Waste water aeration with increased efficiency

Ejectors for waste water aeration

Ejectors for the aeration process
Ejectors for the aeration process

Ideal for plants with highly polluted waste water

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.

Advantages of Körting ejectors

  • 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.

Multi-path ejectors inside aeration tanks

Multi-path ejectors inside aeration tanks

Installation from outside through the tank wall

Installation from outside through the tank wall
Multi-path ejectors inside aeration tanks

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

Regulation of the oxygen supply is achieved solely by altering the air volume flow. A reduced air supply lowers the inlet pressure to the ejector thereby additionally reducing the power intake of the roots blower/compressor. At the same time the specific oxygen efficiency is increased. The result is a nearly constant high oxygen yield over the whole control range of the oxygen transfer system with its maximum value in partial load operation near the design point. Ejectors are designed – depending on the rheological properties of the activated sludge (temperature, dry solids contents) – for a suitable air/water ratio. The improved performance of the oxygen transfer system by simply increasing the air supply as well as its optimal oxygen efficiency in partial load operation are so guaranteed at all times.

Installation from outside through the tank wall

1200 m³ aeration tank for waste water from the paper industry

The ejector shown here 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 in SBR plants

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.

Ejectors in SBR plants
Ejectors in SBR plants

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..

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.

Water jet air compressors for introducing atmospheric air

3 water jet air compressors at the 4000 m3 clear filtrate storage tank of the paper mill

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

Reference projects

Aeration tank of a yeast production works
Aeration tank of a yeast production works Aeration tank of a yeast production works

Discription of the plant

Oxygen transfer system consisting of 8 eight-path ejectors made of PP for the 4450 m³ aeration tank of a yeast production works in Belgium. Based on the tank geometry, the water depth and the oxygen requirements the number, arrangement and execution of the multi-path ejectors were attuned to the case in question. Their installation was effected quite simply by means of two flanges on the pipelines for motive flow and compressed air.

Aeration tank of a petrochemical waste water treatment plant
Aeration tank of a petrochemical waste water treatment plant Aeration tank of a petrochemical waste water treatment plant

Discription of the plant

On January 17th, 2011 Körting Trading (Beijing) Ltd. received an order for the aeration tank belonging to a waste water treatment plant in China: An oxygen introducing and mixing system was to be developed which was to be responsible for the transfer of oxygen in a total of 8 aeration tanks. To ensure a comprehensive aeration and intermixing Körting developed a system consisting of a total of 200 multi-path ejectors made of highly resistant polypropylene. The installation guarantees an optimised oxygen introduction before discharge into the Yellow Sea takes place.

Up to now, local products were utilised as oxygen introducing systems in this sector of industry. However, in China the aspect of energy consumption is being taken more and more into consideration since the Government has encouraged and demanded environmental protection and energy efficiency under the present 5-years plan. Amongst other things, this is shown by the enormously increasing order situation in the waste water treatment sector in China which Körting has been able to register – particularly in the sector of landfill leachate as well as waste water issuing from the petrochemical industry and the paper industry.

Commissioning at the final customer, Hengli Petrochemical (Dalian) Ltd. Co. was done in the end of 2012.

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