Researches on the delimitation between the water aeration and oxygenation processes

Considering that in certain papers the aeration processes are confused with the oxygenation processes, a distinction between them must be made. The paper presents modern water aeration and oxygenation installations. The experimental results obtained for the two categories of installations are highlighted. Theoretical and experimental researches aim to increase the concentration of dissolved oxygen in water; this can be done in two ways: I - by introducing atmospheric air into the water; II - by introducing a gaseous mixture consisting of atmospheric air and oxygen. Version II is better, the results being shown in Figure 11.


Introduction
In order to increase the dissolved oxygen content in water, there are several methods and procedures that do not differentiate between the notion of "water aeration" and " water oxygenation"; there are papers in which the two expressions are confused [1].
Water aeration is achieved by introducing atmospheric air into the water as follows [2] [3]: 1. By mechanical aeration; 2. By pneumatic aeration.
-In the case of mechanical aeration, the air is introduced into the water under the action of rotors, blades, etc. performing surface aeration.
-In the case of pneumatic aeration, the air is sent to diffusers made of plastics, ceramics, etc.; these diffusers are placed below the water level, thus aerating the water with air bubbles.
The aeration equipment's are classified according to the following criteria [4] [5]:  By the way of obtaining the interfacial contact surface: • equipment's that sprays water in the air and equipment in cascade; • equipment's that disperses the gas in the water (deep mechanical aerators, etc.); • mixed equipment's -spray the water in the form of drops and entrain the air through the jet effect upon mass re-entry of the water from the basin (surface mechanical aerators).
 By the movement of the active body of the aeration equipment: • static equipment's (static aerators, ejectors, etc.); • dynamic equipment's (surface or depth mechanical aerators).  Depending on the type of gas used for aeration: • equipment's that disperses air into water (deep mechanical aerators, pneumatic aerators, ejectors, etc.); • equipment's that disperses pure oxygen in water (pneumatic type); • equipment's with ozone or ozone enriched air in water (such as fluid jet pumps).  By the constructive solution: • pneumatic equipment's with porous diffusers, static aerators, etc.; • surface mechanical equipment's, medium or high depth with rotor, brush, etc. ; • mixed equipment's.  By the immersion mode of the dispersion device: • surface equipment's (mechanical surface aerators with rotor or brush); • medium depth equipment's -the dispersion device is located at a depth of 1 ÷ 2 m (INKA type pneumatic devices, medium depth mechanical aerators, etc.); • deep water equipment's -the dispersion device is located at about 3 ÷ 120 m (pneumatic, injector, mechanical deep water equipment's, etc.).

Boundary between water aeration and water oxygenation installations
This delimitation refers to the nature and composition of the gas introduced into the water.
In table 1 a clear distinction is made between the two processes (two installations), namely [1], [2], [3]: I -Aeration installations; II -Oxygenation installations. It is useful to sort the expressions, as follows: I. Water aeration refers only to the introduction of atmospheric air into water (21% O 2 + 79% N 2 ); II. Water oxygenation refers to the introduction of gaseous mixtures in which the oxygen exceeds 21%.
Obviously I + II have the same purpose, namely to increase the dissolved oxygen concentration in water. When increasing the percentage of O 2 introduced into water by more than 21%, the time in which water oxygenation takes place (C 0  C s ) is reduced.

The installation of water aeration
Fine air bubbles generators (FBG) will be used to introduce air into water. From the literature it is known that the rate of oxygen transfer to water increases with decreasing air bubble diameter; the air bubble diameter of is a function of the orifice diameter in the perforated plate of the FBG.
The gas bubbles (atmospheric air or a mixture of gases) can be classified according to the data in figure 1.       The relation (2) was numerically integrated and a calculation program was performed, the results being presented in figure 5, curve 1. In the experimental researches, the dissolved oxygen concentration in the water was measured from 15 in 15 minutes resulting curve 2 (figure 5). Between the two curves (1) and (2) in figure 5 a good coincidence is observed.

Water oxygenation installations
The operating scheme and the experimental results for three water oxygenation installations are presented. 4    In the centre of the photo one can see the oxygen cylinder, and on the right side there is the transparent plexiglass water tank.  In figure 9, on the left side, two oxygen concentrators (1) are observed, each device being equipped with a rotameter. Fig. 9. Overview of the experimental installation for low nitrogen content air supply [5].

Installation that uses low nitrogen air
The atmospheric air extracted from the atmosphere is compressed and sent to the zeolite filters where nitrogen is retained; upon exiting the unit, a gas containing 95% oxygen is obtained. Figure 10 shows the ozone generator (4); at the exit from it the produced ozone is mixed with air and enters the rotameter (7) and then into the FBG (11) [11] [12]. Fig. 10. Scheme of the experimental installation for water oxygenation researches, by introducing into the water tank a gas mixture containing atmospheric air and ozone 1-electrocompressor; 2-compressed air tank; 3-pressure reducer; 4-ozone generator; 5-pipe; 6-thermometer with digital indication; 7-rotameter; 8-water tank; 9-mechanism for the rotation of the oxygen probe in water; 10-oxygen probe; 11-fine bubble generator; 12-manometer with digital indication.

Conclusions
a) The fine bubbles generator with 0.1 mm orifices is original construction and very efficient solution in the process of water aeration and oxygenation.
b) The results obtained in a theoretical way ( figure 5 and figure 11) coincide well with the experimental data.
c) The value of dissolved oxygen concentration in water, at saturation, is reached most rapidly when pure oxygen is introduced into water (curve 6). d) Future researches will follow the use of microtechnologies and nanotechnologies in water aeration processes.