6.3 Water Treatment Process and Technologies

6.3 Water Treatment Process and Technologies

Introduction to Water Treatment

Raw water from surface or groundwater sources contains impurities that make it unsuitable for human consumption. The objective of water treatment is to remove or reduce these impurities to levels prescribed by drinking water quality standards. The treatment process is a sequence of unit operations and processes, each designed to remove specific types of contaminants. The selection and combination of these processes depend entirely on the characteristics of the raw water. This unit details the conventional treatment train and additional specialized processes to produce water that is clear, safe, palatable, and non-corrosive.

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1. Treatment Processes and Purposes

1.1 Screening

• Purpose: The first step in treating surface water. To remove large floating and suspended debris such as leaves, sticks, fish, trash, and other coarse materials that could damage pumps, clog pipes, or interfere with subsequent treatment processes.

• Process:

  • Coarse Screens (Bar Screens): Made of parallel steel bars spaced 20-50 mm apart, placed at the intake. Manually or mechanically cleaned.

  • Fine Screens: Perforated plates or mesh with openings of 5-10 mm, used in smaller plants.

• Significance: A purely physical, preliminary protective measure.

1.2 Plain Sedimentation

• Purpose: To remove suspended solids that are heavier than water (primarily sand, silt, and clay) by gravity settling, without the use of any chemicals. It reduces the load on subsequent treatment units.

• Process: Water is allowed to flow slowly through a large tank called a sedimentation tank or settling basin. The flow velocity is reduced so that particles with a specific gravity greater than 1 settle to the bottom.

• Design Parameters:

  • Detention Time: Usually 4-8 hours.

  • Surface Overflow Rate (SOR): Critical design parameter. $\text{SOR} = \frac{\text{Flow Rate (Q)}}{\text{Surface Area (A)}}$. For plain sedimentation, typical SOR is 20-40 m³/day/m².

  • Weir Loading Rate: Flow rate per unit length of outlet weir.

• Removal Efficiency: Only removes discrete, settleable particles (size > 0.1 mm). Does not remove fine colloidal particles.

1.3 Sedimentation with Coagulation

• Purpose: To remove fine colloidal particles (size 0.001 - 0.1 mm) and color that cannot be removed by plain sedimentation. These particles do not settle due to electrical charges (usually negative) that keep them in suspension.

• Process:

  1. Coagulation: Rapid mixing of a coagulant (e.g., Alum - $\text{Al}_2(\text{SO}_4)_3 \cdot 18\text{H}_2O$, Ferric chloride - $\text{FeCl}_3$) with water. The positively charged coagulant ions neutralize the negative charges on colloidal particles (charge neutralization).

  2. Flocculation: Gentle, prolonged mixing to allow the destabilized particles to collide and aggregate into larger, settleable masses called flocs.

  3. Sedimentation: The heavy flocs settle out in the sedimentation tank.

• Chemical Reactions (Alum): $\text{Al}_2(\text{SO}_4)_3 + 3\text{Ca(HCO}_3)_2 \rightarrow 2\text{Al(OH)}_3 \downarrow + 3\text{CaSO}_4 + 6\text{CO}_2$ The gelatinous, sticky aluminum hydroxide $\text{Al(OH)}_3$ precipitate enmeshes fine particles.

• Design: Sedimentation tanks here are designed with higher SOR (40-60 m³/day/m²) as the flocs settle faster.

1.4 Flocculation

• Purpose: As described above, to promote the aggregation of destabilized (coagulated) particles into visible, settleable flocs.

• Process: Achieved in a flocculation chamber with slow-speed mechanical paddles (10-30 rpm) or by baffled channels that create gentle turbulence.

• Key Parameter: Velocity Gradient (G). It measures the intensity of mixing. Flocculation requires a G value of 20-80 s⁻¹ and a detention time of 20-40 minutes.

1.5 Filtration

• Purpose: The final polishing step to remove any remaining suspended particles, floc carryover, and most pathogens (bacteria, protozoan cysts). It produces crystal-clear water.

• Process: Water is passed through a porous granular medium (usually sand). Removal mechanisms include straining, sedimentation, interception, and adsorption within the filter bed.

• Types of Filters:

  1. Rapid Gravity Sand Filter (Most Common):

     • Bed: Layers of sand (60-75 cm) supported by gravel.

     • Operation: Water flows downward by gravity. Rate: 5-7 m³/hr/m².

     • Cleaning: When head loss becomes excessive, filter is backwashed by reversing the flow (water + air scour) to wash away trapped impurities.

  2. Slow Sand Filter:

     • Bed: Finer sand, deeper bed (0.8-1.2 m).

     • Operation: Very slow rate (0.1-0.3 m³/hr/m²). A biologically active layer called the schmutzdecke forms on top, which is key to pathogen removal.

     • Cleaning: By scraping off the top few cm of sand periodically.

  3. Pressure Filter: Enclosed in a steel cylinder; used for industrial supplies or small communities.

1.6 Disinfection

• Purpose: The most critical treatment step for public health. To destroy or inactivate pathogenic microorganisms (bacteria, viruses, protozoa) to prevent waterborne diseases.

• Criteria for an ideal disinfectant: Effective against all pathogens, provide residual protection, safe, measurable, economical.

• Common Methods:

  1. Chlorination (Most Widely Used):

     • Chemical: Chlorine gas ($\text{Cl}_2$), Sodium hypochlorite ($\text{NaOCl}$ - bleach), Calcium hypochlorite ($\text{Ca(OCl)}_2$ - powder).

     • Mechanism: Chlorine hydrolyzes to form Hypochlorous acid ($\text{HOCl}$), a powerful disinfectant. $\text{Cl}_2 + \text{H}_2\text{O} \leftrightarrow \text{HOCl} + \text{HCl}$

     • Key Terms:

       • Chlorine Demand: Amount of chlorine consumed by reacting with impurities (organics, Fe, Mn, etc.) before disinfection begins.

       • Breakpoint Chlorination: Adding chlorine until the demand is satisfied, after which free chlorine residual appears.

       • Free Chlorine Residual: The concentration of $\text{HOCl}$ and $\text{OCl}^-$ remaining after contact time. Essential for continued protection in the distribution system.

     • Contact Time: Typically 30 minutes in a contact tank.

     • Dosage: Usually 1-2 mg/L to maintain a residual of 0.2-0.5 mg/L at the farthest tap.

  2. UV (Ultraviolet) Radiation:

     • Mechanism: UV light at 254 nm damages the DNA/RNA of microorganisms, preventing reproduction.

     • Advantages: No chemicals added, no taste/odor, effective against chlorine-resistant pathogens like Cryptosporidium.

     • Disadvantages: No residual disinfection power, effectiveness reduced by turbidity.

  3. Ozonation:

     • Chemical: Ozone ($\text{O}_3$), a powerful oxidant generated on-site.

     • Advantages: Very strong disinfectant, improves taste/odor, no harmful by-products like trihalomethanes (THMs).

     • Disadvantages: High cost, no residual, can produce bromate as a by-product.

  4. Boiling: A reliable household-level method.

1.7 Softening

• Purpose: To remove hardness-causing ions (Calcium $\text{Ca}^{2+}$ and Magnesium $\text{Mg}^{2+}$) from water.

• Processes:

  1. Lime-Soda Process (Chemical Precipitation):

     • Chemicals: Lime ($\text{Ca(OH)}_2$) and Soda ash ($\text{Na}_2\text{CO}_3$).

     • Reactions:

       • For Carbonate Hardness (with lime): $\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \rightarrow 2\text{CaCO}_3 \downarrow + 2\text{H}_2\text{O}$

       • For Non-Carbonate Hardness (with soda ash): $\text{CaSO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{CaCO}_3 \downarrow + \text{Na}_2\text{SO}_4$

     • Followed by sedimentation and recarbonation (to stabilize the water).

  2. Ion Exchange Process:

     • Medium: Synthetic resin beads (zeolite). Sodium ions ($\text{Na}^+$) on the resin exchange with $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ in water. $2\text{R-Na} + \text{Ca}^{2+} \rightarrow \text{R}_2\text{-Ca} + 2\text{Na}^+$

     • Regeneration: When exhausted, resin is regenerated with a concentrated brine ($\text{NaCl}$) solution.

     • Produces soft water but increases sodium content.

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2. Miscellaneous Treatments

These are additional processes applied to address specific water quality issues.

2.1 Aeration

• Purpose: To remove dissolved gases (like $\text{CO}_2$, $\text{H}_2\text{S}$ causing taste/odor) and to add oxygen (for oxidation of iron/manganese or for taste improvement).

• Process: Bringing water into intimate contact with air.

• Methods:

  • Cascade Aerators: Water flows over a series of steps.

  • Spray Aerators: Water is sprayed into the air through nozzles.

  • Diffused Air Aerators: Air is bubbled through water.

  • Cone Aerators: Water is spread in a thin film over inverted cones.

2.2 Removal of Iron and Manganese

• Problem: Causes reddish-brown/black stains, metallic taste, and promotes bacterial growth.

• Principle: Iron and manganese are soluble in their reduced forms ($\text{Fe}^{2+}$, $\text{Mn}^{2+}$) but insoluble when oxidized ($\text{Fe}^{3+}$, $\text{Mn}^{4+}$).

• Process:

  1. Aeration/Oxidation: To convert soluble forms to insoluble oxides/hydroxides. $4\text{Fe(HCO}_3)_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{Fe(OH)}_3 \downarrow + 8\text{CO}_2$

  2. Sedimentation & Filtration: To remove the precipitated oxides.

2.3 Removal of Color, Odor, and Taste

• Sources: Natural organic matter (humic/fulvic acids from soil decay), algae, industrial waste, chlorine by-products.

• Treatment Methods:

  1. Activated Carbon Adsorption (Most Effective):

     • Mechanism: Granular Activated Carbon (GAC) or Powdered Activated Carbon (PAC) has a vast surface area that adsorbs organic compounds causing taste, odor, and color.

     • Application: PAC can be added during coagulation. GAC is used as a filter medium or in separate contactors.

  2. Oxidation:

     • Aeration: For volatile compounds (e.g., $\text{H}_2\text{S}$).

     • Pre-chlorination: But can form THMs.

     • Ozonation: Very effective for taste and odor removal.

     • Potassium Permanganate ($\text{KMnO}_4$) Dosing: Oxidizes specific odor-causing compounds.

  3. Coagulation-Sedimentation: Removes color-causing organic colloids.

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Summary of the Conventional Water Treatment Plant (WTP) Flow Diagram

Raw Water Intake → Pump House → Screening → [Optional: Pre-chlorination/Aeration] ↓ Mixing Chamber (Rapid Mix - Coagulant added) ↓ Flocculation Chamber (Slow Mix) ↓ Sedimentation Tank (Clarifier) ↓ Rapid Gravity Sand Filters ↓ Disinfection (Chlorine Contact Tank) ↓ Clear Water Reservoir ↓ Pumping to Distribution System

Conclusion: Water treatment is a multi-barrier approach where each process targets specific contaminants. The conventional "coagulation, flocculation, sedimentation, filtration, and disinfection" train is designed to handle typical surface water. Understanding the purpose and mechanism of each unit operation allows engineers to select, design, and operate treatment systems that reliably transform raw water into a safe, palatable public health commodity.