The primary goal of water treatment is to remove contaminants and make water safe for drinking, cooking, and other uses.

Since pure water is rarely found in nature, it often contains impurities. These impurities can be classified into three progressively finer categories: suspended solids, colloidal solids, and dissolved substances. Different methods of treatment or reduction to acceptable limits are needed for each category.

In most cases, more than one treatment process is required to achieve the desired changes in water quality, so treatment plants usually consist of a chain of processes that operate in sequence.

Differ types Nature of Impurities in Water

Pure water is very rare, and all water contains some degree of contamination. The impurities in water can be classified as follows:

1. Floating and Large Suspended Solids: Includes items such as leaves, twigs, and other debris.
2. Suspended Solids: Includes small particles like silt, clay, and microorganisms.
3. Colloidal Solids: These are finer particles that are not easily settled, such as clay and microorganisms.
4. Dissolved Solids: Includes minerals causing hardness, salts, and some organic compounds.
5. Dissolved Gases: Such as carbon dioxide and hydrogen sulfide.

In many situations, it becomes necessary to add substances as part of the treatment process. For example, coagulants are used for the removal of turbidity, oxygen for biological processes, and chlorine for disinfection.

Water Impurities and Types of Treatment Processes

Water impurities can be broadly classified into three categories: physical, chemical, and biological Treatment Process. Each type of impurity affects water quality and requires specific treatment methods for removal.

1. Physical Impurities

Physical impurities refer to particulate matter and suspended solids present in water. These include soil particles, sand, silt, and other organic and inorganic materials that are not dissolved in water.

What are the Effects:

These impurities can cause turbidity, making the water appear cloudy and reducing its aesthetic quality. They can also clog pipes and filters, reducing the efficiency of water treatment processes.

What are the Removal Methods:

1. Screening: Removal of large debris.
2. Sedimentation: Settling of particles due to gravity.
3. Filtration: Use of sand filters, rapid gravity filters to trap and remove particles.
4. Gas Transfer: Utilized to remove dissolved gases and volatiles.
5. Flocculation: Adding chemicals to form flocs that settle out of suspension.

2. Chemical Impurities

Chemical impurities are dissolved substances in water, which can include salts, heavy metals, organic compounds, and other chemicals. Common examples are chloride, nitrate, lead, arsenic, and pesticides.

What are the Effects:

These impurities can have significant health impacts. For instance, heavy metals like lead and arsenic are toxic and can cause serious health problems, including neurological disorders and cancer. Excessive nitrates can cause methemoglobinemia (blue baby syndrome) in infants.

Note: Nitrates: Can cause methemoglobinemia (blue baby syndrome) in infants

What are the Removal Methods:

1. Chemical Precipitation: Adding chemicals to form insoluble precipitates.
2. Ion Exchange: Removing specific ions from the water.
3. Membrane Filtration: Using reverse osmosis, nanofiltration to remove dissolved substances.
4. Adsorption: Utilizing activated carbon to remove organic compounds and some inorganic chemicals.
5. Coagulation: Adding coagulants to form larger particles from dissolved and colloidal impurities.

3. Biological Impurities

Biological impurities consist of microorganisms such as bacteria, viruses, protozoa, and algae. Common pathogens include Escherichia coli (E. coli), Giardia, and Cryptosporidium.

What are the Effects:

These impurities can cause waterborne diseases such as cholera, dysentery, and gastrointestinal infections. They pose a serious health risk, particularly in areas with inadequate water treatment.

What are the Removal Methods:

1. Chlorination: Adding chlorine to kill microorganisms.
2. UV Radiation: Using ultraviolet light to inactivate pathogens.
3. Ozonation: Using ozone gas to destroy microorganisms.
4. Biological Treatment: Utilizing beneficial microbes to outcompete harmful ones.
5. Membrane Filtration: Using microfiltration and ultrafiltration to physically remove pathogens.

Typical Groundwater Treatment Process

1. Raw Water Intake: Groundwater is pumped from underground aquifers.
2. Screening: Initial screening to remove large debris such as leaves, twigs, and other large particles.
3. Aeration: Air is introduced into the water to release dissolved gases and oxidize metals like iron and manganese, making them easier to remove.
4. Coagulation and Flocculation: Coagulants (like alum or iron salts) are added to the water. These chemicals cause small particles to clump together into larger particles, known as flocs.
5. Sedimentation: The water flows into a sedimentation basin where the heavy flocs settle to the bottom and are removed.
6. Filtration: The water is then filtered through layers of sand, gravel, and sometimes activated carbon to remove any remaining suspended particles and microorganisms.
7. Disinfection: Chemicals such as chlorine or chloramine are added to the water to kill any remaining bacteria, viruses, or other pathogens.
8. Storage: The treated water is stored in large tanks or reservoirs until it is needed for distribution to homes and businesses. For the Better Understanding You can also refer the Flow Diagram Below,

Raw Water Intake
↓
Screening
↓
Aeration
↓
Coagulation & Flocculation
↓
Sedimentation
↓
Filtration
↓
Disinfection
↓
Storage

The choice of a particular sequence of treatment units depends on the quality of the raw water and the desired quality of the treated water.

Water Treatment for Different Water Sources

Groundwater and Surface Water with Low Turbidity

For groundwater and surface water with storage, having turbidity less than 10 NTU and being free from color and odor, plain disinfection is sufficient for drinking purposes.

Groundwater with Excess Iron, CO2, and Odorous Gases

For groundwater containing excessive iron, dissolved carbon dioxide, and odorous gases, the recommended treatment processes are:

• Aeration
• Flocculation and Sedimentation
• Rapid Sand Gravity Filtration
• Disinfection

Groundwater with Only CO2 and Odorous Gases

If the groundwater contains only dissolved CO2 and odorous gases, aeration and disinfection are sufficient.

Surface Water with Moderate Turbidity

For surface water with turbidity less than 50 NTU and where sufficient land is available, the treatment process will be:

• Sedimentation
• Slow Sand Filtration
• Disinfection

Highly Polluted Surface Water

For highly polluted surface water laden with algae and other microorganisms, the treatment sequence will be:

• Prechlorination
• Aeration
• Flocculation and Sedimentation
• Rapid Gravity Filtration
• Post Chlorination (Disinfection)
• Supply

Key Points to Remember

1. The last unit in any treatment plant will always be the disinfection unit.
2. Copper sulfate (CuSO₄) and chlorine (Cl₂) are used to kill algae.
3. Sodium nitrate (NaNO₃) facilitates the growth of algae.
4. When using a slow sand filter in the treatment of water, coagulation and flocculation will never be used prior to it.