Lab 4: Determination of Optimum Dose of Coagulant by Jar Test Apparatus

• Jar Test Apparatus (6 paddles with variable speed control)

• Six 1000 mL glass beakers

• Pipettes (1 mL, 2 mL, 5 mL)

• Nephelometer (for turbidity measurement)

• pH meter

• Magnetic stirrers (if required)

• Measuring cylinders

• Stopwatch

• Aluminum Sulfate (Alum) Solution

• Distilled water

• Water sample

Coagulation and Flocculation in Water Treatment

Coagulation is the chemical process of destabilizing suspended colloidal particles in water by neutralizing their surface charges using a coagulant. These particles, due to their similar negative charges, repel each other and remain suspended. Coagulants neutralize these charges, allowing particles to come closer.

Flocculation follows coagulation. It is a gentle mixing process that encourages the destabilized particles to collide and aggregate into larger, settleable particles known as flocs.

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Types of Coagulants

1. Aluminum-based:

Aluminum Sulfate (Alum) – $\text{Al}_2(\text{SO}_4)_3\cdot18\text{H}_2\text{O}$

Polyaluminum Chloride (PAC)

2. Iron-based:

Ferric Chloride ($\text{FeCl}_3$)

Ferric Sulfate ($\text{Fe}_2(\text{SO}_4)_3$)

3. Natural Coagulants:

Moringa oleifera seed extract

Chitosan (from shellfish waste)

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Alum and Its Role in Coagulation

Aluminum sulfate ($\text{Al}_2(\text{SO}_4)_3\cdot18\text{H}_2\text{O}$), commonly known as alum, is the most widely used coagulant in water treatment. It is effective in destabilizing colloids and forming flocs that can easily settle. For proper floc formation, the raw water must contain sufficient alkalinity (bicarbonates).

Key Chemical Reactions

When alum is added to water, it dissociates into $\text{Al}^{3+}$ and $\text{SO}_4^{2-}$ ions:

Dissociation in Water:

\[ \text{Al}_2(\text{SO}_4)_3 \cdot 18\text{H}_2\text{O} \rightarrow 2\text{Al}^{3+} + 3\text{SO}_4^{2-} + 18\text{H}_2\text{O} \]

In the presence of calcium bicarbonate (natural alkalinity), alum reacts to form insoluble aluminum hydroxide, which forms flocs:

\[ \text{Al}_2(\text{SO}_4)_3 + 3\text{Ca(HCO}_3)_2 \rightarrow 2\text{Al(OH)}_3 \downarrow + 3\text{CaSO}_4 + 6\text{CO}_2 \]

If alkalinity is low, it can be supplemented by lime or soda ash:

With Lime ($\text{Ca(OH)}_2$):

\[ \text{Al}_2(\text{SO}_4)_3 + 3\text{Ca(OH)}_2 \rightarrow 2\text{Al(OH)}_3 \downarrow + 3\text{CaSO}_4 \]

With Soda Ash ($\text{Na}_2\text{CO}_3$):

\[ \text{Al}_2(\text{SO}_4)_3 + 3\text{Na}_2\text{CO}_3 + 3\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 \downarrow + 3\text{Na}_2\text{SO}_4 + 3\text{CO}_2 \]

These reactions demonstrate the importance of sufficient alkalinity for optimal coagulation performance. The optimal pH range for alum coagulation is 6.5 to 8.5.

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Principle of Coagulation

Alum dissociates in water to form $\text{Al}^{3+}$ and $\text{SO}_4^{2-}$ ions. The $\text{Al}^{3+}$ ions neutralize the negatively charged colloidal particles in the water, allowing them to clump together and form microflocs, which further agglomerate into larger, settleable flocs.

Principle of the Jar Test

The Jar Test is a laboratory procedure used to estimate the minimum effective coagulant dose. It replicates the coagulation and flocculation steps under controlled conditions:

1. Rapid Mixing: (~100 rpm for 1-2 minutes) – Ensures uniform distribution of coagulant.

2. Slow Mixing: (~30-40 rpm for 20-30 minutes) – Promotes collision and aggregation of destabilized particles into flocs.

3. Settling: (~20-30 minutes) – Allows flocs to settle at the bottom, leaving clarified water above.

1. Each of the six beakers was filled with 500 mL of water sample.

2. The turbidity of the given water sample was measured.

3. All jars (beakers) were placed on the floc tester and magnetic stirrer bars were placed in each jar.

4. The prescribed dose of coagulant was added to each jar using a pipette. One jar was kept without coagulant as a control sample.

5. The sample was stirred rapidly for 2 minutes at 100 rpm.

6. After the rapid mixing stage, the speed was reduced to 30 rpm for 20 minutes, and then the stirrers were removed.

7. The mixture was allowed to stand for 20 minutes to allow settling.

8. The turbidity of the clarified water was determined.

9. The best dose of coagulant was found.

Each beaker contained 500 mL of sample water:

1% alum solution = 10 mg of alum per mL

Sample volume per beaker = 500 mL

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Conversion Formula:

\[ \text{Alum dose (mg/L)} = \frac{\text{Alum in mg}}{\text{Volume in L}} = \frac{\text{Dose in mL} \times 10}{0.5} \]

\[ \text{Alum dose (mg/L)} = \text{Dose in mL} \times 20 \]

The results demonstrate that at 30 mg/L of alum, maximum turbidity removal was achieved, reducing turbidity from 114 NTU (in untreated water) to just 3 NTU. This is well within the WHO guideline of <5 NTU for safe drinking water and meets the DWSS standard range of 1-5 NTU.

Turbidity reduction at the optimum dose indicates successful charge neutralization and formation of settleable flocs. However, when doses exceeded the optimum (e.g., above 30 mg/L), turbidity began to increase again, likely due to charge reversal, colloid restabilization, or floc disintegration. This behavior emphasizes the importance of dose optimization.

From a treatment plant perspective, 30 mg/L is both effective and economical. Higher doses would:

• Increase chemical consumption and operating costs

• Require larger storage and handling capacity

• Produce more sludge, complicating disposal

• Risk elevated residual aluminum in treated water, which can affect health and filter performance

Accurate dosing based on jar test results allows for efficient, cost-effective, and compliant treatment, ensuring the water is safe and clear without unnecessary chemical use.

The jar test experiment identified 30 mg/L as the optimum alum dose, reducing turbidity from 114 NTU to 3 NTU, well within WHO and DWSS safe drinking water standards. This dose is both effective and safe, ensuring low turbidity and acceptable residual aluminum levels. To prevent overdosing, treatment plants should implement control strategies such as online turbidity monitoring, automated dosing systems, and regular jar testing. These measures help maintain precise coagulant dosing, ensuring efficient treatment, reduced chemical waste, lower sludge production, and consistent water quality.

• Ensure accurate measurement of alum doses using calibrated pipettes or syringes.

• Calibrate the pH meter and nephelometer before use for reliable readings.

• Use clean and dry glassware to avoid contamination and ensure consistent results.

• Maintain uniform mixing speeds during rapid and slow mixing phases.

• Avoid disturbing the settled flocs while collecting supernatant for turbidity testing.

• Adjust and monitor pH (ideally between 6.5 and 8.5) for optimal coagulation.

• Prepare fresh alum solution to maintain its effectiveness.

• Wear safety gear (gloves, goggles) when handling chemicals.