Aeration refers to the use of atmospheric air to supply an oxygen demand, almost always relating to the biological treatment of waste water based on cultures. Most, though not all, organic materials can serve as nutrients for bacterial colonies that oxidize contaminants producing biomass, water and carbon dioxide, The naturally occurring biological systems of streams and rivers utilize residues as nutrient sources, with the oxygen needs supplied from the air by mass transfer through the free liquid surface. Enhancement of the natural aeration rate is needed if there are significant levels of pollution.
Waterfalls and cascades increase the purification capacity of a stream. Drops or jets that impact a free water surface with velocities above about 2 m s−1 entrain air and carry bubbles below the liquid surface. There is a bi-phasic region of intense turbulence, though of short residence times. Gas that accumulates and coalesces below this escapes in a rising bubble cloud surrounding the plunging point of a vertical jet. In terms of energy use, plunging jets are efficient [Bin (1994)]. The entrainment and subsequent mass transfer depend on the velocity at impact, as well as the length and degree of disturbance to the falling jet. Installations of practical size (with jets of say 2 cm diameter) can achieve atmospheric oxygen take up rates in the order of 3 kg per kWh of power consumed (see Plunging Liquid Jets).
Most waste treatment systems use these aerobic biological processes. Oxidation ditches are often used if there is space. The water is repeatedly aerated as it circulates in an oval or labyrinthine channel while a biologically active sludge is maintained. The circulation imposes a cycle of repeated oxygenation and rest that can be exploited to assist the removal of nitrates and other oxidizing materials.
Various mechanical devices that directly disturb the liquid surface have been used to enhance the contact between liquid and gas. The most common designs use rotating paddles mounted at or just below the free surface. These are shaped so that they scoop water from the surface and project it upwards and outwards. The sprayed water takes up oxygen as it passes through the air and entrains more as it impacts with the surface. Another design uses horizontally-mounted cylindrical beaters to thrash the water surface. Like plunging jets, these aerators can generate large-scale circulation of the water in the channel, mixing the bulk liquid and suspending the active sludge. Unfortunately, the energy efficiency falls in large scale installations: values fall below around 1 kg O2 per kWh.
A different approach is used when air is injected beneath the surface of the water to form a bubble plume. In large effluent ponds, the plume is unconfined and induces large scale circulation. The volume of water QW brought to the surface by injection of a volume of gas QG at a depth H is approximately given by the dimensionless relationship /(g H5Og) = 3.4 × 10−3.
The circulation is driven by buoyancy forces and at first approximation is independent of the bubble size distribution. However, the oxygen transfer does depend on the bubble size, so some installations use venturi ejectors or static mixers mounted above the air injection nozzles to ensure that the gas stream is well broken up.
When the bubble column is confined in a pipe, the water can be subjected to a more intense level of oxidation. Bubble columns are widely used for fermentation and in biotechnology, in general, since they are suitable for large systems with relatively slow kinetics or large volumes of process fluids ѕ important factors in waste water purification. One such type is the Deep Shaft. Top-to-bottom circulation is induced in divided deep (or tall) columns, typically 1-5 m diameter and 100 m deep. Once an initial circulation has been established with start-up air, the main air supply is introduced into the downcomer. The air is carried to the bottom of the shaft by the momentum of the circulating fluid before it enters the riser. The pressure increases the driving force for mass transfer, though at the cost of some loss of surface area. Water velocities in a typical installation are of the order of 2 m s−1, giving liquid circulation times of about two minutes.
Respired CO2 is removed at the top of the shaft. A vacuum degasser ensures that the sludge separates satisfactorily in the classifier. Circulation instability is avoided by retaining a small injection of air directly into the riser.
Bin (1993) “Gas Entrainment by Plunging Liquid Jets,” Chem. Engrg. Sc., 48, 3585-3630, Pergamon. DOI: 10.1016/0009-2509(93)81019-R
Deckwer, W. D. (1992) “Bubble Column Reactors,” John Wiley.
Goossens, L. H. J. and Smith, J. M. (1982) “The Mixing of Ponds with Bubble Columns,” Proc. 4th European Mixing Conference, Cranfield, UK.