Fundamentals of coagulation and flocculation

Chemical addition to wastewater has become common over the last decades. Terms like coagulant, flocculant, colloidal are tossed around in discussions, but do you know what they are and what role they serve in the removal of the contaminants from your wastewater stream?

Colloidal particles found in wastewater are usually the most challenging portion of your insoluble contamination to remove. Colloidal particles are of size 0.01 µm to 1.0 µm and usually have a net negative surface charge. Due to their small size, the net negative electrical charge exerts repelling forces on other colloidal particles, resulting in forces that are of greater magnitude than the settling or attraction forces. These repulsion forces create what can best be described as a suspension of the colloidal particles resulting from an equilibrium of the magnitude of the net negative charge that they exert on each other. This phenomenon is called Brownian motion.

Brownian motion can be disturbed by the addition of a coagulant. The main task of a coagulant is to destabilize the equilibrium between colloidal particles. Being a positively charged particle, the addition of a coagulant will result in the neutralization of the net negative surface charge of the colloidal. Therefore, colloidal particles will aggregate around the coagulant creating what is called perikinectic flocculation. Perikinetic flocculation is simply the process in which particles of size 0.01 µm to 1.0 µm aggregate to form particles of greater size, around 10 µm. Inorganic coagulants on the market are divided in two categories which are the iron salts and the aluminum salts. The Iron salts includes ferric sulfate and ferric chloride. On the other hand, the most common aluminium salts are aluminium sulfate and polyaluminium chloride.

Organic coagulant available at Aquasan are separated into 2 different families: Polyamines and polyDADMACS. These polyelectrolytes also have the capacity to destabilize the equilibrium between colloidal particles while having a broader range of charge density and molecular weights.

Once we reached particle size of 10 µm, we can introduce another element that will promote the formation of aggregates, the flocculant. Flocculants are usually organic polymers, charged positively or negatively to enhance the flocculation reaction. During flocculation or macroflocculation, adding a velocity gradient is needed to allow the particles to fully interact with the flocculant. The purpose of this process is to create larger particles that can be easily removed by particle-separation procedure such as gravity sedimentation and filtration. Macroflocculation can’t occur until the colloidal particles reach a size of 1 to 10 µm as the polymer is inefficient at disturbing the Brownian motion and won’t be able to aggregate the colloidal particles.

All this theory can be a bit daunting, but ultimately, it is important to understand the role of each of the elements. Colloidal particles refer to the particles that reach equilibrium in the solution, these particles won’t settle or bind together. Coagulation is the process in which we destabilize this equilibrium, creating large particles. Coagulation needs to occur before flocculation or else the flocculent won’t be able to aggregate the colloidal particles. The flocculation simply creates large particles that allows the removal of the contamination by means of settling or separation.