User:Jigchen L. Norbu/sandbox

Coagulation (Water Treatment)
Coagulation is used in both raw water and wastewater treatment to remove colloids. It is used in conjunction with flocculation, as a preliminary or intermediary step between other treatment processes like filtration and sedimentation. Iron and aluminium salts are the most widely used coagulants but salts of other metals such as titanium and zirconium have been found to be highly effective as well.

Mechanism
The colloids in the water or wastewater are mostly anionic whereas the coagulant added is cationic. When the coagulant is added to the water, it neutralizes the colloids, decreases the repulsive forces and thus, allows them to aggregate and form floc.

Factors Affecting Coagulation
Coagulation is affected 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

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. The microfloc formed after coagulation further undergoes flocculation and is allowed to settle. Then the turbidity of the samples are measured and the dose with the lowest turbidity can be said to be optimum.

Streaming Current Detector (SCD)
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. Coagulant aids (polymers that bridge the colloids together) are also often used to increase the efficiency of the process