Relating zeta potential to jar testing

Waterplant.jpg Jar testing simulates the coagulation / flocculation process in a water treatment plant. In a set of beakers, a defined amount of raw plant water is mixed with varying amounts of coagulant. Each beaker receives an increasing incremental dose of coagulant and is then stirred for a set amount of time to mimic plant operation. The formation of floc during this test simulates plant operation. Observations from the floc formation indicate a preferred range of coagulant dosage. Thus, jar testing can help identify the parameters that promote an appropriate final water quality.

While this jar test is simple to perform, it does require time, typically about 2-3 hours, including preparation and wait for settlement. Since water quality can change seasonally (summer algae growth, flood events, forest fires) the jar test has to be repeated regularly.

A significantly faster method is the measurement of zeta potential expressed in milli Volt (mV). Here, the water is exposed to an electric and scientifically supported field, and the movement of colloidal particles indicates their net charge under the specific water conditions. We present details of the zeta potential method and compare results with jar testing. At the example of alum (Aluminium Sulfate, Al2(SO4)3) as coagulant, we show that the optimal dose is related to a favorable zeta potential range of -8mV to +3mV. ​

Not shown here, the use of ferric (Iron Chloride or Iron Sulfate) typically correlates with the same zeta potential range to optimize water treatment plant operation.

Each coagulant chemistry has an optimal pH range where it is most effective, so it is also important to monitor pH in the plant and maintain the similar pH during the jar testing.

James Hogan*, Ana Morfesis**, Ulf Nobbmann**​
*City of Sacramento, Department of Utilities, 301 Water Street, Sacramento, CA 95811​
** Malvern Panalytical, 117 Flanders Road, Westborough, MA 01581

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Abstract

Jar testing simulates the coagulation / flocculation process in a water treatment plant. In a set of beakers, a defined amount of raw plant water is mixed with varying amounts of coagulant. Each beaker receives an increasing incremental dose of coagulant and is then stirred for a set amount of time to mimic plant operation. The formation of floc during this test simulates plant operation. Observations from the floc formation indicate a preferred range of coagulant dosage. Thus, jar testing can help identify the parameters that promote an appropriate final water quality.

While this jar test is simple to perform, it does require time, typically about 2-3 hours, including preparation and wait for settlement. Since water quality can change seasonally (summer algae growth, flood events, forest fires) the jar test has to be repeated regularly.

A significantly faster method is the measurement of zeta potential expressed in milli Volt (mV). Here, the water is exposed to an electric and scientifically supported field, and the movement of colloidal particles indicates their net charge under the specific water conditions. We present details of the zeta potential method and compare results with jar testing. At the example of alum (Aluminium Sulfate, Al2(SO4)3) as coagulant, we show that the optimal dose is related to a favorable zeta potential range of -8mV to +3mV. 

Not shown here, the use of ferric (Iron Chloride or Iron Sulfate) typically correlates with the same zeta potential range to optimize water treatment plant operation.

Each coagulant chemistry has an optimal pH range where it is most effective, so it is also important to monitor pH in the plant and maintain the similar pH during the jar testing.

Objective

In a water treatment plant, raw water is treated with a coagulant or flocculant. This added component leads to sedimentation of the floc. For optimization of this process, a minimal amount of flocculant should be added for optimal clarification of the water. Coagulant dosing determination has traditionally been based on the jar testing procedure. We compare zeta potential results to the traditional testing method.

Jar testing

Several beakers or jars are filled with water, and varying amounts of coagulant is added. A defined routine of stirring and waiting follows to imitate full-scale plant operation [1,2]. The floc is visually observed by a trained operator to determine the optimal condition. Jar testing is like a mini pilot plant of the water treatment facility.

Jartesting 800.jpg
Figure 1: Jar testing at 6 different conditions. From left to right: 10 ppm alum to 60 ppm alum​

Zeta potential

Water is exposed to an electric field. Charged particles in the water will move in this field, according to the sign and strength of their charge, also called the zeta potential. The actual signal is detected via phase analysis light scattering.

The technique works for particles up to 100microns – and down to a few nanometers. This makes it suitable for early usage in water treatment plants, before visible floc has formed.

Formation of particles due to aggregation and flocculation is enhanced when the net charge of the particles is close to zero. [3]​

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Method and discussion

Jar testing was performed using a six-paddle stirrer jar test analyzer in 2-liter B-KER jars (Phipps & Bird). Raw Sacramento River water was pumped from the Intake structure at the Sacramento River Water Treatment Plant, chlorinated, and then aliquoted into each jar. Several microliters of neat or undiluted alum (Aluminum Sulfate Al2(SO4)3) was pipetted onto PTFE septa, then set on top of each jar, and dropped into solution at the same time. After, each jar was flash-mixed at a rate of 300 RPM for 20 seconds, followed by immediate sampling of approximately 10 mL of solution from the top of each jar with syringes.

Zeta potential measurements were performed using the Zetasizer Nano ZS (Malvern Instruments). The final alum concentrations were 10, 20, 30, 40, 50, and 60 ppm in each of the 6 jars, respectively.

The samples in the jars were stirred for a total of 45 minutes (40 RPM for 15 minutes and 20 RPM for 30 minutes). After the stirring had stopped, flocculation was allowed to form for 10 minutes. We then compared the observed floc to an internal flocculation guide, and took settled water turbidity to determine the jar with the lowest turbidity after settling.

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Figure 2: Zeta potential as a function of added alum. The optimal condition at 30ppm is near the iso electric point.

Results

At an optimal alum dose of 30 ppm we find that the appearance of the floc was also optimal for settling. It was determined that the jar with 30 ppm alum dose also had a zeta potential close to 0 mV and corresponded with the lowest settled water turbidity.
  
The zeta potential was -20 mV for the raw water (typically negative due to presence of organic matter particles) and increased with higher alum concentration. For the 60ppm alum water, the zeta potential had increased to +8 mV.

The optimal zeta potential window for alum is between -8 to +3mV [3]. This coincides with the 30ppm condition in this jar test example.

Summary

Varying raw water quality requires optimization of water treatment plant operation. The traditional jar test mimics the plant treatment process, yet requires time for stirring, resting, sedimentation and a trained operator with the skill and experience to find the optimal coagulant dosage. Zeta potential is a simple physical technique based on light scattering. It assesses the net charge of particles, and close to neutral charge the formation of floc is favored.

With zeta potential the amount of optimal coagulant may be determined in less time and with less effort than jar tests. This methodology can also be incorporated into the plant optimization process as a fully automated, on-line solution to enable in-process plant optimization and coagulant dose control. This system is known as Malvern Panalytical's Zetasizer WT.

References

[1] “Jar Testing: Getting Started on a Low Budget” D Pask. On Tap: National Environmental Service Center. Volume 2 Issue 2, 4-6 (1993)
[2] “Jar Testing” Z Satterfield, Tech Brief - National Environmental Services Center. Volume 5, Issue 1 (2005)
[3] “The Role of Zeta Potential in the Optimization of Water Treatment” U Nobbmann, A Morfesis, J Billica, K Gertig. NSTI Nanotech. Volume 3, 605-607 (2010)

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