The chemical physical treatments are processes that allow the removal of pollutant substances present in the water, generally poorly biodegradable, using chemical additives. These additives, commonly called "reagents", in particular conditions of pH, react with the pollutants, making them insoluble and favoring their separation by coagulation, precipitation and/or neutralization.
Coagulation: the colloidal particles (organic and inorganic), present in the effluent to be treated, generally have a net negative surface charge, that, due to the action of mutual repulsion between electrical charges of the same sign, maintains the particles in stable suspension within the liquid. Coagulation process destabilizes these surface charges, through the dosage of chemical reagents, in order to allow the aggregation of the colloidal particles in the micro-flakes.
Flocculation: is a process, that, by the dosing of chemical reagents of organic nature, increases the dimension of particles because of collisions between them, facilitating their separation from the water by sedimentation.
Generally, it takes place in tanks equipped with slow agitator with blades, to encourage the collision of particles and simultaneously to preserve the flakes formed. Here, irons or aluminum salts are used; subsequently polyelectrolyte is dosed to better the aggregation of suspended solids in the sludge flocks and promote their sedimentation.
But it is important to consider that the use of chemical reagents involves also a net increase of dissolved constituents (the TDS value is higher of 20-25%), an increase of the operating costs for the purchase of chemicals, an increase of danger in handling/storage of the reagents and a high production of toxic sludge difficult to dispose. In addition, the use of iron/polyelectrolyte and lime is very dangerous for any next treatment with membranes.
As a counterparty, this treatment also implies advantages if used as a system for increasing the efficiency of solids removal in the sedimentation tank, for the removal of phosphorus, heavy metals or inorganic compounds, difficult to separate with other processes.