Flocculation is widely employed in the purification of drinking water as well as sewage treatment, stormwater treatment and treatment of other industrial wastewater streams.  Flocculation refers to a process where a solute comes out of solution in the form of floc or "flakes." The term is also used to refer to the process by which fine particulates are caused to clump together into floc. The floc may then float to the top of the liquid, settle to the bottom of the liquid or can be readily filtered from the liquid.

The terms flocculation and coagulation are sometimes used interchangeably. However, it is more accurate to use the term coagulant for a chemical that contributes to molecular aggregation, rather than particular aggregation. Usually dissolved substances are aggregated into microscopic particles by a coagulant and then these particles may be flocculated into a macroscopic floc with a flocculant. In general, coagulants will have higher net charge and a lower molecular weight than flocculants.

Many flocculants are multivalent cations such as aluminium, iron, calcium or magnesium. These positively charged molecules interact with negatively charged particles and molecules to reduce the barriers to aggregation. In addition, many of these chemicals, under appropriate pH and other conditions, react with water to form insoluble hydroxides which, upon precipitating, link together to form long chains or meshes, physically trapping small particles into the larger floc.

Other factors such as pH, temperature and salinity can induce flocculation or influence flocculation rates.

Flocculation is a time-dependent, slow process that directly affects clarification efficiency by providing multiple opportunities for particles suspended in water to collide through gentle and prolonged agitation. The process takes place in a basin equipped with a mixer that provides gentle agitation. This agitation must be thorough enough to encourage inter-particle contact but gentle enough to prevent disintegration of existing flocculated particles. Effective flocculation is important for the successful operation of the sedimentation process.

Several issues regarding flocculation should be evaluated by water systems to ensure optimal operation of flocculation basins, including mixing and time, inlets and outlets and basin circulation. 

Proper flocculation requires long, gentle mixing. Mixing energy must be high enough to bring coagulated particles constantly into contact with each other, but not so high as to break up those particles already flocculated. Water systems should assess the following:

• How many stages are present in the flocculation system?  Three to four are appropriate to create plug flow conditions and allow desired floc formation.
• Is the mixing adequate to form desired floc particles?

If water passes through the flocculation basin in much less time than the volumetric residence time, the influent stream has short-circuited. Inlet and outlet turbulence is often the major source of destructive energy in flocculation basins that contributes to short-circuiting. To avoid this, water systems should evaluate the following:

• Do basin outlet conditions prevent the breakup of formed floc particles? Port velocities should be < 0.5 fps. The velocity gradient at any point from the flocculation basin to the sedimentation basin should be less than the velocity gradient in the last flocculation stage.
• Do inlet conditions prevent the breakup of formed floc particles? Inlet diffusers improve the uniformity of the distribution of incoming water. Secondary entry baffles across inlets to basins impart headloss for uniform water entry.
• What size are the conduits between the rapid mix basin and the flocculation basin? Larger connecting conduits help reduce turbulence.

Baffles are used in flocculator basins to direct the movement of water through the basin. Baffling near the basin inlet and outlets improves basin circulation and achieves more uniform circulation. Water systems may think about the following items when evaluating flocculation:

• Is current baffling adequate? Can baffling be added to improve performance or does existing baffling require repair? Serpentine baffling is better than over/under. Baffling should allow headloss through opening to prevent short-circuiting and to allow plug flow conditions.
• Induced velocity in floc chambers should vary from 2 fps in the first stage to 0.25 fps in the last stage. Velocity through openings in the baffle should be slightly less than the induced velocities.

There are over 50 different chemicals commonly used in treatment of potable water. This document lists some of the common chemicals and their process usage or function. Operators must refer to quality standards maintained by the American Water Works Association for technical information and recommendations for safe handling, storage, feeding and use of these chemicals.

1. Polymers used as flocculants generally have a high molecular weight and have a charge that is positive, negative (anionic) or neutral (nonionic).
2. The purpose of a flocculant is to bridge and enmesh the neutralized particles into larger floc particles, and they are generally fed at a dosage of less than 1 mg/L.
3. Flocculants should be fed at a point of gentle mixing (e.g., diffuser pipe across a flocculation basin) to prevent breaking apart the long-chained organic molecules.
4. Product density and solution strength information can be obtained from the individual polymer manufacturers.

The following chemicals are used as flocculants:

alum
aluminium chlorohydrate
aluminum sulfate
calcium oxide
iron(III) chloride
iron(II) sulfate
sodium aluminate
sodium silicate

The following natural products are used as flocculants:

Chitosan
Moringa oleifera seeds
Papain
A species of Strychnos (seeds)
Isinglass

1. Polymers used as filter aids are similar to flocculants in both structure and function.
2. Filter aid polymers are typically fed at dosages less than 0.1 mg/L; when fed in higher concentrations, they can contribute to filter head loss and short filter run times.
3. Filter aid polymers are fed at a point of gentle mixing (e.g., filter influent trough).
4. Product density and solution strength information can be obtained from the individual polymer manufacturers.

This information is from EPA's Handbook, Optimizing Water Treatment Plant Performance Using the Composite Correction Program, 1998 Edition, (EPA/625/6-91/027), Appendix M "Chemical Feed Guidelines," pp. 237-243.   Refer to the EPA Handbook, 1998 Edition, (EPA/625/6-91/027), page 241-243, for preparation of feed solutions.