Researches on increasing the dissolved oxygen concentration in stationary waters

. This paper presents a way to solve the transfer rate equation of oxygen in water. After running a calculation program, the graph of the function that indicates the increase of the dissolved oxygen concentration in the water as a function of time is plotted. Constructed in an original manner, the experimental installation allows the measurement of the increase of the dissolved oxygen concentration in water. The experimental obtained results are compared with the theoretically obtained data.


1Introduction
The oxygen quantity that is dissolved in wateris a function depending on salinity, the temperature and the pressure [1].
Hot water has less O 2 than cold water, salt watercontains less O 2 than fresh water, and lower-pressure waterhas less oxygen than high-pressure water. Recommended dissolvedO 2 degreefor fishes is from 5.0 mg / dm 3 to 9.0 mg / dm 3 . In the case that thedissolved O 2 degree is under 3.0 mg / dm 3 , due to lack of oxygen, the fishes can die. If the dissolved oxygen level is over 9.0 mg/dm 3 , it can also be deadly to fishes. Figure 1 shows thatO 2 can be: -O 2 bound to H 2 -free oxygen, namely: oxygen dissolved in water. The oxygen solubility in wateris a function depending on the atmospheric pressure, the temperature, water turbulenceandthe magnitude of the air-water area.
Each water type source has its own biological and physicochemical specific featuresand differ from one area to anotheraccording to the composition of the mineralsin the covered region, the contact duration, the temperatureand the climatic regime.
The method of oxygenation of water is made on the basis of the transfer between air and water;from the air, oxygen is transferred through various methods. The air bubbles emitted by a system that forms it are placed in a water tank. Water oxygenation systems that generate very fine air bubblesare the most effective. From the literature [2] [3], it is known that the rate of O 2 transfer to water is higher as the diameter of the air bubbles decreases; the diameter of the air bubble depends on the diameter of the orifice in the perforated plate of a FBG.
The air bubbles can be arranged in as follows ( Figure 2): Theperformance of awater oxygenation system is specified by the following two parameters [4] [5]: -the water oxygenation efficiency; -the water oxygenation efficiency. Themethodof aeration (oxygenation) of water is a fundamental field of Technical Thermodynamics, a discipline materialized by teaching the following courses: Mass and heat transfer, Gas Dynamics, Technical Thermodynamics.

Solving the oxygen transfer rate equation
In water oxygenationmethod, the temperature influences both the oxygen conditions, the dissolved O 2 saturation concentration and the mass transfer coefficient. The saturation concentration values are directly influenced by the water temperature [6].
The dissolved O 2 transfer rate in waterdepending on the temperature is given by the relation [7]: After integrating relation 1 applying the limit condition C = C 0 for τ = 0, it results [6], In the hypothesis that C <C s , after preforming the integration, it results: From the limit condition, the constant C is obtained: C = C 0 whenτ = 0 (4) resulting: Introducing5 in 3 one can obtain: From relation (6) one can obtain: In equation (7), the following must be known: C 0 , C s , ak L . For the determination ofthevalues of [8], [9] was elaborated, presented in figure 3.

The presentation of the experimental installation
In figure 5 one can see the sketch of the experimental installationfor measuring theconcentration of the dissolved O 2 in water, in time. The air compressed by the electrocompressor (1), traversethe flowmeter (6) and subsequently goes inthe FBG (13).
During the experiment, the following were measured: air flow rate, air pressure and the concentration of the dissolved oxygen in water.
The experimental installation (figure 5)comprises a series of devices mounted on the panel (8): -digital thermometer withhigh accuracy, with thermistor sensor; -differential pressure gauge, supplied with a piezoresistive transducer; -oxygenometer with polarographic probe and microprocessor with digital board. The electromechanical machineryfor actuating the oxygenometer probe (11), (12), is supplied with a pecker motor; this machineryadmitsthe cycle of the oxygenometer probe in the water mass, with a speed of 0.38 m /s [10] [11]. Figure 6 shows an overview of the experimental stand, designed and constructed in theLaboratory of the Department of Thermotechnics, Engines, Thermal and Refrigeration Equipment's. One can observe that on the leftwards of the image (figure 6), there is a computer that controls the actuation machineryof the oxygenometer probe; next to the computer is an electro-compressor with a compressed air tank and the devices panel. To the right of the image, is the water tank, made of clear plexiglass.

Experimental researches, obtained results
The steps to be followed in experimental researches are the following: 1. Introduce compressed air into the fine bubble generator and observe if the 152 orifices with Ø 0.1mm are not blocked; 2. The tank is filled with water (H = 500 mmH 2 O) and measure t H2O , C 0 , t air ; 3. The FBG (fine bubble generator)is introduced in water and the time at which the experienceis measured; 4. Introduce compressed air into the FBG and after 15 minutes pull out the FBG from the tank and insert the oxygenometer probe. This operation is performed every 15 minutes, until τ = 120 minutes is reached; 5. From previous researches [12][13] [14], it was found that after a time of τ = 120 minutes, the value of C 0 tends to C s ; 6. Finally, clean the oxygenometer probe and drain the water. Following the measurements, the data in table 1 were obtained. In conformity with the data in table 1, the curveform figure 7 was drawn. From figure 7, one can observe that, with the passage of time the value of C 0 tends to C s . Figure 8 compares the modification of the dissolved O 2 concentration in water in the two cases: 1-graph plottedaccording to the theoretical data; 2-graph plottedaccording to theexperimental data.   1. In the paper, the differential equation of the oxygen transfer rate in waterwas solved, by numerical integration.
2. A computation program was performed and the curve C = f(τ) was drawn; the theoretical results are similar to those obtained in other papers on the field of study.
3. The paper presents an original installation, designed and built-intosee the change in the dissolved O 2 concentration in water. 4. Experimental researches has demonstrated the accuracy of theoretical calculations; for the graphs C = f (τ), the difference between theoretical and experimental values is small.