Report Writing : Sample

OBJECTIVE :
The objective of this test is to determine the chemical coagulation of water sample and thereby
• to determine the optimum doses of coagulant which wll be use to remove colloidal particles by forming larger flocs.
REAGENTS :
• Standard Alum solution
• Standard Arsenic solution (if as removed is to be demonstrated)
APPARATUS :
• Coagulation (stirring) device
• PH meter
• Turbidity meter
• Glass beakers (1000 ml; 6 nos)
PROCEDURE :
• We have to determine PH and turbidity of the water to be treated.
• We have to fill six 1000 mL beakers each with 500 mL water to be treated.
• We have to add required coagulant (standard alum soluiton) to each beaker.
• We have to mix the samples in the beaker with the help of the stirring device. The sample has to be subjected to one minute of rapid mixing followed by 15 minutes of slow mixing ( about 40 rpm)
• We have to allow the flocs to settle down for about 15 minutes. We have to observe the characteristics of the flocs and the settling rates.
• We have to collect the supernatant from each beaker and measure PH and turbidity of each.
• We have to plot PH versus alum dose in a graph paper and we have to observe the effect of alum dose on PH.
• We have to plot turbidity versus the coagulant(alum) dose in a graph paper. We have to determine optimum dose of alum from the plot.
ASSIGNMENT ON DETERMINATION OF CHEMICAL COAGULATION (ALUM COAGULATION)
QUESTION NO. 1:
What do you understand by coagulation and flocculation? Which coagulants are most commonly used for water and wastewater treatment.
ANSWER :
COAGULATION :
The term coagulation is used to describe the process by which the charge on particles is destroyed .
Surface water generally contains wide variety of colloidal impurities that may cause the water to appear turbid and may impart color to the water. Colloidal particles that cause color and turbidity are difficult to separate from water because the particles will mot settle by gravity and are so small that they pass through the pores of most common filtration media. In order to be removed, the individual colloids must aggregate and grow in size so that they can settle by gravity. Chemical agents are used to promote colloid aggregation by destroying the forces that stabilize colloidal particles. The process of destroying the stabilizing forces and causing aggregation of colloids is referred to as chemical coagulation.
FLOCCULATION :
The term flocculation is used to determine the aggregation of particles into larger units.
The chemicals used for the purpose of coagulation is called coagulants.The most common coagulants used in water and wastewater treatment are aluminum and ferric salts as alum, ferric chloride and ferric sulfate.
ASSIGNMENT ON DETERMINATION OF BIOCHEMICAL OXYGEN DEMAND (BOD)
QUESTION NO. 1: What do you understand by coagulation and flocculation? Which coagulants are most commonly used for water and wastewater treatment.
ANSWER:
Various types of color and turbidity producing colloidal impurities do not settle by gravity and are so small that they pass through the pores of most common filtration media. The process of removing these by destroying their stabilizing forces and causing aggregation using chemical agents is referred to as chemical coagulation.
"Coagulation" is the process by which the charge on particles is destroyed, and 'flocculation" is the aggregation of particles into larger units.
The chemicals used for the purpose of coagulation are called coagulants. The most common coagulants used in water and wastewater treatment are -¨ aluminum and ferric salts such as alum,ferric chloride and ferric sulfate.
QUESTION NO. 2:
Why addition of alum may result in a drop in pH value. Discuss the affect of alum dose on pH from your experimental results.
ANSWER :
The common metal salt alum (aluminum sulfate) is a good coagulant for water containing appreciable organic matter. The chemical formula used for commercial alum is Al2(S04)3 . 14 H2O. Once dissolved in water, aluminum forms hydroxo-complexes and solids [e.g., Al(OH)3(s), Al(OH) +2, Al(OH)2+, Al(OH)4- ; Eqs. 1-5] and as a result pH of water is lowered, especially if alkalinity of water is low.
Al2(SO4)3 . 14 H2O (alum) = 2Al +3+ 3 SO4 –2 (1)
Al +3+ 3H2O = Al(OH)3(s) + 3 H+ (2)
Al +3+ H2O = Al(OH) +2 + H+ (3)
Al +3+ H2O = Al(OH)2+ + 2 H+ (4)
Al +3+ H2O = Al(OH)4- + H+ (5)
Theoretically, each mg/l of alum consume approximately 0.50 mg/l (as CaCO3) of alkalinity. For water with low alkalinity, this may result in significant reduction in pH that may interfere with formation of aluminum hydroxide flocs. In water treatment with alum coagulation, residual aluminum that may be present in water after the coagulation process is a cause of concern.
Remedy: If the alkalinity is insufficient, coagulant aids such as *lime ,Ca(OH)2, *soda ash (Na2CO3), *activated silica and *poly-electrolytes are used to provide the necessary alkalinity.
We did not measure pH during our experiment. So we cannot discuss the affect of alum dose on pH in this experiment.
QUESTION NO. 3: What is the primary mechanism by which heavy metal ions are removed during coagulation.
ANSWER:
Coagulation with alum and ferric chloride or ferric sulfate is also widely used for removal of heavy metal ions (e.g., lead, arsenic) from water. In this process heavy metal ions are primarily removed by adsorption (and subsequent precipitation) onto coagulated flocs of metal (either aluminum or iron) hydroxides. Coagulation with alum and ferric chloride/sulfate have been successfully used for removal of arsenic from water.
DATA & CALCULATION SHEET:
EXPT NO. : 12
EXPT NAME : DETERMINATION OF CHEMICAL COAGULATION(ALAM COAGULATION)
Alum Dose
mg/L Alum Dose
mg/500 ml Alum solution Added ml Turbidity
NTU
10 5 0.5 4.6
20 10 1.0 4.9
30 15 1.5 5.8
50 25 2.5 6.5
80 40 4.0 7.1
100 50 5.0 12.7
150 75 7.5 4.6
200 100 10.0 3.9
250 125 12.5 3.5
300 150 15.0 1.14
From the plot of the turbidity versus the coagulant (alum) dose,(in a plain graph paper) the optimum dose of alum =
Result :
The optimum dose of alum =
DISCUSSION :
On the basis of experiment and probable sources of errors :
• In this experiment we are asked to determine PH, color and arsenic concentration of the water to be treated. But we did not measured these parameters.
• The whole experiment was done by the lab technicians, so less error might occur.
• The lab technicians did not take precise measurement of volume of sample, coagulant added the sample.
• Collecting the supernatant from each beaker we measured only the turbidity, but not the PH
• During the 15 minutes of settling down, the sample might be disturbed by some means. For this reason, the result was subjected to error.
On the basis of Result:s
Optimum dose of alum =
Comments :

• The shape of the plot of measured turbidity and the dose of coagulant is almost opposite to that of the typical shape of the curve.
• From the curve we cant get the optimum dose of alum.

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