Chapter 8 Jar TestSDC

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  CHAPTER 8: JAR TESTING 1. Introduction Coagulation and flocculation are important unit processes used in water and wastewater treatment. Coagulation involves the addition of chemicals (coagulants) during relatively intense mixing to destabilize naturally occurring particles and macromolecules and/or to precipitate additional particles. In flocculation, a period of less intense mixing is used to promote the aggregation of destabilized particles into larger flocs that can be removed subsequently by sedimentation and/or filtration. During coagulation and flocculation, various dissolved ions and molecules may be adsorbed by particles or may be precipitated, depending on the type and concentration of species considered and on the overall solution chemistry. A useful laboratory experiment for evaluating coagulation and flocculation condition is the jar test.   This test involves an examination of the effects of coagulant addition, mixing, and settling on water quality parameters such as turbidity, color, total organic carbon, pH and alkalinity. The jar test is often used during the design of treatment facilities and in the routine operation of treatment plants. Jar tests are used primarily to determine the optimal chemical dosages for the removal of turbidity (particles), color, and organic matter and to assess the need for pH adjustment. The test can also be used to examine issues such as: the effect of powdered activated carbon addition; the effect of mixing intensity and duration on coagulation/flocculation; the effect of detention time on floc sedimentation; the kinetics of  particle aggregation; and the removal of trace constituents. 2. Theory Characteristically the insoluble particles in natural or waste water are very small and suspended in the solution (colloidal). The suspended stability of such particles is due to both  their small size and to the electrical charge (usually negative) on their surface causing them to repel their neighboring particles. To promote the removal of these suspended solids requires chemical coagulation and/or flocculation. Adding coagulants to the water creates a chemical reaction in which the repulsive electrical charges surrounding colloidal particles are neutralized, allowing the particles to stick together creating clumps or flocs. The aggregation of these  particles into larger flocs permits their separation from solution by gravity. The two most commonly used metallic coagulant are alum (Al 2 (SO 4 ) 3 .14H 2 O) and Ferric Chloride (FeCl 3 ). Alum reacts with alkalinity according to the following reaction Al 2 (SO 4 ).14H 2 O + 6HCO 3-  < = = > 2Al(OH) 3 (s) + 6CO 2  + 14H 2 O + 3SO 42-  This reaction decreases the pH. Then lime, Ca(OH) 2 , or soda, Na 2 CO 3 , could be added to ensure sufficient alkalinity and pH is not drastically reduced. The most important factor in coagulation is pH and dose, which could be determined by Jar test.  3. Materials and Methods Equipment    Jar Test apparatus (Phipps and Bird, Richmond, VA)    3-way pipetting ball    Pipet    Turbidity Meter (HACH RATIO/XR Turbidimeter, Loveland, CO ) Reagents    Alum      Sulfuric acid and NaOH for pH adjustment   Procedure The jar test is conducted on a sample of natural surface water to estimate an optimum dosage of alum for removal of suspended matter and to find the Turbidity in the sample water. The coagulation process is simulated using Jar Test apparatus (Phipps and Bird, Richmond, VA) at room temperature ranging from 22  –   25 o C. Optimum alum dosage    Add 1 or 2 liters of water sample to each vessel in the jar test apparatus.    There are 6 vessels in the test. The first vessel is blank. Do not add alum or adjust the pH in that vessel. The pH in the last 5 vessels should have been adjusted to the optimum pH by the TA.    Collect 30 mL of water sample to determine the turbidity of water sample before flocculation using Turbidity Meter.     Alum (Al 2 (SO 4 ) 3 .14H 2 O) is used as coagulant at 4, 6, 8, 10, 12 mg/L by adding 4 ml, 6ml , 8 ml ,10 ml, 12 ml of 1000 mg/L alum solution to last 5 vessels, followed by rapid mixing at 100 rpm for 1 minute, flocculation at 30 rpm for 30 minutes and settling for 1 hour.    Observe the flocculation process and record the time when you see the floc forming and when the coagulation process is completed for all 6 vessels. Also record settling time.    Collect 30 mL of water samples to determine the Turbidity samples in all 6 vessels after settling.    Measure the pH and of samples in all 6 vessels after settling. Each group may do only 2-3 vessels and share the data with the other group working on the same apparatus, making sure that all 6 vessels are covered.
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