The flocculation and clotting are two processes within the water clarification stage. Both processes can be summarized as a stage in which the particles coalesce into small masses called floc or flocs, whose specific weight differs markedly to water and may precipitate or float. Originally mineral origin, are introduced in the market increasingly synthetic and polymer, which have greater advantages.

The flocculation is a process by which, with the addition of substances called flocculant, the colloidal substances in the water stick together, facilitating subsequent filtration and decantation. It is a step of purifying surface water source and treatment of domestic and industrial wastewater, with a large presence in the treatment of pools. The compounds concerned are the suspended solids, colloidal particles (less than one micron) and dissolved substances (less than several nanometers). The flocculation process is preceded by coagulation, so we often speak of coagulation-flocculation processes. These facilitate the removal of suspended matter and the colloidal particles, and avoids the use of disinfectants.

In the water treatment process, in order to get a better floc formation which in turn bring even greater performance on the steps of decantation and filtration, and ultimately the best possible quality of treated water, which are often used flocculation assistants, the products commonly known as 'Polyelectrolytes' or 'Polymers', or more precisely 'polyacrylamides'. These high molecular weight polymers may exhibit anionic properties, cationic or nonionic according to its manufacturing and uses for which they apply. Its use is important in circumstances and times when flocculation is more difficult, such as changes in water quality to be treated, and low temperatures.

A very small fraction of polyacrylamide polymer, known as residual acrylamide or free monomer of acrylamide, it's not become part of the floc, it's very soluble in water and remains dissolved therein, that is, becomes a contaminant of the treated water. The polymerized acrylamide is not toxic, but instead the monomer can cause various health problems. Therefore, the various regulations require manufacturers of these polymers to limit the content of free acrylamide. The maximum level is 250 mg. acrylamide monomer per kg. of product (0.025%).

The current European Directive for water consumption sets a parameter value of acrylamide in water, 0.1 g / l. This ceiling would be reached if the applied dose of the polymer, exceeds 0.4 p.p.m. The doses usually applied in the treatment of drinking water does not exceed 0.2 ppm.

At present there are many facilities along water treatment, have introduced a line of treatment of sludge generated (in clarifiers and filters) in order not to spill the channels these muds (hydroxides sludge), separating one part material semisolid by dehydration and recovering by recirculating a header clarified water. From another point of view, recycling and recovery of water, if an optimal clarified water is not achieved, may increase the risk of including and increase, besides acrylamide, certain microorganisms (Cryptosporidium) water to be treated. The recycling of this water can ultimately compromising the ability of a treatment plant to supply safe drinking water.

In the process of treating sewage sludge generally employed acrylamide polymers similar to those used in the flocculation of water with doses much higher than this, such that the free monomer of acrylamide is incorporated into the water if the recirculation is being carried out, increasing the concentration of acrylamide in the treated water, this could even exceed the value regulations (0.1μg / l).

For the calculation of acrylamide contained in the water, enough with dividing the total number of different polymers used, as in the sludge plant (applying the percentage of free acrylamide) by the volume of water treated at the same time .