Determination of optimal coagulant dosage using jar test

DETERMINATION OF OPTIMUM COAGULANT DOSE USING JAR TEST APPARATUS

AIM:- To determine the optimum coagulant dose.

APPARATUS:- Jar test apparatus and Nephelo turbidity meter.

THEORY:- In plain sedimentation, very fine suspended particles of size 0.006mm to 0.002mm are not removed, since they required a detention period of 10 hours to 4 days which is impracticable. In addition to this fine suspended particle, water also contains electrically charged colloidal particles which are continuously in motion and never settle down due to gravity. It has been found that, the above mentioned impurities can be removed by sedimentation with coagulation.

            It has been found that when certain chemicals (i.e. coagulants) are added to water an insoluble, gelatinous precipitate is formed. This precipitate during its formation and descent through the water absorbs very fine suspended and colloidal impurities there by reducing the turbidity of the water.

RELEVANCE:-

            Coagulation of raw water using the optimum coagulant dose removes colloidal impurities from the water. These colloidal impurities are normally associated with organic matter containing pathogenic bacteria which are responsible for water borne diseases. The chemical coagulation also makes the process of disinfection more effective. Coagulation also removes objectionable colour, taste and odour’s from water. Usually the dose of Alum varies between 5mg/lit for relatively clear water to about 85 mg/lit for very turbid waters. The average dose is about 20mg/lit.

CHEMICAL REAGENTS:-

ALUM SOLUTIONS:- Dissolve 1.0 gram of Alum in 1 lit of distilled water so that each ml. of Alum solution contains one milligram of Alum.

PROCEDURE:- Take 2 lit of sample water in all the six jars of the apparatus. Then add Alum solution in each of the six jars in varying amounts. The range of Alum dose depends upon the turbidity of the raw sample water. The normal range of Alum dose varies between 15ppm to 60ppm. Add the Alum solution to each of the six jars as per the tabular column shown in observation sheet. After adding different amounts of Alum solution in the all six jars, place the jars on the platform provided and fix the stirring paddle to the connecting rod which rotates by a gear and spindle system, with thee help of electric motor, the paddles are rotated at a speed of 30-40 rpm for about 2 minutes. The speed of the paddles are then reduced to a minimum so as to cause flocculation and stirring is continued to about 20-30 min. the rotation of paddles are then stopped and floc is allowed to settle for 30min. then pippet out the supematant from each jar and measure the turbidity using nephelometer.

GRAPH:- Plot a graph between the coagulant dose applied and turbidity of coagulated sample, by taking turbidity value on Y-axis and Alum dose on X-axis; as shown below. Then determine optimum coagulant dose from the graph which corresponds to minimum turbidity.

OBSERVATIONS:

Jar No.	Alum dose in mg/lit	ml of Alum solution to be added in 2000ml

CALCULATIONS:-

RESULTS:-

INTERPRETATION OF RESULTS:-