Energy and cost efficiency of biological treatment plants
Every biologically working sewage plant needs a compressed air station for the oxygen supply of the aeration basin. This station only works at the highest possible energy efficiency level if excessive plant pressure through the subordinated system, excessive intake temperatures due to insufficient aeration and deaeration, and increased operating hours due to insufficient maintenance, are excluded. The following examples show that these criteria can have a positive or negative effect on the energy balance of a blower.
Increased intake temperature
As the air for compression and cooling of a blower comes mostly from the installation room, the room temperature also has an influence on the energy balance of the plant. When there is an increase in the room temperature in the installation room the temperature of the air to be compressed also increases. This not only leads to an increased temperature of the compressed air and consequently to a reduced oxygen volume in the compressed air, but also to a lower degree of filling and to inferior cooling of the compressors. The consequence is that: the compressor will take longer to convey the required oxygen volume. Excessive room temperature can occur in the following situations: (1) if the aeration and deaeration orifices are incorrectly positioned or are too small; (2) if the air- flow is not supported by thermostatically- controlled fans or if their capacity is insufficient; (3) if the roof of the compressor room is insufficiently protected against solar radiation; (4) if the compressed air pipes inside the station are not isolated or else are insufficiently isolated.
Increased plant pressure
Prior to delivery, each blower is set to a predefined pressure. The air compressed to this pressure is mostly channelled via a closed circular pipe with several stubs to the airstones installed in the basin. Incorrectly triggered diaphragm control valves at the entry of the basin can lead to in- creased pressure in the supply system. Then the compressors have to run against this pressure, which leads to higher energy expenditure and thus to extra costs. Another criterion: the slide valves are triggered via maintaining a constant pressure. The sup- ply pipes are closed automatically as soon as the required oxygen content in the basin has been achieved. Frequency controlled blowers are shut down with a certain delay. In case of a too lengthy delay, first of all the blowers continue running at full load against the closed, or partly closed, slide valves. Air has to escape via the pressure valves, which may lead to air losses and in the worst case scenario to damage of the blower stages.
The back pressure can also increase when the airstones in the basin become aged due to the presence of chemical components in the waste water, or the pipings and/or airstones get clogged up over time, for example with detached adsorption material from the blower silencers. For Aerzen positive displacement blowers built since 1995, as well as Aerzen rotary lobe compressors of the new Delta Hybrid series, this risk does not exist. Here, the base support is formed as a pulsation silencer, with noise reduction achieved by means of air deflections instead of via adsorption material. Examples show that a pressure increase of 50 mbar may result in additional energy expenditure, depending on blower design, of between 5 and 10 per cent, and consequently in extra costs potentially amounting to several thousand Euros.
The energy balance can also be negatively influenced if maintenance is not carried out according to manufacturer’s instructions. A classic example of this is a pressure valve damaged by increased resistance, caused by the above mentioned criteria, or an air filter which was replaced too late.
If, just to save 50 Euros, this air filter is not replaced, but is only disassembled and blown out insufficiently and then reinstalled, the energy consumption of the blower will already increase. A standard value: a reduction of the pressure on the suction side by 10 mbar will reduce energy costs by approximately 1 per cent. A new filter will show a pressure loss of only around 5 mbar, but contaminated filters can show a pressure loss of more than 30 mbar.
As compressed air stations usually consist of several compressors the extra energy costs can add up quickly. This can be avoided by maintaining all blowers in accordance with instructions, through the service divisions of the manufacturer and/ or the operator. After all, the energy costs of a compressor over its service life amount to up to 90 per cent of the total costs of the plant. Only around 10 per cent are attributable to investment and maintenance costs.