Automated Dewatering using the Streaming Current Monitor: Success Story
In 1998, the Water Environment Research Foundation (WERF) began a study at the request of WERF subscribers. The study, originally entitled "Optimization of Thickening and Dewatering through Automation", was published in December 2001, and is available from WERF under the title of "Thickening and Dewatering Processes: How to Evaluate and Implement an Automation Package." The Instrumentation Testing Association, assisted with evaluation and selection of components that were tested. One of these was the Streaming Current Monitor (SCM) that measures the electrokinetic polarity and intensity of a liquid. This paper presents a summary of results, and some of the problems Chemtrac Systems, Inc., overcame by applying simple design principles to the SCM and sampling system.
The addition of polymers aids in dewatering of process solids. The cost of added polymers is very expensive so optimizing the polymer addition rate can save operating costs. Solids have either a positive (cationic) or negative (anionic) charge. When a polymer of the opposite charge to the solids is added to the solids process stream, the solids begin to bind into the large polymer molecules. The binding reaction releases water and produces large solid particles. The released water (filtrate or centrate contains unused polymer of which the SCM measures the charge polarity and intensity or zeta potential).
SCMs have been used to control coagulant/polymer dosing in potable water treatment plants for clarification and polishing for many years. Thousands of plants have improved process performance and reduced treatment costs using the SCMs. It is now the standard method of control. This type of control has not yet evolved without the "frustrations" of introducing a "new idea" or "technology" to a very traditional based industry. Even though the SCM was used successfully on sludge dewatering devices (centrifuges, belt filter presses, gravity belt thickeners, and dissolved air flotation systems) 20 years ago, the wastewater industry has not adopted this type of dewatering control. There are several reasons why this technology has not been accepted:
1. Many fluids being sampled in a wastewater treatment plant (WWTP) present several challenges for sensors and signal processors. Solids, debris, microbiological fouling, corrosive conditions (air and water) all make reliable sensors difficult design problem.
2. Dewatering processes have been operated manually due to large variations in sludge feed, solids concentration and other process variables.
3. Many operators have low confidence in automatic control systems due to their past experiences with instrument reliability and repeatability.
Streaming Current Measurement
Streaming current is the electrical phenomena produced when a liquid is forced through a capillary tube causing a pressure drop. This is due to displacement of mobile ions surrounding particles. It is important to note that for one directional flow through the capillary, the signal generated is DC.
The amplitude of this signal is related to the zeta potential of a colloid or surface. Zeta potential is difficult to measure on-line, and is usually done with a laboratory instrument. The SCM is an on-line instrument capable of continuously measuring the charge (net positive and negative ions), of a fluid. The functional components are shown in figure 1.
As the liquid is passed over the top of the reciprocating piston, fluid is drawn into and expelled from the annulus (which forms a capillary). Typical clearance between the piston and cylinder walls is 0.005 inches. The piston oscillation (4hz) produces a sinusoidal fluid flow, an oscillating pressure drop, and a sinusoidal signal. Electrodes embedded at both ends of the cylinder wall detect the ionic flux flow (nominally termed the streaming current). This signal is then processed producing various analog and digital outputs suitable for process monitoring and control.
The reciprocating piston design is much more functional than the one-directional DC generated capillary flow.
1. The small clearance and reciprocating piston movement keeps the surface "scrubbed" at the same time the signal is being generated. This ensures repeatability and reduces signal "drift".
2. The generated signal (4HZ, AC) is a defined frequency. This is very important in isolating the signal, and preventing electrical and/or electromagnetic inferences. Also since the signal is sinusoidal and polar (negative and positive), the isoelectric (neutral) point is clearly defined.
This is the first filtrate produced on the BFP which enables the polymer control system to have a very fast response. A one-inch diameter, 6 feet long sample line was used to feed filtrate to the sensor. A liquid head of about 6 feet provided a sample flow rate of 10 gpm. There were no strainers, filters, or "loops" in the sample line. Potable water was used to flush the sensor and sample line on a regular schedule.
Use of the SCM to define and maintain the optimum polymer dose is straightforward. The operator adjusts the polymer pump to the minimum possible value and still maintains the desired cake dryness. As previously stated, in this plant cake dryness is the top process priority. In some plants, particle capture or mass throughput might be the main objective.
Process Results
The controller turned down polymer flow when a concentrated polymer batch was being fed, and turned it up on weak polymer batches. The SCM automatically adjusted polymer pumps when solution strength varied, or sludge feed solids changed. The large variances in percent of pump output occurred coincident with switching of polymer feed tanks that occurred approximately every 45 minutes. It was found that there was up to a 50% difference in polymer solution concentrations between the two separate polymer tank systems. It was not uncommon for the polymer pump output to fluctuate between 15 and 60% to maintain a consistent filtrate charge as shown in the graph in Figure 3.
Polymer savings were achieved without sacrificing the process priority of high cake solids. The SCM enabled operators to realize savings of about 14%, or 1.5 pounds per dry ton. During the study, average sludge feed flow increased along with cake solids at the same time that polymer dose decreased. This was most likely due to a more consistent polymer feed rate.
Reliable streaming current measurements depend upon advances in instrument design and technology. High flow rates through the sensor encourage faster response to changes and large flow through sensors minimizes fouling by particulate matter. Automatic cleaning and flushing reduce operations and maintenance staff troubleshooting. Through the combined efforts of many people and organizations, instrumentation and automation of a complex process has achieved some success.
About the Author
Bob Bryant is the founder and president of Chemtrac Systems, Inc. His career in water treatment began over 33 years ago with Nalco Chemical Company after 8 years as an Air Force Officer. In the early eighties he was instrumental in taking electrokinetic measurement, (streaming current), from the laboratory to the field. This on-line measurement has become the standard coagulant control method in thousands of water treatment plants around the world. His vision of making streaming current work in dewatering systems has now been fulfilled. Bob has several instrument design patents on streaming current and other water quality monitors. He has written over 20 articles for various water and wastewater publications, and he is a member of ITA, AWWA, WEF, WERF, American Filtration Society, and the Technical Association of the Pulp and Paper Industry (TAPPI).
For more information contact Chemtrac Systems Inc., 800-442-8722, www.chemtrac.com
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