February 2011, Vol. 23, No.2

Operator Essentials

What every operator should know about polymers

Paul J. Kemp

Click here for a PDF of this article. 

Knowledge

Principle

A practical consideration

Polymer

A synthetic organic material mixes with or dissolves in water and alters the behavior of suspended material in the water. Polymers are a chain of repeating chemical groups linked together end-to-end.

This type of material can be used to enhance the performance of clarifiers, thicken process solids into sludges, or condition sludges for dewatering.

Suspended solids

Suspended solids consist of undissolved material mixed in a liquid medium.

Particles with the same charge repel each other, preventing contact and thereby preventing settling.

Destabilization

This is the step in which the static charge that keeps suspended solids suspended is neutralized by the polymer and/or coagulant.

This requires a material that has an ionic charge
(cationic = [+], anionic = [–]) that is the opposite of the one present on the suspended solids.

Coagulation

Coagulation is the process that results from destabilization that allows suspended particles to accumulate into bigger clumps.

Think of this process as making a snowball. By pressing snowflakes together, they stick together to make a “single” bigger clump.

Coagulant

Coagulants can be inorganic metal salts or synthetic organic polymers. Coagulants focus on destabilization and serve to allow coagulation.

Inorganic materials are commonly alum, polyaluminum chloride, and ferric chloride or sulfate. Organic coagulants will vary.

Flocculation

Flocculation is the process of gathering coagulated solids together to form much larger clumps.

Think of the polymer strands as a long fishing line with hooks distributed along its length. The hooks grab coagulated solid clumps, and because the strand is so long, it tangles around the clumps like spaghetti does around meatballs.

Coagulant aid

A coagulant aid is a secondary material that works on coagulated material to enhance or perform flocculation.

Sometimes, a coagulant aid is needed when the coagulated materials (the “snowballs”) are too fine or not dense enough (too light) to settle on their own.

Molecular weight

Molecular weight is a measure of the size (length) of a polymer molecule. Technically, molecular weight is the mass in grams of 6.02 × 1023 molecules of a single chemical.

During manufacturing, polymer chains don’t all come out the same length. Molecular weights are reported as one of three categories: high (10,000 to 100,000 g), very high (100,000 to 1 million g), or extremely high (more than 1 million g).

Charge density

Charge density is a measure of the fraction of a polymer that has active sites on the polymer molecule.

Using the fishing line analogy again, charge density would refer to how far apart the hooks are spaced.

Chemical components

The main ingredient in polymers is polymerizable organic chemical that constitutes the molecular “backbone,” the part that links the rest of it together. Inorganic products are usually iron or aluminum salts. (Some salts of aluminum are polymeric.)

Polymers are named for the chemically reactive groups that are chained together to make them.

Using the fishing line analogy, polymers are named for the type of hooks mounted on the line.

For example, a polyacrylate is based on acrylic acid and will carry a negative charge. A polyamine has an ammonia molecule hanging out in the open and will carry a positive charge. A nonionic polyacrylamide has an ammonia molecule built into it and will not have a charge, but it has areas that are more positive than the rest of the polymer.

Jar test

This is the single most useful tool an operator can have to work with polymers and coagulants. It enables an operator to compare products, compare dosages, and set operating conditions for any process involving suspended solids management.

When working on clarifier performance, or sludge thickening, the jar test procedure is the best predictor of what will happen in process.

Jar testing technique is a topic that requires considerable attention but is extremely easy to learn and use (see article, p. 44).

Percent active

When process chemicals are provided as solutions, they consist of a product that affects a process, and something to carry it — called a “vehicle.” The amount of a mixture that is process-reactive is given as a percent of active ingredient.

This parameter is important if a product is to perform reliably in a process. This value should be verifiable by batch/lot number certification upon receipt of a shipment of material. There also are laboratory procedures that can confirm this value; however, they are not simple.

Retained sample

Retained samples are valuable to confirm that what you expect is what you receive. Samples from polymer screenings and from preceding shipments are essential for quality assurance (QA). Polymer suppliers will make samples available to prospective users at no cost. They usually will provide the sample that they used to perform the screening.

Polymer samples have a shelf life, usually in the 6-month range. It is important to perform QA testing on the first incoming shipment following any bid using the samples retained from the polymer screening that preceded the bid.

Thereafter, samples should be kept from each shipment to check against the next batch received. Some procurement contracts require a QA test to be performed before accepting a shipment. An operator familiar with the process should be the one to perform the test.

Viscosity

Viscosity is a measurement of thickness, or resistance to flow.

Each product will have a specified viscosity range. Instruments are available to measure this parameter. It is useful as a QA aid when checking a new shipment of material but is not a good substitute for a jar test.

Fourier transform infra red (FTIR)

FTIR spectra sometimes are offered by suppliers with each batch of material as a means of verification that a received material is indeed what was expected.

FTIR is more sensitive to the vehicle content of a product than to the active ingredient. It is not a good substitute for a jar test when comparing a newly delivered polymer with a reference sample.

 

Paul J. Kemp is assistant general manager of the Guam Waterworks Authority (Tamuning, Guam).

 

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