Document Type : Original Article
Authors
1 Department of Water Engineering Faculty of Agriculture Science University of Guilan
2 1- Associate Professor, Dept. of Water Engineering, University of Guilan, Rasht, Iran. Tel./Fax: +98 131 6690456; P. O. Box: 41635-1314. E-mail: esmaeili@Guilan.ac.ir and esmaeili.varaki@yahoo.com
3 Water Science and Engineering, Faculty of Agricultural Science, University of Guilan
4 Department of Water and Environmental Engineering, Caspian Sea Basin Research Center, University of Guilan, Rasht, Iran
Abstract
Extended Abstract
Introduction
Water quality changes with the penetration of water from the surface of the earth to the subsurface layers, which are accompanied by the passage of subsurface of earth crust with different chemical composition. The most important issues is the increase in the concentration of iron and manganese. Therefore, almost underground water sources have significant amounts of iron and manganese. At high concentrations, those may cause the growth of iron and manganese bacteria, increase pathogenic microorganisms, create bitter taste and unpleasant smell in water, create red and brown spots on appliances and blockage water distribution systems. One of the main steps in the water purification process is the removal of iron and manganese ions. Among these methods, the use of oxidation is more widely used due to less operational costs. Use of an aerator is the main part of this method. Stepped weirs are one of the economical devices for the aeration of water that has acceptable performance. In this research, application of a pyramid-shaped stepped weir with and without sill on increase the dissolved oxygen concentration under various flow discharge was investigated experimentally.
Experimental Setup and procedure
Experiments have been carried out on a stepped-pyramidal weir model in the Hydraulic Modeling Laboratory at of University of Guilan. The physical model of the stepped-pyramid weir is made of PVC panels with a constant slope of 1:2 and the number of steps is 6, with dimensions of 4 cm high and 8 cm long. The weir was installed on Iron’s tank with dimensions of 1.5 m length and 1.5 m in width and 1 m height. A centrifugal pump device with maximum flow rate of 8 l/s was used to provide desired flow discharge. In order to measure the flow rate, an ultrasonic flow meter was used with an accuracy of ±0.01 l/s.
In each geometry of the weir and desired flow rate, first the concentration of dissolved oxygen in the water supply tank was reduced to 2 mg/l using Na2SO3 solution, then variation of dissolved oxygen concentration was measured by 2 oxygen meters (DO meter) model AL20Oxi manufactured by Aqualytic company, which was installed on both sides of the tank, until the dissolved oxygen concentration reached on its initial level.
In this research, totally 30 tests were carried out to consider effect of the stepped-pyramidal weir on increasing the dissolved oxygen concentration.
Results and discussion
The performance of stepped-pyramidal weirs with different geometries at h/yc equal to 7.4 (flow discharge of 2 l/s) for the first step indicates that for stepped-pyramidal weir without sill, the falling jet from the steps have gradually decreased in thickness and it falls discontinuously in steps 5 and 6. By installing the sill at the end of each step, although turbulence and mixing of air and water occurred by formation of hydraulic jump but due to the reduction in the thickness of the falling jet, the intensity of mixing is low and, the performance of stepped-pyramidal weirs with end sill reduced. For the mentioned flow rate, the time to reach DO to from 2 to 7 mg/l for different stepped-pyramidal weir geometries is 857 seconds on average which injected 2.3 mg.l/s dissolved oxygen into the water content of recipient tank.
By increasing the flow rate to h/yc equal to 5.6 (flow discharge of 3 l/s) for the first step, the thickness of the falling jet and corresponding flow velocity increases and the overall performance of the stepped-pyramidal weir to increasing dissolved oxygen improved. For mentioned flow discharge, the time of increase of DO to reach its initial level is 735 seconds on average, which dissolved oxygen injected by rate of 2.5 mg.l/s into the water content of the recipient tank.
by further increase of flow discharge to 6 l/s (h/yc equal to 6) turbulence and mixing of air and water intensified by formation of hydraulic jump. Also, in the mentioned flow and relative critical depth, more turbulence was observed at the end of each steps along with the sill or labyrinth sill. The results show that, with the exception of the SG3 geometry (stepped-pyramidal weir with sill and labyrinth with the space of 4h), the other structures had similar performance. The results showed that the time duration of DO concentration from 2 to 7 mg/l is 395 seconds on average, the lowest value of which was related to SG2 geometry (stepped-pyramidal weir with sill) which could inject dissolved oxygen on an average of 2.3 mg.l/s into water content of the recipient tank.
At the maximum h/yc equal to 3.2 (flow rate of 7 l/s) for the first step, the mixing of air bubbles and turbulence of flow on each step causes the labyrinth sill with small length could better performance due to more separation of falling jet of over passing. The time duration to reach the DO concentration from 2 to 7 mg/l is 294 seconds on average. The SG5 geometry (stepped-pyramidal weir with labyrinth having internal space of h) had the best performance. In general, dissolved oxygen is injected into water content of the recipient tank at an average of 2.7 mg.l/s.
Conclusion
The results showed that at the low thickness of overpassing flow with h/yc> 10, simple stepped-pyramidal weir had better performance than other geometries and could inject 2.9 mg.l/s of dissolved oxygen into the water. The comparison of the results indicates that as the flow rate increases, due to the increase of turbulence and mixing of air bubbles into overpassing flow at each step, the performance of the steps with a simple and labyrinth sill improves, so that their performance to inject dissolved oxygen is from 2 to 3.3 mg.l/s. In general, among the different geometries of the stepped-pyramidal weir, the SG2 geometry had the best performance.
Experiments have been carried out on a stepped-pyramidal weir model in the Hydraulic Modeling Laboratory at of University of Guilan. The physical model of the stepped-pyramid weir is made of PVC panels with a constant slope of 1:2 and the number of steps is 6, with dimensions of 4 cm high and 8 cm long. The weir was installed on Iron’s tank with dimensions of 1.5 m length and 1.5 m in width and 1 m height. A centrifugal pump device with maximum flow rate of 8 l/s was used to provide desired flow discharge. In order to measure the flow rate, an ultrasonic flow meter was used with an accuracy of ±0.01 l/s.
In each geometry of the weir and desired flow rate, first the concentration of dissolved oxygen in the water supply tank was reduced to 2 mg/l using Na2SO3 solution, then variation of dissolved oxygen concentration was measured by 2 oxygen meters (DOmeter) model AL20Oxi manufactured by Aqualytic company, which was installed on both sides of the tank, until the dissolved oxygen concentration reached on its initial level.
In this research, totally 30 tests were carried out to consider effect of the stepped-pyramidal weir on increasing the dissolved oxygen concentration.
Results and discussion
The performance of stepped-pyramidal weirs with different geometries at h/yc equal to 7.4 (flow discharge of 2 l/s) for the first step indicates that for stepped-pyramidal weir without sill, the falling jet from the steps have gradually decreased in thickness and it falls discontinuously in steps 5 and 6. By installing the sill at the end of each step, although turbulence and mixing of air and water occurred by formation of hydraulic jump but ue to the reduction in the thickness of the falling jet, the intensity of mixing is low and, the performance of stepped-pyramidal weirs with end sill reduced. For the mentioned flow rate, the time to reach DO to from 2 to 7 mg/l for different stepped-pyramidal weir geometries is 857 seconds on average which injected 2.3 mg.l/s dissolved oxygen into the water content of recipient tank.
By increasing the flow rate to h/yc equal to 5.6 (flow discharge of 3 l/s) for the first step, the thickness of the falling jet and corresponding flow velocity increases and the overall performance of the stepped-pyramidal weir to increasing dissolved oxygen improved. For mentioned flow discharge, the time of increase of DO to reach its initial level is 735 seconds on average, which dissolved oxygen injected by rate of 2.5 mg.l/s into the water content of the recipient tank.
by further increase of flow discharge to 6 l/s (h/yc equal to 6) turbulence and mixing of air and water intensified by formation of hydraulic jump. Also, in the mentioned flow and relative critical depth, more turbulence was observed at the end of each steps along with the sill or labyrinth sill. The results show that, with the exception of the SG3 geometry (stepped-pyramidal weir with sill and labyrinth with the space of 4h), the other structures had similar performance. The results showed that the time duration of DO concentration from 2 to 7 mg/l is 395 seconds on average, the lowest value of which was related to SG2 geometry (stepped-pyramidal weir with sill) which could inject dissolved oxygen on an average of 2.3 mg.l/s into water content of the recipient tank.
At the maximum h/yc equal to 3.2 (flow rate of 7 l/s) for the first step, the mixing of air bubbles and turbulence of flow on each step causes the labyrinth sill with small length could better performance due to more separation of falling jet of over passing. The time duration to reach the DO concentration from 2 to 7 mg/l is 294 seconds on average. The SG5 geometry (stepped-pyramidal weir with labyrinth having internal space of h) had the best performance. In general, dissolved oxygen is injected into water content of the recipient tank at an average of 2.7 mg.l/s.
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