INTRODUCTION

Generally, we encounter very fine and charged clay like particles in water treatment, which should be detached before we continue for further processes. When water is passed through a sedimentation tank, these impurities do not settle down by gravity. The cause being that these are charged particles, they repel each other and just stay.

Coagulation is the process of addition of a chemical to de-stabilize a stabilized charged particle.

Flocculation is a slow mixing technique which promotes agglomeration and helps the particles to settle down.

The presence of fine charged particles increases the turbidity of the water, which is undesirable, and hence these impurities are to be removed. Therefore, which will be using a chemical, which separates as soon as it added to water and helps in the process of "Coagulation". In the current experiment we are using "Alum" [Al₂(SO₄)₃ 18H₂0] as the clarifying agent.

Coagulation removes not only turbidity, but also colour, odour producing substance, microorganisms, algae, phosphate and taste. Molecules dissociate to yield SO₄^2- and Al³⁺ when alum solution is added to water. In order to neutralize the charge these charged species combine with the charged colloidal particles. A complete description of the charge removal can be found in the web which will be based on two basic definitions "Stern potential" and "Zeta Potential".

Through the slow mixing or so called "Flocculation" a process known as agglomeration occurs which combines the charged particles into a compact whole and helps in the settling of the particle. That is the reason why we have step of "slow mixing" in this experiment. Excess dosage of alum may give excess aluminium in water and increases turbidity. Due to less dosage of alum, all dissolved solids may not get coagulated. Hence it is essential to have an optimal dosage so the turbidity is not increased due to alum.

Jar Test :

Jar test is a common laboratory method used to determine the optimum operating conditions for water or wastewater treatment. This process allows changes in pH, variations in coagulant or polymer dose, alternating mixing speeds, or testing of different coagulant or polymer types, on a small scale in order to predict the working of a large-scale treatment operation.

Jar Testing Apparatus :

The jar testing apparatus consists of six paddles, which stir the contents of six one-litre containers. One container act as a controller while the operating conditions can be varied among the residual five containers. An rpm gage at the top-centre of the device allows for the constant control of the mixing speed in all of the containers.

Relevant Indian Standard for Alum Coagulation Test:

1. IS 3025 (Part 50)-1986: Method of Sampling and Test (Physical and Chemical) for Water and Wastewater: Jar Test (Coagulation Test), First Revision.