Public water systems often use a series of water treatment steps that include coagulation, flocculation, sedimentation, filtration, and disinfection. Municipal wastewater is defined as wastewater from households or a mixture of wastewater from households and of industrial origin as well as precipitation water.
Municipal Water Treatment Processes
There are 2 steps
1) Removal of suspended matter.
2. Removal of microorganisms- Disinfection.
Type of impurity
- Floating matter( leaves, wood pieces)
- Suspended (Clay, Sand)
- Fine suspended inorganic matter
- Micro organisms & colloidal impurities
- Pathogenic bacteria
The process to be employed
- Screening
- Plain sedimentation
- Sedimentation with coagulation
- Filtration
- Disinfection
1) Removal of the suspended matter:
Screening:
- The raw water is passed through screens that contain large a number of holes where floating matter is retained Sedimentation:
- Suspended impurities are removed by allowing the water to stand undisturbed for a few hours (about 2-8 hr) in big tanks (5m deep).
- Due to the force of gravity, most of the particles settle down at the bottom of the tank.
- About 70-75% of the suspended matter can be removed. Sedimentation with coagulation:
- Plain sedimentation can’t remove finely divided silica, clay, and organic matter.
- Sedimentation with coagulation is a process of removing fine particles by the addition of chemicals (coagulants) before sedimentation.
- Coagulant aids (lime, fuller’s earth, poly electrolytes) are added to increase the efficiency of the process.
- Generally, coagulants are added in solution form with the help of mechanical flocculation through agitation.
- Substantial reduction of bacteria also takes place during this process. (O2 i.e. released by some coagulants destroys some bacteria, breaks up some organic compounds, and partial removal of color & taste-producing organisms.)
- It is the process of clarification of water by passing the water through a porous material, which is capable of retaining coarse impurities on its surface & in the pores. [Porous material used – filtering media, equipment used – filter]
- Common materials used as filtering media: quartz sand(0.5- 1.0mm),crushed anthracite(0.8-1.5mm), porous clay
- Slow sand filtration (2gal/Sqft/hr) is generally employed in municipal water treatment.
- A typical sand filter consists of a tank with abed containing fine sand (top layer), coarse sand, and coarse gravel (bottom layer).
- It is provided with an inlet for sedimented water and under a drain channel at the bottom for the exit of filtered water
- Sedimented water is distributed uniformly over the bed and flows slowly through various layers.
- The rate of filtration slowly decreases due to the retention of impurities in the pores.
- The top layer is scrapped and replaced with clean sand to increase the efficiency of the process.
2. Removal of microorganisms- Disinfection:
Removal of pathogenic (Disease-causing microorganisms) is known as disinfection.
a) Boiling
When water is boiled the harmful bacteria and viruses cannot survive at this temperature But this process can be applied only to households, municipalities cannot apply.
b) Bleaching powder(CaOCl2):
By adding bleaching powder disinfection of water is done
CaOCl2+H2O → Ca (OH)2 + Cl2↑
Cl2 +H2O →HCl + HOCl
(Hypo chlorous acid which kills germs)
When bleaching powder is added to water first cl2 is liberated along with Ca(OH)2Cl2 reacts with water& forms Hcl&Hocl( Hypochlorous acid). HOCl is a germicide that kills bacteria or germs present in water.
Limitations:
- It is unstable, and difficult to store
- It introduces calcium in water which increases the hardness of water when used in excess
c) Addition of chlorine – (Chlorination)
Disinfection is done by the addition of liquid chlorine or gaseous chlorine. Chlorine produces hypochlorous acid [HOCl] which kills microorganisms.
Cl2+H2O→HCl+HOCl
HOCl→ H+ +OClHOCl → Kills Germs
Initially, it was found that nascent oxygen[o] from Hocl [Hypochlorous acid] is killing the microorganisms, but later on, two scientists found that Hocl is causing the death of microorganisms. Chlorine is a good disinfectant at a PH of 6.5.
Chlorination depends upon
Time of contact: The number of Microorganisms destroyed by chlorine per unit of time is proportional to the number of microorganisms remaining alive. So death rate is maximum at starting.
The temperature of water: Higher the temperature, the rate of reaction is faster & killing of microorganisms increases.
PH value of water: Lower the PH value, the reaction is faster & a small contact period is required.
Advantages:
- Effective & economical
- It requires very little space.
Disadvantages:
- Excess chlorine produces an unpleasant odour&taste.
- Free chlorine should not exceed 0.1-0.2ppm
- It is more effective below 6.5 &less effective at higher PH values.