14.4 WATER TREATMENT METHODS | Environmental Chemistry Water | Chemistry 11

This document outlines the essential methods for treating water, categorized into raw water treatment for potable use and wastewater treatment for environmental discharge. Understanding these processes is crucial for ensuring public health and environmental sustainability.

14.4.1 Raw Water Treatment

Raw water is sourced from groundwater, rivers, or lakes and is intended for drinking or municipal purposes. This water is typically contaminated with grit, pathogens, and other impurities, necessitating comprehensive cleaning before use.

Steps in Raw Water Treatment:

Screening:
- This initial physical step involves the removal of large debris such as sticks, leaves, pebbles, and soil. Large screens or grates are used to prevent these materials from entering subsequent treatment stages.
Coagulation and Flocculation:
- To remove finer suspended particles, coagulants are added. Common coagulants include aluminum sulfate (alum, $A l_{2} (S O_{4})_{3}$ ) or ferric sulfate ( $F e_{2} (S O_{4})_{3}$ ).
- These chemicals neutralize the charges of suspended particles, causing them to clump together into larger, heavier particles called flocs.
- Coagulation: The rapid mixing process where chemicals are added to destabilize particles.
- Flocculation: The slower mixing process that encourages the collision and growth of flocs.
Sedimentation:
- The water, now containing larger flocs, is allowed to stand in large tanks for 12 hours or more.
- Due to gravity, the heavy flocs and other solid particles settle down at the bottom, forming a layer of sludge, which is then removed.
Filtration:
- Following sedimentation, the water is passed through various filter media.
- Common filters include layers of sand, gravel, and sometimes activated charcoal.
- This step removes remaining fine particles, turbidity, and many pathogens that did not settle during sedimentation.
Killing the Pathogens (Disinfection):
- To ensure the water is safe for consumption, microorganisms like bacteria, viruses, and parasites must be inactivated or killed to prevent waterborne diseases.
- Common disinfection methods include:
  - Chlorination: Addition of chlorine ( $C l_{2}$ ), hypochlorite ( $Cl O^{-}$ ), or chlorine dioxide ( $Cl O_{2}$ ). Chlorine reacts with water to form hypochlorous acid ( $HOCl$ ), which kills pathogens by oxidizing their cellular components. A key drawback is the formation of toxic trihalomethanes (THMs) when chlorine reacts with dissolved organic matter. Halogens→
  - Ozonation: Treatment with ozone ( $O_{3}$ ), a more powerful oxidizing agent than chlorine. It does not produce harmful chlorinated by-products but provides no residual disinfection in the distribution system and is more expensive to produce.
  - UV Treatment: Exposure to ultraviolet (UV) radiation, which damages the DNA/genetic material of microorganisms, preventing reproduction.

Other Raw Water Treatment Methods

Additional processes may include:

pH Adjustment: To optimize coagulation or disinfection efficiency and ensure water is not corrosive.
Membrane Filtration: Such as microfiltration, ultrafiltration, nanofiltration, or reverse osmosis for very fine particle and contaminant removal.
Ion Exchange Treatment: For softening water (removing $C a^{2 +}$ and $M g^{2 +}$ ions) or removing specific ions.
Aeration: To remove dissolved gases (e.g., hydrogen sulfide $H_{2} S$ , which causes odors) and oxidize certain dissolved metals (e.g., soluble $F e^{2 +}$ to insoluble $F e^{3 +}$ , and $M n^{2 +}$ to insoluble forms) that can then be filtered out. It also strips volatile organic compounds (VOCs).
Desalination: For removing salts from saline water sources (e.g., seawater) to produce fresh water.

14.4.2 Waste Water Treatment

Wastewater, originating from domestic, commercial, or industrial sources, contains various pollutants that can severely contaminate water bodies if discharged untreated. Treatment is essential to protect aquatic ecosystems and public health.

Stages of Wastewater Treatment:

Primary Treatment:
- This physical process focuses on removing large solids and suspended particles.
- It typically involves:
  - Screening: Removal of large debris (similar to raw water treatment).
  - Grit Removal: Allowing sand, gravel, and other dense inorganic solids to settle.
  - Sedimentation: Water flows slowly through large tanks, allowing lighter organic solids to settle out as primary sludge.
Secondary Treatment:
- Biological processes are employed to break down dissolved and colloidal organic matter that remains after primary treatment.
- Common methods include:
  - Activated Sludge: Microorganisms (activated sludge) are cultivated in aerated tanks, consuming organic pollutants. The sludge is then separated from the treated water.
  - Trickling Filters: Wastewater is sprayed over a bed of media (e.g., rocks, plastic), where a biofilm of microorganisms grows and degrades organic matter.
Tertiary Treatment:
- This is an advanced treatment stage designed to further purify water by removing specific contaminants not adequately addressed by primary and secondary treatments.
- It removes nutrients like nitrates ( $N O_{3}^{-}$ ) and phosphorus compounds ( $P O_{4}^{3 -}$ ), which are responsible for eutrophication (excessive algal growth) in receiving water bodies.
- Methods include advanced filtration, chemical precipitation, and biological nutrient removal.
Disinfection:
- Similar to raw water treatment, this step kills remaining bacteria and other pathogenic organisms in the treated wastewater before it is discharged into water bodies or reused.
- Methods include chlorination, ozonation, and UV treatment.
- This final step ensures the treated effluent meets regulatory standards (e.g., NEQS in Pakistan) before environmental discharge.

Summary: Raw Water vs. Wastewater Treatment

Feature	Raw Water Treatment	Wastewater Treatment
Goal	Produce potable (drinking) water	Remove pollutants before discharge
Key Steps	Screening → Coagulation → Sedimentation → Filtration → Disinfection	Primary → Secondary → Tertiary → Disinfection
Biological Stage	Not typically used	Secondary treatment (activated sludge, trickling filters)
Nutrient Removal	Not typically required	Tertiary treatment removes $N O_{3}^{-}$ , $P O_{4}^{3 -}$

14.4.1 Raw Water Treatment

Steps in Raw Water Treatment:

Screening:

This initial physical step involves the removal of large debris such as sticks, leaves, pebbles, and soil. Large screens or grates are used to prevent these materials from entering subsequent treatment stages.

Coagulation and Flocculation:

To remove finer suspended particles, coagulants are added. Common coagulants include aluminum sulfate (alum, $A l_{2} (S O_{4})_{3}$ ) or ferric sulfate ( $F e_{2} (S O_{4})_{3}$ ).
These chemicals neutralize the charges of suspended particles, causing them to clump together into larger, heavier particles called flocs.
Coagulation: The rapid mixing process where chemicals are added to destabilize particles.
Flocculation: The slower mixing process that encourages the collision and growth of flocs.

Sedimentation:

The water, now containing larger flocs, is allowed to stand in large tanks for 12 hours or more.
Due to gravity, the heavy flocs and other solid particles settle down at the bottom, forming a layer of sludge, which is then removed.

Filtration:

Following sedimentation, the water is passed through various filter media.
Common filters include layers of sand, gravel, and sometimes activated charcoal.
This step removes remaining fine particles, turbidity, and many pathogens that did not settle during sedimentation.

Killing the Pathogens (Disinfection):

To ensure the water is safe for consumption, microorganisms like bacteria, viruses, and parasites must be inactivated or killed to prevent waterborne diseases.
Common disinfection methods include:
- Chlorination: Addition of chlorine ( $C l_{2}$ ), hypochlorite ( $Cl O^{-}$ ), or chlorine dioxide ( $Cl O_{2}$ ). Chlorine reacts with water to form hypochlorous acid ( $HOCl$ ), which kills pathogens by oxidizing their cellular components. A key drawback is the formation of toxic trihalomethanes (THMs) when chlorine reacts with dissolved organic matter. Halogens→
- Ozonation: Treatment with ozone ( $O_{3}$ ), a more powerful oxidizing agent than chlorine. It does not produce harmful chlorinated by-products but provides no residual disinfection in the distribution system and is more expensive to produce.
- UV Treatment: Exposure to ultraviolet (UV) radiation, which damages the DNA/genetic material of microorganisms, preventing reproduction.

Additional processes may include:

pH Adjustment: To optimize coagulation or disinfection efficiency and ensure water is not corrosive.

Membrane Filtration: Such as microfiltration, ultrafiltration, nanofiltration, or reverse osmosis for very fine particle and contaminant removal.

Ion Exchange Treatment: For softening water (removing

C a^{2 +}

and

M g^{2 +}

ions) or removing specific ions.

Aeration: To remove dissolved gases (e.g., hydrogen sulfide

H_{2} S

, which causes odors) and oxidize certain dissolved metals (e.g., soluble

F e^{2 +}

to insoluble

F e^{3 +}

, and

M n^{2 +}

to insoluble forms) that can then be filtered out. It also strips volatile organic compounds (VOCs).

Desalination: For removing salts from saline water sources (e.g., seawater) to produce fresh water.

14.4.2 Waste Water Treatment

Stages of Wastewater Treatment:

Primary Treatment:

This physical process focuses on removing large solids and suspended particles.
It typically involves:
- Screening: Removal of large debris (similar to raw water treatment).
- Grit Removal: Allowing sand, gravel, and other dense inorganic solids to settle.
- Sedimentation: Water flows slowly through large tanks, allowing lighter organic solids to settle out as primary sludge.

Secondary Treatment:

Biological processes are employed to break down dissolved and colloidal organic matter that remains after primary treatment.
Common methods include:
- Activated Sludge: Microorganisms (activated sludge) are cultivated in aerated tanks, consuming organic pollutants. The sludge is then separated from the treated water.
- Trickling Filters: Wastewater is sprayed over a bed of media (e.g., rocks, plastic), where a biofilm of microorganisms grows and degrades organic matter.

Tertiary Treatment:

This is an advanced treatment stage designed to further purify water by removing specific contaminants not adequately addressed by primary and secondary treatments.
It removes nutrients like nitrates ( $N O_{3}^{-}$ ) and phosphorus compounds ( $P O_{4}^{3 -}$ ), which are responsible for eutrophication (excessive algal growth) in receiving water bodies.
Methods include advanced filtration, chemical precipitation, and biological nutrient removal.

Disinfection:

Similar to raw water treatment, this step kills remaining bacteria and other pathogenic organisms in the treated wastewater before it is discharged into water bodies or reused.
Methods include chlorination, ozonation, and UV treatment.
This final step ensures the treated effluent meets regulatory standards (e.g., NEQS in Pakistan) before environmental discharge.

Feature	Raw Water Treatment	Wastewater Treatment
Goal	Produce potable (drinking) water	Remove pollutants before discharge
Key Steps	Screening → Coagulation → Sedimentation → Filtration → Disinfection	Primary → Secondary → Tertiary → Disinfection
Biological Stage	Not typically used	Secondary treatment (activated sludge, trickling filters)
Nutrient Removal	Not typically required	Tertiary treatment removes $N O_{3}^{-}$ , $P O_{4}^{3 -}$