Ejectors for waste water aeration
In its construction form the ejector for the aeration process corresponds to that of a jet ejector. Because organically polluted wastewater requires a high amount of oxygen and also because of the increasing height of modern biological waste water cleaning plants it is energetically more efficient to pre-compress the air mechanically to the hydrostatic pressure prevailing at the installation point of the ejector and to supply it so to the suction connection.
This waiving of an appreciable pressure in the ejector reduces the necessary motive pressure. At the same time, more favourable mixing ratios (suction flow : motive flow) are achieved. Motive nozzles of Körting ejectors are equipped with a non-clogging spiral. So the motive jet disperses the air already at a low motive pressure in a myriad of fine bubbles which are then mixed vigorously together with the motive flow in the mixing zone.
This air/water mixture is injected into the aeration tank with a high turbulence. In this way the ejector guarantees optimum oxygen supply and a total intermixing of the tank contents. Even with a high biomass concentration in the waste water it is still possible to attain flow velocities which prevent deposits on the tank floor.
Waste water aeration with increased efficiency
There are many different solutions for the aeration of industrial and communal wastewater. However, most solutions quickly reach their limits when dealing with highly contaminated wastewater. Ejectors are an exception here. But what are their advantages and what characterises them?
Design according to requirements
The numerous layout and arrangement possibilities of ejectors and their differing operating methods with atmospheric air intake or compressed air provide ideal basic conditions for their application in large and small water treatment plants.
When equipping an aeration tank with Körting ejectors there will not be any moving parts in the tank which would require maintenance. The ejector’s mechanical stress corresponds to that of a pipeline with a higher flow velocity inside. Roots blowers/compressors and pumps can be installed outside the tank for easy maintenance.
Installation inside aeration tanks
Once the tank geometry, water depth and oxygen requirements have been determined then number, execution form and arrangement of the multi-path ejectors can be adapted to the case in question. Their mounting can be effected quite simply by two flanges in the pipelines for motive flow and compressed air. Installation of compact units on the tank floor guarantees comprehensive aeration and complete intermixture of the waste water.
Design
The relatively short individual and detachable ejectors are connected to a manifold casing and supplied with motive flow from below and with compressed air from above. The jet flow is inclined towards the floor and the angle of exit is adapted individually to the installation conditions. As material we use polypropylene which has excellent resistance to chemical attacks. For particularly demanding application conditions the motive nozzles as well as also the mixing zones can be made of chrome-nickel steel. When there are high calcium carbonate concentrations in the aeration tank the choice of materials must always be adapted to the operating conditions.
Oxygen efficiency
As oxygen transfer not only depends on bubble size (contact surface between gas and water) but to the same measure on the renewal of the gas bubble contact surfaces, ejectors – with their permanent circulation of the waste water – can so achieve a far larger oxygen efficiency than other aerators. With their inclined flow direction towards the floor Körting ejectors utilise the respective tank depth completely as entrance depth. Extensive oxygen supply tests in pure water (ATV M-209) acc. to the oxygen adsorption method form the data basis for designing Körting ejectors. All measurements were executed on full-scale plants and confirmed in numerous inspection tests..
Control range and oxygen yield
The ejector is made of stainless steel and is mounted „through-the-wall“. Its connections for motive flow and compressed air are located outside the aeration tank. These comparatively large units are equipped with a second downstream mixing zone in which additional liquid is sucked in, so intensifying the intermixing action in the tank. If required, the ejector can be equipped with a shut-off device downstream of the motive nozzle so that the motive nozzle can be checked at any time without having to empty the tank.
This equipment option allows for a permanent application of the oxygen transfer system, even with extreme concentrations of calcium carbonate in the waste water. Although the relatively large ejectors are inferior regarding oxygen efficiency to our multi-path ejectors with finer air bubbles, their 100% availability is often the decisive criterion for our customers. It is therefore not remarkable that many plants have been realised in the paper industry – sometimes also as a retrofitting action to replace inadequate installations.
Oxygen efficiency
At the usual entrance depths of 5...8 m it is possible to achieve an oxygen efficiency OC20 in clean water of 11 g O2/Nm3 • m. In partial load operation – at a reduced air amount – the oxygen efficiency can be boosted above this value as far as 14 g O2/Nm3 • m. Depending on entrance depth and oxygen requirements approx. 400-500 Nm3/h of compressed air can be injected into the tank by one single ejector.
Ejectors are exceptionally suited for the aeration and intermixing of so-called „SBR“ plants (Sequenced Batch Reactor). These aeration plants (with retaining operation) in which, amongst other things, the biological processes of nitrification and denitrification take place consecutively in the same tank, require a complete intermixing of the tank contents with and without air supply. Through the installation and special arrangement of ejectors patented for this process in the SBR reactor they can be utilised for the simultaneous aeration of waste water and intermixing during the aeration phase as well as for a selective circulation – without air supply – during the mixing phase.
Recirculation of the waste water through the ejectors takes place automatically at the end of the aeration phase. Both phases can be alternated as often as required so that the independence of the waste water treatment process from feed conditions realised in the SBR process will be supported.
The ejectors, responsible for completely intermixing the tank contents during the mixing phase, are operated with the same circulation pumps used during the aeration phase. When the ejectors convey liquids, the power intake of the circulation pumps is utilised to an optimum – even during the mixing phase.
Through individual adaptation of the flow channels (motive nozzle, mixing zone and diffuser) each water jet air compressor with atmospheric intake is custom-made for its application conditions (filling height, oxygen introduction). The maximum oxygen introduction at optimal energy efficiency is therefore always ensured.
Its installation can take place in the aeration tank as well as through the tank wall. The motive side of the water jet air compressor is connected to a circulation pump suited for the operation via a short pipeline. The suction side is carried above the filling height by means of a pipe or flexible hose pipeline.
Compact aerator
The so-called “compact aerator” is a combination of a water jet air compressor and a waste water submersible motor-driven pump mounted on a combined base frame. The effort required for installation of this compact unit is extremely low.
Compact aerators are utilised predominantly for:
- cost-efficient retrofitting of aeration tanks
- installation in mixing and equalisation tanks
- during conversion or for peak load coverage in aeration tanks
Your advantages
- maintenance-free - no moving elements
- high oxygen efficiency - Fine bubbles create large contact surfaces between air and water and high turbulence renews these contact surfaces
- no deposits - The intensive jet flow directed towards the tank floor prevents deposits of biomass
- straightforward control of oxygen supply
- non-clogging construction - The nozzle diameter defines the narrowest flow cross-section
- no sealing problems - When the plant is inactive, water can enter the air pipeline without negative effects and when re-started, the ejector’s entraining effect expels any liquid in the pipe
- design acc. to requirements - Various sizes can be designed and adapted to requirements
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