Coagulation and Flocculation | Activated Sludge Process | Wastewater Treatment Plant

 COAGULATION AND FLOCCULATION

The chemicals added to wastewater during treatment are chosen to perform specific functions. One of the most important of these tasks is to convert colloidal particles, chemical components, and waste particles suspended in solution so that they do not settle out into particles large and heavy enough to settle out of the water in sedimentation basins. Many of the dissolved and fine suspended particles are useful in the biological treatment process, but many others are either harmful to biology or useless to it, and thus pass through the system untreated unless chemical reactions are created to aid in removal.

To remove these particles and substances, it is necessary to first convert these very small suspended particles and dissolved materials into larger suspended particles that will settle out in a clarifier or sedimentation basin. It is also preferable that all of this occurs relatively quickly in order to reduce treatment time and thus reactor volume requirements.

There are two physical issues here. The first involves the removal of hydrophilic particles (those that thrive in the presence of water), while the second involves the removal of hydrophobic particles (those that are not happy living in the presence of water). Furthermore, they all have such a high surface area to mass ratio that if left untreated, they will simply not settle.

These particles are kept apart from each other in nature and in wastewater by repulsive electrical charges. When poles of the same charge are placed near each other, these repulsive forces act similarly to magnets. When the appropriate chemicals are added, the repulsion forces are overcome by a suppression action on the external electrical charges, and the particles begin to bind. Polymers are other chemicals that form chemical "strands" to which colloidal particles attach until the strand becomes heavy enough to settle. When sufficient charges are suppressed or the strands become long and heavy enough, the particles gain mass and settle out of the water in sedimentation basins.

Coagulation is the chemical process of converting submicron particles to suspended matter (or, in essence, the destabilization of the particles by suppression of the charged surface layers), whereas flocculation is the aggregation of the destabilized particles into a large enough mass to settle. In wastewater treatment, the terms are often used interchangeably, as in "coagulation/flocculation," or they are referred to collectively as "coagulation." If the coagulated particles are to be removed from the wastewater, these two steps must be followed by a sedimentation step. As a result, it's usual to refer to the complete process as "coagulation/flocculation/sedimentation" after adding this third phase to the chain.

Coagulation is a very fast process, whereas flocculation is a much slower process. It is therefore advantageous to ensure that the chemicals come into contact with the colloids as soon as possible. This is typically accomplished with a rapid mixer, often an in-line mixer, which violently mixes the chemical into the water in 30 to 60 seconds of contact time. The mixture is then transferred to the flocculation basin, where the chemicals and wastewater are stirred slowly and gently. This slow, gentle stirring allows the chemicals to suppress the electrical charges, causing the particles to collide and attach without adding so much energy to the system that the combined particles are ripped apart again. This step usually takes about 30 minutes to complete. The water enters a quiet settling zone in a sedimentation basin at the end of the flocculation period, where it sits for 2 to 4 hours to allow the coagulated and flocculated masses to settle out. The settled matter is mechanically removed from the reactor's bottom and handled as waste sludge.

These three distinct steps must be carried out in the order listed. To a point, the longer the flocculation and sedimentation steps, the better the overall colloidal removal rate. Longer detention times have a point of diminishing returns, and those are close to the time limits mentioned earlier. It is also worth noting that this process does not generally remove all colloidal particles, so some form of filtration is required to effectively remove the remainder. Filtration without prior sedimentation is also possible, but it quickly clogs the filter and causes major maintenance issues.