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

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1 Analysis and Design of Cascade Aerator Construction for Mettur Water Treatment Plant Kokila.K 1, R.Divya 2 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 pretreatment, 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, CO 2 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 , 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. The typical analysis of a cascade aerator has been performed on the basis of quantity of water intake. I. INTRODUCTION: The main objective of treating water intended for public water supplies is to produce a 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 we undergo aeration process using cascade aerators. Aeration brings water and air in close contact in order to remove dissolved gases (such as carbon dioxide). 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 to enhance the transfer of gases and increase oxidation. This makes the impure source of water in greater rate of purity approximately 60 to 70%.The aeration processes is carried out through various types of aerators. II. TYPES OF AERATORS: Gravity Fountain Injection or Mechanical Diffused Chemicals removed or oxidized by aeration Constituents commonly affected by aeration are: Volatile organic chemicals, such as benzene (found in gasoline), or trichloroethylene, dichloroethylene, and perchloroethylene (used in dry-cleaning or industrial processes). Ammonia Chlorine Carbon dioxide Hydrogen sulfide Methane Iron and Manganese ISSN: Page 42

2 Design criteria 6 Velocity of water in thecollecting channel 7 CO 2 removal efficiency 8 H 2 S removal efficiency Cascade Aeration Typical value 1 Number of steps 3 to 6 normally (maximum could be More than 10, in fact more the number, higher the efficiency) 2 Space m²/m³.h requirement 3 Head required m 4 Tread of step cm 5 Rise of step cm 0.6 m/s m/s 20-45% 35%. In this method, the water is made to fall through certain height (1 to 3m) over a series of steps (3 to 10) with a fall of about 0.15 to 0.3 m in each step. The structure so formed is known as a free fall Aerator. The simplest type of a free fall aerator is known as a Cascade Aerator. Such aerators are widely used as water features. They will take large quantities of water in a comparatively small area at low head, are simple to be kept clean and can be made of robust and durable materials with a long life. The plates can be made of cast iron, or of RCC, or timber, or even of glass. The aerator should preferably be installed in open air. However, for protection against air pollution, freezing and algal growth, it can be installed in a small house having plenty of louvered air inlets. The cascade aerators are efficient in raising dissolved oxygen content of water, but not for CO 2 removal, which is removed only in the range of 60 to 70%. Weirs and waterfalls of any kind are, of course, cascade aerator. Where a stream passes over an artificial or naturally occurring obstacle, a large contribution occurs to the selfpurification of the river water, due to increase in DO, which accelerates the process of decomposition 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, each receiving basin should have a pool of water of depth 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 to the steps, especially at low discharge rates, the efficiency is reduced. The space requirement is typically of the order of 0.5 m² per 1m³/h water treated. The oxygen transfer efficiency should be as 2.5 kg O 2 / kwh. Operating considerations: Aeration raises the dissolved oxygen content of the water. If too much oxygen is injected into the water, the water becomes supersaturated. fall into two categories. They either introduce air to water, or water to air. The water-in-air method is designed to produce small drops of water that fall through the air. The air-in-water method creates small bubbles of air that are injected into the water stream. All aerators are designed to create a greater amount of contact between air and water to enhance the transfer of gases and increase oxidation. Figure 1 Process of Cascade aerator ISSN: Page 43

3 Figure 2 Cascade aerator plan layout Cascade aerator typical design criteria: (ratio between module 0f elasticity of concrete and steel) can be used to determine the stresses in steel and concrete. It used a factor of about three times with respect to cube strength for concrete and a factor of safety of about 1.8(with respect to yield strength) for steel. Even though the structures designed by this method have been performing their functions satisfactorily for many years, it has three major defects. Figure 3 Cascade aerator in Mettur water treatment plant Objective of project The main objective is to produce a supply water that is chemically and bacteriological safe for human consumption. In this project, the construction of cascade aerator for effective treatment of water has been done. Scope of project The scope of the study covers the design of cascade aerator for water treatment plant, Mettur using a new logic of increasing the number of steps for effective splitting of Water molecules 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 method accepted by National Codes of practice for design concrete sections. It assumes that both concrete and steel acts together and are perfectly elastic at all stages so that the modular ratio 1. Method deals only with the elastic behavior of the member; it neither shows its real strength nor gives the true factor of safety of the structure against failure. 2. The modular ratio itself is an imaginary quantity. Because of creep and nonlinear stress- strain relationship, concrete does not have a definite modulus of elasticity as in steel. The design moment and shear in the structure are calculated by elastic analysis with the characteristic loads. The stress in concrete and 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 of concrete. But this method forms the part of limit state design for a serviceability condition. General design requirements according to the Indian standards, code of practice (IS: Part-II, 1965) Plain concrete structures Plain concrete members of reinforced concrete liquid structures may be designed against structural failure by allowing tension in plain concrete as per the permissible limits for tension in bending specified in IS This will automatically take care of failure due to cracking. However nominal shear reinforcement in accordance with the requirements of IS: 456 shall be provided for plain cement concrete structural members. ISSN: Page 44

4 Permissible stress in concrete for strength calculation Reinforceme nt Permissible tensile stress in the reinforcement Near water face Away from water face Mild steel 100 N/mm² 125 N/mm² Fe N/mm² 200 N/mm² Fe N/mm² 245 N/mm² In strength calculation the usual permissible stresses, in accordance with IS456:2000 is used. Where the calculated shear stresses in concrete above exceeds the permissible values, reinforcement acting in conjunction with diagonal compression in concrete shall be provided to take the whole of the 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 avoid cracking 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 sliding layer are complied with, or the reservoir is to be used only for the storage of water or aqueous liquids at or near ambient temperature and the circumstances are such that the concrete will never dry out. and concrete and the corresponding allowable stress in concrete. Stresses in the reinforcement The following working stresses are adopted A steel reinforcement not less than 0.3 percent of the cross section shall be provided in each principle direction. IV. RESULTANT COMPARISON S. N O PARAMETE R METTUR WATER TREATMEN T PLANT Cascade FROM THIS PROJEC T Cascade 1 Type of Aerator 2 No. of Trays 4 Nos. 9 Nos. 3 Speed of 1450 rpm 1450 rpm water 4 Head 60 m 60 m 5 Water 348 m/hr. 783 m/hr. treatment capacity 6 Purity rate 60% 75% IV. STRESS DIAGRAMS Permissible stress in steel reinforcement for resistance to cracking When the steel and concrete are assumed to act together for checking the tensile stresses in concrete for avoidance of cracking the tensile stresses in steel is limited by the requirement that the permissible tensile stress in concrete is not to exceed so that tensile stress in steel is equal to product of modular ratio of steel Figure 4 Stress contour for M x ISSN: Page 45

5 M y Figure 5 Stress contour for of increasing the steps or trays in cascade aerator for 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 it is expected to obtain the water treatment capacity on 783 m/hr& the purity rate in about 75%. Here we analyzed and designed 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 designed.m25 and Fe415 have been used to design all the components of the aerator. REFERENCES The Staad 3D model is shown below and analyzed 1. Baruth EE (Ed.) Mc Graw Hill, AWWA ASCE Water Treatment Plant Design, 4t Edition. 2. J. K. Edzwald, editor. New York: McGraw-Hill, AWWA (2010b). Water Quality & Treatment: A Handbook on Drinking Water, 6th Edition. 3. We collected some details from Mettur Water Treatment Plant regarding our project. 4. Dr B.C.Punmia, AshokKumar Jain, Arun Kumar Jain, IS & IS Design aids for Reinforced concreter.c.c. Designs (Reinforcedconcrete structures), 5. N.Krishnaraju, Design of Reinforced concrete structures. 6. S.K.Garg on Types of -Water supply Engineering. 7. Dr.FahidRabah-Environment Engineering, Water Treatement. General design requirements according to the Indian standards, code of practice 1. IS 875 (Part -I, Part-II, Part-III)-Wind & Seismic Loads 2. IS 1893 (Part 1) : 2002-Earthquake Resistant Structures 3. I.S 496:2000, Plain and Reinforced 4. SP-16,Design aids for RC to IS I.S 3373 (Part II, Part-III and Part IV-1967) Figure 6 Deflection diagram V. CONCLUSION The Design of Cascade Aerator for Mettur water treatment plant has been done successfully. The calculations we have done using working stress method are within safe limits. In our project we introducing a new logic ISSN: Page 46