INFLUENCE OF INFLOW-OUTFLOW SYSTEMS FOR NATURAL … · Stormwater Management Manual for Malaysia...

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 10, Issue 06, June 2019, pp. 324-335, Article ID: IJCIET_10_06_032

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=10&IType=6

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication

INFLUENCE OF INFLOW-OUTFLOW SYSTEMS

FOR NATURAL MIXING IN A CIRCULAR TANK

Zahiraniza Mustaffa*, Syed Muzzamil Hussain Shah, Wan Norgayah Wan Mohd Noor

and Ebrahim Hamid Hussein Al-Qadami

Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS,

32610, Seri Iskandar, Perak, Malaysia

Marlinda Abdul Malek

Institute of Sustainable Energy (ISE), Universiti Tenaga Nasional, 43000, Selangor, Malaysia

*Correspondence Author

ABSTRACT

A tank that is normally equipped with an inlet and outlet system would exhibit

different response in the mechanics of inflows and outflows. This then allows natural

mixing to take place. An effective natural mixing would then be defined when the

natural flow processes taken place in the tank would somehow clean the storage water

to the most optimum level. This paper presents the influence of inflow and outflow

orientation in creating an effective natural mixing inside of a tank. The study involves

the response of different inlet-outlet configuration systems in imitating natural

cleaning. The cleaning herein is defined through mixing process taken place in the

tank. A circular tank which may be translated as a retention pond in real life was

filled with clay (kaolin), replicating a turbid storage. The cleanliness through mixing

was measured by means of Total Suspended Solid (TSS) test. With this regards, nine

different inflow-outflow orientations were tested. It was noticed that the lowest level of

inflow (I3) and highest level of outflow (O1) gave the best results in producing effective

natural mixing. On the other hand, poor mixing was noticed when the configuration of

the inlet and outlet were kept in series.

Key words: circular tank; inflow-outflow configuration; mixing; TSS; kaolin

Cite this Article: Zahiraniza Mustaffa, Syed Muzzamil Hussain Shah, Marlinda

Abdul Malek, Wan Norgayah Wan Mohd Noor and Ebrahim Hamid Hussein Al-

Qadami, Influence of Inflow-Outflow Systems for Natural Mixing in a Circular Tank,

International Journal of Civil Engineering and Technology 10(6), 2019, pp. 324-335.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=6

1. INTRODUCTION

A stormwater retention pond is used to store excess stormwater runoff for a certain period of

time before releasing it to the environment. This is to prevent flood from occurring especially

at the urban areas due to poor drainage system. The importance of a retention pond is that it

reduces flood peak discharge by temporary storing the excess water which is then gradually

Influence of Inflow-Outflow Systems for Natural Mixing in a Circular Tank


released. The stormwater runoff is temporarily stored in a retention pond before distributing

to the nearby rivers. In other words, the retention pond dampens the flow of the stormwater

runoff before going to the river.

The retention pond is also designed with an inlet and outlet system that can produce

natural mixing inside the pond. Natural mixing describes the transport of sediments within a

water body through longitudinal, transverse, and vertical movements. Thus, it allows natural

behavior of the water in breaking the trapped or unwanted particles within the water body.

When a storage tank is highly contaminated for not being able to flow (static condition), thus

pollutions may occur in the tank. There should be some kind of natural processes to take place

within this water body, without much technical efforts being put in place to clean these

storage tanks. As such, natural mixing seems to be favorable and economical from the

environmental point of views.

By far, there has not been any written procedure that explains which inflow and outflow

orientation is best in producing effective natural mixing. The mixing that occurs inside the

pond is one of the critical components of a distribution system and can pose significant

challenges [1]. There are studies regarding enhanced mixing in a retention pond for example

by using a RainJet which is a man-made device [2], however, studies on the inflow and

outflow mechanism and how it affects the mixing characteristics into a retention pond which

is a natural event are sparse. By creating a natural mixing occurrence in the retention pond

with the purpose of cleaning or reducing the sediments in the pond, the results gained would

be beneficial to the authorities involved because it would be environmentally friendly and

relatively cheap which would be practical for implementation. Although there is an Urban

Stormwater Management Manual for Malaysia (MSMA 2nd

Edition) which is provided by the

Department of Irrigation and Drainage (DID), Government of Malaysia, but MSMA does not

properly describe the effect of mixing characteristics of inflows (stormwater) into a retention

pond.

Herein an experiment was conducted in the Hydraulics Laboratory of Universiti

Teknologi PETRONAS, Malaysia involving mixing of flows in a circular tank, which may in

certain context acts as a retention pond having geometrically similar shape to that of the tank.

However, the experimental study was tested under a range of inflows from 0.0001305 to

0.0001341 m3/s in identifying the best inlet-outlet configuration, with the intention to produce

the most effective natural mixing through the measurement of TSS removal.

Hypothetically, by logic people assume that a particular configuration of the inlet and out

orientation can ensure effective natural mixing, however, such hypothesis has never been

experimentally justified. Therefore, the technical results attained herein through experimental

investigations would support whether such assumptions made by the people are true or untrue.

2. LITERATURE REVIEW

A retention pond is a basin that is designed to provide stormwater reduction and catch runoff

water from higher elevation areas [3]. Figure 1 shows the mechanism of the stormwater runoff

entering the retention pond before going to a river through a pipe, whereas Figure 2 shows a

simple case of inlet stormwater without unit for remediation.

Zahiraniza Mustaffa, Syed Muzzamil Hussain Shah, Marlinda Abdul Malek,

Wan Norgayah Wan Mohd Noor and Ebrahim Hamid Hussein Al-Qadami


Figure 1. Mechanism of stormwater runoff.

Figure 2. Simplest case of inlet stormwater without unit for remediation [4].

A retention pond is also similar to a wet-detention pond because it retains water at a

certain level inside the pond. The design of a retention pond or a wet-detention pond is

provided in the MSMA, Malaysia. Based on this, the design criteria of a detention pond that is

not classified as dams must be designed with the following aspects which have been lined by

pond water depth, embankment top widths, side slopes, bottom grades and freeboard [5]. An

example of typical detention ponds in Malaysia from MSMA are highlighted in Figure 3.

Figure 3. Typical detention ponds [5].

Other than catching runoff from higher elevation areas and to retain stormwater, the

retention pond also has other advantages and disadvantages of its own. Table 1 summarizes

some of the advantages and disadvantages of a retention pond.



Table 1. Advantages and disadvantages of a retention pond.

Advantages Disadvantages

Can cater for all storms.

Good removal capability of urban

pollutants.

Can be used where groundwater is

vulnerable, if lined.

Good community acceptability.

High potential ecological, aesthetic

and amenity benefits.

May add value to local properties.

No reduction in runoff volume.

Anaerobic conditions can occur without

regular inflow.

Land take may limit use in high density

sites.

May not be suitable for steep sites, due to

requirement for high embankments.

Colonization by invasive species could

increase maintenance.

Perceived health and safety risks may

result in fencing and isolation of the pond.

The way how mixing mechanism occur plays a vital role for natural cleaning of a

retention pond. Mixing occurs when the initial energy of an inflow into a reservoir pushes the

more stationary lake water ahead of it. The inflow continues to push the lake water ahead until

the initial momentum is substantially dissipated by river bottom shear forces and by the

pressure gradient across the interface between the water masses. The turbulent kinetic energy

of the inflow is usually sufficient to keep the water completely mixed vertically to prevent the

settling of some materials [6]. Figure 4 shows a plunge point and separation point move

upstream and downstream defining the transition zone.

Figure 4. Pooling and mixing at the plunge point [6].

There are numerous parameters that could affect the mixing process. Since it is the study

of fluids in motion, therefore focus has been given at the ways different forces affect the

movement of liquids [7]. For instance, the flow velocity which is a vector quantity refers to the

rate at which an object changes its position or in easier term it is the speed with direction [8].

Turbidity is the cloudiness or haziness of a fluid caused by suspended solids that are generally

invisible to the naked eye. Fluids can contain suspended solid matter consisting of particles of

many different sizes. While some suspended material will be large enough and heavy enough

to settle rapidly to the bottom of the container if a liquid sample is left to, very small particles

will settle only very slowly or not at all if the sample is regularly agitated or the particles are

colloidal. These small solid particles cause the liquid to appear turbid [9]. Sedimentation is the

natural process in which materials such as stone and sand is carried to the bottom of a body of

water and forms a solid layer. This is due to their motion through the fluid in response to the

forces acting on them. These forces can be due to gravity, centrifugal acceleration or

electromagnetism. When rain or snow falls onto the earth, it moves according to the laws of

gravity. A ration of the precipitation seeps into the ground to replenish groundwater.




However, most of it flows downhill as runoff. Runoff is extremely important because it keeps

rivers and lakes full of water, but it also changes the landscape by the action of erosion.

Runoff occurs during storms, and much more water flows into the rivers during storms. The

Total suspended solids (TSS) are from the influence of suspended particles in a water body

causing its turbidity and transparency [10]. The TSS are present in water that can be trapped

by a filter. It includes varieties of materials which makes it a problem when it is highly

concentrated.

The mixing mechanism varies based on the pond types and shapes as well as their

conditions. Herein the mixing characteristic in a lake, rectangular and circular tank have been

lightly discussed as shown in Table 2.

Table 2. Mixing Mechanism.

Mixing Characteristics Remarks / Outcome

Figure 5. Schematic of the Mixing

Processes in a Lake [11].

Wind is responsible for waves and

currents which is the dominant

energy for mixing.

Boundaries are vital due to the

shear that develops between the

ambient flow and non-slip

condition.

Inflow and outflow create kinetic

energy.

Figure 6. Mixing in a Rectangular Tank

[2].

The RainJet ensures that the side

walls and the center area of the

tank floor are kept free from

deposits due to the mixing force

provided by the installed

equipment.

Figure 7. Mixing in a Circular Tank [2].

RainJet will start building the

inertia of the circular movement

which will clean the stormwater

tank.

3. MATERIALS AND METHODS

The typical concrete design for a retention pond is usually rectangular and circular. However,

to achieve the significant degree of control over pond shape and dimensions is hard because

of the needs of taking the natural topographic of the site into considerations. In order to

choose the suitable shape of the tank to replicate pond condition for the experiment, Table 3

shows the advantages and disadvantages of a rectangular and a circular tank.



Table 3. Advantages and disadvantages of a retention tank.

Type of Tanks Advantages Disadvantages

Figure 8. Rectangular Tank.

Better use of land. Problems with removal

of waste, difficult to

clean and poor mixing

especially at the corner

of the tank.

Figure 9. Circular Tank.

Good mixing of the water, high

ratio of tank volume and can be

used to rapidly concentrates and

remove settleable solids.

Poor use of land area.

The experiment was carried out using a circular tank made of plastic with the volume of

300 L. The water was always kept at 250 L at the beginning of the experiment, then one litre

of kaolin was added and mixed inside the tank. The inlet-outlet configurations were designed

based on the selections shown in Table 4. Once the inlet-outlet configuration was chosen in

the circular tank, inflow was released into the tank and the time was measured for a period of

90 min. The first sample was taken at the outlet of the tank, then the second sample was taken

at the collection point X inside of the tank. The side and plan views of the circular tank are

shown in Figure 10 and 11, respectively. Samples were taken at every 15 minutes interval.

The samples were then brought for TSS testing.

Table 4. Different combinations of inlet-outlet configurations.

No. Inlet-outlet Configuration

1. I1,O1

2. I1,O2

3. I1,O3

4. I2,O1

5. I2,O2

6. I2,O3

7. I3,O1

8. I3,O2

9. I3,O3




Figure 10. Side view (a) Schematic diagram of circular tank and (b) Experimental set-up (not to

scale).

Figure 11. Plan view (a) Schematic diagram (b) Experimental set-up (not to scale).

4. RESULTS

Sample were collected from the inside (collection point X) and outside (outlet) of the tank.

The samples were taken at the lowest point inside of the tank, however it does no reach the

bottom of the tank. This was due to decrease of water level based on its respective outlet and

to maintain the position where the samples were taken. Samples were also taken from outside

of the tank as it indicates the amount of solids that were able to reduce. The graph for the

amount of TSS attained both from the collection point X and the outlet for few configurations

have been discussed as shown in Figure 12, 13 and 14, respectively.



Figure 12. Graph of TSS vs time for I1O1.



Among the samples collected from the collection point X and the outlet, it was noticed

that the amount of TSS collected from the collection point X for all the cases was high. Table

5 shows the percentage difference for all the configurations so that the most effective inlet-

outlet orientation can be classified.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 20 40 60 80 100

TS

S, m

g/

L

Time, min

CollectionPoint X

Outlet

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 20 40 60 80 100

TS

S, m

g/

L

Time, min

CollectionPoint X

Outlet

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 20 40 60 80 100

TS

S, m

g/

L

Time, min

CollectionPoint X

Outlet




Table 5. Percentage difference for collection point x and outlet.

Outlet Inlet

I1 I2 I3

O1 276% 201% 365

O2 72% 369% 0.43%

O3 49% 28% 3.10%

From the above table, a noticeable difference can be seen between the samples collected

from the collection point X and the outlet. However, it can be said that for configuration I3O2,

a difference of only 0.43% was noticed, whereas the maximum difference of 369% was found

for the configuration I2O2.

The time interval used for the experiment was 15 minutes for the duration of 90 minutes.

There was a significant decrease shown at the first 45 to 60 minutes duration for most of the

experiments where the readings start to become constant. The mixing characteristics obtained

for several combinations are further discussed in the following section. The mixing for I1O1,

I1O2 and I1O3 has been shown in Figure 15, whereas the mixing effective region during these

configurations has been highlighted in Table 6. The mixing for I2O1, I2O2 and I2O3 has been

shown in Figure 16, whereas the mixing effective region during these configurations has been

highlighted in Table 7. Similarly, the mixing for I3O1, I3O2 and I3O3 has been shown in Figure

17, whereas the mixing effective region during these configurations has been highlighted in

Table 8.

Figure 15. Graph of TSS vs time for I1O1, I1O2 and I1O3.

Table 6. Mixing effective region for I1O1, I1O2 and I1O3.





Table 6, 7 and 8 shows the observation (schematic view) and the actual view (plan view)

of mixing inside the tank. Through observation, it was found out that the mixing occurred

similarly to what the arrow is shown in the schematic view. The graph for I1 shows that O1

showed poor mixing because of its low TSS value compared to O2 and O3. On the other hand,

O2 showed best effective mixing throughout the period of 90 minutes. Similarly, the graph for

I2 shows that O2 showed poor mixing because of its low TSS value compared to O1 and O3.

However, O1 showed best effective mixing throughout the period of 90 minutes. Lastly, the

graph of I3 shows that O3 showed poor mixing because of its low TSS value compared to O1

and O2. However, O1 showed the best effective mixing throughout the period of 90 minutes.

5. CONCLUSIONS

The research that has been carried out highlights that effective natural mixing does happen in

the circular tank under different configurations. Different choice of inflows and outflows

come out with different TSS results. Thus, it can be observed that which particular

configuration is the best by having the highest TSS values. The most effective mixing

occurred when the configuration was set to I3-O1, while the least effective mixing occurred

when the configuration of the inlet and outlet were kept in series, for instance, I1-O1, I2-O2 and

I3-O3.

ACKNOWLEDGMENTS

This research was supported by Universiti Teknologi PETRONAS (UTP) Internal Grant

(URIF 0153AAG24), the Technology Innovation Program (Grant No.: 10053121) funded by

the Ministry of Trade, Industry & Energy (MI, Korea) and the iRMC Bold2025, Universiti

Tenaga Nasional, Malaysia (Grant Code: RJO 1043 6494).

REFERENCES

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Carnegie, PA, 2011.

[2] P. D. Jensens. “Design of Stormwater Tanks Recommendations and Layouts”. Grundfos

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http://www.grundfos.com/content/dam/Global%20Sit

/Industries%20%26%20solutions/waterutility/pdf/Stormwater_Tanks-lowres.pdf



[3] E. Hendi, A. Y. Shamseldin, B. W. Melville and S. E. Norris. "Experimental Investigation

of the Effect of Temperature Differentials on Hydraulic Performance and Flow Pattern of

a Sediment Retention Pond”. Water Science and Technology, 77 (12): 2896-2906, 2018.

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Brookfield, VT : A.A. Balkema, 2000.

[5] Department of Irrigation and Drainage (DID), Malaysia. “Urban Stormwater Management

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[7] J. Myers. “What is Hydrodynamics?”. WiseGeek, Conjecture Corporation, 2018.

Available: http://www.wisegeek.com/what-is-hydrodynamics.htm

[8] T. Henderson. “Speed and Velocity”. The Physics Classroom – Physics Tutorials, 2018.

Available: http://www.physicsclassroom.com/class/1dkin/u1l1d.cfm

[9] M. A. Mallin, S. H. Ensign, T. L. Wheeler and D. B. Mayes. “Pollutant Removal Efficacy

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http://www.ifh.uni-karlsruhe.de/lehre/envflu_II/Students/ocen689ch9.pdf

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