Analysis and Design of Cascade Aerator Construction for Mettur Water Treatment Plant

7/23/2019 Analysis and Design of Cascade Aerator Construction for Mettur Water Treatment Plant

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National Conference on Research Advances in Communication, Computation, Electrical Science andStructures (NCRACCESS-2015)

ISSN: 2348 – 8352 www.internationaljournalssrg.org Page 42

Analysis and Design of Cascade Aerator Construction for Mettur

Water Treatment Plant

Kokila.K 1, R.Divya2

Assistant Professor, Civil Engineering

Bharathiyar Institute of Engineering for Women

Abstract: Generally water can be treated in

treatment plants for removing harmful substances

present in it. The treatment process includes pre-

treatment, aeration, coagulation, flocculation,

sedimentation, filtration, fluoridation,

conditioning and disinfection. In our project we

concentrate on aeration process using cascade

aerators. The cascade aerators are efficient in

raising dissolved oxygen content of water, CO2

removal and a large contribution occurs to the

self-purification of water due to increase in DO,which accelerates the process of decomposition of

organic matter. The working stress method

approach of design has been used and

recommendations of IS 456-2000, IS 875(Part III),

IS 1893 (Part I):2002 and SP-16 have been

adhered too. Here we analysis and design a

Cascade Aerator for Mettur water treatment

plant having a height of 21.6m using Staad Pro v8i

and Auto Cad 2010 respectively. The elements of

the aerator such as top and bottom ring beams,

slab, column, plinth beams, footing & foundation

are to be designed.M25 and Fe415 have been used

to design all the components of the aerator. Thetypical analysis of a cascade aerator has been

performed on the basis of quantity of water

intake.

I. INTRODUCTION:

The main objective of treating waterintended for public water supplies is to producea supply of water that is chemically and

bacteriologically safe for human Consumption.

The treatment process includes pre-treatment,aeration, coagulation, flocculation,

sedimentation, filtration, fluoridation,conditioning and disinfection. In our project weundergo aeration process using cascade aerators.

Aeration brings water and air in close contact inorder to remove dissolved gases (such as carbondioxide).

It oxidizes dissolved metals such as iron,hydrogen sulfide, and volatile organic chemicals(VOCs).

Aeration is often the first major process at the t reatment plant. During aeration,constituents are removed or modified before

they can interfere with the treatment processes.All aerators are designed to create a greater

amount of contact between air and water toenhance the transfer of gases and increaseoxidation. This makes the impure source ofwater in greater rate of purity approximately 60

to 70% .The aeration processes is carried outthrough various types of aerators.

II. TYPES OF AERATORS:

Aerators

Gravity Fountain Injection orMechanical Aerators DiffusedAerators

AeratorsChemicals removed or oxidized by aeration

Constituents commonly affected byaeration are:

Volatile organic chemicals, such as benzene (found in gasoline), or

trichloroethylene, dichloroethylene, and perchloroethylene (used in dry-cleaning orindustrial processes).

Ammonia

Chlorine

Carbon dioxide

Hydrogen sulfide

Methane

Iron and Manganese

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Cascade Aeration

In this method, the water is made to fall

through certain height (1 to 3m) over a series ofsteps (3 to 10) with a fall of about 0.15 to 0.3 min each step. The structure so formed is knownas a free fall Aerator. The simplest type of a freefall aerator is known as a Cascade Aerator. Such

aerators are widely used as water features. They

will take large quantities of water in acomparatively small area at low head, are simpleto be kept clean and can be made of robust anddurable materials with a long life.

The plates can be made of cast iron, orof RCC, or timber, or even of glass. The aeratorshould preferably be installed in open air.

However, for protection against air pollution,freezing and algal growth, it can be installed in asmall house having plenty of louvered air inlets.The cascade aerators are efficient in raisingdissolved oxygen content of water, but not for

CO2 removal, which is removed only in therange of 60 to 70%.

Weirs and waterfalls of any kind are, ofcourse, cascade aerator. Where a stream passes

over an artificial or naturally occurring obstacle,a large contribution occurs to the self-

purification of the river water, due to increase in

DO, which accelerates the process ofdecomposition of organic matter. In a cascade

aerator, usually, the rate of flow may vary between 20 to 100 m³/h per m length of weir. To

allow entrained air to mix in the water, eachreceiving basin should have a pool of water ofdepth 0.3 to 0.5 m.

Weirs with serrated edges perform better, as they help to break water flow into

separate jets. If the water is allowed to cling tothe steps, especially at low discharge rates, theefficiency is reduced. The space requirement is

typically of the order of 0.5 m² per 1m³/h watertreated. The oxygen transfer efficiency should beas 2.5 kg O2 / kWh.

Operating considerations:

Aeration raises the dissolved oxygencontent of the water. If too much oxygen is

injected into the water, the water becomessupersaturated. Aerators fall into two categories.

They either introduce air to water, or water toair. The water-in-air method is designed to

produce small drops of water that fall throughthe air. The air-in-water method creates small

bubbles of air that are injected into the water

stream. All aerators are designed to create agreater amount of contact between air and water

to enhance the transfer of gases and increaseoxidation.

Figure 1 Process of Cascade aerator

Design criteria Typical value

1 Number of steps 3 to 6 normally(maximumcould be More than 10,

in fact more the number,

higher the efficiency)2 Space

requirement0.015 - 0.045 m²/m³.h

3 Head required 0.50 - 3.0 m

4 Tread of step 20 - 40 cm

5 Rise of step 20 - 40 cm

6 Velocity of waterintheCollecting

channel

0.6 m/s - 0.9 m/s

7 CO2 removal

efficiency

20 - 45%

8 H2S removal

efficiency

35%.








Figure 2 Cascade aerator plan layout

Cascade aerator typical design criteria:

Figure 3 Cascade aerator in Mettur water

treatment plant

Objective of project

The main objective is to produce asupply water that is chemically and

bacteriological safe for human consumption.In this project, the construction of

cascade aerator for effective treatment of waterhas been done.

Scope of project

The scope of the study covers the designof cascade aerator for water treatment plant,Mettur using a new logic of increasing thenumber of steps for effective splitting of Watermolecules and by reducing the usage of Alum in

Water treatment.

III. SPECIFICATIONS

Working stress method

This method of design was evolved

around in 1990 and was theoretical methodaccepted by National Codes of practice fordesign concrete sections. It assumes that both

concrete and steel acts together and are perfectlyelastic at all stages so that the modular ratio

(ratio between module 0f elasticity of concreteand steel) can be used to determine the stresses

in steel and concrete. It used a factor of aboutthree times with respect to cube strength forconcrete and a factor of safety of about 1.8(withrespect to yield strength) for steel.

Even though the structures designed bythis method have been performing theirfunctions satisfactorily for many years, it has

three major defects.

1. Method deals only with the elastic behavior of the member; it neithershows its real strength nor gives the

true factor of safety of the structureagainst failure.

2. The modular ratio itself is an

imaginary quantity. Because ofcreep and nonlinear stress- strain

relationship, concrete does not havea definite modulus of elasticity as insteel.

The design moment and shear in the

structure are calculated by elastic analysis withthe characteristic loads. The stress in concreteand steel in the sections are calculated on the

basis of elastic behavior of composite section.I.S 456 recommended a modulus of elasticity of

concrete which varies with the strength ofconcrete. But this method forms the part of limitstate design for a serviceability condition.

General design requirements according to the

Indian standards, code of practice (IS: 3370-

Part-II, 1965)

Plain concrete structures

Plain concrete members of reinforcedconcrete liquid structures may be designedagainst structural failure by allowing tension in

plain concrete as per the permissible limits fortension in bending specified in IS456-2000. This

will automatically take care of failure due tocracking. However nominal shear reinforcement

in accordance with the requirements of IS: 456shall be provided for plain cement concretestructural members.








Permissible stress in concrete for strength

calculation

In strength calculation the usual permissible stresses, in accordance with

IS456:2000 is used. Where the calculated shearstresses in concrete above exceeds the

permissible values , reinforcement acting in

conjunction with diagonal compression inconcrete shall be provided to take the whole ofthe shear.

Stresses due to dry shrinkage or temperature

change

Stresses due to drying shrinkage or

temperature change may be ignored provided

that adequate precautions are taken to avoidcracking of concrete during the construction

period and until the reservoir is put into use.

The recommendations as regards the provisions of joints and for the suitable slidinglayer are complied with, or the reservoir is to be

used only for the storage of water or aqueousliquids at or near ambient temperature and the

circumstances are such that the concrete willnever dry out.

Permissible stress in steel reinforcement for

resistance to cracking

When the steel and concrete areassumed to act together for checking the tensile

stresses in concrete for avoidance of crackingthe tensile stresses in steel is limited by therequirement that the permissible tensile stress inconcrete is not to exceed so that tensile stress insteel is equal to product of modular ratio of steel

and concrete and the corresponding allowablestress in concrete.

Stresses in the reinforcement The following working stresses are

adopted

A steel reinforcement not less than 0.3 percentof the cross section shall be provided in each

principle direction.

IV. RESULTANT COMPARISON

S.

N

O

PARAMETE

R

METTUR

WATER

TREATMEN

T PLANT

FROM

THIS

PROJEC

T

1 Type ofAerator Cascade Cascade

2 No. of Trays 4 Nos. 9 Nos.

3 Speed ofwater

1450 rpm 1450 rpm

4 Head 60 m 60 m

5 Water

treatmentcapacity

348 m/hr. 783 m/hr.

6 Purity rate 60% 75%

IV. STRESS DIAGRAMS

Figure 4 Stress contour for “Mx”

Reinforcement

Permissible tensile stress in thereinforcement

Near waterface

Away fromwater face

Mild steel 100 N/mm² 125 N/mm²

Fe 415 170 N/mm² 200 N/mm²

Fe 500 205 N/mm² 245 N/mm²








Figure 5 Stress contour for

“My”

The Staad 3D model is shown below and

analyzed

Figure 6 Deflection diagram

V. CONCLUSION

The Design of Cascade Aerator forMettur water treatment plant has been donesuccessfully. The calculations we have doneusing working stress method are within safelimits. In our project we introducing a new logic

of increasing the steps or trays in cascade aeratorfor decreasing the usage of alum in treatment

process. At present in Mettur water treatment plant, the water treatment capacity is 348 m/hr&the purity rate is about 60%. From our project itis expected to obtain the water treatmentcapacity on 783 m/hr& the purity rate in about

75%. Here we analyzed and designed a CascadeAerator for Mettur water treatment plant havinga height of 21.6m using Staad Pro v8i and Auto

Cad 2010 respectively. The elements of theaerator such as top and bottom ring beams, slab,column, plinth beams, footing &foundation aredesigned.M25 and Fe415 have been used todesign all the components of the aerator.

REFERENCES

1.

Baruth EE (Ed.) Mc Graw Hill, AWWA ASCEWater Treatment Plant Design, 4t Edition.

2. J. K. Edzwald, editor. New York: McGraw-Hill,AWWA (2010b). Water Quality & Treatment: AHandbook on Drinking Water, 6th Edition.

3. We collected some details from Mettur WaterTreatment Plant regarding our project.

4. Dr B.C.Punmia, AshokKumar Jain, Arun KumarJain, IS 456-2000 & IS 456-1978 Design aids forReinforced concreteR.C.C. Designs

(Reinforcedconcrete structures),5. N.Krishnaraju, Design of Reinforced concrete

structures.6. S.K.Garg on Types of Aerators-Water supply

Engineering.

7.

Dr.FahidRabah-Environment Engineering, WaterTreatement.

General design requirements according to the Indianstandards, code of practice

1. IS 875 (Part -I, Part-II, Part-III)-Wind & Seismic

Loads

2. IS 1893 (Part 1) : 2002-Earthquake ResistantStructures

3. I.S 496:2000, Plain and Reinforced4. SP-16,Design aids for RC to IS 456-2000

5. I.S 3373 (Part II, Part-III and Part IV-1967)




Documents

Analysis and Design of Cascade Aerator Construction for Mettur Water Treatment Plant