Low-Cost Design of Stepped Cascade Aeration System

International Journal for Humanities, Science and Technology Vol 1 (3) 2012

Low-Cost Design of Stepped Cascade Aeration System Sanjib Moulick1, K. Ravikumar2, Mukesh Goel2 and Ashutosh Das2 1

Department of Agricultural & Food Engineering, IIT Kharagpur, Kharagpur - 721302.

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Centre for Environmental Engineering, PRIST University, Thanjavur – 613403 aerators (developed for other hydraulic structures) (Avery and Novak, 1978; Thene, 1988; Holler, 1970; Pruel and Holler, 1969; Markofsky and Kobus, 1978; Nakasone, 1987; Rindels and Gulliver, 1993; Foree, 1976; Tsivoglou and Wallace, 1972; Wilhelms and Smith,1981; Watson et al, 1981). These equations are mostly empirical and limited to certain situations. The hydraulics behind the stepped cascade aerators plays a major role in oxygen transfer mechanism (Chanson and Toombes, 2002). A stepped cascade flow is characterized by a succession of free-falling nappes (i .e., nappe flow) at low flow rates. At large flow rates with identical chute geometry (step height, mean slope), the water skims over the pseudo-invert formed by the step edges (i.e., skimming flow). For an observer on the chute bank, a skimming flow has the same appearance as a smooth-invert chute flow. For a range of flow rates in between, a transition flow pattern is observed. It is characterized by significant splashing and flow deflections at certain steps (Chanson and Toombes, 2002). Stepped chute hydraulics is not simple, because of different flow regimes, most importantly, strong flow aeration, very-strong turbulence and interactions between entrained air and turbulence (Chanson and Toombes, 2002). The findings of Chanson and Gonzalez (2005) during dimensional analysis for physical modeling of air-water transfer for stepped cascades emphasized that physical modeling of stepped chutes based upon a Froude similitude is more sensitive to scale effects than classical smooth-invert chute studies.

Abstract Since eons various methods for the purification of water have been devised. The aim of this project is to portray one such very effective method that has been used since ages and to analyze the various parameters like salinity, dissolved oxygen and temperature that influence the oxidation and hence the purification. The experimental work is substantiated with theoretical values and establishes the fact that chutes increase the dissolved oxygen in the water and increase significantly the rate of energy dissipation taking place along the chute and reduce the size of the required downstream energy dissipation basin. It also makes use of the latest technology available by making use of RCC gabions that are sturdy. Introduction Stepped channels and spillways are used since more than 3,000 years. Recently new construction materials (e.g. RCC, gabions) have increased the interest for stepped chutes. The steps significantly increase the rate of energy dissipation taking place along the chute and reduce the size of the required downstream energy dissipation basin. Stepped cascades are used also for in-stream reaeration and in water treatment plants to enhance the air-water transfer of atmospheric gases (e.g. oxygen, nitrogen) and of volatile organic components (VOC). The process by which oxygen present in air becomes entrained in the water is called aeration . In waste water treatment processes, air is either mechanically bubbled into the treated water or water is allowed to flow turbulently over the rocks, steps or any hydraulic structure as a means of adding atmospheric oxygen before discharging to a stream or river. In fact, post-aeration is necessary at many facilities in order to comply with the minimum dissolved oxygen concentration as per the State’s Water Pollution Control Regulations. Some of the hydraulic structures that are employed for the aeration purposes are weirs, spillways, gated sills, ogee crests, gated conduit outlets, stepped cascades and tainter gates. However, one of the most cost effective post-aeration designs is a stepped -cascade aerator. Stepped cascades have been used for a long time for the purpose of energy dissipation and aeration. There are many existing equations in use for modeling the oxygen transfer at stepped cascade

Aeration The process by which oxygen present in air becomes entrained in the water is called aeration. The water is brought in intimate contact with air, so as to absorb oxygen and to remove CO2 gas. It may also help in killing bacteria to a certain extent. It also helps in removing H2S gas and iron and manganese to a certain extent from the treated water. In waste water treatment processes, air is either mechanically bubbled into the treated water of water is allowed to flow turbulently over the rocks, steps or any hydraulic structure as a means of adding oxygen before discharging to a stream or river. Post- aeration is necessary at many facilities in order to comply with the minimum dissolved

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oxygen concentration in the State's Water Pollution Control Regulations. Some of the hydraulic structures that are employed for the aeration purposes are weirs, spillways, gated sills, ogee crests, gated conduit outlets, stepped cascades and tainter gates.

artistic cascade type aerator. 11. These can help in maintaining the water oxygen levels in water and remove dissolved iron and manganese, remove CO2 and H2S gases as well as the color and tastes caused by volatile oils, etc. released by algae and other microorganisms.

In waste water treatment processes, air is either mechanically bubbled into the treated water of water is allowed to flow turbulently over the rocks, steps or any hydraulic structure as a means of adding oxygen before discharging to a stream or river. Post-aeration is necessary at many facilities in order to comply with the minimum dissolved oxygen concentration in the State's Water Pollution Control Regulations. Some of the hydraulic structures that are employed for the aeration purposes are weirs, spillways, gated sills, ogee crests, gated conduit outlets, stepped cascades and tainter gates.

12. Raw water reaching the plant can be pumped into the aerator tank through a 450 mm dia pipe and will outflow through 4 nos of 250 mm dia pipes. Material Balance Equation: The material balance equation relating to instantaneous rate of change in DO concentration (dCt/dt) to the rate of oxygen mass transfer between air voids and water is, dCt/dt = kL. a(Cs-Ct) ------------------1

This process is explained as follows

where kL the liquid mass transfer coefficient; a- the air/water interfacial surface area per unit volume of water; Cs- the DO concentration at saturation and Cc the DO concentration at time t.

l. In this method, the water is made to fall through a certain height (1-3m) over a series of steps with a fall of about 0.15 to 0.3 m in each step.

The term kLa is known as the overall mass transfer coefficient and is a function of temperature, liquid viscosity, turbulence or degree of mixing, time of exposure, and in the case air entrainment, bubble size( Avery and Novak, 1978).

2. Cascade aerators are widely used as water features. 3. These take large quantities of water in a comparatively small area at low head. 4. These are simple to clean and can be made of robust and durable materials with a long life.

Cs is related to existing partial pressure of O2 (P02 ) in air by Henry's law

5. The plates can be made of cast iron or of RCC or of timber, or even of glass.

Cs =kHPO2------------------------------2 Where kH is Henry's law constant for 02 and it is a function of temperature and ionic strength of water (Standard Methods, 1992)

6. The aerator should preferably be installed in open air. 7. For protection against air pollution, freezing and algal growth, it can be installed in a small house having plenty of louvered air inlets.

Integrating eqn.l between the limits of 0 and t for time, and C0 and C1 for DO concentration results in the equation of gas transfer in which 'r' is commonly called the deficit ratio ( Avery and Novak, 1978).

8. Cascade aerator are efficient in raising dissolved oxygen content of water but not of carbon dioxide removal which can be removed only in the range of 60-70%

Cs-Ct/(Cs-C0) = 1-1/r------------------3 For oxygen transfer at cascades, the DO concentration upstream (Cu) and downstream (Cd) of the cascade are used in place of C0 and C1 in eqn.3 or eqn.4 to calculate ‘r’ and 'E'.

9. This aids in the self- purification of the river water, which occurs due to increase in DO which accelerates the process of decomposition of organic matter.

E = Cd – Cu/(Cs – Cu) = 1-1/r Materials and Methods

10. Since raw water does not contain too much color and odour, only nominal aeration is proposed by constructing an

The working principle is when water flows through the series of steps oxidation takes place due

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to the fall from certain height and also due to the area of contact surface, thereby the amount of dissolved oxygen increases as the water progresses. The process was repeated and checked for various conditions of flow.

through the pipes, the flow rate was controlled by using a gate valve provided next to the pump. With valve fully open the discharge was calculated by using a vessel of known volume (V) and time (t) taken for vessel to be filled. The velocity of the flow in the first step is determined by inserting a pitot tube and the velocity is calculated using the formula V = (2gh)1/2

Description of the setup: The cascade part was designed with the following dimensions: Rise= 0.5m; Tread= 0.5m; Total rise= 1.5m. The steps had been made of GI sheets of gauge 1mm thick. Angle sections were used at the edges for supporting the series of steps. All pipe fittings were done using PVC pipes of diameter 1 inch and 2 inches. Pitot tubes were used for measuring the velocity of the water at required places. The type of pump used was jet pump (Capacity: 0.5 HP Speed: 1800 rpm). Water analyzer kit (Systronics, 387) was used for determining the salinity, temperature and dissolved oxygen. The experimental setup is shown in the figure below (Fig.1).

Now the sample water collected in the beaker was taken and the values of salinity, temperature and dissolved oxygen were recorded. As the water was allowed to progress through the series of steps and finally collected in barrel. In the final step the velocity of the water was again measured using pitot tube and the values of salinity, temperature and dissolved oxygen were noted down and compared with the initial values. The same process was repeated by adjusting the valve for 6 turns,9 turns,12 turns and l4 turns from the valve fully open so that the discharge is varied and the corresponding velocities at the final step is noted. Then the values of the two velocities were compared and the values of salinity, temperature and dissolved oxygen were found out using the water analyzer kit and compared with the initial values.

Experimental procedure: Initially the barrel was filled with water and a small quantity of the same was taken in a beaker and analyzed for salinity, temperature and dissolved oxygen. Now as the water in the barrel was pumped

Fig1. Experimental Set up Results and Discussion Results of the experiments are presented in Tables 1 to 6. Efficiencies were calculated for each case, as the flow rate and times were varied at different turns of valve openings. It can be inferred from the above analysis.

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1. 2.

As the water flows through the series of steps the oxygen content is found to be higher than the initial value due to the surface contact with atmosphere.. As the discharge was varied, the flow rate could be controlled. Lesser the discharge more the time taken for the water to flow through the series of steps and, as a result, more the contact time and subsequent contribution to the rise in dissolved oxygen content.

Table 1: Measurement of salinity, temperature and dissolved oxygen Description Sample Water

Salinity (ppm) 2850

Temperature (oC) 25

Dissolved Oxygen (ppm) 0.2

Table 2: Value fully open Description

Salinity (ppm)

Temperature (oC)

First step Second step

2650 2670

27.9 27.3

Dissolved Oxygen (ppm) 0.5 0.7

Velocity (cm/s) 54.40 52.24

V = 74.74*10-3 m3; T = 54s ; Q = 1.38*10-3 m3/s Efficiency

= (Cd – Cu)/(Cs - Cu) = (0.7-0.5)/(7.9-0.5) = 2.7 %

Table 3: 6 Turns (closing) of the valve from fully open Description

Salinity (ppm)

Temperature (0C)


2690 2660

26.9 26.7

Dissolved (ppm) 0.5 0.8

Oxygen


Oxygen


Oxygen


V = 74.74*10-3 m3 ; T = 62s ; Q = 1.21*10-3 m3/s Efficiency = (Cd – Cu)/(Cs - Cu) = (0.8-0.5)/(8.1-0.5) = 9.95% Table 4: 9 Turns (closing) of the valve from fully open Description

Salinity (ppm)

Temperature (oC)


2670 2680

26.6 26.4


V = 74.74*10-3 m3 ; T = 62s ; Q = 1.21*10-3 m3/s Efficiency = (Cd – Cu)/(Cs - Cu) = (0.3-0.2)/(8.1-0.2) = 1.27% Table 5: 12 Turns (closing) of the valve from fully open Description

Salinity (ppm)

Temperature (oC)


2690 2680

26.9 26.6

V = 74.74*10-3 m3 ; T = 64s ; Q = 1.17*10-3 m3/s Efficiency = (Cd – Cu)/(Cs - Cu) = (0.8-0.2)/(8.1-0.2) = 7.6%

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Table 6: 14 Turns (closing) of the valve from fully open Description

Salinity (ppm)

Temperature (oC)


2670 2680

26.6 26.4


Oxygen


V = 74.74*10-3 m3 ; T = 343s ; Q = 0.22*10-3 m3/s Efficiency = (Cd – Cu)/(Cs - Cu) = (2.3-1.8)/(8.1-1.8) = 7.9% Nakasone, H. (1987). “Study of aeration at weirs and cascades.” Journal of Environmental Engineering, Jan/Feb 1987, Vol. 113(1), pp 64-81.

Conclusion The research work opens up the scope of using the most traditional way of water treatment, i.e., aeration and estimates the optimized configuration for DO-enrichment, with detailed technical estimation. In fact, the given guidelines can be used for creating least cost-aeration in waterbodies or treatment of wastewater or with minor modification of existing hydraulic structures.

Preul, H.C., and Holler, A.G.(1969). “Reaeration through low dams in the Ohio River.” Proceedings of the Industrial Waste Conference, Purdue University, South Bend, Ind., 1249-1270. Rindels, A.J., and Gulliver, J.S. (1991). “Oxygen transfer at spillways.” Air-water mass transfer, Proc., 2nd Symp on Gas Transfer at Water Surfaces, Steven Wilhems and John Gulliver, eds., American Society of Civil Engineers, Reston, Va, 524-533.

Acknowledgement The authors express their sincere thanks to our B. Tech. (Civil) students Jehoshuah Sylvester, Johnson, Mohameed Ameen, Sanya Atlaf and Sivashanmuganathan for their assistance in carrying out this work.

Srikanth Koduri and Brian David Barkdoll (2002). “Evaluation of Oxygen transfer at stepped cascade aerators.” Conference Proceeding of World Water & Environmental Resources Congress 2003 and Related Symposia, June 23.26, 2003, Philadelphia, Pennsylvania.

References Avery, S.T., and Novak, P. (1978). “Oxygen transfer at hydraulic structures.” Journal of the Hydraulics Division, American Society of Civil Engineers, 104(1), 1521-1540.

Thene, J.R. (1988). “Gas transfer at weirs using the hydrocarbon gas trace method with headspace analysis.” M.S.Thesis, University of Minnesota, Minneapolis.

Chanson, H., and Gonzalez, C.A. (2005). “Physical modeling and scale effects of air –water flows on stepped spillways.” Journal of Zhejiang University Science, 6A (3):243-250.

Tsivoglou,E.C., and Wallace,J.R. (1972). “Characterization of stream reaeration capacity.” USEPA Report No. R3-73-012, U.S. Environmental Protection Agency, Washington,D.C.

Chanson, H., and Toobes, L. (2002). “Experimental study of gas-liquid interfacial properties in a stepped cascade flow.” Environmental Fluid Mechanics 2:241-263.

Watson, C., Walter, W.R.., and Hogan, S.A. (1978). “Aeration performance of low drop weirs.” Journal of the Hydraulics Engineering, American Society of Civil Engineers, 124(1), 6349.

Foree, E.G. (1976). “Re-aeration and velocity prediction for small streams.” Journal of the Environmental Engineering Division, American Society of Civil Engineering, 102(5), 937-951.

Wilhelms, S.C., and Smith, D.R. (1981). “Reaeration through gated conduit outlet works.” Technical Report E-81-5, U.S. Army Corps of Engineers, Waterways Experiment Station, Vickburg, MS.

Holler, A.G. (1970). “Reaeration of discharge through hydraulic structures.” Project Report, U.S. Army Corps of Engineers, Ohio River Division, Cincinnati, OH. Markofsky,M., and Kobus, H. (1978). “Unified presentation of weir-aeration data.” Journal of Hydraulics Division, American Society of Civil Engineers, 104(4),462-468.

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