A technical and economic analysis of aeration systems in the Control of Eutrophication in the Water Supply

“A technical and economic analysis of aeration systems in the Control of

Eutrophication in the Water Supply”

Gafsi Mostefa(1), Kettab Ahmed(2), Djehiche Abedelkader(1)

(1)

Research Laboratory of Civil Engineering: RLCE, ResearchTeam. Water Resources University of Ammar Telidji Laghouat, Laghouat , Algeria. Email:m.gafsi@mail.lagh-univ.dz;msgafsi@yahoo.fr;

a.djehiche@mail.lagh-univ.dz; djehichea@yahoo.fr

(2) _{Laboratory Research in Water Science:LRW-Water/ENP. National Polytechnic School El Harrach,}

Algeria. Email: kettab@yahoo.fr;lrs eau@netcourrier.com

Abstract

The techniques of restoration of lakes or the prevention against eutrophication are numerous

(chemical, biologic, mechanical…). Due to their excessive cost and the relatively insignificant

income of some of these techniques, the process of dynamic aeration is one of the most promising methods.

Several techniques of the control of nutritional elements are selected for this study: artificial destratification by the bubble plume, partial (or total) lift hypolimnetic aerator, bubble plume oxygenation and Speece Cone oxygenation in the hypolimnetic aeration. Each of these methods has its advantages and inconveniences. A technical and economic analysis established by different researches reveals that the hypolimnetic oxygenation is the most favorable for the control of nutrional elements. In hypolimnetic aeration systems, the aeration system by bubble plume appears to be the most economic and perhaps the most simple among the systems used in Standley lake (Colorado, U.S.A), even as other researches select Speece Cone aeration system. We found that the aeration by oxygen limits the nitrogen saturation and in contrast the aeration creates it. We demonstrate that the most efficient hypolimnetic aeration system is the bubble plume diffuser; although an accidental destratification may occur. We show as well that the destratification can be used in winter because the temperature of the lake is not modified. However, the hypolimnetic aeration is used in summer in order to avoid the homogenization of the lake temperature during this period.

The work presented in this paper is a review of aeration techniques and their impacts on water reservoirs. As well, this study concentrate on the economic and technical sides associated to these aeration systems.

Keywords: artificial aeration, destratification , hyolimnetic aeration, dissolved oxygen,

thermal stratification, bubble plume, lake.

1. Introduction

The artificial aeration of oxygen-depleted lake waters is one of many methods used for remediation [1, 2]. It is undoubtedly the most frequently used technique [1, 3], due to its relatively low cost and ease of deployment [1, 2]. There are two main types of artificial aeration of lakes: destratification and hypolimnetic aeration [2, 4]. In the first case, the entire lake is mixed, usually by the release of compressed air from a perforated air line laid along the bottom of the lake. In the second case, the objective is to maintain thermal stratification, while oxygenating the hypolimnion [2, 5]. These restoration techniques can be used separately or in

combination [2, 6]. In the case of separately used systems [6], an artificial mixing of the water column during the cold season and an input of oxygen into the hypolimnion during summer in order to preserve the stratification are assured .Combined systems can be switched between mixing mode using coarse air bubbles, and hypolimnetic oxygenation or aeration mode using fine oxygen or air bubbles. Every diffuser is operated using air or oxygen during the summer and air during the winter [2, 7]. Each one of these methods has its advantages and disadvantages. We found that the aeration by oxygen limits the nitrogen saturation and in contrast the aeration creates

it. We demonstrate that the most efficient hypolimnetic aeration system is the bubble plume diffuser;

although an accidental destratification may occur. We show as well that the destratification can be used in winter because the temperature of the lake is not modified. However, the hypolimnetic aeration is used in summer in order to avoid the homogenization of the lake temperature during this period.

The work presented in this paper is a review of aeration techniques and their impacts on water reservoirs. As well, this study concentrate on the economic and technical sides associated to these aeration systems.

2. Aeration systems

Three oxygen input devices are commonly used for hypolimnetic aeration (figure 1) [2, 8]:  Airlift hypolimnetic aerator (figure 1-a): Full-lift hypolimnetic aerators typically consist of a

vertical riser tube, a diffuser inside the bottom of the tube, an air air-water separation chamber at the top of the riser, and one or two return pipes, called downcomers [2, 8]:

 Bubble-plume oxygenator (figure. 1-b): Bubble-plume diffusers are generally linear or circular and inject either air or oxygen at relatively low gas flow rates [9]. These systems are most suitable for deep lakes where the bulk of the bubbles dissolve in the hypolimion and the momentum generated by the plume is low enough to prevent significant erosion of the thermocline [2, 8].

 Speece Cone oxygenator (figure 1-c): The system consists of a source of oxygen gas, a conical bubble contact chamber, a submersible pump, and a diffuser that disperses highly oxygenated water into the hypolimnion [2, 8].

a b

Based on a study of Standley lake (Colorado: USA), McGinnis and Little (1997), described a technical and economic analysis of these three systems (figure 3) in order to select the most appropriate aeration mechanism for a specific lake, thus optimizing both the design and operation to ensure the greatest oxygen transfer efficiency (see tables 1, 2 and 3) [2, 8].

Table 1. Partial-lift hypolimnetic aerator [2, 8].

Variable and predicted performances: Values

-Air flow (Nm3/s) 0.12

-Height of ascending tube (m) 12.2

-Diameter of ascending tube (m) 3.10

-Flow of drained water (m3/s) 1.17

-Increase in oxygen concentration (g/m3) 4.60

-Efficiency in oxygen transfer (%) 16

-Total oxygen transfer (16 aerators) (kg/jour) 464

- Oxygen transfer by aerator (kg/day) 7400

Table 2. Bubble plume [2, 8].

Variable and predicted performances: Values

-Oxygen flow (Nm3/s) 0.069

-Initial diameter of bubbles (mm) 2.5

-Length of the diffuser (m) 2.500

-Initial speed of the plume (m/s) 0.038

-Height of the plume rise 1.5

-Efficiency in oxygen transfer (%) 93

-Total oxygen transfer (kg/day) 7400

Table 3. Speece Cone [2, 8].

Variable and predicted performances: Values

-Oxygen flow (Nm3/s) 0.068

-Initial diameter of bubbles (mm) 2.0

-Imposed water flow (m3/s) 1.3

-Detention time of bubble (min) 2.0

-Increase in oxygen concentration (g/m3) 66

-Total oxygen transfer (kg/day) 7400

-Efficiency in oxygen transfer (%) 94

Results

McGinnis and Little (1997) showed that the bubble plume diffuser is the most economic system as well as being the most simple of these three systems. Their conclusions were based on the following [2, 8]. In the Partial-lift hypolimnetic aerator, the efficiency of oxygen transfer is the

lowest (16%). For the Speece Cone, and the bubble plume, the efficiency of oxygen transfer is very similar (94% and 93% respectively). In addition they found that a high value of the water velocity must be maintained in the Speece Cone in order to ensure that the bubbles would not reach equilibrium with the water which may lead to accumulation of bubbles and coalescence in the cone leading to a decrease in the total efficiency.

Oxygen absorption efficiencies for the bubble plumes and Speece cones were evaluated by Richard E. Speece in 1994. He showed that in impoundments and shallow reservoirs with depths less than 30 m the fine bubble diffusers require a rise height of about 30 m within the hypolimnion to ensure efficient oxygen transfer to the hypolimnion. He highlighted that the bubbles must be maintained in contact with the water for approximately 100 seconds to accomplish oxygen absorption efficiencies more than 80%. An example of an oxygen transfer device is the Downflow Bubble Contact Oxygenator Speece Cone, or Speece Cone, developed by the author. Consequently, very efficient oxygen transfer can be achieved. The author concluded that free rising bubble plumes are to be avoided in order to maintain stratification [2, 10].

Mark H. Mobley made a survey in 1997 on a diffuser aeration system. In this report he described how the Tennessee Valley Authority (TVA) had developed a proficient and economical aeration diffuser design that operated successfully. By spreading the gas bubbles over a very large area in the reservoir, the oxygen is rapidly transferred, and temperature destratification and sediment disturbance is minimized [2, 11, 12]. The author shows that the reservoir water displayed significant increases of dissolved oxygen in the hypolimnion and no substantial disruption of thermal stratification (figure 2).

Finally, the author mentioned that TVA line diffuser can represent a benefic resolution for meeting difficult dissolved oxygen requirements at hydropower projects. As well, because of the high oxygen transfer efficiency and operational suppleness, set costs are reduced.

3. Compared results of destratification and hypolimnetic aeration systems

Richard J. Ruane and his co-workers (1977) have compared the two methods of aeration (Table 4) applied to the Patrick Henry Dam (south of Holston river, USA) [13, 14].

Table 4: Application domain of the aeration methods for the Patrick Henry dam [13, 14].

Conditions under which the aeration technique must be applied

Destratification

hypolimnetic Aeration with diffusion

Mechanic.

Pumping air Diffusion Air Oxygen

Cold water - - + +

Strong increase in DO - - ? ?

Minimal effect on the

production of energy + + + +

Minimal increase in the

dissolved nitrogen ? ? ? +

Small cost + + -

-+ Indicates a positive effect on the particular aeration method - Indicates a negative effect on the particular aeration method

? Indicates an unknown effect for the shown condition

Report

Ruane and his co-workers (1977) reported that destratification and hypolimnetic aeration are not applicable to the Patrick Henry dam. They explained that the first method could increase the water temperature and consequently lead to ominous effects on the pisciculture, since the dam is classified as a coldwater fishery. In addition, the hypolimnetic aeration with air causes nitrogen supersaturation, and the aeration with oxygen is not promising due the high cost of oxygen [15, 16].

In order to increase the oxygen content in the hypolimnion, oxygen gas can be used instead of air. The advantage of using oxygen gas is that a compressor is not necessary, and that oversaturation with nitrogen is avoided [13, 14].

4. Conclusion

This study allows us to conclude that:

 The aeration by oxygen limits the nitrogen saturation; in contrast the aeration by air

creates it.

 The most efficient hypolimnetic aeration system is the bubble plume diffuser; although an

accidental destratification may occur. In shallow reservoirs, these systems should be avoided, because it can entrain the colder, hypolimnion water and carry it through the thermocline into the epilimnion and to the surface by the momentum induced by the bubble plume.

 Destratification can be used in winter because the temperature of the lake is not modified.

However, the hypolimnetic aeration is used in summer in order to avoid the homogenization of the lake temperature during this period.

 For reservoirs and dams, summer destratification is not profitable due to the warming-up

of the reservoir water which can be a problem. .

However, it should be noted that in the case of lakes or reservoirs used for fish farming, the hypolimnetic oxygenation limits the amount of nitrogen introduced as compared with systems that aerate using air, and guarantees the preservation of the fish.

References

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