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Research on the Influencing Factors of Hydraulic Efficiency in Ditch Wetlands
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Procedia Engineering
Procedia Engineering 00 (2011) 000–000 Procedia Engineering 28 (2012) 759 – 762
www.elsevier.com/locate/procedia
2012 International Conference on Modern Hydraulic Engineering
Research on the Influencing Factors of Hydraulic Efficiency in Ditch Wetlands Liang LIUa, Haoyun HUb, Jihong QIc, a* a
College of Hydraulics and Electric power, Hebei University of engineering, Handan, Hebei 056021, CHINA b Department of science and technology, Hebei University of engineering, Handan, Hebei 056038, CHINA c Kexin College, Hebei University of engineering, Handan, Hebei 056038, CHINA
Abstract The hydraulic retention time is an important factor that affects treatment performance of constructed wetlands, and it should be taken into consideration in hydraulic optimization design. In these days, the theoretical retention time is more considered to design the constructed wetlands. But the water doesn’t flow the wetlands uniformly and stagnant water area exists in the wetlands. So the hydraulic retention time is less than or equal to the theoretical retention time and it will cause the overestimation of designation. In this article, the effective retention time and hydraulic efficiency were put forward to optimize the treatment performance and to improve hydraulic efficiency of the ditch wetlands. And the factors which influence the hydraulic efficiency (t10/T) were studied. The experiment results of ditch wetlands verified that the total ratio between the ditch length and ditch width is one of the key factors influencing the hydraulic efficiency (t10/T). The bigger the ratio is, the bigger the value of t10/T is. And the flow is more close to ideal Piston flow. The growth rate of t10/T begins to slow down along with the ratio (L/W) increasing.
© © 2012 2011 Published Publishedby byElsevier ElsevierLtd. Ltd.Selection and/or peer-review under responsibility of Society for Resources, Environment and Engineering Keywords: ditch wetlands; hydraulic retention time; hydraulic efficiency; agricultural non--point source pollution
1. Introduction In recent years, point source pollution has been controlled relatively, so non-point source pollution of the water environment is concerned by more and more people[1-2]. A large number of studies show that non point source pollution, especially agricultural non point source pollution, has become the main factor
* Corresponding author. Tel.: +86 310 3124286 E-mail address: [email protected]
1877-7058 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Society for Resources, Environment and Engineering doi:10.1016/j.proeng.2012.01.804
760 2
Liangname LIU /etProcedia al. / Procedia Engineering 28 (2012) 759 – 762 Author Engineering 00 (2011) 000–000
that affects the water environmental quality [3-4]. Agricultural non point source pollution refers to the nutrients such as nitrogen and phosphorus, pesticide and other organic or inorganic substances pollution. This pollutants cause massive contamination to the water source and agricultural ecology by agricultural surface runoff and leakage. The pollutions include fertilizer pollution, pesticide pollution, straw burning pollution, poultry raising pollution, agriculture film pollution and natural disaster pollution. There is dispersion, concealment, randomness, uncertainty, extensiveness and difficulties to monitor in agriculture non-point source pollution and it has become the main pollution source in surface water and underground water. Field drainage ditches with complex ecological structure and unique ecological features have double effects of water conveyance and artificial wetland. The transport and transformation of agricultural non point source pollution in drainage ditches are the keys to control agricultural non point source pollution. The hydraulic retention time reflects the chemical reaction time of pollutants in the wetland. The treatment effect and hydraulic retention time of wetland system are close related [5-6]. If the retention time is too short, it would influence the effect of biochemical reaction. So the hydraulic retention time in wetland is a very important design factor. In these days the theoretical retention time is used to design the constructed wetlands usually. But the water doesn’t flow uniformly in the ditch wetlands and it could be stagnant in some areas of the wetlands. So the real hydraulic retention time is less than the theoretical retention time and this will cause the overestimation of designation. In this article, the effective retention time (t10) and hydraulic efficiency (t10/T) are put forward to optimize the design of ditch wetlands and to improve hydraulic efficiency of the ditch wetlands. And the factors which influence the hydraulic efficiency are studied. The experiment results of ditch wetlands verify that the total ratio between the ditch length and ditch width is one of the key factors influencing the hydraulic efficiency (t10/T). 2. The effective retention time (t10) and hydraulic efficiency (t10/T) 2.1. The effective retention time In order to ensure efficiency of ditch wetland in treatment farmland drainage, the contact time between farmland drainage and the microbial of ditch wetland is expected to be the theoretical retention time. But in fact, it is unrealistic that the retention time of pollutants in the ditch wetland is all the theoretical retention time. The flow in ditch wetland cannot achieve the ideal plug flow, so the retention time of the pollutions in ditch wetland is less than the theoretical retention time. This will reduce the efficiency of ditch wetland in treatment farmland drainage. In order to have a more accurate representation to the residence time of pollutants in ditch wetland, the effective retention time (t10) is put forward instead of the theoretical retention time in this article. The retention time of pollutions in ditch wetland can be expressed by Residence-time Density Function f(t) and Cumulative Residence-time distribution Function F(t). The definition of f(t) : it is m that the gross of pollutions flowing into ditch wetland by entrance at time t . It is n that the gross of pollutions flowing out ditch wetland by exit at time t+△ t. The ratio between the n and m is f(t)△ t. So it is: (1) 0 f ( t )dt 1 The definition of F(t) : it is m that the gross of pollutions flowing into ditch wetland by entrance at time 0 . It is n that the gross of pollutions flowing out ditch wetland by exit at time t. The ratio between the n and m is F(t). So it is: (2) F ( t ) f ( t )dt t10 is the t while F(t)=0.1(that is 10%). The definition of t10 in dealing polluted water with ditch wetlands: 90 percent pollutions can reach the
0
761 3
Liang LIU name et al. // Procedia – 762 Author Procedia Engineering Engineering 28 00 (2012) (2011) 759 000–000
Residence-time in ditch wetlands. That is the time when 10 percent of gross pollutions flowing into ditch wetlands flow out ditch wetland by exit. This is the reason using t10 as the design basis in the ditch wetlands. If it is the ideal plug flow (Piston flow), t10/T equals 1 and t10/T equals 0.1 if it is CSTR. The value of t10/T for ordinary reactor varies from 0.1 to 1. The value of t10 can be determined by tracer experiment in the application. 2.2. The hydraulic efficiency (t10/T) The flow can make full use of ditch wetlands volume and the effective residence time T10 would increase under the condition of constant flow if the hydraulic conditions and hydraulic efficiency of ditch wetlands are improved to make the flow approaches to the ideal plug flow. Let t10/T denote the evaluation index of hydraulic efficiency, then the flow is closer to plug flow as the value of t10/T is larger. It is significant to increase the value of t10/T for improving the treatment effect of ditch wetlands. The sufficient hydraulic retention time is needed to ensure the water quality of outflow from ditch wetlands. Here HRT be denoted by t10. So, if the theoretical retention time (T) is invariable, that is to say, the volume of the ditch and the inlet flow are invariable, the increase of value of t10/T can increase the effective hydraulic retention time t10 and enhance the water quality of outflow from ditch wetlands. And if the t10 is invariable, the increase of value of t10/T can decrease the theoretical retention time (T). So if the volume of the ditch is invariable, the quantity of disposed wastewater in the unit time can increase. Now we give an example to illustrate the theory above. (1) There is a ditch wetland. Here the theoretical retention time T=12H and t10/T=0.2, then the effective hydraulic retention time is: t10=0.2×12=2.4h. If the t10/T increases to 0.6, the effective hydraulic retention time is: t10=0.6×12=7.2H. It is three times of original. (2) Suppose that the quality of water flowing out ditch wetland approach the aim (the flux of inflow is Q), the t10 needs to be 12 hours. Let t10/T of the ditch wetland be 0.2, then the theoretical retention time is: T=12H/0.2=60 hours. And if t10/T increases to 0.6, then the effective hydraulic retention time is: T=12H/0.6=20 hours. It is only 1/3 of original. It indicates that the ditch wetland volume can reduce to 1/3 of original under the condition of keeping the flux of inflow is invariable. So the ditch wetland volume can reduce by increasing the value of t10/T or the inflow can enhance by increasing the value of t10/T under the condition of keeping the quality of outflow is invariable. 3. Experimental Table 1. Relationship of t10/T and ratio between ditch length and ditch width ratio between length and width
t10/min
T/min
t10/T
1:25
17
40
0.43
1:50
52
82
0.63
1:75
96
125
0.77
1:100
143
166
0.86
1:150
225
250
0.90
The factors that influence the t10/T of ditch wetlands have the ratio between the ditch length and ditch width, the density of aquatic plants in ditch wetlands, the number of the corners in ditch wetlands, the lining material of ditch and so on. In all the factors, the ratio between the ditch length and ditch width is
762 4
Liangname LIU /etProcedia al. / Procedia Engineering 28 (2012) 759 – 762 Author Engineering 00 (2011) 000–000
the most important one. In this article, Tracer experiment was done in fore ditches in the farmland of Xupu in order to make certain of the relationship between t10/T and the ratio between the ditch length and ditch width. The experimental results are shown in Table 1. The experiment results of ditch wetlands verify that the ratio between the ditch length and ditch width has an effect on the value of t10/T. The bigger the ratio is, the bigger the value of t10/T is. And the flow is more close to ideal Piston flow. The ratio should be enough in order to improve the hydraulic efficiency (t10/T). As shown in figure 1, the growth rate of t10/T began to slow down along with the ratio (L/W) increasing. 4. Conclusion Ditches are extremely common in farmland and have an impotent effect on both agricultural production and ecological environment. The ditch wetlands have many functions such as irrigation, drainage, and protection of ecological environment. So the ditch wetlands are concerned by experts gradually. In this article, the effective retention time (t10) and hydraulic efficiency (t10/T) are put forward to optimize the design of ditch wetlands and the factors which influence the hydraulic efficiency are studied by Tracer experiment in several ditches wetland. The experiment results of ditch wetlands verify that the total ratio between the ditch length and ditch width is one of the key factors influencing the hydraulic efficiency (t10/T). The bigger the ratio between the ditch length and ditch width is, the bigger the value of t10/T is. So the ratio between the ditch length and ditch width should be enough to improve the hydraulic efficiency (t10/T). Acknowledgements Supported by NSF of Hebei Province (D2009000827; C2011402042) and the Research Fund for Youth of Higher Education of Hebei Province (2011247) References [1] GUO Hong-peng, ZHU Jing-ya, YANG Yin-sheng, et al. Research Status and Development of Technologies for Controlling Agricultural Non-point Source Pollution[J]. Transactions of the CSAE, 2008,24(4):290-295. [2] LONG Tian-yu, LI Ji-cheng, LIU La-mei, et al. Adsorbed Non-point Source Pollution Load of Jia-ling River Basin [J]. Environment Science, 2008,29(7):1810-1817. [3] YIN Fu-cai, ZHANG Zhi-yuan. Survey of Chaohu Lake Eutrophication Research [J]. Journal of Lake Sciences, 2003, 15(4):377-384. [4] LI Qiang-kun, LI Huai-en, SUN Juan, et al. Nitrogen Loss in Qingtongxia Irrigation Area[J]. Journal of Agro-Environment Science, 2008, 27(2):683—686 [5] LU Xue-qiang. HRT Measurement of Wetland Project for Wastewater Treatment[J]. Arid Environmental Monitoring, 1999, 13(2):74-75. [6] Persson J, Somes N L G, Wong Y H F. Hydraulics Efficiency of Constructed Wetlands and Ponds [J].Water Sci Technol,1999,40(3):291-300.
Procedia Engineering
Procedia Engineering 00 (2011) 000–000 Procedia Engineering 28 (2012) 759 – 762
www.elsevier.com/locate/procedia
2012 International Conference on Modern Hydraulic Engineering
Research on the Influencing Factors of Hydraulic Efficiency in Ditch Wetlands Liang LIUa, Haoyun HUb, Jihong QIc, a* a
College of Hydraulics and Electric power, Hebei University of engineering, Handan, Hebei 056021, CHINA b Department of science and technology, Hebei University of engineering, Handan, Hebei 056038, CHINA c Kexin College, Hebei University of engineering, Handan, Hebei 056038, CHINA
Abstract The hydraulic retention time is an important factor that affects treatment performance of constructed wetlands, and it should be taken into consideration in hydraulic optimization design. In these days, the theoretical retention time is more considered to design the constructed wetlands. But the water doesn’t flow the wetlands uniformly and stagnant water area exists in the wetlands. So the hydraulic retention time is less than or equal to the theoretical retention time and it will cause the overestimation of designation. In this article, the effective retention time and hydraulic efficiency were put forward to optimize the treatment performance and to improve hydraulic efficiency of the ditch wetlands. And the factors which influence the hydraulic efficiency (t10/T) were studied. The experiment results of ditch wetlands verified that the total ratio between the ditch length and ditch width is one of the key factors influencing the hydraulic efficiency (t10/T). The bigger the ratio is, the bigger the value of t10/T is. And the flow is more close to ideal Piston flow. The growth rate of t10/T begins to slow down along with the ratio (L/W) increasing.
© © 2012 2011 Published Publishedby byElsevier ElsevierLtd. Ltd.Selection and/or peer-review under responsibility of Society for Resources, Environment and Engineering Keywords: ditch wetlands; hydraulic retention time; hydraulic efficiency; agricultural non--point source pollution
1. Introduction In recent years, point source pollution has been controlled relatively, so non-point source pollution of the water environment is concerned by more and more people[1-2]. A large number of studies show that non point source pollution, especially agricultural non point source pollution, has become the main factor
* Corresponding author. Tel.: +86 310 3124286 E-mail address: [email protected]
1877-7058 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Society for Resources, Environment and Engineering doi:10.1016/j.proeng.2012.01.804
760 2
Liangname LIU /etProcedia al. / Procedia Engineering 28 (2012) 759 – 762 Author Engineering 00 (2011) 000–000
that affects the water environmental quality [3-4]. Agricultural non point source pollution refers to the nutrients such as nitrogen and phosphorus, pesticide and other organic or inorganic substances pollution. This pollutants cause massive contamination to the water source and agricultural ecology by agricultural surface runoff and leakage. The pollutions include fertilizer pollution, pesticide pollution, straw burning pollution, poultry raising pollution, agriculture film pollution and natural disaster pollution. There is dispersion, concealment, randomness, uncertainty, extensiveness and difficulties to monitor in agriculture non-point source pollution and it has become the main pollution source in surface water and underground water. Field drainage ditches with complex ecological structure and unique ecological features have double effects of water conveyance and artificial wetland. The transport and transformation of agricultural non point source pollution in drainage ditches are the keys to control agricultural non point source pollution. The hydraulic retention time reflects the chemical reaction time of pollutants in the wetland. The treatment effect and hydraulic retention time of wetland system are close related [5-6]. If the retention time is too short, it would influence the effect of biochemical reaction. So the hydraulic retention time in wetland is a very important design factor. In these days the theoretical retention time is used to design the constructed wetlands usually. But the water doesn’t flow uniformly in the ditch wetlands and it could be stagnant in some areas of the wetlands. So the real hydraulic retention time is less than the theoretical retention time and this will cause the overestimation of designation. In this article, the effective retention time (t10) and hydraulic efficiency (t10/T) are put forward to optimize the design of ditch wetlands and to improve hydraulic efficiency of the ditch wetlands. And the factors which influence the hydraulic efficiency are studied. The experiment results of ditch wetlands verify that the total ratio between the ditch length and ditch width is one of the key factors influencing the hydraulic efficiency (t10/T). 2. The effective retention time (t10) and hydraulic efficiency (t10/T) 2.1. The effective retention time In order to ensure efficiency of ditch wetland in treatment farmland drainage, the contact time between farmland drainage and the microbial of ditch wetland is expected to be the theoretical retention time. But in fact, it is unrealistic that the retention time of pollutants in the ditch wetland is all the theoretical retention time. The flow in ditch wetland cannot achieve the ideal plug flow, so the retention time of the pollutions in ditch wetland is less than the theoretical retention time. This will reduce the efficiency of ditch wetland in treatment farmland drainage. In order to have a more accurate representation to the residence time of pollutants in ditch wetland, the effective retention time (t10) is put forward instead of the theoretical retention time in this article. The retention time of pollutions in ditch wetland can be expressed by Residence-time Density Function f(t) and Cumulative Residence-time distribution Function F(t). The definition of f(t) : it is m that the gross of pollutions flowing into ditch wetland by entrance at time t . It is n that the gross of pollutions flowing out ditch wetland by exit at time t+△ t. The ratio between the n and m is f(t)△ t. So it is: (1) 0 f ( t )dt 1 The definition of F(t) : it is m that the gross of pollutions flowing into ditch wetland by entrance at time 0 . It is n that the gross of pollutions flowing out ditch wetland by exit at time t. The ratio between the n and m is F(t). So it is: (2) F ( t ) f ( t )dt t10 is the t while F(t)=0.1(that is 10%). The definition of t10 in dealing polluted water with ditch wetlands: 90 percent pollutions can reach the
0
761 3
Liang LIU name et al. // Procedia – 762 Author Procedia Engineering Engineering 28 00 (2012) (2011) 759 000–000
Residence-time in ditch wetlands. That is the time when 10 percent of gross pollutions flowing into ditch wetlands flow out ditch wetland by exit. This is the reason using t10 as the design basis in the ditch wetlands. If it is the ideal plug flow (Piston flow), t10/T equals 1 and t10/T equals 0.1 if it is CSTR. The value of t10/T for ordinary reactor varies from 0.1 to 1. The value of t10 can be determined by tracer experiment in the application. 2.2. The hydraulic efficiency (t10/T) The flow can make full use of ditch wetlands volume and the effective residence time T10 would increase under the condition of constant flow if the hydraulic conditions and hydraulic efficiency of ditch wetlands are improved to make the flow approaches to the ideal plug flow. Let t10/T denote the evaluation index of hydraulic efficiency, then the flow is closer to plug flow as the value of t10/T is larger. It is significant to increase the value of t10/T for improving the treatment effect of ditch wetlands. The sufficient hydraulic retention time is needed to ensure the water quality of outflow from ditch wetlands. Here HRT be denoted by t10. So, if the theoretical retention time (T) is invariable, that is to say, the volume of the ditch and the inlet flow are invariable, the increase of value of t10/T can increase the effective hydraulic retention time t10 and enhance the water quality of outflow from ditch wetlands. And if the t10 is invariable, the increase of value of t10/T can decrease the theoretical retention time (T). So if the volume of the ditch is invariable, the quantity of disposed wastewater in the unit time can increase. Now we give an example to illustrate the theory above. (1) There is a ditch wetland. Here the theoretical retention time T=12H and t10/T=0.2, then the effective hydraulic retention time is: t10=0.2×12=2.4h. If the t10/T increases to 0.6, the effective hydraulic retention time is: t10=0.6×12=7.2H. It is three times of original. (2) Suppose that the quality of water flowing out ditch wetland approach the aim (the flux of inflow is Q), the t10 needs to be 12 hours. Let t10/T of the ditch wetland be 0.2, then the theoretical retention time is: T=12H/0.2=60 hours. And if t10/T increases to 0.6, then the effective hydraulic retention time is: T=12H/0.6=20 hours. It is only 1/3 of original. It indicates that the ditch wetland volume can reduce to 1/3 of original under the condition of keeping the flux of inflow is invariable. So the ditch wetland volume can reduce by increasing the value of t10/T or the inflow can enhance by increasing the value of t10/T under the condition of keeping the quality of outflow is invariable. 3. Experimental Table 1. Relationship of t10/T and ratio between ditch length and ditch width ratio between length and width
t10/min
T/min
t10/T
1:25
17
40
0.43
1:50
52
82
0.63
1:75
96
125
0.77
1:100
143
166
0.86
1:150
225
250
0.90
The factors that influence the t10/T of ditch wetlands have the ratio between the ditch length and ditch width, the density of aquatic plants in ditch wetlands, the number of the corners in ditch wetlands, the lining material of ditch and so on. In all the factors, the ratio between the ditch length and ditch width is
762 4
Liangname LIU /etProcedia al. / Procedia Engineering 28 (2012) 759 – 762 Author Engineering 00 (2011) 000–000
the most important one. In this article, Tracer experiment was done in fore ditches in the farmland of Xupu in order to make certain of the relationship between t10/T and the ratio between the ditch length and ditch width. The experimental results are shown in Table 1. The experiment results of ditch wetlands verify that the ratio between the ditch length and ditch width has an effect on the value of t10/T. The bigger the ratio is, the bigger the value of t10/T is. And the flow is more close to ideal Piston flow. The ratio should be enough in order to improve the hydraulic efficiency (t10/T). As shown in figure 1, the growth rate of t10/T began to slow down along with the ratio (L/W) increasing. 4. Conclusion Ditches are extremely common in farmland and have an impotent effect on both agricultural production and ecological environment. The ditch wetlands have many functions such as irrigation, drainage, and protection of ecological environment. So the ditch wetlands are concerned by experts gradually. In this article, the effective retention time (t10) and hydraulic efficiency (t10/T) are put forward to optimize the design of ditch wetlands and the factors which influence the hydraulic efficiency are studied by Tracer experiment in several ditches wetland. The experiment results of ditch wetlands verify that the total ratio between the ditch length and ditch width is one of the key factors influencing the hydraulic efficiency (t10/T). The bigger the ratio between the ditch length and ditch width is, the bigger the value of t10/T is. So the ratio between the ditch length and ditch width should be enough to improve the hydraulic efficiency (t10/T). Acknowledgements Supported by NSF of Hebei Province (D2009000827; C2011402042) and the Research Fund for Youth of Higher Education of Hebei Province (2011247) References [1] GUO Hong-peng, ZHU Jing-ya, YANG Yin-sheng, et al. Research Status and Development of Technologies for Controlling Agricultural Non-point Source Pollution[J]. Transactions of the CSAE, 2008,24(4):290-295. [2] LONG Tian-yu, LI Ji-cheng, LIU La-mei, et al. Adsorbed Non-point Source Pollution Load of Jia-ling River Basin [J]. Environment Science, 2008,29(7):1810-1817. [3] YIN Fu-cai, ZHANG Zhi-yuan. Survey of Chaohu Lake Eutrophication Research [J]. Journal of Lake Sciences, 2003, 15(4):377-384. [4] LI Qiang-kun, LI Huai-en, SUN Juan, et al. Nitrogen Loss in Qingtongxia Irrigation Area[J]. Journal of Agro-Environment Science, 2008, 27(2):683—686 [5] LU Xue-qiang. HRT Measurement of Wetland Project for Wastewater Treatment[J]. Arid Environmental Monitoring, 1999, 13(2):74-75. [6] Persson J, Somes N L G, Wong Y H F. Hydraulics Efficiency of Constructed Wetlands and Ponds [J].Water Sci Technol,1999,40(3):291-300.