Research Paper-water Treatment Plant.docx

Contents Introduction ............................................................................................................................................ 2 Background ............................................................................................................................................. 2 Drinking water quality standards ............................................................................................................ 3 Project requirements .............................................................................................................................. 4 Measurement of Water Quality .............................................................................................................. 4 Design of Conventional Water Treatment Facility .................................................................................. 6 Flash mixing & Slow mixing (Coagulation and Flocculation)................................................................... 7 Coagulant Dosage ............................................................................................................................... 7 Control of pH....................................................................................................................................... 8 Rapid Mixing Tank Design ................................................................................................................... 8 Design of flocculation basin .................................................................................................................... 9 Design of Sedimentation Tank .............................................................................................................. 10 Filtration................................................................................................................................................ 11 Disinfection ........................................................................................................................................... 11 Design Summary ................................................................................................................................... 13 References ............................................................................................................................................ 14

Introduction Potable water is perhaps the most basic human need. The provision of adequate potable water for the humanity is among the most challenging endeavours, especially in the presence of today’s increasing pollution levels. The general criteria upon which water is assessed is the odour, clarity, and taste. Also, the concentration levels of some toxic chemicals such as sulphides, sulphates, and chlorides determine the type of treatments needed to meet the municipal potable water requirements. There are several factors that influence the design of a water treatment facility. Among those, are the water source nature, the concentration of minerals, and the quality that is aimed for, of the effluent water. Clearly, the quality of the effluent water is mostly based on the end use, which is not limited only to the human consumption of potable water, but also includes irrigation, power generation, and industrial use. Of course, the end use decides which water source is most appropriate and requires less sophisticated procedures to produce cost effective and adequate amounts of treated water.

Background Again, the water source is a great indicator of how polluted the raw water is. Thus, through several sampling tests of the water source, the environmental engineer can determine the treatment processes required to get to the desired water quality. The general criteria which correlates the water source with the level of pollution, and consequently the required water treatment processes are summarized in the following table: Table (1): Water treatment processes based on the type of water source and level of pollution [1]

Water source Ground Water

Pollution Level

Water treatment processes

No pollution and Chlorination high content of minerals. No pollution and Iron and manganese high content of chlorination. manganese and

removal

and

iron. With pollution. Surface water

Ground or surface water

Coagulation, settling, filtration, and chlorination. With almost no  Slow sand filtration and chlorination. pollution and low  Coagulation, settling, filtration, and turbidity. chlorination.  Membrane filtration, and chlorination. With moderate  Pre-chlorination, coagulation, settling, level of pollution. filtration, and chlorination.  Membrane filtration, and chlorination.

With high level  Coagulation, settling, sand filtration, of pollution. ozonation, GAC filtration, and chlorination. With high  Softening with a soda ash/lime reactor hardness and a filter or an ion exchange resin.

Drinking water quality standards The rules dominating the quality of the drinking water have developed over the years. The federal regulations, in almost every part of the world follow four different rules: 1234-

The water treatment rule The total coliform rule The lead and copper rule The contaminant rule

These rules have been reinforced by the USEPA, United States Environmental Protection Agency, which has developed drinking water standards, that are in accordance with the above-mentioned rules, as shown in the table below: Table (2): Potable water standards [2]

Contaminant Aluminum Chloride Color Copper Corrosivity Fluoride Foaming agents Iron Manganese Odor pH Silver Sulfate

Maximum Contaminant Level 0.05 to 0.2 mg/L 250mg/L 15 color units 1mg/L Neither corrosive nor scale forming 2.0 mg/L 0.5 mg/L 0.3 mg/L 0.05 mg/L Three threshold odor numbers 6.5 to 8.5 0.1 mg/L 250mg/L

Total Dissolved Solids Zinc

500mg/L 5mg/L

Project requirements The Lake major plant’s capacity is 94,000 m3/day as shown in table 5. Although the plant capacity seems to be quite large, the average production of the treatment plant is about 40,100 m3/day. However, the design of the treatment plant will target the total plant’s capacity, which is quite larger than the average daily production in order to avoid fatigue and weariness of the plant’s equipment. Table (3): Summary of water supply plants [3]

Measurement of Water Quality Perhaps, the most significant factor of water quality is the D.O, which stands for dissolved oxygen. Although the latter is not easily dissolved in water, it is very essential for the aquatic life. It is noteworthy that the amount of dissolved oxygen in water decreases with the increase of temperature levels. At its maximum, D.O could get as high as 9 mg/L of water. A lot more important than the amount of dissolved oxygen is the rate at which dissolved oxygen is consumed. A small rate of D.O consumption, which is referred to as BOD, biochemical oxygen demand, indicate a clear water source. BOD is can be defined as a

measure of the oxygen amount required by the bacteria to decompose the organic matter. Since there is not enough data on the characteristics of the raw water, it is assumed that the latter has the following water qualities:

Parameter Magnesium Calcium HCO3pH Flow rate TSS Fine particles Color BOD Nitrogen Phosphate

Unit mg/L as CaCO3 mg/L as CaCO3 mg/L as CaCO3

mg/L mg/L mg/L

Value 70 200 150 6.0 150,000 900 High High 100 20 5

Fecal coliform

#/100ml

10,000

m3/d mg/L

Design of Conventional Water Treatment Facility In this report, the conventional treatment method of the surface water has been selected to convert the raw water from the Lake Major into the desired drinking water that the end user can make use of. Below is an outline that illustrates the treatment processes that the raw water should pass through to meet Halifax municipal water qualities listed in table 4.

Figure (1): Design of water treatment facility

Almost in every part of the world, and regardless of the water source and its water quality, the raw water needs to be pass through different treatment stages, of course with varying degrees, before it can be pumped into the distribution system. Many different types of contaminants and impurities are in contact with the surface water, and may result in catastrophic health diseases if dissolved in our drinking water. As previously mentioned, the quality of the raw water decides the levels of treatment that it may need to match the standards of the drinking water. In this project, judging from the water characteristics listed in table 4, it is assumed that the raw water of the Lake Major is a surface water source with moderate level of pollution. Thus, the conventional treatment processes, diagrammed in figure 1, will be adopted to get to the desired water qualities, summarized in table 4.

Flash mixing & Slow mixing (Coagulation and Flocculation) The purpose of flash or quick mixing is to produce a mixture with high levels of homogeneity between the influent water and the coagulants. The addition of coagulants aids in the destabilization process of the particles in collision. In other words, rapid mixing is defined as the dispersion of coagulants in the body of influent water. Below is a table that summarizes the most important parameters of our prosed flash mixing basin: Table 4: Design parameters of rapid mixing tank [4]

Mixing intensity 600–1000 s −1 Detention times 1–2 min Power/unit volume 1–2 hp/ft 3/s (26.3–52.6 kw/m 3/s) Basin dimensions (diameter) 3–10 ft (0.9–3 m)

The raw water that enters the proposed system is deemed to have a high level of turbidity, which is caused by movement and collision of different particles. These particles seem to be in constant motion, due to the electrostatic charges that they carry, as they enter the plant. An easy way to reduce the turbidity level from 0.3 to less than 0.03 is by adding alum, which will neutralize the charges, and mitigate collision among the charged particles. The addition of alum to the inlet water will aid in producing flocs, sticky and large particles that can be filtered easily. The process of neutralizing the colloidal particles is defined as coagulation, where as the formation of larger flocs is called flocculation. These two processes are elementary for treatment of raw water, and they constitute the first two stages of our water treatment plant design, thus we need to start our design with the calculation of three important factors, the alum dosage, the velocity gradient, and the hydraulic detention time.

Coagulant Dosage From a practical point of view, Alum is used most effectively when the raw water lies within the range of 5-7.5 [5]. Most water treatment facilities use alum at a dose that ranges from 5

mg/L to 50 mg/L[5]. In our case, we will assume an alum dose of 20 mg/L. Therefore, the daily amount of alum needed to cover the average daily production of the plant is: (94000 m3/day) x (1000L/m3) x (20 mg/L) = 1880 kg of Alum per day

Control of pH When Alum is added to the raw water, in the presence of high levels of alkalinity, the following chemical reaction takes place in the rapid mixing tank:

However, when an inadequate amount of alkalinity is available, the following reaction takes place, which significantly reduces pH levels:

Rapid Mixing Tank Design The plant’s capacity is Q= 94000m3/day. We will assume 2 min detention time for rapid mixing, a velocity gradient equal to 600 sec-1, a water depth of 2.5 m, and a total of 4 basins. A radial flow type turbine is utilized for rapid mixing. V= (65.27/4) x2= 32.6 m3 A=32.6/2.5=13 m2 Length= √13 ≈ 3.6𝑚

4𝑥13

Tank equivalent diameter= √

𝜋

= 4.1 𝑚

Geometric ratio D/T H/D H/T B/D

Allowable range 0.8 0.19 3.13 0.61 1

0.14-0.5 2.0-4.0 0.28-2.0 0.7-1.6

Radial Impeller Diameter 1.1 1.4 0.27 0.34 2.27 1.79 0.61 0.61 0.76 0.6

Choose a radial impeller diameter of 1.1 m. Input power= 6002 𝑥1.002𝑥10−3 𝑥32.6 = 11759 𝑊

11759

1 3

Rotational speed= [5.7𝑥1000𝑥1.15 ] = 1.1 𝑟𝑝𝑠 = 65.2 𝑟𝑝𝑚

Design of flocculation basin Remember that the maximum capacity of the water treatment plant is 94,000m3/day, as mentioned in table 3. We will assume that the dimensions of the flocculation tank, based on trial and error analysis, are 83 ft in length, 42 ft in width, and 14 ft in depth. In this project, the total power input is taken as 201 ft-lb/sec and the dynamic viscosity of water at 40 F is 3.228 x 10-5 lb-sec/ft2. From the dynamic viscosity, input power, and the tank volume, the velocity gradient is calculated as follows:

𝑃 201 𝐺=√ =√ = 11.3 𝑠𝑒𝑐 −1 𝑉𝜇 83𝑥42𝑥14𝑥3.228𝑥10−5 Where, G=Velocity gradient in sec-1 µ=

Dynamic viscosity in lb-sec/ft2

V=Tank volume in m3 or ft3

On the other hand, the hydraulic detention time is calculated as follows:

𝑉

𝑡=𝑄=

83𝑥42𝑥14𝑥0.0283168 94000

= 0.0147 𝑑𝑎𝑦 = 21.5 𝑚𝑖𝑛

Where, t=hydraulic detention time in min Q=Design capacity m3/min

The general acceptable design value of G ranges between 30 to 60 sec-1 [6]. To match the general acceptable standards, the power input is increased to 500 ft-lb/sec, whereas the flocculator’s dimensions are reduced to 50 ftx30ftx10 ft. Repeating the same calculations, would result in increasing G up to 32 sec-1 and reducing t down to 6.5 mins. Again, to check our design, the typical values of the term Gt varies from 104 to 105[6]. In our case, Gt was found to be 1.25x104, which matches the general requirement standards.

Design of Sedimentation Tank Sedimentation basins, also called clarifiers, are generally rectangular in shape with a pattern of upward flow. There are four different types of sedimentation tanks: inlet, outlet, settling, and sludge storage, shown in the following figure.

For this project, assume 25 clarifiers, with the following dimensions: 24.4 mx4.88mx4.57 .

Detention time= (24.4x4.88x4.57)/(94000/25)=0.145 day= 208 mins Overflow rate= (3760/(24.4x4.88) = 32 m/day Horizontal Velocity= (3760/(4.88x4.57)= 169 m/day Weir Loading rate= Q/2.5 w= 3760/(2.5 x 4.88)= 308m/day

Filtration The water that exits the clarifier may not be totally free of flocs. The turbidity of water leaving the sedimentation tank generally has turbidity levels that range from 1 to 10 TU [6]. Thus, to decrease turbidity down to 0.2 TU, a rapid sand filter must be utilized. Sand filtration is the separation of suspended and colloidal particles through the passage of water into a soil media. In this project, we will utilize 10 rapid sand filter which has the following with a loading rate of 200 m/d Surface Area= 94000/200=470 m2 Surface Area per Filter= 47 m2 (10 filters) Assume one filter is out of service >> Surface Area= 470/9=52 m2 Cost minimization suggests that the best length to width ration is 3:6 3WxW= 52 >>>>>>>>>>>>>>> W=4.16 m and L= 12.5 m

Disinfection Disinfection is the process of destroying disease causing pathogens. Among the most commonly used disinfectants is chlorine. Disinfection through the addition of chlorine is called chlorination. Upon the addition of chlorine to water the following reaction take place:

The recommended chlorine dosages are summarized in the following table: Table 5: Recomendded chlorine dosages for various systems [5]

Waster water type

Chlorine dosage (mg/L) to yield 0.2 mg/L free residual after 15 min of contact

Raw:

6-12

Fresh to stale

12-25

Septic Settled:

5-10

Fresh to stale

12-40

Septic

3-6

Effluent chemical precipitation Trickling filter

3-5

Normal

5-10

Poor Activated Sludge

2-4

Normal

3-8

Poor sand filter

1-3

Normal

3-5

Poor

Assume a chlorine dosage of 5 mg/L. Total amount of chlorine per day is: (94000 m3/day) x 1000L/dayx 5 mg/L= 470 kg/d

Design Summary To summarize, the water treatment processes implemented in this project are coagulation, flocculation, sedimentation, filtration, and finally disinfection. An alum daily amount of 1880 kg should be mixed and dispersed in the coagulation basin. We will have a total of four rapid mixing tanks, one flocculation tank, 25 clarifiers, and a total of 10 rapid sand filters of which are 9 in service. Finally, chlorination has been selected as our disinfection process , with a chlorine daily requirement of about 470 kg/d.

References [1]

K. Tomono and Y. Magara, "Environmental and health aspects of water treatment and supply," in Design of water treatment facilitiesParis, France: Eolss Publishers.

[2]

American Society of Civil Engineers, & American Water Works Association. (1998). Water treatment plant design (3rd ed.). New York: McGraw-Hill.

[3]

Halifax Water, "Integrated Resource Plan", Advocate Printing, Canada, 2017.

[4]

Robert A. Corbitt: Standard Handbook of Environmental Engineering, Second Edition. WATER TREATMENT, Chapter (McGraw-Hill Professional, 1999), AccessEngineering

[5]

Peavy, H., Rowe, D., & Tchobanoglous, G. (1985). Environmental engineering (Mcgraw-hill series in water resources and environmental engineering). New York: McGraw-Hill.

[6]

Weiner, R., Matthews, R., & Vesilind, P. (2003). Environmental engineering (4th ed.) [4th ed.]. Burlington: Elsevier. (2003). Retrieved October 17, 2018, from INSERTMISSING-DATABASE-NAME.