Chemical Feed: The Trouble with Polymers
By Greg Kriebel
Summary: Coagulants and flocculants are just two types of chemicals that may be used in water treatment to help remove unwanted contaminants. Here is a discussion of various polymers and chemical feed considerations.
Polyelectrolytes, or polymers with long chain organic molecules, are used in water and wastewater treatment plants to attract and adsorb suspended solid particles to make the solids easier to remove. Activated polymer molecules possess a myriad of charged sites to attract suspended solids of opposite charge, and the resultant floc is settled by gravity in a clarifier or sedimentation basin. Introduced into a sludge stream through a polymer blending and feed unit, a polymer solution helps condition the sludge to enhance the de-watering process.
Although higher molecular weight makes them very effective, polymers can be difficult to mix and feed into a treatment process. Other typical water/wastewater treatment chemicals such as alum, ferric chloride (used for coagulation) and sodium hypochlorite (used for disinfection) can be easily diluted or applied directly to the process from the storage container. To be effective, however, polymers must be “activated,” hydrated, and extended prior to dilution and introduction to the process stream.
Polymers are commonly used to remove colloidal suspensions from surface waters and to condition municipal wastewater sludges to enhance the de-watering process. Lower cost metallic salts such as alum or ferric chloride are frequently used in the coagulation process to neutralize the negatively charged suspended particles and begin the coagulation process. High molecular weight polymers—flocculants—are fed into the process to bridge these neutralized particles to form larger particles—flocs— which settle faster. Use of metallic salts in the coagulation process can contribute to objectionable levels of residual metal content in the treated water and in some cases where a high dose rate is required, there will be an excessive amount of sludge produced resulting in increased treatment costs. Feeding a low to medium molecular weight polymer is a viable alternative or adjunct to the use of metallic salts.
There are numerous industrial applications as well. Polymers applied to the papermaking process can improve the fiber retention on paper machines or improve the dry strength of paper when recycled materials are used. In another application, polymers may be used in cooling towers, boiler waters, heat exchangers, etc., as an antiscalant or dispersant. Used in these applications, the polymers either prevent the formation of process-hindering, scale build-up or keep the scale particles from adhering to pipes or heat exchange surfaces.
When polymers make initial contact with water, the outer surface of the polymer particles becomes very sticky. If the particles aren’t properly dispersed prior to and during the initial wetting phase, agglomerations or fish-eyes will be formed. Agglomerations make it more difficult and time-consuming for water to penetrate and successfully hydrate and activate the bound-up polymer. Pumping neat (concentrated) polymer into a tank of water and using a high-speed mixer properly disperses the polymer and prevents clumping, or formation of agglomerations. Once activated, however, polymers are extremely fragile. In their concentrated form, polymers are like a coiled spring. But, when the molecules are uncoiled and extended, the polymer molecules become very fragile and are susceptible to fracture by any high-shear device. High-speed mixers used to keep the sticky polymer particles separated can fracture the activated polymer strands and render them less effective in forming settleable flocs. To compensate for reduced effectiveness, plant operators often feed more polymer than necessary, increasing their chemical costs. One option for avoiding fractured polymer molecules is low-speed, low-shear mixing. Unfortunately, this requires excessively large tanks to allow for the slow dissolution of the inevitable agglomerations formed. Such a system requires the batching of polymer to be started hours before the diluted polymer solution is needed for the process, and greatly increases the capital costs of equipment and facilities.
A better option is a liquid polymer blending and feed unit. An ideal polymer feed system should include a means of introducing the neat (meaning as delivered) polymer to the water so as not to form agglomerations, and incorporate a two-stage or tapered mixing system in its design. The first stage produces the high shear/high energy to disperse and wet the polymer molecules, often referred to as inversion. Polymer feeder manufacturers have developed various ways of introducing polymer to the dilution water to prevent formation of fish-eyes or agglomerations. One such method is to draw the polymer out in a ribbon-like thin sheet into a high-energy water stream. Research has shown that when polymer is introduced into the water in this fashion, it will be instantly and thoroughly wetted into solution. These wetted and extended polymer molecules may be easily fractured if they remain in the high-energy zone for any extended period of time. Providing a second, low-shear zone or tapered mixing regime will complete the blending of the polymer with dilution water without damaging the activated and fragile polymer strands.
One for every occasion
Polymers are available in a variety of different forms and concentrations. Developing an understanding of the different characteristics is essential in properly evaluating process design.
- Dry polymers—a powder form similar to table salt or sugar: Considered 100 percent active when calculating for process design. Typical shelf life of dry polymers is several years, making them ideal for quantity purchase and storage.
- Emulsion polymers— a liquid form, oil-based polymer product with a milky opaque appearance: Viscosities range from 100-2,000 centipoise (cps)—similar to motor oil. Average content of 40 percent active assumed for calculating process design. Typical shelf life of emulsion polymers is 4-6 months.
- Dispersion polymers—a liquid form, oil-based polymer product: Viscosities range from 100-2,000 cps (again, similar to motor oil). Average content of 50 percent active assumed for calculating process design. Typical shelf life of dispersion polymers is 4-6 months.
- Solution polymers—known as polyamines and polydmac; used for coagulation only, primarily in water plants: Solution polymers are a liquid form, water-based polymer product with a clear appearance. Viscosities range from 2,000-10,000 cps (similar to honey). Average content of 10 percent active assumed for calculating process design. Typical shelf life of solution polymers is 4-6 months.
- Mannich polymers—a liquid form, formaldehyde-based product with a clear/milky appearance: Viscosities range from 10,000-50,000 cps (similar to gelatin). Average content of 5 percent active assumed for calculating process design. Typical shelf life of Mannich polymers is several weeks.
Which polymer to use is the decision of the clarifier, filter or de-watering equipment vendor. A decision is dependent on water or wastewater characteristics, changes in water or wastewater characteristics, bench test results, and a comparison of savings against ease of use.
Concentrations & accessories
Applied solution strengths vary by application, but typical concentrations range from 0.05-0.1 percent for clarification and 0.25-0.5 percent for de-watering. Compact liquid polymer blending units are commercially available to meet most polymer feed needs with capacities of up to 600 gallons per hour (gph) of neat polymer and 12,000 gph of dilution water. Units typically include all of the components necessary for a successful polymer feed system. In addition to the activation and blending devices, other components such as built-in, on-off water control, visual indication of dilution water flow, adjustments for water and neat polymer flow, and a low water flow switch should be included. Typical accessories to round out the system include a calibration kit to verify neat polymer pump rate, hand pump to assist the pump in priming the viscous polymer, and auto-flush of the polymer blending unit and feed lines.
Today’s high molecular weight polymers can represent a significant part of a treatment plant’s chemical cost. Properly mixed and activated polymer can result in improved process performance and reduced chemical costs, making proper feeding of these chemicals of particular interest to water and wastewater treatment plant operators.
About the author
Greg Kriebel is product manager of Semblex Polymer Preparation and Feed Equipment for Severn Trent Services, of Ft. Washington, Pa. He holds a bachelor’s degree in environmental engineering from Temple University. His 23 years of industry experience includes field engineer, project management, sales representative and product manager. He can be reached at (215) 997-4048, (215) 997-4062 (fax) or email: email@example.com