Online Zeta Potential Measurement Provides Water Treatment Control, Cost Reduction

Online zeta potential measurements can provide real-time water quality monitoring and support effective process control under all circumstances. The value of online measurement is illustrated through the experiences of Aurora Water, which is using zeta potential at one facility as both an offline and online tool for monitoring and controlling water treatment processes.

Oct 23rd, 2014
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By Bill Fulbright, Alon Vaisman, Hart Krumrine, Kirk Watson, and Ana Morfesis

Converting raw water into a potable liquid is a complex process. Water purification facilities typically use a variety of chemical and physical treatments to remove organic, inorganic and bacterial contaminants in order to purify water to the point where it is safe for human use.

Online zeta potential measurement is helping Aurora Water to optimize water treatment processes.

Control of a water treatment plant (WTP) can be difficult because contamination of the water supply can originate from many different sources. Both particulate and microbiological contaminants are a concern, and although surface water is particularly at risk, even deep wells can be contaminated by a variety of sources. Further, water sources are at risk from bacteria, heavy rainfall, chemical and reused water contamination, and weather changes such as spring snow melts, natural disasters and even forest fires.

Both the chemistry and microbiology of the raw water entering the WTP can exhibit marked variability as a result of these types of external influences, which then require rapid changes to the coagulation dosage in the process. Many facilities find it challenging to respond rapidly to sudden fluctuations in the raw water composition, and this often results in over treatment and excessive use of costly chemical additives.

This article considers the use of online zeta potential measurements to provide real-time water quality monitoring and to support effective process control under all circumstances. The value of online measurement is illustrated through the experiences of Aurora Water, which is using zeta potential at one facility as both an offline and online tool for monitoring and controlling water treatment processes.

Controlling Flocculation

Traditional water treatment processes begin with a pretreatment where chlorine dioxide is added to the water as a pre-oxidant and activated carbon may be used to control odor and taste. In the next stage, coagulants are added to a rapid mix tank so that organisms, bacteria and unwanted particulates can be bound into large agglomerates. In the following stage, sedimentation can be used to separate the agglomerated contaminants from the water.

Sedimentation relies on gravity to separate the suspended particles. The incoming raw water contains naturally- suspended material that is usually small (< 1000 ) and therefore will tend to separate slowly. Sedimentation can therefore be a lengthy process. By encouraging flocculation, the rate of sedimentation can be substantially increased, speeding up the process and improving efficiencies.

A faster and more efficient process is a direct filtration process, which is used in Aurora Water Treatment Facilities. Using a direct filtration process means that the water passes through the rapid mix where coagulants are added, and the water is then processed directly through a filtration system; there are no sedimentation basins in this type of process. The filtration process is now the primary separation method, and the coagulation and flocculation step must be very well controlled for this process to be effective.

As most water impurities tend to carry an anionic charge, dosing with positively-charged additives, such as aluminum sulfate, encourages flocculation. The resulting flocculent (floc) can then either settle, after which any residual suspended material is transferred directly to the filtration system (traditional water treatment), or it is directly sent to the filtration system (direct filtration water treatment). Controlling the nature of the floc is important to ensure successful sedimentation and/or effective downstream processing through filters.

For instance, an insufficiently aggregated floc may break up, resulting in incomplete sedimentation and carryover to the filter, or it may break up in the filter. Retaining control of the initial coagulation and flocculation process in the face of variable raw water quality is therefore crucial for optimizing treatment and chemical usage, as well as ensuring water safety.

Several manual techniques are routinely used to study and monitor flocculation. Specifically jar tests and turbidity are routinely used in WTPs. Jar tests are used to simplistically simulate the traditional flocculation process. The measurement involves collecting a raw water sample from the process, dosing it with a known concentration of coagulant, and then assessing floc formation under standard conditions. Turbidity measurements on the other hand, quantify the amount of cloudiness in the water and are typically made using bench top turbidity meters.

Both jar and turbidity methods require water to be sampled manually before and after each process change. This is both time consuming and introduces operator and operator-to-operator variability within the analysis. In the case of the jar tests, results are obtained after two hours. Moreover, this time delay means that the results generated may not give an accurate representation of the process conditions at the time of treatment. Real-time, automated analysis is therefore an attractive strategy to free operator capacity and exert greater control over process conditions.

Introducing Zeta Potential

A number of WTPs now use laboratory-based zeta potential measurements to monitor and predict flocculation.1, 2 Zeta potential is a measure of the magnitude of electrostatic or charge repulsion at the boundary layer surrounding a particle and is one of the fundamental parameters known to affect a system's stability.

Zeta potential is measured using the technique of electrophoretic light scattering (ELS), which correlates directly with coagulation performance (see Fig. 1). An electric field is applied across a sample, and particles migrate towards the electrode of opposite charge at a velocity proportional to the magnitude of their zeta potential. The velocity at which the particle moves is measured using the technique of laser Doppler anemometry. The frequency shift, or phase shift, of an incident laser beam caused by the moving particles is measured to determine particle mobility. This is converted into zeta potential values, using the known viscosity of the dispersant and applying Smoluchowski or Huckle theories.3

Fig. 1. Zeta potential is a parameter that indicates particle charge and can be used to assess the tendency of a system to coagulate.

Zeta potential is an innate characteristic of colloidal particles and is easily measured by advanced analytical systems. Unlike jar or turbidity tests, zeta potential measurement is objective and less vulnerable to the variability associated with conventional techniques. Further, zeta potential measurements are made in 30 seconds to 1 minute. Monitoring the zeta potential of water in treatment streams provides more time-relevant information on plant conditions. This allows more confident calculation of the coagulant concentration required to maintain optimal flocculation conditions. Online zeta potential analysis offers further improvements in terms of rapid, real-time data acquisition and provides a route to reducing the reliance on manual analysis.

Moving Zeta Potential Online

The desire for real-time water monitoring has driven the development of several online analysis systems within the industry with varying degrees of success. Early attempts at online analysis centered on streaming current detectors (SCD). These detectors work by immobilizing a charged particle and measuring the electric current produced as charged particles in the water flow past it -- the streaming current. Theoretically, this method should provide reliable information to help regulate and control the coagulation process, and streaming current should correlate to some extent with zeta potential.

However, in practice, many operators have found these systems to be inaccurate and unreliable. Many variables can affect the SCD charge measurement4 and lead to erratic readings, particularly at high flocculent concentration.5 As a result, many WTPs consider SCD too subjective to rely upon for process control.6 Online zeta potential measurement technology provides an alternative solution to the problem of automated flocculation monitoring. Initial trials conducted with the Zetasizer Nano from Malvern Instruments, for example, demonstrate the feasibility of this approach.

Case Study: Aurora Water Improves Efficiency of Water Flocculation Processes

Aurora Water provides water to approximately 340,000 people in the metropolitan Denver-Aurora, Colo., area. Since early 2012, Aurora Water has been using an online zeta potential measurement system (Zetasizer WT from Malvern Instruments) in one of its WTPs. Figure 2 illustrates how the analyzer is integrated directly into the process line.

Figure 2: Direct Filtration Schematic

Extensive experience with lab-based zeta potential systems has shown that the flocculation process proceeds optimally if zeta potential is maintained within the +3 mV to -3 mV range. If the zeta potential becomes too negative, the flocculent dosage is increased; if the zeta potential rises too far above zero, the chemical additive concentration is reduced. A lab-based Zetasizer Nano is also employed within the facility to conduct measurements to validate the online system results.

Figure 3: Online Zeta Potential Measurements Following PLC Shutdown

In February 2013, the plant suffered a major fault on one of its programmable logic controllers (PLCs). Once the problem had been fixed, the plant was reset and operation continued. However, once it was switched back on, a significant rise in zeta potential was recorded. This was verified using lab-based zeta potential analysis. Further evidence of a developing problem was an increase in suspended particles, which had not sedimented effectively. As such, insufficient sedimentation resulted in contaminant breakthrough and led to the filters being taken offline.

As this change could not be attributed to physical or chemical variations within the water, it was assumed that the recorded incoming water flow rate was lower than the volume actually entering the plant. This would result in the ratio of chemicals added to the water being higher than required and would account for the positive zeta potential reading. In order to rectify this, the amount of coagulant was lowered. This appeared to move the zeta potential in the correct direction, and further reductions in coagulant addition were made (see Fig. 3).

Overall, coagulant addition was reduced by around 17 percent, bringing zeta potential back down to the required levels. Meanwhile, effluent particle concentrations were also reduced, turbidity readings were improved and filter run times became longer. The combination of all these results confirmed that the plant was moving back toward an efficient operating regime.

However, further investigation revealed that the water flow rate had not changed significantly following shutdown. Troubleshooting activities revealed that the rapid mixer designed to encourage flocculation had not restarted following the reboot. Once the rapid mixer was switched back on, a strong negative shift in zeta potential was immediately observed (see Fig. 4). This suggested that the rapid mixer was having a detrimental effect on the process -- a finding supported by evidence in published literature.7, 8 The decision was made to leave the rapid mixer off, resulting in significant energy savings, adding to the chemical savings already reported.

Figure 4: Zeta Potential Measurements Following Reset of Rapid Mixer

The availability of real-time information during this brief investigation was highly valuable in monitoring the fluctuations in zeta potential. If the plant had been reliant on less timely analysis, then downstream issues would have been the main indicators of a problem, which would have greatly delayed the plant operator's ability to rectify the situation. Having online measurement allowed the operators to make swift and effective alterations to the plant and rapidly assess their impact without upsetting plant performance. The high measurement frequency and instantaneous feedback of the automated Zetasizer WT system offers advanced plant control, delivering reduced manual labor costs and enhanced chemical dosing.

Fulfilling the Online Potential

While benchtop zeta potential measurements are already widely used for water treatment analysis, the possibility of online analysis has real benefit for those looking to take a more efficient approach. Experience at Aurora Water shows that incorporating an online Zeta potential analyzer -- directly linking its output to plant SCADA -- helps plant operators make confident decisions and maintain stable operational conditions. Use of zeta potential as a laboratory method of coagulation control -- and the later addition of automatic, online analysis capability -- enabled the plant to optimize operations, reduce chemical costs by 15 to 20 percent and bring automated coagulation control one step closer to reality.

About the Authors: Bill Fulbright is a plant operator at Aurora Water; Hart Krumrine is the chief plant operator at Aurora Water; Kirk Watson is the water treatment superintendent at Aurora Water; Alon Vaisman is the product development manager – Process Systems for Malvern Instruments; and Ana Morfesis is a technical/scientific advisor for Malvern Instruments.

References

1. E. L.Sharp, et al. "The Application of Zeta Potential Measurements for Coagulation Control: Pilot Plant Experiences from UK and US Waters with Elevated Organics," Water Sci. Technol., Water Supply 2005, 5 (5), 49-56.
2. A. Morfesis, et al. "Role of Zeta (ζ) Potential in the Optimization of Water Treatment Facility Operations," Ind. Eng. Chem. Res., 2009, 48 (5), pp 2305-2308.
3. Zeta potential in 30 minutes - Malvern Instruments technical note.
4. S. Dentel, K. Kingery. "AWWA research foundation report: Evaluation of Streaming Current Detectors," p. 25.
5. S. Dentel, A. Thomas, K. Kingery. "Evaluation of the streaming current detectors II: Continuous flow tests," Water Research, April 1989, 23 (4), pp 423-430.
6. A. Adgar, C. Cox, C. Jones. "Enhancement of coagulation control using the streaming current detector," Bioprocess and Biosystems Engineering, August 2005, 27 (5), pp. 349-357.7. A. Amirtharajah, K. M. Mills. "Rapid-mix design for mechanisms of alum coagulation," Journal AWWA, April 1982, pp 210-216.
8. James K. Ezwald. "Coagulant mixing revisited: theory and practice," Journal of Water Supply: Research and Technology-AQUA April 2013, 62 (2), pp 67-77.

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