L-9 AERATIONUNIT-II- BY PROF S S JAHAGIRDAR

Environmental Engineering -I

INTRODUCTION

Aeration is used to remove tastes and odours, and

to remove minerals such as iron and manganese from water, and

to remove carbon dioxide from the water.

OBJECTIVES OF AERATION

1. Removes taste and odours caused due to organic gases.

2. Increases dissolved oxygen content

3. Removes Hydrogen Sulphide and odour due to it.

4. Decreases CO2 content and raises pH.

5. Removes Iron and Manganese.

6. Due to agitation bacteria may get killed.

7. Can be used for mixing chemicals

HOW DOES AERATION WORK?

Aeration is the intimate exposure of water and air.

It is a way of thoroughly mixing the air and water so that various reactions can occur between the components of the air and the components of the water.

Scrubbing action

Oxidation

Aeration removes or modifies the constituents of water using two methods - scrubbing action and oxidation. Scrubbing action is caused by turbulence which results when the water and air mix together. The scrubbing action physically removes gases from solution in the water, allowing them to escape into the surrounding air.

Scrubbing action will remove tastes and odors from water if the problem is caused by relatively volatile gases and organic compounds.

Oxidation is the other process through which aeration purifies water.

When air is mixed with water, some impurities in the water, such as iron and manganese, become oxidized.

Once oxidized, these chemicals fall out of solution and become suspended in the water.

The suspended material can then be removed later in the treatment process through filtration.

EFFICIENCYThe efficiency of the aeration process

depends almost entirely on the amount of surface contact between the air and water.

This contact is controlled primarily by the size of the water droplet or air bubble.

The goal of an aerator is to increase the surface area of water coming in contact with air so that more air can react with the water.

IRON AND MANGANESE REMOVAL

4Fe(HCO3)2 + O2 + 2H2O = 4Fe(OH)3- + 8CO2

ferrous bicarb

onate

oxygen water ferric

hydroxide

carbon

dioxide

2Mn(HCO3)2 + O2 = 2MnO2 + 4CO2- + 2H2O

manganese

bicarbonate

oxyg

en

manganese

dioxide

carbon

dioxide

water

Iron and manganese in well waters occur as soluble ferrous and manganous bicarbonates. In the aeration process, the water is saturated with oxygen to promote the following reactions:

The oxidation products, ferric hydroxide and manganese dioxide, are insoluble.

After aeration, they are removed by clarification or filtration.

TYPES OF AERATOR

1. Free fall or gravity aerator

i. Cascade aerator

ii. Inclined apron aerator

iii. Slat tray aerator

iv. Gravel bed aerator

2. Spray aerators

3. Air diffusers

4. Mechanical aerators

1. CASCADE AERATOR

Simplest of free fall aerator

Can be constructed of concrete or metal

Height is 1 to 3 m.

50 to 60% CO2 reduction

2. INCLINED APRON AERATOR WITH RIFFLE 2. INCLINED APRON AERATOR WITH RIFFLE PLATESPLATES

Water falls along inclined plane/apron.

Breaking up of water due to aprons cause agitation

Agitation increases contact of air and water

3. SLAT TRAY AERATOR3. SLAT TRAY AERATOR

Consists of closely stacked super imposed wood slat-trays

Water is sprayed evenly on top tray.

Water trickles from one tray to the other.

Air is supplied at the bottom with the help of blower (Air flow is in upward direction).

Ventilator discharges air and gases

4. GRAVEL BED AERATOR4. GRAVEL BED AERATOR

Water is sprayed at the top and it trickles down the bed of coke or anthracite.

Thickness of bed is 1 to 1.5 m

TRICKLING BEDS

3-4 trays are kept filled with coke or stone

Thickness of bed is – 0.5 to 0.6 m

Distance between trays is 0.5m

Water is sprayed from distribution pipe at the top.

OBJECTIVE QUESTIONSOBJECTIVE QUESTIONS

1. Air diffusion is type of _______ system. (chlorination/aeration/flocculation/sedimentation).

2. Iron and Manganese are removed by ____________ action in aeration.

3. ___________ aerator is commonly used in water treatment plants.

4. Diffused and mechanical aerator are mostly used in _____________ treatment.

THEORY QUESTIONSTHEORY QUESTIONS

Q1. Write note on objectives of aeration. Also enlist types of aerators. (Dec 2011, 5 marks)

Q2. Enlist different types of aerators and discuss with neat sketch any one in detail.