International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
[(]
Municipal Wastewater Treatment Plant for Thingazar Creek (Mandalay)
Shin Nuan Kyin, Daw Aye Aye Thant
Abstract— This paper presents the wastewater treatment process for Thingazar creek (Mandalay). The quality and quantity of wastewater from this creek is studied and the suitable wastewater treatment system is determined. The wastewater samples from Thingazar creek are collected from six locations along the creek and analysed at the Laboratory of Water and Sanitation Department, Mandalay City Development Committee (MCDC) and ISO TECH in Yangon. From the test results, the highest BOD concentration of Thingazar creek is 125 mg/l and the lowest content of DO value is 1.5 mg/l. Float method is used to calculate the flow velocity of Thingazar creek. The wastewater discharge of Thingazar creek is determined by calculating the cross-sectional area of the creek and flow velocity. Based on the test results and the discharge of municipal wastewater, the appropriate flowsheet is selected including treatment processes such as screening, grit chamber, aerated lagoon and waste stabilization pond. These processes are designed according to the respective design criteria to obtain the acceptable wastewater quality for discharging to the Kan Daw Gyi Lake. The waste stabilization is made only as facultative pond. After the treatment of municipal wastewater by using the waste stabilization pond system, After the treatment of municipal wastewater by using the waste stabilization pond system, the BOD content becomes the acceptable BOD quality of 20 mg/l for discharging into inland surface water
Index Terms—municipal wastewater, wastewater treatment, BOD, DO.
I. INTRODUCTION
Water is one of the most abundant compounds found in nature, covering approximately three-fourths of the surface of the earth. As everybody knows the fresh air, clean land and pure water are the fundamental sources for life of human, plants and animals. No one can live without breathing air and using water. When clean water is essential for all on earth, water pollution has always been a major problem throughout the world.
At the present time, as growth rate of population accelerates, usages of water increase. The more water consumption, the more wastewater is discharged. These vast quantities of untreated wastewater flow into drains leading to streams, lakes or rivers. So, the environment becomes polluted more and more because of larger amount of untreated wastewater. The discharge of raw wastewater into the aquatic environment may cause serious damages to many forms of life as a result of oxygen depletion in the receiving water bodies. Additionally, this discharge poses a potential risk for the transmission of a large number of water related diseases.
The unsound development, economic growth, the environmental pollution and ecological degradation are more and more serious. In fact, the environmental cleanness and water pollution control are essential for public health. To complete this necessity, wastewater is required to be treated properly so that the chances of disease transmission can reduce and consequently, leads to improvement of general public health.
After doing treatment, this water returned to the environment as clean and safe water which can be used for land irrigation, fish production, recreation and etc. This study reveals design of municipal wastewater treatment plant for Thingazar creek. Wastewaters from the western part of Mandalay City are discharged into Thingazar creek and it enters into Kan Daw Gyi Lake. Kan Daw Gyi Lake located in Chanmyathazi Township, Mandalay is now being used for recreation. Therefore, this study aims to control the water pollution of Kan Daw Gyi Lake for the preservation of a healthy environment. The location of the study area is shown in Figure 1.
Figure. 1. Location of the study area
II. Wastewater Characteristics
Wastewater characteristics studies are conducted to determine the physical, biological, and chemical characteristics and the concentrations of constituents in the wastewater, and the best mean of reducing the pollutants concentrations. Physical examination includes tests fod determining pH, color, turbidity, total solids, suspended solids and dissolved solids. Physical impurities do not have a direct relationship with health but produce many indirect consequences. Chemical examination of raw wastewater is presented as dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total hardness, total alkalinity, phosphate, iron and chloride. The chemical impurities are important in both design and process of wastewater treatment. In order to be characterized the wastewater samples from Thingazar creek are collected from six locations and tested at the laboratories of MCDC and ISO Tech. The wastewater collection points are Chan Thar Gyi Ale Baung bridge, the 26th bridge, the 35th bridge, the 41st bridge, the Gate exit of Thingazar and the entrance of Kan Daw Gyi. The samples are collected from two feet in depth and middle of the width of the creek. The test results are compared to the tolerance limits for discharging into surface water. They are described in Table I. The results show that BOD, turbidity and total hardness are greater than the acceptable limits for discharging into surface water. On the other hand, it is also found that DO are less than the acceptable limits.
table I
Result of wastewater characteristics
III. Site Selection For Treatment Plant
For this study, the location and requirement of site selection for the treatment plant of Thingazar creek concerns the availability of land area. Because of adequate availability of land area, the location of the treatment plant is selected before draining into Kan Daw Gyi Lake.
IV. Flowsheet for Wastewater Treatment
Depending on the constituents that must be removed (reduced) and increased, an almost limitless number of different flowsheet can be developed using unit operations and processes. The selected flowsheet for treatment of Thingazar creek is shown in Figure 2. In the first step, municipal wastewater lets flowing into the screens to remove coarse solids from wastewater. For removing heavier organics, grit chamber is used. In the secondary treatment, waste stabilization is used. This process includes aerated lagoon and facultative pond. Finally, treated effluent is disposed into Kan Daw Gyi lake.
Figure 2. Flowsheet for Treatment of Thingazar Creek
V. Design Calculation
In order to design the wastewater treatment, it is necessary to determine the design discharge.
A. Calculation of Design Discharge
In this study, float method is used to calculate the flow velocity of Thingazar creek. The float method is to time how long it takes for a buoyant object to travel a specific distance. The creek is not wadeable so the width and depth of the creek segment is estimated and these measurements are recorded and the average depth is also determined. The depth at 1 or 2 feet increments across the creek is recorded. The average depth is achieved by adding all these depth measurements together and divided by the total measurements taken. The depth measurements of Thingazar creek are described in Table II. The cross-sectional area of the creek is estimated by multiplying the total width by the average depth. A minimum of 50 feet along the streambank is measured off or marked by a tape. Float measurements are taken at the same location. The float is gently and slightly released before the upstream end of the measured segment. This is done so the float is moving at the speed of the stream when timing begins. The time when the float crosses the upstream end of the measured segment is counted and stopped when it crosses the downstream end (using stopwatch or digital watch). Table III shows the recording time of float measurement for five runs. Then, creek flow is calculated by multiplying the velocity and the area of the creek taking into account of a correction factor of 0.85 [1].
table II
Measurements of creek depth
No. / Depth1 / 4′8″
2 / 5′6″
3 / 6′4″
4 / 6′10″
5 / 6′10″
6 / 6′11″
7 / 7′0″
8 / 7′0″
9 / 7′0″
10 / 7′0″
11 / 7′0″
12 / 6′11″
13 / 6′10″
14 / 6′10″
15 / 6′9″
16 / 6′6″
17 / 5′8″
18 / 5′6″
19 / 4′9″
Total / 121.83 ft
table III
recording time
Run / Recording Time of Float Measurement (sec)1 / 387
2 / 317
3 / 338
4 / 307
5 / 281
Total / 1630 sec
Length of reach = 50 ft
Width of creek = 39 ft
Average depth =121.83/19 = 6.412 ft
Cross sectional area, A = width × depth
= B × D (1)
= 39 × 6.412 = 250.068 ft2
Average float time = 1630/5 = 326 sec
Velocity, V = 50/326 = 0.153 ft/s
Creek flow, Q = A ×V (2)
=Area × Velocity × Correction factor
= 250.068 × 0.153× 0.85
= 32.52 ft3/s
Q = 79719 m3/d (0.923 m3/s)
B. Design of Bar Screen
Screening is the first unit operation of this study in wastewater treatment plant. A screen is a device with openings generally of uniform size. The screening elements consist of parallel bars. The design calculation for bar screen according to the design criteria for parallel bars summarized in Table IV are as follows [2].
Table IV
Design Criteria for Parallel Bars
Bar sizeWidth (mm)
Depth (mm) / 5-15
27-75
Clear spacing (mm) / 25-50
Slope from vertical (deg) / 30-45
Velocity (m/s) / 0.3-0.6
Allowable head loss (mm) / 150
Maximum flow, Q = 79719 m3/d
= 0.923 m3/s
Desired velocity through the screen, at ultimate flow,
V= 0.6 m/s
Net area of screen opening required, A = (3)
A = = 1.54 m2
Using rectangular bars in the screen having 15 mm width, 75 mm height and 50 mm clear spacing.
Number of opening required = = 411
Number of bar = 411 – 1 = 410
Total area required for screen =1.54 + (4101575 10-6)
= 2.0 m2
Assume the inclination of screen to the horizontal at 45 degree
The gross area of screen = 2.0Sin 45 = 1.414 m2
Velocity through the clear screen, V = 0.6 m/s
Velocity above the screen, v = 0.6 = 0.46 m/s
Using the equation of continuity,
Head loss through the screen is usually given as
= 0.0729 (V2 – v2) (4)
= 0.0729 (0.62 – 0.462)
= 0.011m
= 11 mm < 150 mm (OK)
BOD5 influent, Li =125 mg/l
Assume efficiency in BOD5 removal = 5 % (5-10%)
BOD5 effluent, Le = 125 – (1250.05) = 119 mg/l
C. Design of Grit Chamber
Grit chamber are intended to remove the grit present in the wastewater. Grit chamber are designed to maintain a velocity as close to 0.3 m/s as practical. A grit chamber consists of 10 to 18 metres long narrow open channel with a depth of liquid between 1 to 1.3 m. The detention periods for grit chambers may vary from 45 to 90 seconds. A detention period of 60 seconds is usually adopted [2].
Flow rate = 0.923 m3/s
Average velocity, V = 0.3 m/s
Surface area, A = = = 3.08 m2
Assume width = 2 x depth
A = D B
3.08 = D 2 D
D = 1.24 m < 1.3m (maximum)
B = 2 1.24 = 2.48 m (Take 2.5m)
Assume detention time = 60 s
Length = velocity detention time (5)
= 0.3 60 =18 m
Length of tank = 18 m
Width of tank = 2.5 m
Take depth of tank = 1.24 m
BOD5 influent, Li = 119 mg l
Efficiency in BOD5 removal = 10% (10-20%)
BOD5 effluent, Le = 119 – (1190.1) = 107.1 mg/l
E. Design of Aerated Lagoon
Completely mixed aerated ponds, usually followed by facultative ponds, are used for the first stage treatment of high-strength for pretreatment of wastewater. In the aerated lagoons, the depth varies from 2.5 to 4 m and the land requirements are much less because of the greater depths and smaller detention time needed for the stabilization of organic matter. The detention time of aerated lagoon is 2 to 10 days. Oxygenation requirements are the order of 0.7 to 1.3 kg per kg of BOD5 removed and the power requirements vary from 0.75 to 2.25 kW per 1000 m3. The specification of surface aerator are shown in Table V [5]. The BOD removals are only of the order of 75 to 85 %.
Table V
Specification Of Surface Aerator
Model / Motor(Hp) / Speed Output (rpm) / O2Transfer rate KgO2/d / Preferred liquid depth (m)
Cit-SF02 / 2 / 110-140 / 40-90 / 1.6-2.3
Cit-SF03 / 3 / 110-140 / 60-130 / 1.8-2.4
Cit-SF05 / 5.5 / 100-120 / 110-250 / 2.0-2.9
Cit-SF07 / 7.5 / 99-120 / 160-340 / 2.2-3.2
Cit-SF10 / 10 / 99-100 / 210-450 / 2.3-3.3
Cit-SF15 / 15 / 85-99 / 320-680 / 2.6-3.7
Cit-SF20 / 20 / 85-90 / 430-910 / 2.85-4.1
Cit-SF25 / 25 / 60-90 / 540-1140 / 3.0-4.3
Average flow, Q = 79719 m3/d = 79.719 mld
BOD5 influent, La = 107.1 mg/l
BOD removal efficiency, E = 75%
E = 100 (6)
75 = 100
Effluent BOD, L = 26.78 mg/l
Assume detention time, t = 2d (2-10d)