process air.”