البوليمرات المستخدمة فى معالجة الصرف الصناعى السائل/ Ill. WATER SOLUBLE WASTE TREATMENT POLYMERS

Ill. WATER SOLUBLE WASTE TREATMENT POLYMERS

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Synthetic polymers used for water treatment began to be widely used in the 1960's and have become a vital tool in the treatment of wastewater and potable water.

Although the basic chemistry of these chemicals has not changed radically, new uses for them are being discovered all the time.

General Concepts

A. Structure

The word polymer stems from two Latin terms: mer, meaning unit and poly, meaning many. That is, a polymer is "many units".

More specifically, a polymer is a chain or network of single units (monomers) strung together.

These chains can be linear, branched, or crosslinked. In water treatment applications, linear and branched polymers are most frequently encountered.

Crosslinked polymers are usually only partially soluble in water and are, therefore, not very useful.

B. Molecular Weight

Another way of categorizing polymers is by their molecular weight.

This is simply a measurement of how large or long the chain is. Molecular weights for water treatment polymers range from a few hundred thousand to tens of millions.

High molecular weight polymers, those whose molecular weight is above 1 million, are shear sensitive meaning that their chains can be broken into smaller fragments by excessive mixing.

This can be detrimental to their usefulness as we will see later.

C. Charge and Charge Density

A final way of identifying polymers is by their charge, both the sign of the charge and its magnitude.

Nonionics,as you would expect, contain no charge-bearing groups (they are not polyelectrolytes).

These polymers are typically homopolymers of acrylamide.

Anionics, when dissolved in water, are negatively charged. Anionic polymers are usually copolymers (polymers containing two types of monomer units) of acrylamide and acrylic acid, sodium acrylate or another anionic monomer.

The charge is located on a pendant group sticking off from the polymer chain backbone. The charge on these polymers is pH sensitive; they function best at a pH above 6.

Finally, cationics become positively charged when dissolved. Cationics can be copolymers of acrylamide with a cationic monomer, cationically modified acrylamide or a polyamine.

The cationic charge in these polymers is derived from nitrogen in the form of a secondary, tertiary or quaternary amine group.

Those containing secondary or tertiary amines are sensitive to pH.

The charge on these polymers drops off quickly as the pH rises above 6.

In addition, they are susceptible to attack by chlorine.

Polyquaternary amines are pH insensitive and function well over a broad pH range. They are also chlorine resistant.

In these polymers, the charge can be located on a pendant group or may be in the backbone of the polymer chain.

D. Viscosity

Viscosity, the measure of the resistance to flow of a liquid, is directly related to several properties of polymers.

The first of these is the concentration of the polymer in solution. For a given charge density and molecular weight, the higher the concentration, the higher the viscosity.

Another property which affects viscosity is the molecular weight. For a given charge and concentration, the higher the molecular weight, the higher the viscosity.

Another property is charge density. For a given molecular weight and concentration, higher charge densities give higher viscosities.

Finally, the structure of the polymer chains has an effect on the viscosity.

Branched polymers often give a creamy or syrupy viscosity.

Linear polymers will give a stringy or "leggy" type of viscosity.

By measuring this "stringiness" (known as pituity or elongational viscosity), a relative measure of the polymer's linearity can be obtained.

E. Dissolution of Polymers

When polymers are made, they are in coiled chains.

This is particularly true of high molecular weight polymers.

When they are put into solution, the charged areas on the chain repel each other and force the chain to uncoil.

As this occurs, the viscosity of the solution increases.

It is very important to recognize that this process takes time; more time is required for high molecular weight polymers.

Since the charge affects the speed at which the chain uncoils, higher charged polymers will uncoil faster than low charged products.

Indeed, nonionic polymers may never fully uncoil since they carry no charge.

The ability of the polymer to do its job hinges on it being completely uncoiled. Therefore, it is important to allow an ageing period before polymers are used.

There are several factors that impact on the dissolution of polymers, especially high molecular weight polymers.

Dissolved solids, hardness, and other impurities can inhibit complete dissolution since they shield the polymer's charged groups from repelling each other.

Because of this, softened or deionized (distilled) water is preferred.

General Types of Polymers

A. Dry, Powdered or Granular Polymers

The majority of these polymers are made overseas in Europe or Japan although there are now some domestic manufacturers as well.

These polymers benefit from the fact that they are essentially 100% active so you are not paying for the cost of shipping water.

However, they suffer from the drawback of being difficult to put into solution and they require special feed equipment for making up large amounts of diluted polymer.

Typically, these polymers are put into solution by the use of eductors or automated dilution systems.

Improper dilution of dry polymers can result in the formation of "fisheyes".

This is a very descriptive term for globs of polymers that are wetted on the exterior of the particle but dry on the inside.

The gelatinous coating slows down the dissolution process considerably and can result in the plugging of other auxiliary equipment.

Another drawback to dry polymers is that they will pick up moisture in the air and will solidify if they become too moist.

Care must be taken so they will be stored in a very dry location and kept free from moisture.

A final concern with dry polymers is due to their dustiness.

Nuisance dust and the hazard of residual (toxic) acrylamide monomer in the dust are both OSHA hazards and employees working with these materials should be made aware of them.

Dry polymers can be nonionic, anionic or cationic and can have a wide range of charge densities.

They are typically high molecular weight materials (MW > 1 million).

Use dilutions of dry polymers are limited by viscosity so the upper range is about 1 to 2%. Usually it is under 1% to permit adequate mixing of the solution.

B. Liquid or Solution Polymers

Solution polymers are solutions of water soluble polymers in water.

They benefit from the advantage of being relatively easy to put into dilute solution, requiring no sophisticated equipment.

However, since they are solutions, you are shipping water with them which increases the cost.

Additionally, high molecular weight polymers are limited by viscosity so they are frequently very dilute and very viscous.

However, some polymers such as polyamines and Mannich polymers are available only in this form.

Solution polymers are typically cationic and can have a wide variety of charge densities and molecular weights.

The concentration of these products can range from 2% to 70% depending upon the nature of the polymer.

Use dilutions of solution polymers are generally less than 10% to permit adequate control over feedrates.

However, for the lower molecular weight materials, prior dilution is not absolutely necessary if adequate mixing conditions exist.

C. Emulsion Polymers

Emulsions are liquids comprised of oil droplets dispersed in water or water droplets dispersed in oil.

Water soluble emulsion polymers belong to the latter, water-in-oil, group.

The polymer is concentrated in the water phase.

Emulsion polymers benefit from the fact that they are very easy to put into solution and are quite concentrated (25% to 50%, typically) even though they usually have very high molecular weights.

Also, their low bulk viscosity and liquid form makes them very easy to handle, especially in automated systems.

They can be diluted by a variety of methods ranging from simply pouring them into the vortex of mixing water to sophisticated dilution systems which require very little manpower to operate.

Emulsions suffer from the drawbacks that they are not 100% active and, because they are emulsions, they will separate to some extent over time.

However, they are easily reconstituted by brief mixing. Some products on the market are considered to be microemulsions.

Microemulsions are inherently stable and this minor problem is overcome.

On the other hand, some of the microemulsions currently on the market require post-dilution pH adjustment and they are quite expensive so there may be a trade-off to achieve this added stability.

Emulsion polymers can be nonionic, cationic or anionic.

They can have a wide variety of charge densities and are usually medium to high molecular weight.

Dilution levels of these products are limited by viscosity so the upper limit is usually 2% to 3%.

In practice, however, it is usually better to dilute to 0.5% to 1.0%. This permits the full dissolution of the polymer.

If lower dilutions are to be used, they should be diluted from this stock solution.

Specific Types of Polymers

The following section describes some specific types of polymers that are available for applications.

A. Quaternized Polyacrylamide copolymers

These products are available in dry, emulsion and solution forms. They are copolymers of acrylamide with a cationic monomer.

As mentioned above, they are relatively insensitive to pH.

However, pHs above 10 should be avoided if possible since this pH will allow slow degradation of the polymer through hydrolysis.

B. Mannich Polymers and Quaternized Mannich Polymers

Mannich (pronounced "manic") polymers are produced by performing the Mannich reaction on the homopolymer of acrylamide.

The process results in a highly charged, high molecular weight cationic polymer.

They are quite inexpensive and in some applications extremely cost-effective.

However, they have a short shelf-life (typically only a few months), are usually extremely viscous (which make them hard to pump, dilute and feed), are very bad smelling (rotten fish comes to mind), and are prone to gelation.

Also, they are tertiary amine-based so they are not chlorine resistant or pH insensitive. Finally, Mannich polymers contain various amounts of residual formaldehyde, a known carcinogen.

These drawbacks have limited the use of Mannich polymers in many applications.

Mannich polymers are sometimes quaternized to make them more useful.

However, this adds significantly to their cost and makes them even more viscous than their unquaternized precursors.

C. Polyamines

These cationic solution polymers are often referred to as polyquaternary amines or simply polyamines.

However, it should be noted that the term polyamine is used to refer to any chemical containing more than one amine group, including those which are not quaternized.

These products are very versatile.

They are typically low to medium molecular weight, can be linear or branched and are usually >20% active.

Polyamines are used in a variety of applications from oil emulsion breaking to paint detackification.

They go easily into solution, have quite long shelf lives, do not have a repulsive odor and are chlorine resistant and pH insensitive.

D. Poly (Diallyl Dimethyl Ammonium Chloride) Polymers

These polymers are usually referred to as DADMAC or DMDAAC polymers.

They are similar to polyamines in their characteristics with the additional advantage that they can be copolymerized with other monomers such as acrylamide.

E. Other Cationic Polymers

There are a variety of other polymers which are available. However, this list is composed of products which are less common and usually find niche uses.

It is included here merely for reference:

Polyethyleneamines and Polyethylenimines
Cationic Starches
Melamine/formaldehyde polymers
Unquaternized polyamines
Modified tannins and gums