Polymer comes in five forms: 1) Mannich, 2) Liquid, 3) Emulsion, 4) Dry, and 5) Gel. The choice of polymer to use for a given application is based on safety, ease-of-handling, product availability, and economics, among other factors.
1. Mannich polymers contain about 2 to 10% active polymer solids, have a high molecular weight, which we want for a sludge dewatering operation, and are extremely viscous. Like all polymers, they need to be diluted to a solution strength of 1% or less before using. Given issues with viscosity, low polymer solids content, and the formaldehyde required to make the polymer, this polymer option is not recommended. In order to prepare the dilute polymer solution, two tanks are required: 1) a mix tank for mixing and polymer aging and 2) a day tank for feeding the dilute polymer solution to the point of application.
2. Liquid polymers contain about 10 to 50% active polymer solids, tending more toward the lower solids range, have a low to medium molecular weight, and have a much lower viscosity than Mannich polymers. Generally, liquid polymers have a lower shelf life than Mannich polymers with the reactivity of some products lasting less than 90 days. In order to prepare the dilute polymer solution, two tanks are required: 1) a mix tank for mixing and polymer aging and 2) a day tank for feeding the dilute polymer solution to the point of application.
3. Emulsion polymers contain about 25 to 50% active polymer solids, have a high molecular weight, viscosities similar to liquids, and an excellent shelf life of up to three years. Emulsion polymers can be diluted to 1% or less “on-the-fly,” using static mixers, thus requiring no additional tankage for aging and feeding. An example of a polymer makedown system that I use often is shown in Figure 1. The polymer dilution panel is small in size, light in weight, and can be mounted anywhere. This polymer dilution and chemical feed system represents the lowest cost equipment option for all of the polymer forms described here, requiring minimal maintenance which consists primarily of cleaning the water flow meter and periodically replacing the static mixers. The float in the water rotameter is the only “moving” part in the polymer dilution panel, further minimizing maintenance requirements.
4. Dry polymers can contain active polymer solids as high as 95% and can be purchased as powders, granules, beads, or flakes. For high-humidity environments, the micro-bead polymer form is recommended. Regardless of the form the dry polymer is in, special, sophisticated polymer makedown equipment is required and the polymer and equipment must be housed in a humidity-controlled environment to minimize caking of the polymer and plugging of the polymer makedown system.
5. Gels are a tough, rubbery solids that are semi-clear to milky in appearance. Produced with gamma radiation instead of chemical catalysts, gels are actually solutions of high molecular weight polyacrylamide polymers or co-polymers. Gels are shipped in log form and require special make-up equipment.
Figure 1: Polymer Makedown System
Safety Concerns Associated with Polymer Use
Polymer is slippery, in any of its various forms, whether dry, liquid, Mannich, or emulsion. Any use of a polymer product requires personnel to use special precautions when handling the product. Any spillage of polymer, and any addition of water to any polymer product, creates the potential for slip hazards. These hazards can be minimized when the proper equipment is available for handling and feeding the polymer.
Charge type refers to whether the polymer is cationic (positively charged), anionic (negatively charged), or nonionic (neutral charge, balanced in positive and negative charges) as shown in Table 1. The polymer charge serves two purposes: 1) it provides a means of adsorption onto the particle surface by electrostatic attraction and 2) it causes the polymer molecule to extend and uncoil due to charge repulsion along the length of the polymer chain. The expansion and uncoiling of the polymer allows it to attach to more particles in the waste stream (or sludge).
The chemical “backbone” refers to the basic polymer type that is used. It is referred to as a backbone because of its long branches as shown in Figure 2. In these branches, other species may be added to alter the chemical make-up of the polymer.
Table 1: Form of Polymer and Charge Type
Figure 2: Polymer (Cationic) Backbone
The molecular weight is the measure of the backbone length or chain. In most manufacturer’s literature the molecular weight will be described as being either low, medium, high, or very high. These terms and their relative weights are summarized in Table 2. The best flocculants available today are synthetic, organic polymers with a very high molecular weight. Higher molecular weight polymers will adsorb on, or bridge, several particles at once, forming a large three-dimensional matrix. In general, the higher the molecular weight, the better the flocculant polymer will perform. More specifically, with increasing molecular weight you can anticipate an increase in the rate at which solids will settle in a clarifier.
Table 2: Polymer Molecular Weight Range
Charge density is the measure of the amount of sites on a polymer chain that have the charge associated with that polymer. On a cationic polymer, all positively charged sites would be measured. The remaining sites have neutral or possibly negative charge. Charge density is expressed as a percentage of available sites. The charge level varies from a low 10% to a high of 100%.
Another characteristic used to describe a polymer is the amount of active solids in the polymer. The actual backbone volume in the polymer comprises the active solids. This number is expressed as a percentage of the total volume. Depending on the polymer type, the active solids may be as low as 2% or as high as 95%.