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Environmental Sciences

Procedia Environmental Sciences 13 (2012) 498 - 512 ^^^^^^^^^^^^^^^^^^^

The 18th Biennial Conference of International Society for Ecological Modelling

Variation of Oxygen Transfer along the Rectangular Weir

Crest Distance of Wastewater Treatment Pond

Satreethai Poommai , Kasem Chunkao, Surat Bualerd

The Leam Phak Bia Environmental Study Research and Development Project under Royal Initiaives Petchaburi Province 76100, Thailand College of Environment, Kasetsart University, Bangkok 10900, Thailand

Abstract

Community, instead of industrial factories, has been identified as a big point sources for wastewater in which it also spread out around the Kingdom of Thailand. The government inverted to settle wastewater treatment plants to recover them with a lot of money. Unfortunately, such wastewater treatment systems seemed blur for doing those methodology. To fulfill the wastewater recovery for some desirable water uses, H.M. the King of Thailand has initiated natural purification under the nature by supporting nature processes and simple technology by using aquatic plants, constructed wasteland, and stabilization ponds. One of the very successful projects is "The Laem Phak Bia Environmental Study Research and Development Project under Royal Initiatives Petchburi Province". Our research is performed on this project area, the rectangular weir was introduced to increase the efficiency by oxygen transfer into wastewater as flowing over the weir crest. The experimental setup for five wastewater depths in meters (0.03, 0.04, 0.05, 0.06 and 0.07) is presented in this work. Results found that 0.03-m depth showed the highest efficiency of oxygen diffusion, and it decreased when water depth increase.

© 2011 Published by Elsevier 13. V. Selection and/or peer-review under responsibility of School of Environment, Beijing Normal University.

Keywords.Oxygen Transfer; Community Wastewater; Rectangular Weir.

* Corresponding author. Tel.: +66-2579-2116; fax: +66-2579-3473.

E-mail address: cestp@ku.ac.th.

1878-0296 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of School of Environment, Beijing Normal University. doi:10.1016/j.proenv.2012.01.041

1. Introduction

Community wastewater is a big problem and spreads out in every part of Thailand, especially the dense populated cities such as Bangkok in the central, Chiang Mai in the north, Nakhon Rachasima in the northeast, Haad Yai in the south, and Suphanburi in the west. Incidentally Thai Government has launched to attack such problem by using the engineering devices of wastewater treatment systems with the budget approximately 50,000 million baht (U$ 1,700,000,000) per annum. Unfortunately, the success would be so far from the target of working plans, because of very less efficiency of both the government and private enterprise units lacking of responsibility. In consequence, river, canals, and/or watercourses have been polluted by not only communities both also industrials, agriculture, and transportation since the last 3 decades. This is why water quality of public water sources in all parts of the country has its own constraints to utilize for waterworks. They are so more expeanses to spend the budge for cleaning, especially to eliminate suspended solids, total dissolved solids, chemical, and diseases.

After studying the country wastewater problem for some period of time, His Majesty the King has initiated productive research on wastewater treatment under nature-by-nature processes and simple technology, name as "The King's Royally Initiated Laem Plak Bia Environmental Research and Development Project at Laem Phak Bia Sub-distric, Ban-Laem Distric, Pecthaburi Province" since 1990. There are four community wastewater treatment systems, they are oxidation ponds/stabilization ponds, grass filtration, aquatic plant filtration, mangrove forest, and constructed wetlands. Wastewater has been pumped from Petchaburi to the research site by 1-m HPDE pipe with the distance of 18.5 km. However, wastewater treatment efficiency has met the requirement by upgrading from surface water quality classes 4 and 5 to class 2 and 3 inside Petchaburi municipal area.

Due to oxidation pond wastewater treatment system under nature-by-nature processes some what the same as biological process, more benefits have been received not only well-treated polluted community wastewater, but also it provides herbivore fishes per annum. The better efficiency of such wastewater treatment was noticed that higher dissolved oxygen on surface treated wastewater pond moving over weir crests of the consecutive ponds (a series of five ponds) gained more and more by air diffused process. Additionally, the flow thickness overt weir crest has been accepted as a main factor for diffusing oxygen from the air into water. In theoretical point of view, the lower the oxygen diffusion are dominant because of conduction by concrete weir to wastewater. While the highest thickness of over-weir-crest flow found less oxygen diffusing into water. In other words, air diffusion into wastewater body moving over weir crest has been up to oxygen transfer coefficient. There are a lot of study on oxygen transfer from air into water especially aeration potential of weir such as Gameson [1], Wormleaton and Soufiani [2], Baylar and Bagatur [3-5], Baylar et.al [6-8], Baylar and Emiroglu [9], Emiroglu and Baylar [10-12], Ozkan et.al [13], Kim and Walters [14] and Chern and Yang [15]. Actually, such previous studies of named scientist focused on natural water. This study will focus on community wastewater in order to apply whenever wastewater has to drain over weir crest.

2. Mathematical model development

Natural phenomena of oxygen as a volatile gas with gas-water transfer rate can change the dissolved oxygen (DO) concentration in water when flowing through a hydraulic structure, see Baylar and Bagatur [6], Baylar et.al [8] and Emiroglu and Baylar [11]. The rate of change of dissolved oxygen concentration can be expressed by the model equation

dC / dt = KL (A / V)(CS - C)

where C is the dissolved oxygen concentration (mg/l), Cs is the saturation oxygen concentration (mg/l), KL is the liquid film coefficient for oxygen (t-1), A is the area of air-water surface area (m2) and related with volume of water V (m3), and t is time (s).

Simplifying, let a = A / V which is a specific surface area per unit volume, equation (9) will be

dC / dt = (KL ,a){Cs - C) , (2)

Actually, equation (2) describes the occurrence of oxygen transfer in natural/normal stream water. If wastewater polluted into the stream, some dissolved oxygen is used for organic digestion, then equation (2) becomes

dC / dt = (KL ,a){Cs - C) - R , (3)

When R is Oxygen utilization rate of the biomass, but Cs in equation (3) is specific for process

conditions in the steady-state conditions. Therefore, the rate of oxygen it means that ( dC / dt ) overall is equal to zero. The equation (3) can be written to

KL ■ a = R /(C, - C) (4)

However if given r is deficit ratio (KL ■ a) as obtained from modifying equation (3) it will be

dC / dt = r (Cs - C) - R (5)

In fact, r can be from no oxygen transfer until infinity for saturation oxygen transfer. Furthermore, the oxygen transfer efficiency E can be writer Gulliver et al. [16] by

E = ((Cd -Cu)/(Cs -Cu)) = 1 -exp[-frfKL(A/V)dt] (6)

Gulliver et al. [17] presented r = (Cs - Cu )/(Cs - Cd). Where Cu is dissolved oxygen concentration upstream (mg/l) and Cd is dissolved oxygen concentration downstream (mg/l). The equation (6) can be written to

E = 1 - (1/r) (7)

r = (1 - E)-1 (8)

Gulliver et al. [17] suggested to adjust new value because the oxygen transfer was corrected at temperature 20oC, then equation (13) will be changed into

1 - E20 = (1 - E )1/ f (9)

When E20 is oxygen transfer efficiency at the 20oC, and f is the exponent described by

f = 1 + 0.02103(2" - 20) + 8.261 x 10-5(T - 20)2 (10)

When T is water temperature (oC). Then the equation (9) can be proposed as

r20 = r"f (11)

Where r20 is deficit ratio at 20 oC.

Due to the quantity of wastewater flow as measured by rectangular weir found by derived formula as equation (12) Brooks et al. [18]

Q = 1.86bh3'2 (12)

Where Q is discharge (cms). b is width of weir crest (m). The term h is height of flowing water over weir crest (m). It would be pointed out that when water flow over weir crest, then it drops down to the bottom of the first weir and flowing in another weir with time and can be expressed as dv = dx / dt or

dt = dx / dv (13)

If dt in equation (2) is replaced by equation (13) then the result will be

dC = KL ■ a(Cs - C)(dx / dv) (14)

Since, Q = Axv , Q is discharge, Ax is cross sectional area, and dv = dQ / dAx, then the equation (14) will be

dC = KL ■ a(Cs - C)(dx ■ dAx / dQ) (15)

dC / dx = KL ■ a(C^ - C)(db ■ dh / dQ) (16)

In reality, b is constant, and h is constant because of Q is under steady state, then the equation (16) will be

dC/dx = (KL ■ a(Cs -C)b ■ h)/Q (17)

If equation (12) replaces in equation (17), then it will be obtained as

dC / dx = (KL • a)(C - C)/1.86h3'2 (18)

In fact, C in equation (18) is Cw in water then equation (19) can be adjusted as

dCw / dx = (5.4 x 10-1 )(Kl ■ a)(Cs - Cw )/Jh (19)

dCw = ((5.4X lO-1)^ • a)(Cs -Cw)ljh)dx (20)

dCw l(C -Cw) = ((5.4x 10-1)(Kl • a)l4h)dx (21)

Q dCw l(C, - Cw) = ((5.4 x 10-1 )(KL • a)l jh)dx (22)

-ln\Cs -Cw||C = ((5.4x 10-1 )(KL ■ a)ljh)x\x (23)

- ln |C, - Cd| + ln |Cs - Cu\ = ((5.4 x 10-1)(Kl • a)^^/h )(xd - xB ) (24)

ln|(Cs -Cu)l(Cs -Cd) = (5.4x 10-1)(Kl • a)(xd -xn)ljh (25)

Kl = (ln|(Cs - Cu )l(Cs - Cd )\Jh)l(5.4 x 10-1 • a • (xd - xu )) (26) From a = A l V, then it will be obtained as

KL = (ln|(Cs - Cu )l(Cs - Cd )\4h)l(5.4 x10-1 • (A l V) • (xd - xu )) (27;

Where Xu is represented as a starting point distance of wastewater flow over weir crest, where Xd as a ending point.

3. Methods and procedures

3.1. Three-consecutive oxidation pond with rectangular weir construction

Since each oxidation pond (five of them) has covered very big area, but this research project is really needed small-scale oxidation ponds. To serve need of research, three-consecutive oxidation ponds were well planned to construct with size of 3.83 m3 length*width*depth (4.2*2.4*2.5 m) at the project site as shown in Fig. 1. Only the narrow weir crest as seem was used for this research.

Fig. 1. Map of The King's Royally Initiated Laem Phak Bia Environmental Research and Development Project Laem Phak Bia Sub-distric, Ban-Leam Distric, Petchaburi Province, Thailand.

Rectangular weir at outlet of each oxidation pond was designed bowl-shaped weir crest of turning upside down of with smooth surface as shown in Fig. 2.

(a) (b)

Fig. 2. Schematic diagram showing two points of different oxygen transfer. (a) Bowl-Shaped weir crest with length of 0.225 m. (b) Fixed width of bowl-shaped weir crest for running the experiment

3.2. Installation of equipments and instruments

In principles, dissolved oxygen (DO) is very sensitive, and difficult to measure only if DO meter does not stand still. This is the reason why the research team has fixed measuring points of DO concentration, water temperature, air temperature as shown in Fig. 3.

Fig. 3. Fixed points with installation of equipments and instruments to measure DO concentration.

3.3. Environmental set-up

Continuous flow of community (Petchaburi municipal) wastewater was brought to determine an optimum depth of water over weir crest by setting 5 levels, they are 0.03, 0.04, 0.05, 0.06 and 0.07 m. The experimental data was collected on 13 April 2010, for 24-hour duration, measuring interval of 3 hours. DO meters (model 0xi3310; accuracy of the oxygen measurement:0,5% of measured value+1 digit.) were used to measure DO at fixed-measuring points with every 3 hours. The measuring points and measuring supporters were described in Fig. 3. At the same time with DO measurement, other indicators of environment were measured. These are water and air temperatures, pH, BOD (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand), TKN (Total Kjeldahl Nitrogen), NH3 (Ammonia), NO2-(Nitrite), NO3- (Nitrate), SS (Total Suspended Solids), alkalinity, hardness were measured only at the influent or inflow wastewater. These were analyzed by American Public Health Association(APHA)[19]. Flow velocities were measured by Model FP 111 Flow Probe PN#BA1100 (range : 0.1-6.1 m/s, accuracy : 0.1 m/s).

3.4. Data analysis

Determine the DO upstream and downstream concentration. An optimum water height over weir crest as well as oxidation transfer coefficient were determined. Determine the relationship between flow distance and height of water over weir crest. After analyzing the experimental data, the results were compared with the mathematical model in order to check and apply the final conclusions. Oxygen transfer coefficients were determined.

4. Results and discussions

4.1. Water quality

The experiment of oxidation transfer coefficient has been taken wastewater as influent flowing through three-section oxidation ponds. There were 15 indicators which measured every 3 hours, starting from 09.00, 12.00, 15.00, 18.00, 21.00, and 24.00 on 13 April, and on 14 April 2011 at 03.00 and 06.00. The wastewater qualities are shown in Table1.

It was observed that BOD and COD showed very low quantity. It would be the cause of during wastewater drained through pipe with the distance of 18.5 km. and being digested under anaerobic processes about 5.5-hour travelling time. Normally, BOD concentration as found in drainage system of Petchaburi municipal is more than 100 mg/l. It decreased more than 70 percent as the same number as found in COD shown in Table1.

4.2. Upstream and downstream dissolved oxygen concentration

Measurement of upstream and downstream DO concentration over the weir crest was conducted by 8 times of 24 hours and varying wastewater height over the weir crest. The experimental results are shown in Fig. 4. It can be seen that the downstream DO concentration is higher than upstream DO concentration. The difference between the downstream and the upstream concentration indicated highly potentials for aerated diffusion from air to wastewater. The highest difference occurs at the wastewater height 0.03 m while the lowest difference occurs at the height 0.07 m.

Table1. Wastewater quality as drained out from PDHE pipe that used for the whole experiment

Times of measurement

Indicators Unit -

09.00 12.00 15.00 18.00 21.00 00.00 03.00 06.00 STD (*)

BOD mg/l 22 26 29 28 22 25 26 30 20*

COD mg/l 16 64 64 96 80 48 48 64 -

TKN mg/l 11.48 21.72 12.18 12.61 18.12 19.33 17.56 15.98 35*

NH3 mg/l 0.7937 0.9012 0.8286 0.8723 0.8362 0.8464 0.8362 0.8490 -

NO2" mg/l 0.0103 0.0109 0.0110 0.0108 0.0112 0.0111 0.0109 0.0111 -

NO3" mg/l 0.7941 0.8075 0.8132 0.8162 0.7906 0.8102 0.7878 0.7990 -

SS mg/l 13 17 9 3 6 8 13 17 30*

TDS mg/l 477 475 469 473 471 474 472 476 -

Salinity ppt 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 -

EC ^s/cm 712 709 699 705 701 706 703 711 -

pH - 6.9 7.1 7.2 6.9 6.8 6.9 6.8 6.7 5.5-9.0*

Alkalinity mg/l 199 182 178 176 177 187 193 199 -

Hardness mg/l 104 94 106 94 96 98 90 98 -

Air Temp. oC 33 34 33 31 30 29 25 26 -

Water Temp. oC 32.0 32.2 31.9 31.8 31.7 31.5 30.9 30.9 -

Remark: *Building Effluents Standards; Thai Environmental Regulations from Pollution Control Department (PCD) Ministry of Natural Resources and Environment, Thailand.(http.//www.pcd.go.th/info_serv/en_std_water04.html#s3)

C - C h

Fig. 4. Relationship between d u and water height (h ) over weir crest for various time measured.

4.3. Mathematical model 4.3.1 Free surface curve model

In this section, the mathematical model presented in equation (5) is used to describe the variation of dissolved oxygen concentration across the weir crest. In the model equation, we need to know the free surface area of wastewater A and the wastewater volume V for each case of flows. Thus, we have measured the height of free surface at each point along the weir distance ranging from 0 to 0.225 m. We measured at 9 positions for each case of flows. For example, the scatter plots of the free surface and weir data sets for water depth 0.07 m. are shown in Fig.5. To calculate the surface area and volume of wastewater, we apply the interpolation technique by using polynomial degree three for the free surface while using polynomial degree two for the shape of semi-circular weir. General forms of these polynomials can be expressed by f (x) = ax2 + bx + c , or f (x) = ax3 + bx2 + cx + d . The results of interpolated free surface profiles for each case of wastewater depth are as follows (equation (28)-(32)).

f3 = -0.0017735x3 + 0.030685x2 - 0.1797x + 4.3376 (28)

f =-0.0076964x3 - 0.0010067x2 + 0.063007x + 4.3376 (29)

f = -0.0012027x3 + 0.013701x2 - 0.081509x + 6.1414 (30)

f =-0.0011513x3 + 0.01657x2 - 0.16646x + 7.4687 (31)

f = -0.0020024x3 + 0.045636x2 - 0.41166x + 8.3474 (32)

Subscript denotes to wastewater depth measured at the middle of the weir.

The three-dimensional plots of the free surface and weir for h = 0.07 m are shown in Fig. 6. It is clearly seen that the downstream wastewater depth is small when comparing with the upstream. The free surface area can be approximated by using double integral approximations for each interpolating free surface. As the results, the wastewater volume can be approximated by using triple integral approximations. Approximated free surface area and wastewater volume for each case of wastewater depth are summarized in Table 2. Note that we have measured the flow velocity and we know the weir length (0.225 m), the duration time can then be calculated. These calculated free surface area and volume will be used in the presented model equation (5) to show the variation of dissolved oxygen concentration in space in the next section.

Table 2. Approximated the area of free surface of wastewater and wastewater volume for each case of h. Flow velocity is measured, duration time is calculated.

height; h ; (m) velocity; v ; (m/s) duration; t ; (s) area; A ; (m2)x10-4 volume; V ; (m3) x10-4

0.03 0.43 0.5232 512.88 18.54

0.04 0.54 0.4166 500.85 21.37

0.05 0.70 0.3214 506.22 26.09

0.06 0.87 0.2586 499.01 30.62

0.07 1.20 0.1875 506.62 33.34

Fig. 5. Cross-sectional free surface and weir crest profiles for h 007

Fig. 6. Three-dimensional plots of free surface and weir for h 007 m

4.3.2 Liquid film coefficient for oxygen calculation

An application of equation (27), Cd and Cu in Table 3 were taken to calculate KL as shown the result in Table 4. In fact, KL values in Table 4 would be in the condition of water quality as indicated in Table 1, which were comprised of BOD (22-30 mgll), COD (16-96 mgll), TKN (11-21 mgll), NH3 (0.79-0.90 mgll), NO2 (0.01 mgll), NO3 (0.8 mgll), SS (3-17 mgll), (TDS 469-477 mgll), salinity (0.1 ppt), EC (699-

712 ^slcm), pH (6.7-7.2), alkalinity (176-199 mgll), hardness (90-106 mgll), air temperature (25-34 oC) and water temperature (30.9-32.2 oC).

Table 3. The average of upstream and downstream dissolved oxygen concentration for each case of h from measurement.

height; h ; Upstream DO Downstream DO

(m) concentration; concentration;

Cu ;(mg/l) C, ;(mg/l)

0.03 0.16 0.88

0.04 0.13 0.41

0.05 0.10 0.19

0.06 0.08 0.15

0.07 0.07 0.12

Table 4. The liquid film coefficient for oxygen from calculation

height; h ; liquid film

(m) coefficient for

oxygen; KL

0.03 0.005100

0.04 0.002612

0.05 0.001612

0.06 0.001126

0.07 0.001114

4.4. The dissolved oxygen concentration predictions

Due to the application of equation (25) and following the principles in equation (33), the result was obtained in equation (34).

ln(|Cs -C„|l|Cs -Cd|) = -ln(|Cs -Cd\l|Cs -Cu|) (33)

-ln|(Cs -Cd)l(Cs -Cu)| = (5.4x 10-1)(^l • a)(xd -xu)lVh (34)

|(Cs -Cd)l(Cs -Cu)| = e-(5-4x10-1)(KL■ a)(xd-xuvj; (35)

C -cd\ = \CS -Cu\e-(5-4xl0_1)(KL■a)(xd-xu(36)

It is evident that when Cs - Cd > 0, the calculated equation will be Case 1. Case 1:

Cd = Cs -\Cs -Cu\e-(54x10-1)(KLa)(xd-xu) (37)

If Cs - Cd < 0, the equation will be Case 2.

Case 2:

Cd = Cs + \CS -Cu\e (54x10_1)(klaX*-) (38)

According to Cs values as presented by Ramaswami et al. [20], Cd will be calculated by equation (37). The calculated Cd values were shown in Table 5.

4.5. Model vr/ificatioc

It is an obligation to verify the calculated KL under the comparison with the experimented KL values in Table 5, the comparison of calculated Cd and experimented Cd has been expressed in Fig. 7.

Table 5. The downstream dissolved oxygen concentration from calculation

height; h ; Downstream DO concentration; Cd ;(mg/l) (m) measured calculated

0.03 0.88 0.87

0.04 0.41 0.41

0.05 0.22 0.21

0.06 0.15 0.14

0.07 0.12 0.12

Normal P-P Plot of Regression Standardized Residual

Dependent Variable: MEASURED

Observed Cum Prob

Fig. 7. Verification of calculated d with experimented d .

When simple linear regression was determined the correlation between calculated Cd and experimented Cd it can be illustrated as Cdmm = 1.004C^^ + 0.004, R2 = 1.°° .

5. Conclusions

The main propose of this research is to study the variation in space of dissolved oxygen concentration along the bowl-shape weir. The experimental set up is designed for investigating the increasing of oxygen transfer across the weir crest. The research project has been performed at The King's Royally Initiated Laem Plak Bia Environmental Research and Development Project at Laem Phak Bia Sub-distric, Ban-Laem Distric, Pecthaburi Province (LERD). The experimental setup for five wastewater depths in meters (0.03, 0.04, 0.05, 0.06, and 0.07) is performed. We have also presented a mathematical model to describe the variation of dissolved oxygen. It is found that the coefficient of aeration increases as the wastewater depth decreases. This study would provide some in sign to achieve better understanding the variation in space of dissolved oxygen along a hydraulic structure.

Acknowledgement

This study was funded by The King's Royally Initiated Laem Plak Bia Environmental Research and Development Project at Laem Phak Bia Sub-distric, Ban-Laem Distric, Pecthaburi Province (LERD).

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