In many water treatment plants, the processes of coagulation and flocculation follow pH adjustment and come before sedimentation. Taking place in immediate succession, these two are often used interchangeably, or at times, are referred to as just flocculation. But the coagulation-flocculation step is actually a combination of two distinct, albeit closely related, processes, both of which are explained in detail in the next sections of this post.
Coagulation and flocculation are two essential phases in a water purification system, aimed to remove the cloudiness or turbidity of water. When water has a high turbidity, there is no assurance that it can be completely disinfected. The recommended level of turbidity is 0.1 NTU (Nephelometric Turbidity Unit) although the allowable maximum level is at 0.5 NTU.
When coagulation and flocculation are successfully done, the resulting water is clear, odorless, and colorless.
What are Colloidal Particles?
To have a better understanding of the processes of coagulation and flocculation, an explanation of the nature of the matter that these steps would eliminate is important.
Various clarification systems have been designed to remove all forms of suspended matter in untreated water. In application however, many of these systems fall short of being able to clarify the water because many suspended solids are too small and too dispersed to settle, float, or be filtered out, therefore the need for chemical coagulants. This type of solids is generally known as colloidal particles.
Colloidal particles, also known as nonsettleable solids, are particles suspended in water which are very small in size with diameters ranging from 1 to 1000 nanometers. They remain very stable in aqueous solutions, and are known to be the primary cause of turbidity, bringing about objectionable tastes, colors, and odors in drinking water. Colloidal solids can come from mineral, organic, or biological sources.
One specific characteristic of a colloidal particle is that it carries a negative electrical charge. When numerous particles of the same nature (negatively charged) come together, they tend to repel each other, and hence, remain dispersed while suspended in water.
The purpose of introducing coagulants into the water is to neutralize the negative charges carried by the colloids, allowing them to bond together.
The Process of Coagulation
With the addition of coagulants, the process of coagulation thus commences.
Coagulation is defined as the destabilization of colloids, and is both a physical and chemical process. First, chemical coagulants that are cationic or positively-charged are added to the raw water. The coagulants work to destabilize the negative forces of the colloidal particles so that by neutralizing these colloids, they are now more prone to agglomerate or bunch up upon collision with each other.
During this process of neutralizing, colliding and sticking together, colloidal particles form microflocs, solids that are relatively bigger in size but are still invisible to the naked eye. What should be noticeable now is that the water where the microflocs are supposed to have formed should be clear and colorless.
If cloudiness remains, then the colloids have not been totally neutralized. More chemical coagulants may have to be added for the coagulation to be successful
The amount of coagulants needed is dependent on the zeta potential present in the colloidal system. By definition, the zeta potential is the electrokinetic potential surrounding colloidal solids in a liquid medium. It is indicative of the degree of repulsion between the similarly charged particles which helps them stay in the water.
Colloids with high zeta potential are more stabilized and remain dispersed in water, while those with low zeta potential have a higher tendency to agglomerate. Therefore, the greater the zeta potential, the more coagulants will be needed to overcome it.
To ensure that coagulants are sufficiently dispersed throughout the raw water, high-energy and rapid mixing is required. Good coagulation results not only in the proper dispersion of the chemicals but also in the promotion of particle collision.
In most water treatment systems, there is a rapid-mix chamber where the coagulants are to be added. The recommended contact time in this chamber is from one to three minutes. Over-mixing will not affect the end result as much as insufficient mixing will. In fact, if the coagulants are not adequately mixed into the water, the process would be incomplete and will cause the flocculation to be ineffective as well.
The Process of Flocculation
To further enhance water clarification, flocculation comes into play as soon as the colloidal particles are neutralized by coagulants. You can think of this process as the ‘bridging’ of the submicroscopic microflocs produced by coagulation so that they form larger and visible flocs which can then be easily removed in the succeeding steps of sedimentation and filtration. These larger flocs are also referred to as pin flocs.
During this step, the water flows through to a flocculation basin, typically a tank with paddles that slowly mix the particles together. In contrast to the rapid mixing during coagulation, flocculation requires gentle motions because if the mixing is done too fast, the flocs will break apart when they come into contact instead of adhering to one another.
Aside from the physical mixing, flocculation is further aided by the addition of flocculant polymers, also called coagulant aids, which improve the efficiency of the process. These polymers are of high molecular weight and are either cationic (positively-charged) or anionic (negatively-charged). Because of such densities and ionic charges, they are able to bridge the unstable particles which have previously coagulated. With additional collisions, floc size is increased and the flocs strengthened.
Depending on the type and design of the sytem and equipment, floc formation should take no more than 20 minutes to an hour or so. As the flocs increase in size, the speed of the mixing should gradually taper off, until the ideal floc size and strength is achieved. At this point, the larger, agglomerated particles should easily settle and the water can now be deemed ready for sedimentation.
Coagulants and Flocculants
Coagulants are generally classified into two types: primary coagulants and coagulant aids.
Primary coagulants are responsible for the destabilization or neutralization of colloidal matter in water so that they form microflocs. Thus, primary coagulants are always used during coagulation. Coagulant aids (also known as floc builders) on the other hand, serve to enhance the flocculation process, strengthening and adding weight to the flocs. Because coagulant aids mainly reduce the time required for the formation of larger flocs, these may or may not be used during this step.
Inorganic coagulants which are commonly based on either aluminum or iron are the most widely used, easily available, and cost-effective coagulants. Aluminum-based coagulants include sodium aluminate, alum, and aluminum hydroxide, while iron-based coagulants include iron hydroxide, ferric sulfate, and ferric chloride. These types are known to effectually remove majority of suspended solids, while the volume of flocs produced can also entrap bacteria in the process.
Organic polymer flocculants typically serve as coagulant aids, and are introduced only after inorganic coagulants like alum and iron salts have been added. Two common types of cationic polymers are utilized in water treatment: polyAMINE and polyDADMAC. However, while these organic polymers are effective in forming easy-to-remove flocs, they are usually more expensive than inorganic coagulants.