Most flocculants in use today are synthetic water soluble polymers based on acrylamide and its derivatives and which may include either anionic or cationic groups in a wide range of molecular weights and charge densities. They are supplied as dry powders or in liquid form as liquid dispersions up to 50 percent active, emulsions up to 40 percent and in some cases as low concentration solutions.
Most flocculants in use today are synthetic water soluble polymers based on acrylamide and its derivatives and which may include either anionic or cationic groups in a wide range of molecular weights and charge densities. They are supplied as dry powders or in liquid form as liquid dispersions up to 50 percent active, emulsions up to 40 percent and in some cases as low concentration solutions.
They all are characterized by high solution viscosities at very low concentrations. The correct preparation of the flocculant solution is of paramount importance. This ensures the optimum solution is prepared every time and that expensive polymers are used to their maximum efficiency.
Flocculation
The performance improvement in dewatering processes from using synthetic polymers is achieved by bringing dispersed particles together, increasing the effective particle size of the solid phase. The stability of the suspension is broken and the liquid phase is released.
The term flocculation is derived from the Latin ?flocculus,? literally a small tuft of wool or a loosely fibrous structure. It often is confused with coagulation, although the two mechanisms are quite different, and the two processes may be used together.
Coagulation basically is electrostatic in that it is induced by neutralizing the repulsive charges between particles. Flocculation is brought about by the action of long chain high molecular weight polyelectrolytes where the polymer physically forms a bridge between two or more particles, uniting them into a random three dimensional structure that is loose and porous.
The great majority of flocculants in use today are synthetic water-soluble organic polymers of very high molecular weight consisting of molecules that are long straight chains. In recent years a number of ?structured? polymers also have been introduced. In these the individual polymer molecules are branched and joined into a matrix structure. Structured polymers are only available in liquid forms.
Requirements of flocculant polymers are that they be strongly adsorbed onto the particles and that they are capable of spanning the gap between the particles. Synthetic polymers of high molecular weight are long enough for one end of a single molecule to adsorb onto one particle and the other end onto a second particle. Higher molecular weight polymers can adsorb on several particles at once, forming a three-dimensional matrix.
In general, the higher the molecular weight the better the flocculation and the faster the sedimentation rate. In the case of filtration, however, the lower molecular weight products can be more effective. This is because the flocs formed with high molecular weight products are relatively large, trapping water within the structure and increasing the final moisture content of the filter cake.
Most synthetic flocculants are based on polyacrylamide and its derivatives. Flocculants generally carry either a positive (cationic) or a negative (anionic) charge. Polyacrylamide itself essentially is nonionic and the desired ionic character is produced by copolymerisation with other monomers. Anionic polyacrylamides are produced by copolymerisation with acrylic acid, while for cationic polymers one of several cationic monomers will be used. These charges serve two purposes; they provide a means of adsorption onto the particle surfaces by electrostatic attraction and they cause the polymer molecule to extend and uncoil due to charge repulsion along the polymer chain.
Flocculant Preparation
For a synthetic flocculant to function effectively it must be ?released? from its transported state to be available as a free, dissolved and fully extended single molecule. Solid grade polymers are supplied as small particles which can be considered as hard packed tangles of long polymer chains similar to balls of string. For the individual molecules to be released they must absorb water and uncoil by hydrating and activating their repulsive ionic groups. This process typically requires one hour of solution aging time, although it should be noted that longer periods can be required for low charge products or where the water temperatures are low.
Unfortunately, the wetted polymer on the outer surface of each particle forms initially into a highly viscous gel which resists the passage of free water necessary for hydrating the polymer in the center. It is at this stage that problems with polymer dissolution can occur. If the particles are not completely separated when their surface is wetted, the outside surface of a cluster of particles forms a gel layer through which water cannot penetrate. Thus it is critical that the wetting out device provides good mixing of completely separated particles with the water. It is generally accepted in the industry that this is best achieved with a pneumatic transport system for the polymer. A batch make-up system is also preferable since this ensures that there is no possibility of ?under-age? or partially hydrated particles going forward to the dewatering process.
Liquid grade polymers also require special treatment. Most are supplied as dispersions or emulsions in a mineral oil and the stability of the mixture must be ?broken? to enable the polymer solution to form. Various proprietary types of mixing equipment are supplied for this purpose, with the critical factors being the avoidance of gel blockage formation at the polymer/water interface and the provision of adequate shear during the initial mixing process. Aging is still required but the molecules are generally much less tightly coiled than in dry grades and already are partially hydrated. So 20 minutes aging time would be more typical for these products.
In whichever physical form the polymer initially was supplied, for maximum efficiency, normal practice is to produce the initial solution at between 0.25 percent and 0.6 percent real polymer concentration. Polymer solutions are sensitive to shear once the molecules are hydrated and this sensitivity increases significantly with further dilution. This does vary with different products and the supplier should be consulted if in doubt, but centrifugal pumps should not be used for polymer solutions. Progressive cavity pumps are favored by a majority of users although other low shear designs also can be considered. Generally, it is preferable for any flow splitting or mechanical metering activities to be undertaken before any secondary dilution takes place.
Flocculant Addition
The application of the polymer solution to the sludge is important enough sometimes to make the difference between success and failure of flocculant treatment. The polymers by their nature have very viscous solutions and to be distributed homogeneously throughout the suspension is sometimes a problem. The flocculant is very attractive toward particle surfaces and becomes irreversibly attached, so uneven distribution wastes flocculant in polymer-rich areas. The overall result is for dosage to be stepped up until satisfactory performance is achieved at a much higher cost than would otherwise be required.
The polymer solution should therefore be added at a point of local turbulence either existing or created. This must give sufficient agitation for complete and even distribution of the flocculant but without producing excess shear that would break down the flocs that are formed. It is normal to assist this mixing by dilution of the polymer solution to 0.1 percent or below to reduce the viscosity difference between the liquids that must be mixed. Mixing is further assisted by using multiple addition points, static mixers or one of a number of proprietary systems available. Positioning of the addition point in relation to any other chemical additions and to the residence time before the dewatering process may be another important factor.
Polymer Selection
There is a very wide range of flocculants available and selecting the most appropriate for any particular application can be a daunting task. The broad selection of ionic character and molecular weight band can be achieved from knowledge of the process streams and the type of dewatering process in use. The physical form is likely to be dictated by cost, shipping and physical handling criteria. In general, dry powder grades are less expensive than equivalent liquids but can have more expensive equipment requirements and can involve more manual handling.
Organic sludges normally require cationic polymers for optimum flocculation, with highly biological sludges reacting most favorably to the highly charged cationic products. Inorganic sludges and some organics that previously have been treated with inorganic coagulants, such as aluminum or ferric salts or lime, normally require anionic or nonionic polymers.
Future Developments
Polymer manufacturers are continually developing their products, applications experience and expertise to improve the cost-performance of dewatering processes. Process control equipment now is available for all types of dewatering processes so that automatic adjustments to polymer can be made in response to changes in sludge flow or concentration. Bulk and semi-bulk delivery and handling systems for dry polymers are available as standard items. For remote sites a fully weatherproof outdoor installation can be provided that does not require any building for the flocculant storage, dissolution, dosing or control equipment.
About the Author:
About the Author: Steve Waller is Regional Engineering Manager, Pollution Control, of Ciba Specialty Chemicals. Ciba Specialty Chemicals is a large supplier of flocculants and make-up systems worldwide.
