ABSTRACT
With river water quality deterioration in recent years, an increasing number of river water quality control studies have been conducted. Among relevant methods, aeration and vegetation planting are effective techniques. The combination of aeration and vegetation can improve the purification effect on the water quality. Based on flume experiments, the mass transfer coefficient of dissolved oxygen in rivers with floating vegetation patches of different diameters under hydrodynamics was studied. Large-diameter floating vegetation can effectively reduce the breaking of bubbles and increase the mass transfer coefficient of dissolved oxygen in rivers. According to mechanism analysis, a model of the oxygen mass transfer coefficient in floating vegetated channels was proposed, and a favorable simulation effect was obtained. This type of research could provide a theoretical basis for selecting and arranging vegetation in aeration floating vegetated channels.
HIGHLIGHTS
Explored the effect of different floating vegetation diameters on oxygen mass transfer.
Provided a formula for the oxygen mass transfer coefficient related to the diameter of floating vegetation.
INTRODUCTION
With improvements in human production and living standards, increasingly more pollutants are discharged into rivers, and water pollution in rivers is becoming increasingly serious (Chinyama et al. 2016; Li et al. 2021). Since the beginning of the new century, scholars have directed more attention toward addressing water quality and environmental problems of rivers (Takić et al. 2017; Jiang et al. 2019). As the carrier of pollutants in rivers, vegetation plays an important role in the river environment (O'Briain et al. 2018; Ielpi et al. 2022). The main types of vegetation in river channels include emergent vegetation, submerged vegetation and floating vegetation. Floating vegetation (water lilies, pondweed, American lotus, Eichhornia crassipes, duckweed, and common bladderwort) is often applied in rivers because of its high aesthetic value (Rooney et al. 2013; Han et al. 2018). Moreover, aeration can increase river water reoxygenation and improve the pollutant retention efficiency (Olgac et al. 1976; Chen et al. 2019). Therefore, it is important to study the dissolved oxygen (DO) mass transfer coefficient produced by aeration in floating vegetation channels.
In aeration, bubbles are injected into water through an aeration device at the bottom of the river. In the process of bubbles rising toward the water surface and breaking, DO is transmitted through the interface between bubbles and water. The transmission efficiency has been extensively determined and studied. It is generally considered that the mass transfer coefficient of DO is mainly related to the aeration rate, bubble size and water velocity (Burris et al. 2002; Zoheidi et al. 2017). Fayolle et al. (2010) demonstrated that when the velocity varied between 0 and 0.42 m/s, the total DO transfer coefficient increased by 29%. Gillot et al. (2000) showed that when the velocity reached 0.44 m/s, the oxygen transfer efficiency of the aerator increased by 38%, and the bubble diameter decreased by 24% compared with that in static water. Zoheidi et al. (2017) investigated the relationship between the bubble size distribution and aeration rate. To determine the bubble size distribution during bubble plug transition flow in a narrow rectangular channel, Zhang et al. (2022a) proposed two sets of bubble fitting methods for transition flow.
There have been many studies on the effect of aeration on DO mass transfer in river channels (Gillot et al. 2005; Fayolle et al. 2007), but studies on the DO mass transfer coefficient in floating vegetation river channels have not been conducted. The impact of floating vegetation on bubble transport and the mechanisms of oxygen transfer must still be studied. Based on flume experiments, the oxygen mass transfer efficiency was investigated in this paper under different floating vegetation diameters and hydrodynamic conditions. The main research purposes were (1) to explore the influences of floating vegetation on the distribution and migration of air bubbles, (2) to examine the influence of floating vegetation on the DO mass transfer coefficient, and (3) to propose a prediction equation for the oxygen mass transfer coefficient in floating vegetation channels. This study is helpful for optimizing vegetation planting in floating vegetation flumes and improving water purification.
STUDY METHOD
Experimental setup
Diameter of vegetation d (cm) . | Air flow rate A (N m3/h) . | Discharge Q (L/s) . | Slope . | Water depth h (cm) . | Average velocity Um (cm/s) . | Bubble diameter (cm) . | . |
---|---|---|---|---|---|---|---|
10 | 6 | 10.29 | 0.0005 | 16.82 | 15.30 | 3.11 | 2,875 |
6 | 12.55 | 0.0005 | 19.30 | 16.26 | 3.14 | 3,506 | |
6 | 14.22 | 0.0005 | 20.31 | 17.50 | 3.15 | 3,972 | |
20 | 6 | 10.54 | 0.0005 | 17.35 | 15.19 | 3.07 | 2,944 |
6 | 12.42 | 0.0005 | 18.11 | 17.14 | 3.22 | 3,469 | |
6 | 14.61 | 0.0005 | 20.53 | 17.79 | 3.04 | 4,081 | |
40 | 6 | 10.18 | 0.0005 | 16.23 | 15.68 | 3.08 | 2,844 |
6 | 12.31 | 0.0005 | 18.23 | 16.88 | 3.06 | 3,439 | |
6 | 14.34 | 0.0005 | 20.68 | 17.34 | 3.16 | 4,006 |
Diameter of vegetation d (cm) . | Air flow rate A (N m3/h) . | Discharge Q (L/s) . | Slope . | Water depth h (cm) . | Average velocity Um (cm/s) . | Bubble diameter (cm) . | . |
---|---|---|---|---|---|---|---|
10 | 6 | 10.29 | 0.0005 | 16.82 | 15.30 | 3.11 | 2,875 |
6 | 12.55 | 0.0005 | 19.30 | 16.26 | 3.14 | 3,506 | |
6 | 14.22 | 0.0005 | 20.31 | 17.50 | 3.15 | 3,972 | |
20 | 6 | 10.54 | 0.0005 | 17.35 | 15.19 | 3.07 | 2,944 |
6 | 12.42 | 0.0005 | 18.11 | 17.14 | 3.22 | 3,469 | |
6 | 14.61 | 0.0005 | 20.53 | 17.79 | 3.04 | 4,081 | |
40 | 6 | 10.18 | 0.0005 | 16.23 | 15.68 | 3.08 | 2,844 |
6 | 12.31 | 0.0005 | 18.23 | 16.88 | 3.06 | 3,439 | |
6 | 14.34 | 0.0005 | 20.68 | 17.34 | 3.16 | 4,006 |
Note: .
Measured oxygen mass transfer coefficient
Simulated oxygen mass transfer coefficient
Model accuracy calculation
RESULTS AND DISCUSSION
Floating vegetation, as common riverway vegetation, profoundly impacts hydrodynamic forces and water quality (Li et al. 2021; Rudi et al. 2021). After adding aeration measures, sufficient oxygen will accelerate the retention of pollutants by vegetation and bacteria (Zhang et al. 2022b). The results of this paper showed that floating vegetation is helpful for improving the oxygen mass transfer efficiency in aeration channels, but this was determined by comparing the results with Equation (5). When l is greater than the vegetation diameter d, bubbles will theoretically overflow out of the vegetation area, so the vegetation slightly affects the mass transfer coefficient of DO. Therefore, a reasonable arrangement of aeration devices in line with floating vegetation channels could help to improve the oxygen mass transfer efficiency in rivers.
The change in the oxygen mass transfer coefficient caused by rigid vegetation varies, as rigid vegetation generates a mechanical diffusion coefficient and a turbulent diffusion coefficient formed by eddy currents between vegetation units. Floating vegetation does not yield these two diffusion coefficients, so its impact on DO diffusion is relatively limited compared to that of rigid vegetation.
In real-world cases, floating vegetation can impede the transfer of oxygen at the air‒water interface. Moreover, areas where floating vegetation naturally occurs often experience very low water velocities. These locations are naturally preferred by vegetation, suggesting that they are likely already populated by other forms of natural vegetation (i.e., submerged and emerged). The density of vegetation in these areas may hinder light penetration and photosynthesis, consequently leading to oxygen depletion and increased community respiration. These complexities highlight the challenges associated with the presence of real vegetation. The application of the experimental results is limited by the assumption that the primary source of oxygen in the channel is the aerator positioned at the bottom rather than oxygen diffusion across the air‒water interface.
The distribution of floating vegetation in reality is not as regular as that in this paper, and the effect of irregular floating vegetation on the oxygen mass transfer coefficient must be studied further. Notably, the change in the oxygen mass transfer coefficient impacts the pollutant degradation efficiency in floating vegetation channels, which should be examined further. This paper is based mainly on indoor experiments. To improve the results of this paper, additional outdoor validation experiments should be performed.
CONCLUSIONS
Based on flume experiments and theoretical analysis, in this paper, the influence of vegetation characteristics on the oxygen mass transfer coefficient in floating vegetation channels was studied, and the following conclusions were obtained:
(1) The average flow velocity in the floating vegetation channel exerts a relatively limited impact on the oxygen mass transfer coefficient. With increasing flow velocity, the DO mass transfer coefficient slightly decreases.
(2) According to the diameter of floating vegetation, an improved oxygen mass transfer coefficient equation is established, and the simulation effect of the model is satisfactory. According to theoretical analysis, the simulation effect can be further improved after adding the breaking coefficient.
(3) When l is greater than the vegetation diameter d, floating vegetation exerts a negligible effect on aeration, which also provides a theoretical reference for selecting and arranging floating vegetation in floating vegetation channels.
(4) The effect of irregular floating vegetation on the oxygen mass transfer coefficient should be studied further, and additional outdoor validation experiments should be conducted.
AUTHORS CONTRIBUTIONS
Y.B. analyzed and interpreted the data and was a major contributor in writing the manuscript.
FUNDING
The paper was supported by National Science Foundation for Young Scientists of China (Grant No. 42207099); Scientific Research Foundation of Zhejiang University of Water Resources and Electric Power (xky2022004); Key Technology Research and Development Program of Zhejiang (No. 2021C03019); Zhejiang Provincial Natural Science Foundation of China (LZJWD22E090001); key technology development and application demonstration of comprehensive management and resource utilization of cyanobacteria in Taihu Lake Basin (Key R&D funds of Zhejiang Province: 2021C03196).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.