By Monica Webb,2014-04-14 11:14
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    The objective of flocculation is to bring the particles into contact so that they will collide, stick together, and grow to size that will readily settle. Enough mixing must be provided to bring the floc into contact and to keep the floc from settling in the flocculation basin. Too much mixing will shear the floc particles so that the floc is small and finely dispersed.

     The objective of coagulation (and subsequently flocculation) is to turn the small particles of color, turbidity, and bacteria into larger flocs, either as precipitates or suspended particles. These flocs are then conditioned so that they will be readily removed in subsequent process.

    3+The two most commonly used coagulants are aluminium (Al) and ferric ion

    3+(Fe). Aluminium can be purchased as either dry or liquid alum [Al(SO)•14HO]. 2342

    Commercial alum has an average molecular weight of 594. Liquid alum is sold aas approximately 48.8 percent alum (8.3% AlO) and 51.2 percent water. If it is sold as a 23

    more concentrated solution, there can be problems with crystallization of the alum during shipment and storage.

    When alum is added to a water containing alkalinity, the following reaction occurs:

    - 2-Al(SO)•14HO + 6HCO; 2Al(OH)•3HO (s) + 6CO + 8 HO + 3SO 2342332224

    Such that each mole of alum added uses six moles of alkalinity and produces six moles of carbon dioxide. The above reaction shifts the carbonate equilibrium and decreases th pH. However, as long as sufficient alkalinity is present and CO(g) is allowed to evolve, 2

    the pH is not drastically reduced and is generally no an operational problem. When sufficient alkalinity is not present to neutralize the sulfuric acid production, the pH may be greatly reduced:

    - Al(SO)•14HO ; 2Al(OH)•3HO (s) + 3HSO + 2HO 234232242

    If the second reaction occurs, lime or sodium carbonate may be added to neutralize the acid.

     Two important factors in coagulant addition are pH and dose. The optimum dose and pH must be determined from laboratory tests. The optimal pH range for alum is approximately 5.5 to 6.5 with adequate coagulation possible between pH 5 to pH 8 under some conditions.

     An important aspect of coagulation is that the aluminium ion does not really exist

    3+as Al and the final product is more complex than Al(OH)When the alum is added to 3.

    the water, it immediately dissociates, resulting in the release of an aluminium ion surrounded by six water molecules. The aluminium ion immediately starts reacting with the water, forming large Al•OH•HO complexes. Some have suggested that it forms 24+ [ALOH•28HO]as the product that actually coagulates. Regardless of he actual 8202

    species produced, the complex is a very large precipitate that removes many of the colloids by enmeshment as it falls through the water. This precipitate is referred to as a floc. Floc formation is one of the important properties of a coagulant for efficient colloid removal. The final product after coagulation has three water molecules associated with it in the solid form as indicated in the equations.

     One of the most common methods to evaluate coagulation efficiency is to conduct jar tests. Jar testing is a method of simulating a full-scale water treatment process, providing system operators a reasonable idea of the way a treatment chemical will behave and operate with a particular type of raw water. Because it mimics full-scale operation, system operators can use jar testing to help determine which treatment chemical will work best with their system’s raw water.



    The equipment used include jar test apparatus and beakers


    The materials used include wastewater and aluminium sulphate Al(SO) 243


    1. One liter of wastewater was filled into each of six beakers and labeled from 1 to 6. 2. Beaker 1 was used as control in the experiment. Coagulant, aluminium sulphate

    Al(SO) was weighed as shown below: 243

    Beaker number Quantities of

    coagulants (g)

    1 0.0

    2 0.2

    3 0.4

    4 0.6

    5 0.8

    6 1.0

    3. Gang stirrer was put into the six beakers and run at 250 rpm for 3 minutes concurrently. 4. Then, the gang stirrer was run again at 30 rpm for 30 minutes.

    5. After that, the stirrer was stopped and taken out of the beakers.

    6. The samples were taken from each of the six beakers and tested for its turbidity and pH.

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