determinatin of optimum alum dose

COAGULANT DOSAGE IN A WATER TREATMENT PLANT
INTRODUCTION The main difficulty is to determine the optimum coagulant dosage related
to the influent of raw water. Excessive coagulant overdosing leads to increased treatment costs and public health concerns, while underdosing leads to a failure to meet the water quality targets and less efficient
operation of the water treatment plant.
Manual methods mainly include jar testing

.
Jar testing involves taking a raw water sample and applying different quantities of coagulant to each sample. After a short period of time each sample is assessed for water quality and the dosage that produces the optimal result is used as a set point.
Operators change the dose and make a new jar test if the quality of treated water changes.

OBJECTIVES

To conduct jar test on a natural surface water in order to estimate an optimum dosage of aluminum sulfate or ferric sulfate for the removal of suspended matter , To observe the rate of floc formation and sedimentation.
The dose of coagulant which gives the best floc is the optimum dose of coagulants.

PROCEDURE

A. DETERMINE OF OPTIMUM COAGULANT DOSAGE

1. Collect 20 to 50 liters of a natural surface water. Analyze the water for pH, turbidity, color after filtration, and alkalinity.
2. Alternatively, make up a synthetic water sample for testing. Record both the water temperature and ambient air temperature.
3. Calculate the amount of alkalinity required to react with the maximum dosage of aluminum or ferric sulfate. If necessary, augment the natural alkalinity by the addition of 0.1 N Na2CO3 so that the alkalinity will be at least 0.5 meq/l (25 mg/l as CaCO3).
4. Measure exact 1 liter of water into each jar test reactor. Prepare portions of the aluminum or ferric sulfate solution which will yield 10 to 50 mg/l as Al2O3 or Fe2O3 when added to the sample aliquots.
5. Mix at 50 rpm to ensure water is completely mixed.
6. Measure chemical volumes to achieve desired dose in each reactor.
7. Increase mixing speed to 250 rpm. Add the chemicals to each reactor near the vortex. All reactor should be dosed at the same time.
8. Rapid mix for 1 min.
9. Reduce mixing to 60 rpm for 9 min.; Observe the reactors at 3 min. intervals to detect the formation of flocs. Reduce mixing to 25 rpm for 4 min.; Reduce mixing to 10 rpm for 2 min.
10. Turn off mixers and allow particles to settle for 20 min.
    Measure the turbidity or color, alkalinity, and pH of the liquid in each jar by sampling at the top, taking care not to disturb the sediment in sampling. Measure the depth of sludge in the beaker.

B DETERMINATION OF OPTIMUM pH Repeat the jar test of Part A using the observed optimum dosage of ferric sulfate but adjusting sample pH to 6, 7, 8, and 9 with NaOH or H2SO4 prior to adding coagulant.
Measure final pH, turbidity or color of the supernatant of each sample. Measure the depth of sludge in the beaker.
Plot turbidity or color versus pH.

 

EFFECT OF MIXING (AGGREGATION KINETICS)

Prepare identical (optimum) coagulant dosages for all six beakers.
Use the same rapid mix as before but vary the time of slow mix at 30 rpm. Use 5, 10, 15, 20, 30, and 45 minutes for the six jars respectively. Terminate mixing by carefully lifting the paddle from the beaker at appropriate time.
Allow 30 minutes for settling.
Measure the color or turbidity and pH of the supernatant in each beaker.

 

APPARATUS

Jar test apparatus and beakers
Magnetic stirrer plus magnetic stirring bars
Spectrophotometer or color comparator
Turbidimeter
pH meter
Assorted measurement pipettes (1, 5, 10 ml) and volumetric pipettes to 50 ml
Burette
Glass or plastic funnels
Ringstands and rings

 

MATERIALS

Aluminum sulfate solution, 1 g/l or ferric sulfate solution, 1 g/l
H₂SO₄, 2 liters each, 5 x 10^-2 M, 10^-2 M
NaOH,, 1 liter, 10^-1 M
Sodium carbonate, 1 liter each, 5 x 10^-2 M, 10^-2 M
Indicators: methyl orange, phenolphthalein
Synthetic water sample: Add suspended matter with a kaolin, montmorillonite, illite, or bentonite clay and/or the color with extract from boiled leaves or with instant coffee to raw water. Turbidity might be increased to about 40 turbidity units and color to about 80 color units (Clay suspension sample should be mixed for 3 days).
Buffer solution
Whatman #1 filter paper

REPORT

Prepare tables which facilitate comparisons of coagulant dosages with alkalinity, pH, color, turbidity, and other changes observed. Plot the inverse of turbidity and color versus coagulant dosage as part of the analysis. Plot turbidity versus coagulant dose at different settling times to determine the influence of floc formation and settling characteristics on the selection of coagulant dosage.
Comment on the differences between coagulation with iron and aluminum salts. Define the pH ranges over which each salt should result in effective coagulation.
Determine the rate of aggregation for a particular coagulant dose and pH by plotting the reciprocal of the turbidity versus time. (This would represent a second order reaction with respect to turbidity). Also plot the natural log of the reciprocal turbidity (representing a first order reaction) versus time. Determine which gives the best straight line fit of the data and determine the slope. HAND BOOK : ( From ENVO's Practical Experience of Last Ten Years )

Chemical

PH value


Dose


Aluminium Sulphate(ALUM)


6.5-8.5


70-85 mg/ liter


Ferric Sulphate


8-8.5


35 mg/liter


Ferric Chloride (Used in STP/ETP)


5.5


28-35 mg/liter


Ferrous Sulphate(Used in ETP)


8-8.5


35-70 mg/liter

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Saleem Asraf Syed Imdaadullah
Mobile : 9899300371
311/22,Zakir Nagar,New Delhi-110025
email: saleemasraf@gmail.com
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