Surface aerator
Surface aerators mechanically introduce air and thus oxygen into the water . The oxygen is required by the living things in the water (from bacteria to fish). The most common place of use are aeration tanks in sewage treatment plants .
Two main types have become established, the roller aerator and the centrifugal aerator .
Energy demand
Units for mixing and aerating aeration basins are by far the largest energy consumers in wastewater treatment plants. Depending on the ventilation system, around 50–80% of the energy requirement falls on this procedural step. Pressure ventilation systems generally require additional units for mixing the activated biomass in the activation tank. Surface aerators, on the other hand, primarily do not require any additional agitators, as these can generate sufficient mixing by the mechanical introduction of ambient air. Such purely mechanical surface aerators are so-called roller aerators (also called rotors or bar rollers) or centrifugal aerators. Roller aerators emerged from the Kessen brush roller developed in 1926.
The oxygen input from roller aerators is achieved via paddles mounted radially on a horizontally running shaft. The interface between water and atmospheric oxygen is constantly renewed. The centrifugal aerator uses the same procedural principle. Centrifugal aerators are driven by a vertical axis of rotation. The centrifugal aerators generate a negative pressure through their pumping action, which sucks water from the pool floor and throws it over the water surface. Since surface aerators were not designed for the mixing or acceleration of the water, the specific energy input for mixing is (about 8W. El / m 3 ) as opposed to a pure agitator (approx. 2 - 5 W el / m 3 ) high . The use of agitators and guide plates can reduce the energy input and increase the oxygen yield and input. Furthermore, greater water depths can be reached.
Basics of mechanical surface ventilation
The introduction of oxygen into water is based on the principle of oxygen diffusion from gases into liquids. The interface between the two phases (water - air) is of crucial importance. In addition, the mechanical energy input during surface aeration sprays water into the air and creates wave movements and air bubbles, which also promote the introduction of oxygen. However, the air bubbles have a minor effect (Hunze & Werner, 2004). Surface-active substances such as surfactants , salts and dry matter content can hinder the process of dissolving molecules from a gas bubble into the wastewater. The alpha value describes this process.
Furthermore, the mechanically introduced energy creates a flow field in the basin that prevents biomass from being deposited. For this reason, a primary clarification can theoretically be dispensed with if aerobic stabilization is used. The shape and geometry of the pool must be selected depending on the choice of aerator. Conversely, the ventilation system must be selected accordingly for existing pools. A wide variety of internals have an influence on the flow and entry conditions.
When commissioning surface aerators, it must be checked whether the maximum water level set by the weir leads to operational disruptions. If the power of the engines does not match the water level, short-term failures can occur and, in the long term, greater wear and tear. Movement of the surface of water always creates waves. Another problem with surface ventilation is the build-up of these generated waves. This physical phenomenon is called interference and it describes the superposition of amplitudes in two or more waves. Shafts therefore have a major influence on the stress on the gearbox and drive motor, as they briefly increase the immersion depth and thus generate resistance. This problem can be largely eliminated by using guide walls or brake crosses, but this is only possible after a few attempts with the installed aerators. Apart from that, there are many parameters that influence wave formation. Winter operation with surface aerators can also cause problems. The formation of ice can lead to unbalance, which puts heavy loads on the bearings. This problem can be minimized by installing in bridges and attaching hoods.
Individual evidence
- ↑ a b DWA-M229: Systems for ventilation and mixing of aeration systems. Ed .: DWA Verlag. Part 1, 2017.
- ↑ Hunze, M. & Werner, D .: Handbook for Surface Ventilation Part 1 . Ed .: Passavant Roediger Products GmbH. No. 1 , 2004.
- ↑ a b c F. Leobendorf: Planning and design of ventilation systems . Ed .: Wastewater training and advice.
- ↑ Frey, W .: Requirements for ventilation systems. Dimensioning and operation of sewage treatment plants for nitrogen removal . Ed .: Wiener Mitteilungen. 1993, p. 110 .