4.3 Jar Testing & Process Adjustment Decisions

For water treatment operators, jar testing is the primary tool used to optimize the chemical processes of coagulation and flocculation. Raw water quality is constantly changing due to weather events, seasonal temperature shifts, and algae blooms. Because operators cannot risk experimenting on the full-scale treatment plant, they use laboratory-scale simulations to determine the most effective and economical chemical dosages. On the certification exam, you must understand how to perform a jar test, interpret the results, and make process adjustment decisions.

Coagulation Chemistry and the Zeta Potential

To understand jar testing, you must understand coagulation chemistry. Most suspended impurities in surface water, such as clay, silt, and organic matter, carry a negative electrical charge. These negative charges repel each other, preventing the particles from clumping together and settling out. This repelling force is measured as the zeta potential.

To overcome this repulsion, operators add a chemical coagulant, typically alum (aluminum sulfate). The coagulant dissolves to release highly charged positive ions, which neutralize the negative charges on the suspended particles. This neutralization process is called coagulation. Once the charges are neutralized, the particles can collide and stick together to form larger, settleable particles, a process called flocculation. Sometimes, a synthetic polymer is added as a coagulant aid to bridge the particles and form stronger, heavier floc.

The Jar Testing Procedure

A jar testing apparatus typically consists of a gang-stirrer with four to six paddles, which stir liquid in matching 1-liter or 2-liter beakers. The process simulates the rapid mix, flocculation, and sedimentation basins of the treatment plant:

1. Flash Mix (Rapid Mix): Fill the jars with raw water. Start the paddles at a high speed, typically 100 to 150 RPM, to simulate the flash mix basin. Immediately add different dosages of coagulant to each jar, leaving one jar as a control or baseline. Run the rapid mix for 1 to 2 minutes to ensure the chemicals are thoroughly dispersed and contact the suspended particles immediately.
2. Flocculation (Slow Mix): Reduce the paddle speed to 20 to 40 RPM and stir for 15 to 20 minutes. This slow mix simulates the flocculation basin, providing gentle agitation that promotes particle collisions, allowing them to grow into visible floc. If paddle speeds are too high, the shear forces will break the fragile floc apart; if too low, the particles will settle prematurely in the jar.
3. Sedimentation (Settling): Turn off the stirrer completely. Allow the water to remain still for 30 to 45 minutes, simulating the sedimentation basin. During this period, the floc particles settle to the bottom of the jar.

Observing and Evaluating Jar Tests

During and after the test, the operator must make several key observations:

• Floc Formation Time: Note how quickly visible floc begins to form during the slow mix. Rapid formation indicates an effective dose.
• Floc Size and Appearance: Evaluate the size of the floc. A good test yields large, distinct particles resembling snowflake-like structures. A poor test may produce pin floc (very small, pinpoint-sized particles that do not settle well).
• Settling Rate: Observe how fast the floc settles when the stirbars stop. Fast settling indicates heavy, dense floc.
• Supernatant Clarity: Examine the supernatant (the clear water above the settled sludge). A successful test yields a crystal-clear supernatant. A hazy or cloudy supernatant indicates incomplete coagulation.
• Turbidity Measurement: Collect a sample of the supernatant from each jar and measure its turbidity using a turbidimeter. The jar that achieves the lowest turbidity with the lowest chemical dose represents the optimal dosage.

Process Adjustments: Dose and pH Control

Based on jar test results, operators make critical dosage adjustment decisions:

• Underdose: If the supernatant turbidity is high and the floc is small or slow to form, the coagulant dose is too low. The operator must increase the coagulant dosage at the plant.
• Overdose: If the operator adds too much coagulant, the excess positive charges will coat the particles and reverse their charge to positive. This overdose causes the particles to repel each other again (charge reversal), leading to poor flocculation, a hazy supernatant, and a waste of treatment chemicals. The operator must decrease the coagulant dosage.

Operators must also monitor pH and alkalinity. Alum coagulation is highly pH-dependent, with an optimal pH range of 5.5 to 6.5. Furthermore, alum is an acidic chemical that consumes alkalinity in the water. For every 1 mg/L of alum added, approximately 0.5 mg/L of alkalinity (measured as calcium carbonate, CaCO3) is consumed. If the raw water has low alkalinity (typically below 30 mg/L), the pH will drop sharply. This drop in pH prevents the alum from reacting, resulting in poor floc formation and dissolved alum passing through the filters, a condition known as alum carryover. To prevent this, operators must add an alkaline chemical such as lime, soda ash, or caustic soda to maintain sufficient alkalinity and stabilize pH.

Table 4.3: Jar Test Troubleshooting and Actions