The Water Treatment Process includes the treatment of water supplied to the household for drinking and other utility purposes and also the wastewater to be disposed of in the water sources.
The Water Treatment Processes involved in both the supply of water and wastewater differ in many ways. Here, we will be discussing the processes included in the drinking water supply.
Drinking Water Treatment
The journey of drinking water starts from the source. It can be anything- a lake, river, or other stream of water except seawater. Treatment of seawater costs too much to make it feasible for drinking, hence it is avoided.
Drinking water treatment means the removal of the pollutants present in the water and the inactivation of the microbes that may cause harm.
Drinking Water Quality Standards
Every country has decided to drink water quality standards. It includes the lower and upper limits of various ions and elements present in water.
Also, it sets the limitations up to which certain contaminants like turbidity, odor, color, and other physical characteristics can be tolerated.
Objectives of the Water Treatment Process
- To remove color and odor from water
- To remove the hardness of water
- To remove the harmful microbes that cause diseases from water
- To remove murkiness and turbidity of the water
- To remove dissolved gases present in water
- To make the water suitable for various purposes like drinking, industrial and many others as per the demand
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Types of Drinking Water Treatment Process
Drinking water treatment is treated in a water treatment plant and supplied to the household via a conveyance system.
Care should be exercised so that the drinking water is not contaminated again during its conveyance. For this, precautions are taken at the treatment plant itself from the start.
The Processes Involved in Water Treatment
The following are the types of processes involved in water treatment,
1. Physical Water Treatment Process
- Dissolved Air Floatation
2. Chemical Water Treatment Process:
3. Physio-Chemical (Conventional) Water Treatment Process:
4. Biological Water Treatment Process:
- Slow Sand Filtration
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Stages In Water Treatment Process
Let us see the drinking water treatment process in detail here and the various processes involved in the same. The water treatment process starts with the collection itself.
There are two types of sources of water. One is the surface water source like a river, reservoir, etc. the other one is a groundwater source like a bore well. The water treatment process differs for these systems considerably.
The impurities present in both the sources belong to a different category and hence the treatment process is different too.
Surface water has a large number of suspended particles and other physical impurities while groundwater has the presence of ions.
These ions make groundwater- hard. However, the ions are lacking in surface water. So, the groundwater required chemical treatment more than physical.
1. Collection of Water
The water is collected from the sources like a lake, rivers, or reservoirs. The water is to be transported from this source to the water treatment plant for the water treatment process. This is included in the collection of water.
Generally, water treatment plants are built near the water source itself.
Screening is done to remove the floating matter from the water during the water treatment process. Surface water contains a large number of suspended particles that increase the unnecessary load on the treatment plant units.
Screening is mostly done at the intake point itself. The large-sized suspended particles like dried leaves, fallen twigs, and other floating debris.
Two types of screens are used for screening:
1. Coarse Screens/ Bar screens
Bar grills are installed and water is allowed to pass through them in this process. 25 mm bars are installed at 75 to 100 mm center-to-center distance. This traps particles of large size as the water flows through them.
Mostly the bars are kept in an inclined position so that they can be cleaned easily with racks to remove the trapped particles.
2. Fine Screens/ Automatic Strainers
An automatic device is fitted in the screens so that the trapped materials can be removed on their own. Such type of screens is called automatic strainers.
A stainless steel wire is used to make the cloth designed specially and it is mounted on t periphery of the revolving drum afterward. It has an arrangement for backwashing too. This ensures that it does not get clogged.
The sedimentation process removes the heavy particles that can settle down under gravity. The weights of the particles increase as they aggregate and then settle down.
A sedimentation tank is so designed that the velocity of the flowing water is reduced. As the water is discharged into the sedimentation tank, the cross-section area of the water flow is in the case and therefore, velocity reduces.
Sedimentation is also of two types: One is Plain sedimentation without the addition of chemicals while the other is sedimentation with coagulation in which chemicals are added.
The efficiency of the sedimentation tank depends upon the following factors:
- Design considerations for the tank
- Shape and size of the sedimentation tank
- Detention period
- Size of suspended particles
- Characteristics of suspended particles
- The flow velocity of water
i. Plain Sedimentation
In this type of sedimentation, the particles aggregate on their own nature and settle down under the force of gravity.
ii. Sedimentation with Coagulation
To speed up the process of sedimentation and to increase efficiency as well, sometimes, chemicals are added to the water. These chemicals are called coagulants.
Coagulants help in the aggregation of the particles, which increases the size of aggregated particles and hastens their settling in the water treatment process.
Types of Sedimentation/ Settling Tank
There are two types of sedimentation or settling tanks as described below:
1. Fill and Draw Type
The water is filled in the fill-and-draw type first and then allowed to remain for a particular duration. Detention time is normally kept 24 hours for the fill and draw-type tank. The suspended particles settle down to the tank bottom in this detention period.
The above-mentioned tank is also known as the Quiescent Tank.
As the retention period is over, the clear water from above is taken out by opening the outlet valve, and the tank is cleared for the suspended particles settled down earlier. To clean the tank, 6-12 hours is required.
Thus, one operation cycle to obtain clear water takes around 30 to 36 hours for the fill and draw-type tank.
2. Continuous Flow Type Tank
The fill and the draw-type tank take up too much time to get one batch of filtered water. This is not feasible when a large population is to be supplied with water.
Taking a look at the factors mentioned above-affecting sedimentation, the velocity of incoming water can be managed without extensive effort.
Hence, in a continuous flow type tank, the water is allowed to move continuously under much less velocity. The suspended particles settle down as the water flows and reach the outlet at the bottom of the tank.
Continuous flow type tanks can be further constructed of two types based on the flow direction of water as follows:
Horizontal flow tank- A rectangular tank is used and the length is generally kept twice its width. The flow of water is in the horizontal direction. Maximum permissible velocity = 0.3 seconds.
Vertical flow tank- A circular or rectangular tank with a hopper bottom. The dimension of these tanks is generally more in the depth. The flow of water is in the vertical direction. Water enters the tank through a centrally located inlet pipe and then it is deflected downwards.
4. Clarification or Sedimentation with Coagulation
The sedimentation with coagulation is termed clarification. It is required to increase the efficiency of sedimentation as stated above during the water treatment process. Plain sedimentation consumed too much time.
How does clarification work?
The colloidal particles suspended in the water have a negative or positive charge around them. Coagulants neutralize this charge and allow these particles to coagulate.
Steps in Clarification in Water Treatment Process:
- Addition of measured amount of chemicals to the water
- Thorough mixing of water
- Formation of precipitates in water
- Formation of flocs which is formed by coagulation initiated by the precipitates
Types of Coagulants used in Water Treatment Process:
- Ferrous sulfate and lime
- Magnesium carbonate
- Sodium aluminate
Alum is most commonly used as a coagulant.
Devices used for Carrying out Mixing of Coagulants:
- Centrifugal pump
- Compressed air agitation
- Mixing basin
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Filtration is one of the most crucial steps of the water treatment process.
The flocs formed during flocculation are not removed entirely by sedimentation. Hence, to remove the finely sized particles and flocs, filtration is required. And the particulate matter which was retained in the sedimentation tank previously is removed, especially the non-settleable particles.
Types of Filters
The following type of filters are available based on the time taken for filtration:
i. Slow Sand Filter
These filters were introduced as early as the 1800s. Slow sand filters are named so because they have taken too much time for filtration. The filtration time is one-twentieth as compared to that of the rapid filters.
The components of a slow sand filter comprise an enclosure tank, fitter media, base material, an under-drainage system, and appurtenances.
An enclosure tank is an open basin tank with a bed slope of 1 in 100 or 1 in 200. The surface area varies from 50 to 1000 cubic meters.
Filter media is formed of sand which should be free from loam and suspended or organic matter.it is laid in 90 to 110-centimeter thickness.
The base material is formed of gravels laid in a 30 to a 75-centimeter thick layer. Under drainage system is formed of a number of lateral drains.
Some appurtenances are also installed for various purposes, like measuring head loss through filter media, controlling the depth of water above filter media, etc.
ii. Gravity Type Rapid Sand Filter
The components of a rapid sand filter comprise an enclosure tank, fitter media, base material, an under-drainage system, and appurtenances.
The enclosure tank is smaller in size compared to that the slow sand filter. Other components are similar in characteristics to that of the slow sand filter.
However, the appurtenances consist of wash water troughs, air compressors, and rate control devices.
Rapid sand filters may be logged frequently, which is why the sand has to be cleaned at frequent intervals. For this, backwashing and surface wash methods are applied.
iii. Pressure Filters
The pressure filter is also a rapid sand filter, however, instead of an open basin, the closed container is used for this, and water is allowed to pass through the filter in pressure. The pressure may be about 3 to 7 kg/cm3
These filters are either horizontal or vertical type based on the direction of water flow.
Automatic pressure filters are also available at present. In this type of filter, backwashing is done automatically.
Issues in Filters:
Because of poor design or lack of proper operation of the filter, many filtration problems arise over sue course of time.
Some of the issues in filters include:
1. Cracking and Clogging of Filter Bed in Filters used in Water Treatment Process
This mostly occurs when the solids accumulate on the top surface of the filter media. The coating on sand grains cracks and the head loss is observed to have been increased.
This results in the penetration of dirty water into the filtered media up to the gravel layer. This decreases the efficiency of the filter.
2. Mud Balls Formation
Conglomerates of floc sand and other binders are termed mud balls. Mud balls are formed near the top of filter media and are resulted from the insufficient washing of filter sand grains.
Mud accumulates on the sand surface forming a dense mat and reducing the effectivity of the filter. This can be avoided by backwashing the sand at regular intervals.
3. Binding of Air
When dissolved gases of water are released in the form of air bubbles, air binding occurs. These air bubbles clog the voids of the filter media and the water may not be able to flow as earlier.
This condition of air binding can be avoided by preventing the warming of water before it is allowed to pass the filter media, control of algae, or avoiding the supersaturation of water with air.
4. Sand Boils
When the water follows the path of least resistance during backwashing, sand balls may be formed. The sand may boil like quicksand, and lifts up sand, and even gravel to the surface. Surface washing or air scouring before backwashing can eliminate fluidity.
5. Sand Leakage
When the layer of fine gravel is displaced during backwashing, sand leakage may take place. Sand leakage is the downward migration of the fines.
With the proper proportions of the level of sand and gravel, sand leakage may be prevented.
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After filtration, the next step of the water treatment process is disinfection. Disinfection includes the inactivation of pathogenic bacteria and other microorganisms that can cause diseases. This step is essential to control water-borne diseases.
A disinfectant is added in this step to filtered water. Skilled operators are required to carry out the process of disinfection so as to maintain the appropriate dosage.
Some of the Important Characteristics of A Disinfectant
- It should be effective in killing the microorganisms in the contact time provided.
- It should be easily available in the market.
- It should not be expensive.
- It should not make the water toxic
- Its application should not be complicated involving a series of processes
- It should not impart any objectionable color to the water
- It should have the ability to remain in the water for a residual amount so that the water is not contaminated again
Types of Disinfectants used in the Water Treatment Process
- Physical treatment- Application of ultrasonic waves, heat, or other physical agents
- Radiation- Application of gamma radiation
- Metal ions- copper, silver
- Alkalis and acids
- Oxidants- Addition of chlorine, bromine, iodine, ozone, chlorine dioxide, etc
The most commonly used disinfection method is chlorination.
Chlorination- Most widely used Disinfection Method
The chlorination process for disinfection has become the most popular method because it is cheap, easily available, and easy to handle.
Chlorine is dissolved in water at a temperature of 49-212 °F. Chlorine gives hypochlorous and hypochlorous acid through the hydrolysis process. Ionization takes place and hypochlorous acid dissociates into hydrogen and hypochlorite ions. These compounds- hypochlorous acid and hypochlorite ions aid in disinfection.
Forms of Chlorine Application to Water:
As bleaching powder ie in hypochlorite form: It was used when chlorination was first introduced, however, it is not stable and its power of disinfection is lost when stored or exposed to air.
As chloramines: These are compounds of chlorine and amino acids. Chloramines were discovered to be disinfectants following the instability of bleaching powder.
The residual of chloramines is more stable than the chlorine residual. These also reduced the objectionable taste in water. Ammonia is added to water before the addition of chlorine.
As Free Chlorine: It can be applied in liquid, gaseous, or even solid form. It is the most popular form of chlorine tablets that are also available.
As Chlorine Dioxide: Chlorine dioxide is more effective in leaving the microorganisms inactive as compared to free chlorine. However, it is only used in special water treatment processes.
It is done before the water enters the treatment units, especially the filter unit. In pre-chlorination, a little amount of chlorine is applied so that the organic matter can be oxidized and a less amount of coagulant is required.
ii. Post chlorination
Post-chlorination is the amount of chlorine added after the water is completely treated and ready to be distributed.
Post-chlorination prevents the contamination of water in the route and also ensures no harmful microbes are introduced to the drinking water.
7. Water Softening
Water softening is done to make the hard water soft. Surface water usually does not contain much hardness.
However, the water taken from underground sources like bore wells contains hardness due to the presence of ions.
The hardness of water prevents the water from forming lather and also causes problems in the plumbing system.
It even affects the taste of water and even food cooked with hard water tastes tough or rubbery.
There are two types of hardness of water and their treatment methods also vary with it.
Types of Hardness and Their Removal
i. Temporary Hardness
Cause: calcium bicarbonate and magnesium bicarbonate ions
Methods of Removal of Temporary Hardness:
- by boiling
- by the addition of lime
ii. Permanent Hardness
Cause: calcium sulfate, magnesium sulfate, calcium chloride, and magnesium chloride ions
Methods of Removal of Permanent Hardness:
- By Lime Soda process
- Zeolite process
- Demineralization or Deionization process
8. Other Treatment Methods
There are a few other characteristics of water to be adjusted as per the quality standards set by the guidelines.
Some of these characteristics of water to be brought under the limit are:
To Remove Color, Odor, and Taste:
This is necessary when objectionable color, odor, or taste is imparted to the water, which may be because of one of the following reasons:
- Organic and vegetable matter
- Dissolved gases beyond a certain limit
- Dissolved mineral matter
- Industrial waste
- Microbes like molds, bacteria, etc.
Some of the methods used to remove this objectionable color, odor, and taste are:
In aeration, air and water are mixed intimately.
It mostly removes the taste because of dissolved gases. It also increases the content of dissolved oxygen in the water.
Free Fall Aerators
There are three types of aerators as follows:
1. Cascade Aerators
It is the simplest type of free fall aerator. A series of steps are constructed and water is allowed to fall through a height of 1 to 3 meters.
2. Slat Tray Aerators
These are not commonly used. It has a series of closely stacked wooden slats in a closed room. It is either circular or square in shape. Water flow is from top to bottom.
3. Gravel Bed Aerators or Trickling Beds:
In this type of free-fall aerator, water is supplied to the top and allowed to fall downwards while air is blown upwards. The water trickles down from the beds of gravel of thickness 1 to 1.5 meters.
4. Spray Aerators
Water is divided into fine streams and droplets in the case of spray aerators and it is allowed to come in intimate contact with the air. Water is sprinkled in the form of a jet with the help of nozzles.
5. Air Diffusion
A perforated pipe network is present in the bottom of the aeration tank through which compressed air is blown. The water travels upwards in this case and the retention time is about 15 minutes.
6. Copper Sulphate Application
Besides removing color, odor, and taste, it also removes algal growth if present in water.
It is available in powder or crystal form and is directly applied to the water when it enters the distribution system.
It is required when the water contains excessive saline concentration. Very less water on the surface of the earth is fresh water and if freshwater is not available, and saline water is to be used, then this step is necessary.
Desalination methods are costly. Some of them are listed below:
- Reverse Osmosis
- Solar evaporation
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Water Treatment Process at a Glance
The water treatment process includes various treatments of water so that it meets the quality standards specified.
There are various types of processes involved in the water treatment process and each process plays a significant role.
It starts with the collection of water from the source. Then it is screened for larger-sized articles. After that, it is allowed to flow through the sedimentation tank where the heavier particles settle down under gravity.
If the particles have a relatively large size, then an appropriate coagulant is added and then sedimentation is carried out which is called clarification.
After that, water is filtered and disinfected as well. For hard water, the water softening process is also carried out depending upon the type of hardness.
U.S. Safe Drinking Water Act
U.S. E.P.A. Environmental Protection Agency has set standards for drinking water quality under the U.S. Safe Drinking Water Act for public water systems.
State health agencies look out that the standards to be implemented in the region.
Around 90 contaminants are listed and divided into 6 groups by EPA. These six groups are as follows:
- Disinfection by-products
- Inorganic chemicals
- Organic chemicals
All countries have their own drinking water quality standards. In India, IS: 10500 describes the standards for drinking water quality.
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