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
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
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
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) -Water treatment plant
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.