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Before we delve into the basics of Streaming Current Detectors (SCDs), it is first important to understand why they are used and what problems SCDs solve. In many water treatment applications, operators may be challenged by any of the following:

  1. Fewer resources and increased operator demands
  2. Raw or influent water with fluctuating turbidity. This is particularly common in plants
  3. Goals to improve quality, specifically reduction of turbidity and improved water color
  4. Goals to reduce costs of chemical usage
  5. Maintaining process control in weather events
SCD Surface Water Treatment

As one example, a customer in a surface water treatment application saw tremendous benefit from the use of implementing a Streaming Current Detector into their process. Heavy rains caused dramatic and rapid fluctuation of raw water turbidity and color (5-400 NTU; 10 - 300 PtCo).

Water Treatment Plant

Prior to using a Streaming Current Detector, this plant regularly:

  • Conducted Jar Tests more than 2 times a day to determine proper Alum dose rate
  • Resulting treated water, effluent, varied from 0.7-1.2 NTU and 5-10 Pt Co; with higher results during rain events
  • Overran chemical budget regularly

After installation and setup of the Streaming Current Detector:

  • Jar Tests totally eliminated - allowing the treatment team to focus on other tasks
  • Complete automated control of Alum dose rate
  • Effluent Water Quality improved: less than 0.7 NTU and less than 1 PtCo, even during weather events
  • Alum cost reduced by 50%, Lime reduced by 50%, Polymer use (for color) totally eliminated. These chemical cost reductions paid for the streaming current detector in less than 1 year of use!
Streaming Current Detector Process

What is a Streaming Current Detector?

Streaming Current is the presence of charged ions moving in a high velocity fluid stream. A Streaming Current Detector detects and amplifies the signal from those charged ions to implement process control. These instruments continuously measure the electrical charge within a water sample. The monitor continuously measures net charge density of the particles in this water sample. This allows for continuous monitoring and control of coagulation processes in water and wastewater treatment and also provides effective polymer control in sludge dewatering processes.

SCD Process Control

How does a streaming current detector work?

A streaming current detector is equipped with a reciprocating plunger in which a water sample is continuously passed through. Charged particles adhere to the plunger and cylinder surfaces. The SCD contains electrodes to induce a current such that ionic charges are transported by fluid velocity and then measured at the electrodes. The charge is measured in 10-9 Amps which is then amplified and processed by the SCD. The ultimate output of the SCD is a 4-20 mA analog output signal that can control the speed of a metering pump to increase or decrease the chemical dosage rate to maintain process control.

What are the different applications in which a streaming current detector would be used?

Streaming current detectors can be used in any treatment process in which charged ions can be used to determine a chemical dose rate. Surface water treatment plants are some of the most prevalent users of Streaming Current Detectors to help optimize coagulant dose rates. In fact, some jurisdictions have regulations in place that require usage of streaming current detectors as the resultant water quality is significantly improved with lower costs compared to traditional methods. SCDs are typically used in Influent clarification, Reverse Osmosis, High Purity, Waste Water Treatment clarification and sludge dewatering.

SCD and Influent Clarification

What is a jar test experiment and how is a streaming current detector used in one?

While traditional methods, such as jar testing, are highly prevalent and useful, automating this task and taking the guesswork out of the process is beneficial not only to the water treatment plant but to the customers the plant serves as well. While jar testing is required to determine the optimum chemical amount, it can be done in conjunction with the SCD upon initial installation to determine proper setpoints. In some instances, usage of SCDs has eliminated jar testing entirely. The elimination of the jar test task enables personnel at the water treatment plants to focus their efforts on other demands.

SCD Jar

How does a streaming current detector work in optimizing a coagulant dosage?

The SCD measures charge of the water and can determine the work not done by the coagulant. By measuring the charge in comparison to setpoint, the SCD then determines if the chemical dose rate needs to increase or decrease to reach and maintain setpoint. There are two types of SCDs: A more simple gain based SCD and a more advanced SCD that contains PID control.

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