Removal of Turbidity Using Aluminium Sulphate

Environmental Pollution And Control Lab

Assignment

Ravi Kumar Sikarwar -B090308CHRohini Biswas -B090660CHSangeetha Ramdas -B090277CHS Vysakh Menon -B090058CH

CONTENTS PAGE

1. AIM 3

2. INTRODUCTION 3

3. PRINCIPLE 5

4. SPECTROPHOTOMETER 5-7

5. MATERIALS AND CHEMICALS REQUIRED 7

6. PROCEDURE 9

7. RESULTS AND DISCUSSIONS 9

8. OBSERVATION TABLE 10

9. CONCLUSION 11

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AIM

Removal of turbidity using aluminium sulphate

INTRODUCTION

Rapid growth of population, urbanization and industrial as well as agricultural activities haveincreased water demand, particularly in recent decades. Water treatment industry is among the most important industries in many countries such as Iran. Coagulation, flocculation, sedimentation, filtration and disinfection are the most common treatment processes used in the production of drinking water. Coagulation/flocculation processes are of great importance in solid-liquid separation practice.

The coagulation process is used to destabilize colloidal material in water by the addition of a chemical agent. It requires rapid mixing to quickly disperse the coagulant and subsequently flocculation process. Flocculation is the formation of aggregates of the destabilized colloidsand requires gentle mixing to allow effective collisions between particles to form heavy flocs which can be removed from water by settlement. Colloids are small suspended particles in water which cannot be settled or removed naturally due to their light weight and stability.

These particles pose some degree of stability and cause water turbidity. Turbidity may contain many contaminants like pathogenic organisms. Many pollutants of concern to human health e.g., metals or some synthetic organic chemicals are also associated with turbidity. Thus, effective turbidity elimination is necessary to ensure removal of many health-related contaminants. In addition effective removal of turbidity may ease subsequent water treatment processes.

Aluminium and iron salts are the most widely used coagulants in Iran as well as many other countries in the drinking water industry. Their effectiveness was evaluated at different pH values and coagulant dosage to find optimal operational conditions for low to highly turbidity waters. The influence of lime, as a coagulant aid, in accompany with aluminium sulphate and ferric chloride was also studied. The interest in coagulation processes and therefore the removal of turbidity lies in the fact that colloids may directly or indirectly threaten the human health. The results of the current study can be used as the baseline data for drinking water treatment facilities which uses these two types of coagulants.

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The 3 processes- flash mix, coagulation and flocculation

The spectrophotometer

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Schematic of Spectrophotometer

PRINCIPLE

The chemistry of coagulation/flocculation consists of three processes - flash mix, coagulation, and flocculation.  Each of these processes is briefly explained below.

 In the flash mixer, coagulant chemicals are added to the water and the water is mixed quickly and violently.  The purpose of this step is to evenly distribute the chemicals through the water.  Flash mixing typically lasts a minute or less. 

During coagulation, the coagulant chemicals neutralize the electrical charges of the fine particles in the water, allowing the particles to come closer together and form large clumps. 

The final step is flocculation.  During flocculation, a process of gentle mixing brings the fine particles formed by coagulation into contact with each other.   Flocculation typically lasts for about thirty to forty-five minutes. 

The chemistry of coagulation and flocculation is primarily based on electricity.  Electricity is the behaviour of negative and positively charged particles due to their attraction and repulsion.  Like charges (two negatively charged particles or two positively charged particles) repel each other while opposite charges (a positively charged particle and a negatively charged particle) attract. 

 Most particles dissolved in water have a negative charge, so they tend to repel each other.  As a result, they stay dispersed and dissolved or colloidal in the water, as shown above. The purpose of most coagulant chemicals is to neutralize the negative charges on the turbidity particles to prevent those particles from repelling each other. 

Coagulants tend to be positively charged.  Due to their positive charge, they are attracted to the negative particles in the water, as shown below. The combination of positive and negative charge results in a neutral, or lack, of charge. 

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As a result, the particles no longer repel each other. The next force which will affect the particles is known as van der Waal's forces.  Van der Waal's forces refer to the tendency of particles in nature to attract each other weakly if they have no charge.  Once the particles in water are not repelling each other, van der Waal's forces make the particles drift toward each other and join together into a group.  When enough particles have joined together, they become floc and will settle out of the water.

THE SPECTROPHOTOMETER

This equipment consists of two main parts: the spectrometer and the photometer. The former produces light of selected colour while the latter measures the intensity of light.

The instrument is arranged such that liquid in the cuvette is placed between the spectrometer beam and photometer. The amount of light passing through the tube is measured by the

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Picture 1: Inner view of Spectrophotometer

Picture 2: Samples at Day 2

photometer which delivers a voltage signal to a display device, usually a galvanometer. The signal changes as the amount of light absorbed by the liquid changes.

The concentration is measured by determining the extent of absorption of light at appropriate wavelengths. This is obtained by Beer’s Law which is given by:

II o

=10−kcl

Where;

I = intensity of transmitted light when coloured compound is added

Io = intensity of transmitted light using pure solvent

k = constant

l = distance light passes through solution

c = concentration of coloured compound

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 MATERIALS REQUIRED

Conical flask (5 no.)

Stirrer

Mixing equipment

Spectrophotometer

CHEMICALS REQUIRED

Aluminium sulphate (Al2(SO4)3.18H2O)

Calcium Carbonate (CaCO3)

Ferric chloride (FeCl3.6H2O)

Distilled water

Sodium hydroxide (NaOH)

Sulphuric acid (H2SO4)

Samples in Agitator

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Weighing Machine

PROCEDURE

1. Prepare 5 samples by adding 300mg, 400mg, 500mg, 600mg and 700mg calcium carbonate respectively to 100 ml distilled water in 5 different conical flasks.

2. Stir the mixture well with stirrer.3. Record the absorbance values of these turbid samples using a spectrophotometer.4. Add 50mg aluminium sulphate and 15mg ferric chloride to each sample.5. Place the samples in the mixing equipment and provide rapid mixing (350 rpm) for 1

minute.6. Then the mixing speed is reduced to 20-30 rpm.7. Necessary adjustments in pH are done using sodium hydroxide and sulphuric acid to

maintain the levels between 4 and 8.8. The samples are allowed to settle for 45 minutes.9. Then the respective absorbance are measured again and compared with the initial

values.

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10. The experiment is repeated for different time intervals of mixing.

RESULTS AND DISCUSSIONS

Removal of turbidity using aluminium sulphate has been studied and noted for 3 days.

There is a decreasing trend in the Absorption of the colloidal solution as time goes on which implies the turbidity is decreasing. As there is turbidity, there will be more particle motion, which implies the absorption will be more.

The Absorption measurement was carried out in an Absorption mass spectrometer and we noted the decrease in turbidity as the solutions became more and more transparent as time passed by. There was one reference cell which was filled with distilled water and the other one was that of the sample which was filled, removed and filled again. Aluminium sulphate coupled with ferrous sulphate has been stable chemical coagulants which effectively helped in the formation of agglomerates.

The agitation helped in speeding up the coagulation process. NaOH and H2SO4 were added periodically when the pH deviated from the desired range of 4-8.

From the table we can see that there was an initial drop in turbidity level on day 1 that was marked by the effective reaction with aluminium sulphate. On the successive days we noted decreasing turbidity’s for respective samples however not that much decrease was noted compared to the one noted on the first.

OBSERVATION TABLE

Sl.No: Volume Of Sample

(ml)

Absorption

Initial Day1 Day2 Day3

1 300 0.987 0.570 0.541 0.538

2 400 1.128 0.522 0.509 0.340

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3 500 1.170 0.514 0.379 0.203

4 600 1.389 0.181 0.163 0.146

5 700 1.432 0.166 0.147 0.132

Absorbance Vs Volume of Sample plot

250 300 350 400 450 500 550 600 650 700 7500

0.1

0.2

0.3

0.4

0.5

0.6

Absorbance Vs Volume of sample

InitialDay1Day2Day3

Volume of Sample

Abso

rban

ce

CONCLUSION

This experiment has helped us understand the behaviour of various chemical coagulants on the turbidity. The concept of flocculation, colloidal dispersion, the chemistry behind flocculation/coagulation has been studied through the course of this project. It is important to remove turbidity from water in order to terminate any pathogenic bacteria that it picked up and to remove colour.

We have familiarised ourselves with some of the common coagulants and studied their order of reactivity. The most commonly used coagulants are alum and ferrous/ferric sulphate.

From the above experiment, we have recorded the absorbance using the spectrophotometer. There has been a decrease in absorbance with each day, signifying the decrease in turbidity

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levels. Also, we have familiarised ourselves with the use of a spectrophotometer, along with the usage of Beer’s Law.

Initial- Samples after the addition of aluminium sulphate and ferric chloride

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Final-(Day3)

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