Why is Coagulant Dose control important?

Turbidity and NOM in the source water feeding a drinking water purification plant, particularly a surface water source, fluctuates over time. This continual change may require adjustment to the coagulant dose, pH, or polymer addition to enable the flocculation process to work efficiently. Careful management of the coagulant dose with accurate and timely monitoring can impact the overall plant performance. Monitoring this based on zeta potential, a scientific measurement of the electrical potential charge between particles provides reliable and relevant data. Particle charge neutralization is needed to enable a strong floc to form and allow the entire process to run smoothly.   

Here are the top 7 reasons to consider laboratory and/or on-line zeta potential systems for coagulation dose control

1. Zeta Potential is a scientifically proven technique

Zeta Potential measures the charge of the particles that need to be filtered. Large particles will tend to settle on their own, so it is the small particles that are in stable suspension and require some assistance to flocculate for filtration to be effective. Zeta potential is a first principle measurement technique (no calibration) of the particle charge, reported in mV. Raw water particles are typically in a zeta potential range of -15mV to -25mV. A good floc can form in the Zeta potential range of -8mV to +3mV. Adding coagulants (which are positively charged) neutralizes the repulsive (negative) charge of the particles in the raw water and enables a strong floc to form.

2. Zeta Potential can provide reliable quantitative data to plant operators for coagulant dosing

While there are many types of sensors and meters available, for both lab and online monitoring, the need for quick quantitative results to enable chemical dose determination is critical. Zeta potential can be used to monitor the incoming raw water just after coagulant addition and rapid mix, and provide the information needed to adjust the coagulant dose accordingly. Driving the dose to a range of zeta potential that works best for the particular plant and chemical use, gives operators a target based on quantitative data.

3. Zeta Potential outperforms other devices typically used to monitor the pre-treatment process:

Many operators use zeta potential to confirm the dosage is correct by analyzing the particle charge itself, rather than relying on other techniques like jar testing, turbidity or streaming current. These techniques are not able to effectively measure the charge of the particles and it is this charge that is the most important factor in building a filterable floc.

  • Streaming Current – Streaming current devices tend to drift and often require re-calibration and are not sensitive to raw water quality changes that may impact the dose required.
  • Turbidity Meter – this is not a measurement of charge and cannot effectively offer a guide for dose control.
  • Jar Testing – again, not a charge measurement, but also very subjective based on user, requires time for sampling, testing, and reporting.
  • Zeta Meter – this is a device that attempts to measure zeta potential manually, requiring the operator to record time movements via a microscope. Very difficult to attain repeatable results, time-consuming and very subjective.

4. The ROI from a lab Zetasizer or the online Zetasizer WT can be significant.

Flashy raw water, multiple raw water sources, storm events, and seasonal effects all contribute to the need for a reliable and fast monitoring system to provide scientific data to the operators so the dosage can be correctly adjusted. Environmental impacts, such as mudslides and wildfires can be overwhelming to operators that are fighting to keep the plant operational. Blindly overdosing coagulant is wasting time and at a huge cost, which municipal budgets can no longer absorb. When employing zeta potential monitoring, typical savings are documented to be about 15%-25% (lab/online). Considering the annual cost of coagulant chemicals used in a typical plant, the savings can have a valuable impact on the plants operating budget. Reported ROI for an online Zetasizer WT is under 12 months.

Another savings is in the generation of sludge, which can be reduced with better coagulant dose control and can add savings of about 10% of disposal costs, contributing to the value of using zeta potential as a charge monitoring solution. It is also evident that filters, in particular membrane filters, require maintenance over time and are affected by the amount of chemicals used in the pre-treatment process. Managing this with predictive analysis can greatly extend filter performance and overall life

5. Continuous measurement 24/7 is needed

The online Zetasizer WT generates a full data report, the result of three tests averaged together, once every few minutes. This equates to over 400 measurements per day, which is then part of the trend evaluation overtime to ensure the optimum dose is being used.

6. Zeta potential delivers additional value-added benefits

The Zetasizer Nano-Z (integrated bench unit in the online Zetasizer WT) produces consistent and reliable results and can be verified with a monthly standards test. It eliminates the need for subjective testing (jar test) to determine coagulant dose. With integrated controls, the online Zetasizer WT functions automatically and continuously, and data is exported directly into the SCADA control system. This real-time reporting can provide the data needed to determine the right dose at any given time, and against any changing raw water conditions (including torrential rains in northern California or a hurricane in Texas).

Coagulation chemicals are also pH dependent, and advanced dosage control schemes using pH, temperature, turbidity and zeta potential can improve precipitation. Using such automated multi-variate control software can take advantage of this technology and drive towards a true IIoT solution, with even greater impact on chemical use, dose control and costs.

7. On-line Zetasizer WT data is integrated with the plant control SCADA system

With connection to the plant SCADA system via Modbus TCP/IP, the operations team understands the data they need in real-time, updated every few minutes. These systems can enable true process optimization supporting today’s Industry 4.0 initiatives.  

Bonus: The Zetasizer and online Zetasizer WT are instruments from Malvern Panalytical

For more than 60 years Malvern Panalytical has been a trusted supplier to scientists and engineers in a wide range of industries and organizations to solve the challenges associated with maximizing productivity, developing better quality products, and getting them to market faster. Within Malvern Panalytical’s portfolio of technologies, the Zetasizer is a world-class instrument supported by years of academic and industrial use, with a huge following and thousands of application-driven publications.  Its use in the water treatment market has been ongoing for over 15 years. Our technical expertise on zeta potential measurements and our automation know-how brings the use of zeta potential to a new standard for water treatment plants.

Further reading