On-Line Control of Coagulants, Polymer Dosing with Streaming Current Technology
Market and political pressures, along with professional pride are making plant operations personnel revisit on a regular basis the general and specific aspects of their treatment facilities.
By T.J. Day
Market and political pressures, along with professional pride are making plant operations personnel revisit on a regular basis the general and specific aspects of their treatment facilities. Water facilities today find such pressures from privatization concerns, security updates and various regulatory oversights adding to their professional burdens in managing modern water treatment plants. These pressures require the use of strategic technology to assist their efforts and provide maximum optimization of the facility without any compromise of public health and safety of the water supply. One of these strategic technologies that can readily assist the modern water facility is Streaming Current Technology.
Traditionally, a grab sample is made from a given process area to determine the instantaneous composition of water or sludge in its conveyance through a facility. Testing of samples in this age is highly accurate using modern laboratory procedures and microprocessor based analytical equipment. However, there is a critical lag in the time the sample is taken and the time the sample is analyzed. Hours to days can pass before the result of a specific test is known and returned to an operator for process adjustment. As a result, on-line analytical equipment is used to provide flow-paced control of the dosing process.
Flow-paced dosing is the adjustment of chemical addition to a batch or process stream as a result of changes in a baseline chemical makeup in a batch or flow of material. This is accomplished by continuous monitoring of a process stream using on-line instrumentation. The equipment changes the chemical additive adjustments up or down based on a pre-set algorithm so the maximum amount of treatment is provided from the minimal amount of chemical introduction. Good electronics packages allow for non-linear dosing variations that can further maximize chemical usage and provide greater cost savings.
This process works well with dissolved and low aggressive chemicals. However, solids, viscous materials and aggressive chemicals can complicate the automation aspect. This is a result of available technology, product demand and endpoints in the capabilities in available manufacturing materials.
Water Treatment Applications
All three of these conditions appear in water treatment applications. As a result, there is a limited choice in the products available to provide flow-paced dosing in water processing environments. Ferric chloride, sodium hypochlorite, polymers in sludge processing and other treatment chemicals fall into the aggressive and viscous categories, providing great challenges to water operators and plant facilities in automating their control loops. Such direct measurements other than pH / ORP or temperature can be difficult. A method for overcoming this situation is the use of a relational measurement property.
Relational measurement is the tracking of a residual parameter that directly reflects the chemical composition of a given flow or batch process in a control loop. The change in this relational measurement is directly proportional to the content of the process flow or batch of interest. One method that can perform all these measurements is the use of Streaming Current Technology as a relational operator for process measurement.
Electrokinetic and Zeta Potential
In order for this method to be successful, the relational quantity measured must be a direct representative of the actual process stream or batch composition. Streaming Current Measurement is such a measurement and is quite representative of any water composite, useable in any water treatment system.
Water being treated generates free ions in its conveyance of other substances. This is a result of the shear of fluid that passes over the given particles held in suspension. This generates an electrokinetic potential that is relational to the content of the water being treated. This potential is generated from the shear of the slipping plane, or fluid, against the particle being conveyed. The electrical potential generated is sometimes referred to as the Zeta potential and is commonly referenced in water treatment. However, Zeta potential is a combination of other forces and tends to be less pure than using Streaming Current.
Although related to Streaming Current, it is found less accurate in continuous process applications. Direct on-line measurement of this Streaming Current potential can be performed using Streaming Current Technology and effectively control the process of interest.
Streaming Current Measurement
Electrokinetic potential in a process stream is the stream of current potentially generated in the process flow or batch. The direct determination of this potential is a direct measurement of treatment requirements. If one considers that these charges occur from material that has not reacted with treatment chemicals, it can readily be deduced that the amount of charge in the system is a direct relational component of treatment needs. Streaming Current Technology is used to measure this potential through the use of Streaming Current Detectors.
How It Works
A sample of the process water or sludge stream is drawn from the continuous reaction by the streaming current detector using a piston. This motion generates an alternating current across the material, by which Streaming Current gets it name. This temporarily immobilizes the charged particles. The streaming current is measured across the immobilized sample. This is directly proportional, or relational to the amount of dosing required.
A control signal is then connected to the dosing control section. This control signal can within seconds control the amount of chemical, increasing or reducing the amount necessary.
This direct relational measurement running continuously can allow plant personnel to adjust their processes real time and maximize treatment with minimal chemical introduction. This is because the analysis performed is continuous and does not require additional laboratory setup or lag time, except for initial calibration or periodic maintenance.
Streaming Current Detector Operations and Maintenance
The SCD analyzer performs a continuous operation and sends a signal on a continuous basis. In some sampling applications, a buildup of residual material may form on the probe. Automatic Jet-Washing of the probe readily cleans it and keeps the analysis continuous and accurate. Automatic, periodic cleaning using a jet wash allows the signal integrity to continue true limiting or eliminating any material build up on the probe. Polymer based measurements will require more periodic jet washing than a more soluble chemical introduction.
The device must be calibrated at start-up to user setup requirements and given periodic maintenance. Like any other electronic measurement device, a Streaming Current Detector requires attention if only a visual inspection to ascertain that its installation is secure, the sample line is properly inserted in the flow and its baseline calibration is strong. Calibration solutions are made using manufacturer's instructions particular to their probe construction.
Streaming Current Detectors are accurate and quite applicable throughout the entire process where chemical dosing, coagulant control and process monitoring is necessary. The relational measurement is quite representative of the needs of the process.
Polymer is a very expensive additive to the sludge portion of water treatment. Even the use of dry polymers can be expensive. Also, the irregular nature of sludge production in any given plant poses a challenge for operators using a batch grab-type testing of their sludge for polymer dosing. Since Streaming Current Technology is continuous, the technology can provide data on the changing characteristics minimizing the dosing and maximizing the process.
In the case where aluminum-based coagulants are used in water processing, Streaming Current Technology is quite effective in minimizing the aluminum residual by minimizing the coagulant dosing. Elevated aluminum level in water supplies is now suspected in contributing or accelerating Alzheimers' disease in elderly patients. One of the best monitors dosing for this is using Streaming Current Technology.
Streaming Current Detectors can be mounted on small skid packages to control chemical dosing providing a total solution package for chemical application. This also provides for more accurate maintenance by concentrating all components of interest in a specified area, making the technicians' job easier. This accelerates a better job in the more conscious worker.
About the Author: Thomas J. Day is the Product Manager for Mechanical and Hydraulic Technology for Milton Roy USA. He holds an undergraduate degree in Management, BS and MS in Engineering and has over 18 years experience in the Environmental Products and Process industry. He has over 35 technical papers published documenting his work in this industry including a textbook on infrastructure management.