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How to design a Surface Water Treatment Plant for your University Coursework

Updated: Dec 5, 2020



What is a Water Treatment Plant?


Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, including being safely returned to the environment. Water treatment removes contaminants and undesirable components or reduces their concentration so that the water becomes fit for its desired end-use. This treatment is crucial to human health and allows humans to benefit from both drinking and irrigation use.



When designing water treatment facilities, the main factors to be considered are the type of water source, finished water quality, the skill of facility operators and the available size of funds.

Sources: Wikipedia

How many stages of treatment units are present in a surface water treatment plant? (e.g lake)


In a surface water treatment plant units used are:

  1. COAGULATION

  2. FLOCCULATION

  3. SEDIMENTATION

  4. FILTRATION

  5. DISINFECTION

Surface water treatment plant process units involved

A multiple barrier approach should be adopted for reliability reasons of safety in the water supply system. In the case of malefaction or maintenance of one unit, the system will be able to continue to operate with the rest of the units being in operation. Similarly, having multiple process units can offer more reliable supplies in terms of removal of pathogenic contamination.


The first stage of the system, adopted from industry standards, a screen will be placed where large solids found in the watercourse could be disposed of the start of the water treatment.



Following the design of the pipe infrastructure and the intake pumps required for water to flow in a sufficient rate into the system, three treatment units of Mixing-coagulation, flocculation and sedimentation will be integrated.


Furthermore, sand filter units are added to the process were appropriate gullet, wash troughs and unit dimensions are specified. Thereafter, after the clean water is filtered, it will pass through a contact chamber where water will get disinfected by the addition of chlorine concentration.


water treatment plant process

Finally, after the water passed through the necessary treatment processes and quality checks, it will be distributed into the wider pipe network of the water system. As a final stage of the process, a clear well storage tank is used for storing the clean water destined to the taps of the consumers, the general public.


The headloss across the units can be set to be 0.8m which is found widely acceptable by common practices. All units are assumed to be constructed with reinforced concrete where walls and slabs of water-bearing structures should be a minimum of 200mm thickness regardless of the loading condition (Kawamura,2000).

Lake Reservoir

WATER DEMAND


Water demand is calculated assuming a litre/capital/day consumption.


1. COAGULATION


Coagulation is the rapid mixing process of adding compound chemicals to the water that promote the clumping of fines found in the lake (e.g. soil particles) into large flocs thus they can be easily separated. Adding of coagulants such as aluminium sulphate (alum), ferric sulphate or sodium aluminate will destabilise colloidal suspensions, neutralising the particle's charge and hence form flocs. Dosages range of 3-60 mg/L of alum are suggested (Kawamura S., 2000).


Tests to determine the dose estimation of coagulants in the tanks will be done through usually 4-8 jar test of 1 Litre of the source water.


Design Steps


Where:

V – chamber volume m3

Q – water demand flow rate across the unit m3/min

t – mixing time (seconds)


Mixing time values are given in a range of 1 – 3 minutes for coagulation rapid mixing (Coagulation Foundamentals,2009)


- t=1 minute can be assumed as a design value. Two units of flash mixers were designed.


Dimensions of the chamber can be calculated using the chamber volume.


Appropriate rapid mixing flat-blade impellers are going to be used in the design. Also, mixers motors such as the one below can be used.

PRO-DO-MIX® ACR agitator (35 rpm)


2. FLOCCULATION


Flocculation is the slow mixing process by which fine particulates are caused to clump together into a floc. The floc may then float to the top of the liquid. The design criteria of the flocculation tank are based on previous studies assumed as common practice in the industry. The calculations are done as follow (Aziz,2019):


Design Steps

Where:

V – chamber volume m3

Q – water demand flow rate across the unit m3/min

t – mixing time (seconds)


- Mixing time values are given in a range of 10-30 minutes for flocculation slow mixing.


- t= 30 minutes was assumed as a design value (‌Mcghee, T.J. 1991). Two or more units of flash mixers can be designed.





3. SEDIMENTATION


Sedimentation is the process of removing solid particles by gravity. As per Britannica, a sedimentation tank allows suspended particles present in the water to settle out of the water as the flow rate through the tank is slow. This process allows a layer of sludge, which is a mass of solid particles coming together to form at the bottom of the tank where it can be removed. Assumptions are used to determine dimensions of the tank based on the retention time (RT) and surface overflow rate (SOR). The calculations are done as follow:


Design Steps

Surface Overflow Rate equation:

Where:

Q - Flow rate (m3/d)

A – Area (m2)


1. SOR is assumed 30 m/day (since we design for coagulated sedimentation from common practice) and rearranging the surface overflow rate formula to get:


2. Then the radius of the tank can be found,


3. Hence the diameter D can be found:



Where:

V- Volume (m3)

Q – Flow rate (m3/d)


1. Next by assuming Retention time (tR) = 2.5h, Volume of the tank can be determined:



Dimensions of the tank can be calculated using the tank volume calculated above.


Typical Cross -section of a circular sedimentation tank example

4. FILTRATION


A filtration unit is built-in into one of the last stages of the water treatment cycle. Filter units aim to remove the suspended solids that were not removed in the previous treatment process. The filters are made out of fine and coarse sand layers were by means of mechanical screening trapping of solid particles in between the grains of sand occur hence purifying and cleaning the water from impurities. Tests of filter performance can be done by assessing the turbidity of a sample of the water.


Then backwash is performed where clean water is blasted from the bottom of the gullet tank to the top to remove the impurities present in the sand and hence clean the filters. Throughout this process water level rises to a point where wash troughs transport the water and dispose it into the gullet were a pipe recirculates the dirty water back into the system for treatment as seen in the video below.


  • Filter beds are normally considered of 0.75m depth (Kawamura S., 2000)



Design Steps

Filter design

Area of filter bed:



Where:

A - area of a bed, m2

Q – maximum day flow rate, m3/q

N – number of beds

q – filtration rate, m3/d*m2


1. Maximum, minimum and standard values of filtration rate – q is underlined in Table below as per common practice in the industry.

Range values of filtration rate - q (m/h)

2. Using the Filter area unit bed equation stated above with the assumptions of q made above, the area can be calculated


3. Assumed filter length (e.g., L= 3 m, the width of a filter unit can be determined.


4. The individual filter intake demand flow rate can be calculated by dividing Q with the numbers of filters present in the design:


Dimensions per filter Unit thereafter can be calculated.


Wash Troughs Design


Wash troughs are used to carry the purified water produced from the method of backwash. The material options are of fibreglass-reinforced plastic (FRP), concrete or stainless steel. Wash throughs have a semi-circular shape to create smooth flow streamlines and to prevent the accumulation of foam and solids. The dimensions of the wash throughs can be determined using the graph below.


Q per filter = (m3/h)

Y = (cm)

W/2 = (cm)


Gullet Design


The filtration unit depth can be calculated by determining the correlation of the velocity head, entry head loss and the pipe diameter of the backwash output. The backwash rates are assumed to be around 1.8 m/h. The calculations are done as follow:


Design Steps

Depth of Filtration unit:


Where:

H – depth at the upstream end, m

h – depth at distance x

Qww – wash water discharge, m3/s

x – length of the gullet, m

g – gravitational acceleration, 9.81 m/s2

b – width of the gullet, m


1. Firstly, calculate the depth at distance – headloss

Where:

U – water velocity, 1.8 m/s

g – gravitational acceleration, 9.81 m/s2


Pipe diameter = 450 mm assumed from standard practice.


2. Design backwash rate (q) is assumed as 20 m/h, hence by rearranging the formula below, Qww could be determined:

Where:

A – Filter total surface area (m2) of filter


3. Finally, calculate H using the equation of depth of Filtration unit to find the depth of the Gullet unit.


A typical sand filter will have a profile such specified in the picture above.



5. DISINFECTION


The unit of disinfection is the last process required of the water treatment plant proposed. Chlorine disinfection is directly associated with the inactivation/killing of pathogenic microorganisms present in the water which are directly related to waterborne diseases affecting public health, and hence need to be taken seriously. Disinfection of water using chlorine was chosen due to being inexpensive, reliable, and relatively safe to handle.





Chlorine contact chamber


A contact chamber is used to allow the chlorine dosage added to the water to mix efficiently with the water body before being distributed to the main network system and then to the consumer. Baffles are provided to promote plug flow to enhance the mixing of chlorine with water as shown in the picture below.

Typical contact chamber

Design Steps


- t10 – time when 90% of the water will be exposed in the disinfection chamber - minutes

- t0 – hydraulic retention time (V/Q) - minutes


t10/t0 = 0.7

t10 = 100 min


Hence t0 could be calculated:



By using rations of L/W = 40 and H=3W:



Clearwater storage tank


The capacity of treated clean water in the clear well storage tank should be sufficient to the water demand of the specified population for a minimum of 24 hours. Hence this water quantity is stored in a cylindrical storage vessel with minimum dimensions determined as follow:


Design Steps

  • The volume required = Q flow demand per day.

  • Prelim dimensions based on L=40W, H=3W


Further Reading Available

  1. Blyth, E.M., Martinez-de la Torre, A. and Robinson, E.L. (2018). Trends in evapotranspiration and its drivers in Great Britain: 1961 to 2015. [online] Available at: https://hess.copernicus.org/preprints/hess-2018-153/hess-2018-153.pdf?fbclid=IwAR3VcnmdFe1ezGkV84PoU5ZqainDfg1iJFjolIVSGmRBM9irv5ykyna5DlU.

  2. Hasenmueller, E.A. and Criss, R.E. (2013). Water Balance Estimates of Evapotranspiration Rates in Areas with Varying Land Use. Evapotranspiration - An Overview. [online] Available at: https://www.intechopen.com/books/evapotranspiration-an-overview/water-balance-estimates-of-evapotranspiration-rates-in-areas-with-varying-land-use

  3. 2020. The Clean Water Team Guidance Compendium For Watershed Monitoring And Assessment 2 State Water Resources Control Board 5.1.3 FS-(RC) 2011. [online] Available at:https://www.waterboards.ca.gov/water_issues/programs/swamp/docs/cwt/guidance/513.pdf

  4. Aziz, Shuokr Qarani & Mustafa, Jwan. (2019). Step-by-step design and calculations for water treatment plant units. Advances in Environmental Biology. 13. 1-16. 10.22587/aeb.2019.13.8.1.

  5. Mukashev, Temirlan. (2015). Water Treatment Facility Design. 10.13140/RG.2.1.1446.3522.

  6. Heydari, Mohammad & ROHANI, Seyed & HOSSEINI, Seyed. (2013). Correlation Study and Regression Analysis of Drinking Water Quality in Kashan City, Iran. Walailak Journal of Science and Technology. 10. 315-324. 10.2004/wjst.v10i3.338

  7. Coagulation and Flocculation Process Fundamentals 1 Coagulation and Flocculation. (2009). [online] Available at: https://www.mrwa.com/WaterWorksMnl/Chapter%2012%20Coagulation.pdf.

  8. ‌ Mcghee, T.J. (1991). Water supply and sewerage. New York, N.Y.: Mcgraw-Hill.


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