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Why surface aeration? Comparison between surface aerators or low-speed turbines and fine bubble bottom aeration.

How to decide ?


Aeration systems can be subdivided into 2 groups: bottom aeration and surface aeration.

Bottom aeration can in turn be subdivided into different systems, the most widely known of which is the fine bubble system. Surface aerators can be subdivided into slow-speed and high-speed aerators. Slow aerators can also be subdivided into vertical shaft aerators (low-speed turbines) and horizontal shaft aerators (brush aerators).

Of the slow-speed aerators, vertical shaft turbines are the most widely used as they offer more efficient oxygenation, the highest machine induced oxygenation capacity and the greatest capacity for mixing and propulsion in oxidation ditches. In this article we compare the two most effective systems, i.e. low-speed vertical turbines and aeration by means of fine bubble diffusers, in order to guide decision makers towards the best choice.

By comparing low-speed turbines (or surface aerators) with fine bubble aeration, we find that slow-speed vertical shaft aerators have numerous advantages. The investment cost is considerably lower. The same goes for maintenance, whereas fine bubble aerators have to be cleaned regularly. The lifetime of slow-speed surface aerators is far higher and they are far more reliable. Their oxygenation efficiency appears to be lower initially, but as the diffusers age, the fine bubble oxygenation efficiency falls away considerably. The oxygen transfer in urban waste water (alpha factor) is higher for surface aerators. Over extended periods, we see that the energy consumption of slow-speed surface aerators is comparable to or even far lower than fine bubble energy requirements. Other significant criteria also have to be taken into account, such as noise, and the aerosols and accessories required in an aeration system. Slow-speed surface aerators can be entirely covered, in order to avoid any nuisance. The current accelerators, ventilators and instrumentation required for fine bubbles are not necessary for slow-speed aerators.

                            station d'épuration de Louvain-la-Neuve
                           Louvain-la-Neuve water treatment station: membrane bioreactor
                                         with covered AIRMAX surface aerators

Various studies have been conducted on the results obtained by both systems. Some of their conclusions are presented below.

The graph below shows the effect of the activated sludge concentration on the Alpha factor, for low-speed turbines, fines bubbles and coarse bubbles (Alpha factor α = specific environment transfer rate (in field conditions / in clean water)).

Effect of MLSS on Alpha Factor

l’effet de la concentration en boues activées

Reference: Bratby, John R. et al, Merits of Alternative MBR Systems,  
                 WEFTEC 2002.

The above graph shows that for high concentration membrane bioreactors (MBR), the energy saving of LOW-SPEED TURBINES is considerable.

The following 3 tables offer examples of the energy calculation of surface aerators (AIRMAX) and fine bubble systems under several hypotheses. The simplified calculation below does not include either the beta coefficient or the (Cs – C), which are identical for low-speed turbines and fine bubbles.

Example of a project calculation
(* = CEBEDEAU recommendations)

(CEBEDEAU: specialist water treatment and management centre -Liège-Belgium).                 

Specific oxygen transfer rate
(SOTR): 200 kg O2/h
under standard conditions



AIRMAX Low-speed Turbine

Fine bubbles


Taking into account the age

200/1 = 200

200/0.9 =222

kg O2/h

Taking into account the α (alpha) factor*

200/0.9 = 222

222/0.6 = 370
(most favourable case)

kg O2/h

Taking into account the efficiency*

222/1.8 = 123

370/2.2 = 168


Taking into account the accessories:
Mixer, ventilator

0       If mixed using the aerator, which is normally the case for all correctly sized square, round or Carrousel basins!

6   Obligation to take into account the accessories for aeration in accordance with the EN 12-255-15 standard


Total electrical power
to provide 200kg O2/h
  under real conditions!!!                              




i.e. +/- 30% energy saving for the AIRMAX low-speed turbine.

Example of a calculation for 2 existing MBRs in Belgium
(MBRs = membrane bioreactors)

Standard aeration efficiency (SAE):
Measurement compliant with the
EN 12 255-15 standard
(measurements at the motor terminals)

AIRMAX Low-speed Turbine
Louvain -la Neuve

(Carrousel: not perfect tank size!)



Fine bubbles







kg O2/ kwh

Taking into account the age

2.0 x 1.00 = 2.0

2.5 x 0.9 = 2.25

kg O2/ kwh

Taking into account the α (alpha) factor

2.0 x 0.85 = 1.7

2.5 x 0.45 = 1.12

kg O2/ kwh

Taking into account the accessories:
Mixer, ventilator







kg O2/ kwh

i.e. +/- 35% energy saving for the AIRMAX low-speed turbine.

Even if an alpha factor of 0.6 is applied for fine bubbles, the slow turbines are still more efficient.

Example of a calculation for a measured BEST CASE at wire power

Standard aeration efficiency (SAE):
Measurements compliant with the
EN 12 255-15 standard, conducted by an approved laboratory

(measured at wire power)

Low-speed Turbine
Saint-Vaast 75 kw

2.4 (test 1 1999)

2.5 (test 2 2010)

Fine bubbles





kg O2/kwh

Taking into account the age

2.4 x 1.0 = 2,4

3.5 x 0.9 = 3.15

kg O2/ kwh

Taking into account the α (alpha) factor

2.4 x 0.9 = 2.16

3.15 x 0.6 = 1.89

kg O2/ kwh

Taking into account the accessories:
Mixer, ventilator





2.16 !

1.8 !

kg O2/ kwh

i.e. +/- 10% energy saving for the AIRMAX low-speed turbine.


All of these considerations lead us to conclude that the slow-speed surface aerator or low-speed turbine is the solution that offers the most energy-efficient, long term advantages for water aeration.

Furthermore, we have to note that the best efficiencies for the slow turbines are obtained when a close collaboration is developed between  the water treatment station designer and the turbines producer.
Moreover, we have to take in account the fact that only the new generation very slow turbines are able to obtain those efficiency levels.

Contrary to some publications turning out to fine bubbles’ advantage, the slow turbine should be recommended for big water purification projects, like it has already been for some big and densely populated towns (Ho Chi Ming Ville, Riyadh, Cairo…).

Alain Mineur
Process engineer

Text revised by Professor Jean-Luc Vazel, FUL (Luxembourg University)

Battice, Belgium, june 2010

Some useful internet links

designer and manufacter of the AIRMAX suface aerator kamps