water treatment Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, inc ...

, coagulation and

flocculation Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from pr ...

involve the addition of compounds that promote the clumping of fine floc into larger floc so that they can be more easily separated from the water. Coagulation is a chemical process that involves neutralization of charge whereas flocculation is a physical process and does not involve neutralization of charge. The coagulation-flocculation process can be used as a preliminary or intermediary step between other water or

wastewater treatment Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environme ...

processes like filtration and

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...

. Iron and aluminium salts are the most widely used coagulants but salts of other metals such as

titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

and

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...

have been found to be highly effective as well.

Factors

Coagulation is affected by the type of coagulant used, its dose and mass; pH and initial

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids can ...

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 Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

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 Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface. Zeta potential is a scientific term for electrokinetic potential in coll ...

, 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 In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...

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. The microfloc formed after coagulation further undergoes flocculation and is allowed to settle. Then the

of the samples is measured and the dose with the lowest turbidity can be said to be optimum.

Microscale dewatering tests

Despite its widespread use in the performance of so-called "dewatering experiments", the jar test is limited in its usefulness due to several disadvantages. For example, evaluating the performance of prospective coagulants or flocculants requires both significant volumes of water/wastewater samples (liters) and experimental time (hours). This limits the scope of the experiments which can be conducted, including the addition of replicates. 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 A microplate, also known as a microtiter plate (''Microtiter'' is a registered trademark in the United States, therefore it should not be used generically without attribution), microwell plate or multiwell, is a flat plate with multiple "wells" ...

, 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 Streaming media is multimedia that is delivered and consumed in a continuous manner from a source, with little or no intermediate storage in network elements. ''Streaming'' refers to the delivery method of content, rather than the content it ...

value of 0 when the charges are neutralized (

cationic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

coagulants neutralize the

anionic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

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.

References

{{reflist Chemical engineering Water treatment Water technology Environmental engineering