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Flocculants: Precipitation and Dewatering
Precipitants, coagulants and flocculants are used in waste treatment to separate unwanted components from water and sludge. Not only do these materials make the task easier, in some cases the separation would be impossible without them. This page is divided up into three sections. The first section deals with inorganic coagulants and treatment chemicals. It presents the products that are available, how they are typically used and any pertinent factors relating to their use.
The second section covers water soluble wastewater treatment polymers. Here, too, we will look at the various products that are used, how they are applied and the various factors which determine their success in waste treatment.
The third section will look at some of the typical waste treatment applications that you are likely to encounter in the field. In this section you will see how the inorganics and polymers discussed in the first section are put to use.
Before we get started, let’s look at some of the terms and concepts that we will encounter when we are doing waste treatment:
Coagulation – Coagulation refers to the destabilization of suspended colloidal materials. Because these particles all carry the same surface charge (usually negative), their mutual repulsion is enough to prevent them from settling. Note that this is different from precipitation. Also, note that this is technically different from flocculation although the two terms are often used synonymously.
Flocculation – Flocculation refers to the bridging between particles by a polymer chain, causing them to form flocs or larger aggregates. These flocs float (flotation) or sink (sedimentation), making them easier to remove from the system.
Precipitation – Precipitation is the insolubilization of dissolved materials. An example is the precipitation of iron by raising the pH to 8 with sodium hydroxide to form an insoluble iron hydroxide precipitate.
Sludge Dewatering – Sludge dewatering is the removal of water from sludge to reduce its volume, lowering hauling costs. This process, typically accomplished with belt filter or screw presses or other equipment, also makes the sludge easier to burn, reducing fuel costs.
Clarifiers – Clarifiers are large cylindrical or rectangular basins which allow the separated solids to settle from the water, permitting clear water to pass over a weir.
Dissolved Air Flotation (DAF) – DAF is the process of passing dissolved air through flocculated wastewater. The tiny bubbles attach to the flocs, floating them to the surface. The resulting raft is then skimmed from the top of the water, allowing the clear water to exit the system.
Electrolytes – Electrolytes are atoms and molecules that, when dissolved in water, ionize and carry a positive (cationic) or negative (anionic) charge. Ferric chloride and aluminum sulfate are examples.
Polyelectrolytes – Polyelectrolytes are electrolytes which have more than one charge-bearing atom in the molecule. Examples are acrylamide copolymers and polyaluminum chloride.
Jar Testing – Jar testing is the process of evaluating various treatments in the laboratory to determine the best method. Often a four or six paddle “gang-stirrer” is used to view several treatments simultaneously.
II. INORGANIC COAGULANTS AND TREATMENT CHEMICALS
Inorganic coagulants can be electrolytes or polyelectrolytes and are typically based on iron (ferric or ferrous), aluminum, calcium, or magnesium. These coagulants all have one thing in common: when they are dissolved in water they generate a highly charged cation useful for destabilizing dispersed solids.
Most wastewaters contain finely divided solids or emulsified liquids that are dispersed due to the mutual repulsion of their surface negative charges. When a highly cationic ion is introduced into this system, it interferes with this repulsive stabilization and allows the particles to come into close contact. This starts the coagulation process. Van der Waals attraction and/or the use of polymeric coagulants completes the process forming larger aggregates which can be further flocculated or separated as is from the waste stream.
The following is a list of the most common inorganic coagulants in use today and a brief overview of some of their features. The best choice for a particular application depends on the system and is usually determined only after jar testing in the laboratory.
Ferric Chloride [FeCI3] – Typically sold in solution form. Applications include phosphate removal, sludge conditioning and dewatering, trace metals removal, and odor control. Solutions are very acidic and corrosive. Available as a solid and in solution (27-43% FeCl3) form.
Ferrous Chloride [FeCI2] – Applications include phosphate removal, odor control, heavy metals removal, controls toxic sulfide generation in anaerobic digesters, oil & grease removal, and sludge conditioning. Available in solution form only (8-14% iron). Very acidic and corrosive.
Ferric Sulfate [Fe2(SO4)3] – Applications include water clarification, decolorization of surface water, sludge conditioning and dewatering, trace metals removal, organics removal (including trihalomethanes), sulfide control, phosphate removal, oil &grease separation and DAF. Available as a solid and in solution (10-13% iron) form. Very acidic and corrosive.
Hydroxylated Ferric Sulfate [Fe5(SO4)7(OH)] – The newest of the iron salts. Billed as a replacement for alum. Sold in solution form. Very acidic and corrosive.
Ferrous Sulfate [FeSO4] – Applications include phosphate removal, trace metals removal, and odor control. Available in solid and solution (5-12% iron) form. Very acidic and corrosive.
Aluminum Chloride [AlCl3] – Applications include metals removal, oil & grease separation and water clarification. Available as a solid and in solution form. Acidic and corrosive.
Aluminum Sulfate [alum, Al2(SO4)3] – Perhaps the most widely used inorganic coagulant. Uses similar to aluminum chloride. Available as a solid and in solution form. Acid and corrosive.
Calcium Chloride [CaCl2] – Infrequently used for metals removal, organics reduction and water clarification. CaCl2 also has great utility for. phosphate removal. Available as a solid and in solution form. Nonhazardous.
Magnesium Hydroxide [Mg(OH)2] – Used for pH control (maximum pH of 9.0 helps prevent overshooting pH target). Helps reduce sludge levels when used as a precipitant. Safe to handle. Contains no heavy metals. Available as a 50% slurry or in solution form. Can have problems with stability of slurry and is slow to dissolve.
Polyaluminum Chloride [PAC] – This describes a wide variety of materials containing more than one aluminum atom in the molecule up to about 13. These materials are typically described by their Al2O3 content and basicity. Al2O3 ranges from about 8% to 25% and basicity is usually between 50 and 70% for most commercial products. Aluminum chlorohydrate is an example of a PAC. Some manufacturers replace part of the chloride content with silicate or sulfate. Some materials are corrosive. Available in solution form only.
Sodium Aluminate – Applications include color removal,, phosphorus removal, lime softening, pH control, and many papermaking applications. Very alkaline and very corrosive. Available in solution form.
There are numerous chemicals which are used in waste treatment for precipitation and to aid in removal of unwanted constituents. The following is a short list of products used mainly for pH adjustment and buffering:
- Sodium Carbonate (Soda Ash)
- Sodium Hydroxide (Caustic Soda)
- Potassium Hydroxide (Caustic Potash)
- Sulfuric Acid
- Calcium Oxide (Lime)
- Magnesium Oxide
There are several products used for metals precipitation: sodium sulfide, sodium polysulfide, dimethyl dithiocarbamate (DTC, also used as a microbiocide), and trimercapto-s-triazine (TMT). Of these, DTC and TMT are the most widely used. These materials form insoluble complexes with dissolved metals. The insoluble complexes can then be settled as is or further treated with coagulants or flocculants prior to removal. The chief advantages of using these materials over precipitation of metal/hydroxide complexes with caustic soda is that they work even on chelated metals. Also they often allow even lower heavy metals residuals in the final effluent.
III. WATER SOLUBLE WASTE TREATMENT POLYMERS
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.
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.
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.
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
IV. WATER TREATMENT MECHANISMS AND APPLICATIONS
Due to the large variety of applications for water soluble polymers, it is nearly impossible to list all of them. In this section, you will find an overview of some of the more common uses.
As we have already discussed, contaminants in water carry a slight surface negative charge which stabilizes them due to electrostatic repulsion. This includes both oil and grease emulsified in wastewater and finely divided solids suspensions. Inorganic coagulants and polymeric flocculants neutralize this charge which allows the particles to come closer together and which destabilizes the suspension.
In addition, charged polymers (polyelectrolytes) can agglomerate the destabilized particles through the “charge patch” or “bridging” mechanisms. The particles then sink or float (depending upon the nature of the contaminant) and are removed from the system.
In a typical treatment scheme, a low molecular weight cationic polymer is fed. often in conjunction with an inorganic coagulant such as alum, aluminum chloride or ferric chloride, to generate small flocs known as “pinfloc”. This is frequently followed by treatment with a high molecular weight anionic polymer which attaches to the now cationic particles and causes even bigger flocs. These larger flocs are then easy to remove.
In metals removal, the metals are often dissolved in the water. Adjusting the pH to 9 or 10 will precipitate the metals as their hydroxides. Also, as previously mentioned, metal precipitants such as DTC or TMT can be used and then the scheme described above is followed.
It is worth noting that nearly every system is different and very few “rules” can be established regarding how to treat them. This is the true challenge of water treatment. The comments made here should be viewed only as guidelines. Thorough jar testing in the laboratory of a variety of products is always necessary to choose the most efficient, cost-effective treatment.
Areas of water treatment which conform loosely to the scheme described above are river water clarification, oil and grease removal, and emulsion breaking.
Another area of water treatment is water clarification where low levels of contaminants are present. These systems are often treated only with high molecular weight anionic or occasionally cationic polymers.
Still another area is sludge dewatering. In this application, sludge of low percent solids is mixed with various polymers and other chemicals to assist with water removal. This allows the sludge to be significantly concentrated so that it can be landfilled or incinerated. Medium molecular weight cationic polymers are most often used in these applications. The charge densities of the polymers used will vary from medium charge for paper mill sludges to high charge for municipal sewage sludge. As stated before, each system is different and there are no hard and fast rules regarding which products will work.
The mechanism for sludge dewatering is believed to be based on the fact that the sludge carries a negative charge. When the cationic polymer attaches to the sludge solids, its charges are shielded and the polymer chains begin to recoil into the shape they formed during the polymerization process. As they coil, they bring the sludge solids closer together, excluding water which is separated from the sludge. The resulting sludge is much easier to dewater on a screw press, filter press or other dewatering device. The medium molecular weight polymers often used in this application appear to be capable of “squeezing” the sludge solids tighter, concentrating the sludge more effectively than a higher molecular weight polymer of the same charge.
As a final note about sludge dewatering, biological sludges such as activated sludge from a waste treatment plant or sludge resulting from an anaerobic biological treatment system are notoriously difficult to dewater. In these cases, inorganics such as ferric chloride or lime are used to assist in the dewatering process. Also, “anionic coagulants” are emerging as a possible solution to this problem.
Another use for water soluble water treatment polymers is in paint detackification. This is a very broad subject and will not be fully covered here. However, a few comments can be made. Paint detackification is the process of removing oversprayed paint from a paint spray booth in a paint shop in an easily managed, non-tacky (detackified) state. The oversprayed paint is usually trapped in a water curtain or shower containing various treatment chemicals, many times including either or both anionic and cationic polymers. The treatments vary from company to company and from application to application. The only rule that we have been able to formulate regarding paint detackification is that every system is different and no treatment seems to work exactly the same on any of them. In some cases an anionic polymer will be added followed by a cationic polymer. Other times the reverse approach is taken. Often, inorganic chemicals such as silicates or aluminates are used.
The wastewater treatment market is being driven by increased public awareness and government regulation. Companies that were not treating their wastewater 10 years ago today find that their local publically owned water treatment (POWT) facility is requiring pretreatment before discharge. In fact, wastewater treatment has seen annual growth of 5 to 11% over the past decade.
Coagulants, flocculants, and precipitants play a central role in wastewater treatment. By familiarizing yourself with the various products that are available, where to use them, and how they are applied, you can tap into this fast growing market and open doors to new water treatment business.Visit Our Home Page