Coagulation (water treatment)

Coagulation is affected only by the type of coagulant used, its dose and mass; pH and initial turbidity of the water that is being treated; and properties of the pollutants present. The effectiveness of the coagulation process is also affected by pretreatments like oxidation. == Mechanism ==

In a colloidal suspension, particles will settle very slowly or not at all because the colloidal particles carry surface electrical charges that mutually repel each other. This surface charge is most commonly evaluated in terms of zeta potential, the electrical potential at the slipping plane. To induce coagulation, a coagulant (typically a metallic salt) with the opposite charge is added to the water to overcome the repulsive charge and "destabilize" the suspension. For example, the colloidal particles are negatively charged and alum is added as a coagulant to create positively charged ions. Once the repulsive charges have been neutralized (since opposite charges attract), van der Waals force will cause the particles to cling together (agglomerate) and form micro floc. == Determining coagulant dose ==

Jar test The dose of the coagulant to be used can be determined via the jar test. The jar test involves exposing same volume samples of the water to be treated to different doses of the coagulant and then simultaneously mixing the samples at a constant rapid mixing time. Furthermore, the analysis of jar test experiments produces results which are often only semi-quantitative. Coupled with the wide range of chemical coagulants and flocculants that exist, it has been remarked that determining the most appropriate dewatering agent as well as the optimal dose "is widely considered to be more of an ‘art’ rather than a ‘science’". As such, dewatering performance tests such as the jar test lend themselves well to miniaturization. For example, the Microscale Flocculation Test developed by LaRue et al. reduces the scale of conventional jar tests down to the size of a standard multi-well microplate, which yields benefits stemming from the reduced sample volume and increased parallelization; this technique is also amenable to quantitative dewatering metrics, such as capillary suction time. Streaming current detector An automated device for determining the coagulant dose is the Streaming Current Detector (SCD). The SCD measures the net surface charge of the particles and shows a streaming current value of 0 when the charges are neutralized (cationic coagulants neutralize the anionic colloids). At this value (0), the coagulant dose can be said to be optimum. == Limitations ==

Coagulation itself results in the formation of floc but flocculation is required to help the floc further aggregate and settle. The coagulation-flocculation process itself removes only about 60%-70% of Natural Organic Matter (NOM) and thus, other processes like oxidation, filtration and sedimentation are necessary for complete raw water or wastewater treatment. == See also ==