Building Services Engineering Department, Technical University of Cluj-Napoca Boulevard December 21, no , , Cluj-Napoca, ROMÂNIA

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1 Applied Mechanics and Materials Submitted: ISSN: , Vol. 656, pp Revised: doi: / Accepted: Trans Tech Publications, Switzerland Online: Analysis of Oxygen Transfer and Dissolved Oxygen Concentration Measurement Tests in a Wastewater Treatment Plant ROMAN Marius-Daniel *, FELSEGHI Raluca-Andreea Building Services Engineering Department, Technical University of Cluj-Napoca Boulevard December 21, no , , Cluj-Napoca, ROMÂNIA * Marius.ROMAN@insta.utcluj.ro, ralu.f_2006@yahoo.com, Keywords: oxygen transfer, dissolved oxygen, measurement, aeration systems, diffusers. Abstract. The efficiency of oxygen transfer depends on many factors including the type, size and shape of diffusers and the tank geometry. In this paper, the effect of the depth of water in the tank and the extension of coverage area of diffusers on each of oxygen transfer capacity, efficiency and dissolved oxygen concentration is tested. Experimental procedure is adopted to evaluate the effect of dissolved oxygen concentration. The results of the case study showed that, both the depth of water and the extent of coverage area of diffuser had a significant effect on the tested parameters. The values of oxygen transfer capacity was 76.7 kg O 2 /h, oxygen efficiency (without agitation): 5.3 kg O 2 /h and oxygen efficiency (with agitation): 4.2 kg O 2 /h. Introduction Aeration systems normally perform two functions in activated sludge wastewater treatment processes, oxygen transfer and mixing. While oxygen transfer may be regarded as the primary function, mixing is also important to ensure a full utilisation of the activated sludge reactor volume and a uniform dispersion of dissolved oxygen throughout the mixed liquor. However, in some applications of the activated sludge process, a mixing input is provided which is independent of the aeration system, thereby relieving the aeration system of its mixing function [1]. Oxygen (dissolved in the mixed liquor) is required for respiration by the micro-organisms in the aeration tank. The activated sludge process depends on the activity of these aerobic microorganisms and consequently, accurate control of oxygen in the aeration tank is vital [2]. Submerged fine-pore air diffusers is the most common way to provide oxygen to the biomass treating wastewater. The bubbles are generated with porous ceramic, plastic or flexible membranes of various shapes (dome, disk, tube, panel). Oxygen transfer Oxygen transfer is described by the value of mass transfer and the saturation concentration of oxygen in the liquid. In many cases too much or too little oxygen in aeration tank is undesirable for different reasons: too much oxygen adds unnecessary cost, to increased power consumption and too little can decrease the metabolism of the microorganisms and the efficiency of the process [3]. The theoretical oxygen requirement can be calculated by assuming that the biodegradable matter is converted to carbon dioxide, water and energy. It is possible to estimate the exact oxygen requirement by writing down a balanced equation: C 6 H 12 O 6 +6O 2 6CO 2 +6H 2 O (1) In reality however, such theoretical calculations become complex because of the diversity of the chemical compounds found in wastewater. If nitrification is required, then the oxygen demand for the convention of ammonia to nitrate must be taken into account. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-10/05/16,04:12:53)

2 Applied Mechanics and Materials Vol NH 4 + 3O2 + nitrifyingbacteria (2) + + 2NO2 + 4H + 2H 2O 2NO + O + nitrifyingbacteria NO (3) Aeration equipment is compared by the amount of oxygen transferred per unit of air introduced to the wastewater under standard operating conditions expressed as kgo 2 /kwh. Residual dissolved oxygen is removed from the liquid by adding sodium sulphite and a catalyst (cobalt chloride). Aeration is started and the rate of oxygenation of the liquid is measured by recording the DO at regular intervals. The oxygen transfer coefficient (K La ) may then be determined [4]. The test method involves the removal of dissolved oxygen (DO) from a known volume of water by the addition of sodium sulfite followed by re-oxygenation to near the saturation level. The DO of the water volume is monitored during the re-aeration period by measuring DO concentration at several different points [5]. The basic equation describing the rate at which oxygen is absorbed by water is: dc dt where: dc dt = = K C C ) (4) La ( st t transfer rate of oxygen to the water (mg/l.t); C t = concentration of oxygen in the water at time (t), (mg/l); C st = saturation concentration of oxygen in water with respect to air in bubble at mean depth, (mg/l); t = the time. The difference (C st C t ) between saturation value and actual concentration of oxygen (C) in the body of the liquid phase is usually called oxygen deficit. The oxygen transfer rate is determined by integrating of this equation. From equation (5), the initial oxygen uptake rate at C t = 0, is: dc dt where: = OC = K La C ) (5) ( st OC = the oxygen transfer capacity of the system, (gro 2 /m 3 water.h). The fraction of oxygen transferred to the water, to pass one-meter cubic of air is expressed as oxygenation efficiency (E) of the diffuser system, which can be written as: OC H E = (6) I where: H = the liquid depth in the tank in meters. I= the aeration intensity, or volumetric air flux per unit area of tank surface. The solubility of oxygen in water is temperature dependent, as: K = K (7) La( T ) T 20 La(20 C) Θ where: Θ is a temperature correction factor typically taken as for aeration devices and T is the temperature at which the test is carried out.

3 488 Monitoring, Controlling and Architecture of Cyber Physical Systems Mass transfer coefficients are usually measured in clean water which does not have the same surface tension as wastewater. Two coefficients are used to compensate for the difference between measured and actual values. These are the α and β values [6]. K La( wastewater ) α = (8) K La( tapwater ) The α value is used to compensate between the measured and actual values of K La with regard to surface tension, tank geometry and mixing intensity. Typical α values for diffused and surface aerators are in the range of and respectively. These values are related to the design of the tank used during the test procedure. β-values are used to account for the differences in the solubility of oxygen to constituents in the wastewater. The presence of salts, particulates and detergents may affect the oxygen transfer rate. The factor is: CS 20( wastewater ) β = (9) C S 20( tapwater ) where: C S20 is the saturation concentration of oxygen at 20 C. Values may vary from 0.7 to 0.98 with a value of 0.95 being commonly used. Monitoring of dissolved oxygen Monitoring the amount of dissolved oxygen in a aeration process is an important factor in determining efficiency of the process. There are a variety of wastewater dissolved oxygen sensor technologies, electrochemical cells ( galvanic or polarographic) or optical sensors with luminescent techniques, each with its own advantages and disadvantages [7]. One of the new ways to accurately measure DO is to use a luminescent sensor (LDO) for online measurement and control of DO. The optical method of measuring dissolved oxygen avoids the disadvantages of traditional electrochemical measurement methods. The LDO principle is based on the physical phenomenon of luminescence. If a combination of a suitable luminophore and a suitable wavelength of light is chosen, the intensity of the luminescence and the time it takes to fade are dependent on the oxygen concentration around the material. Case study Oxygen transfer experiment at the wastewater treatment plant from Gherla s city The wastewater treatment plant from Gherla s city which represents the objectives of investigation was designed for population equivalents. It was conceived as an activated sludge treatment plant with a simultaneous aerobic sludge stabilization. At the beginning of the experimental tests, the installation of optical sensors in the wastewater under the set point values, the measurements started and the aeration system was controlled by the oxygen concentration in two points in the tank, where the dissolved oxygen was measured each 30 seconds till reaching the saturated level, where run was terminated. The operation parameters and geometric configuration were considered as variables according to the purpose of the experiments to identify their influence. The experimental test equipment includes: 1. Aeration type: fine bubble diffusers 2. Number of diffusers: 48 pieces 3. Mixer: 1 piece

4 Applied Mechanics and Materials Vol Electric power: 4.5 KW 4. Blower: 1 piece 4.1. Volume flow: 800 Nm 3 /h 4.2. Differential presure: 570 mbar 4.3. Motor power: 22 KW 4.4. Blower speed: rotations/minute This research aims to achieve the following objectives: to determine the effect of water depth in the tank on each of oxygen transfer capacity (OC) and measurements of the dissolved oxygen concentration. In the aeration tank were installed two oxygen sensors ( see. fig. 1-3 ) Measurement of dissolved oxygen concentration: Evaluation of sensor 1 Aeration tank volume: 4000 m 3 OCL,h g/( m 3 N * m) Water temperature: 14.4 C t90: min Saturation factor with O 2 : mg/l ta/t90: 106 % Motor power: 22 KW E.E. : 0.15 Depth of water he: 4.95 m Air pressure: 1000 mbar Volume flow: 800 Nm 3 /h Evaluation area: t: 30 sec. C t- concentration of oxygen in the water at time (t): mg/l C x - oxygen concentration: mg/l K La(T) - value of oxygen transfer coefficient at test water temperature: /h K La(20 C) value of oxygen transfer coefficient at 20 C : /h OC (20 C) - oxygen transfer capacity at 20 C : kg/kwh Fig. 1 Measurement of dissolved oxygen concentration sensor 1

5 490 Monitoring, Controlling and Architecture of Cyber Physical Systems Table 1 Evaluation of sensor 1 Measurement of dissolved oxygen concentration: Evaluation of sensor 2 Aeration tank volume: 4000 m 3 OCL,h- 19,58 g/( m 3 N * m) Water temperature: 14.4 C t90: min Saturation factor with O 2 : mg/l ta/t90: 100 % Motor power: 22 KW E.E. : 0.14 Depth of water he: 4.95 m Air pressure: 1000 mbar Volume flow: 800 Nm 3 /h Evaluation area: t: 30 sec. C t- concentration of oxygen in the water at time (t): 20, 21 mg/l C x - oxygen concentration: mg/l K La(T) - value of oxygen transfer coefficient at test water temperature: /h K La(20 C) value of oxygen transfer coefficient at 20 C : /h OC (20 C) - oxygen transfer capacity at 20 C : kg/kwh

6 Applied Mechanics and Materials Vol Fig. 2 Measurement of dissolved oxygen concentration sensor 2 Table 2 Evaluation of sensor 2

7 492 Monitoring, Controlling and Architecture of Cyber Physical Systems Measurement of dissolved oxygen concentration: Evaluation of sensor 1 and 2 Fig. 3 Measurement of dissolved oxygen concentration sensor 1 and 2 Table 3 Evaluation of sensor 1 and 2

8 Applied Mechanics and Materials Vol Results and discussions For each measurement, the following procedure was adopted: measurement of DO concentration versus time of aeration and residues. Tables shows the hour, measurement of DO concentration, set point of DO concentration and residues. The average values used in data analysis of this two measurements, were: oxygen transfer: 76.7 kg O 2 /h, oxygen efficiency (without agitation): 5.3 kg O 2 /h and oxygen efficiency (with agitation): 4.2 kg O 2 /h. Energy consumption obtained 3.9 kw for the mixer and 14.4 kw for the blower. However, the effect of decrease in the depth of water on the oxygenation capacity (OC) can be explained by the mechanism of exchange of the oxygen from the gas to liquid. During this phase, the transfer is occurring by the processes of bubble formation, release, and ascension. This rate of oxygen transfer is dependent on the relative rate of ascent, bubble size, partial pressure of oxygen, temperature, and driving force (the difference between the liquid film oxygen concentrations in equilibrium with the gas bubble and the bulk-liquid dissolved oxygen content) [8]. Additionally, the oxygen transfer is influenced by the dispersion and coalescence characteristics produced through turbulent recirculation patterns existing in the tank. The dissolved oxygen concentration is varying not only along the tank but also as function of time as is shown in figures 1, 2 and 3. In figure 1 and table 1, all measurements were made at water temperature of 14.4 C at every 30 secundes, with a set point for oxygen concentration from mg/l to mg/l, measurements obtained were from 23.1 mg/l to mg/l. The residues were between -0,62 and 0,99. In figure 2 and table 2, all measurements were made at water temperature of 14.4 C at every 30 secundes, with a set point for oxygen concentration from mg/l to mg/l, measurements obtained were from mg/l to mg/l. The residues were between and 0.8. Figure 3 and table 3 shows how oxygen concentration decrease from 23.1 respectively to respectively in 1 hour and 37 minutes. As shown in this figures the influence of this LDO sensors in the wastewater proved to obtain measurements close to the set points for oxygen concentration. Conclusions At a fixed airflow rate, increasing the level of water depth in the tank significantly increase each of oxygen transfer capacity (OC) in the system. This effect can be explained by the influence of elongation the bubbles paths which lead to increase its detention time in the tank. If the airflow rate is kept constant, oxygen transfer capacity (OC) is directly proportional to submergence. At 0.5 m submerges, the oxygenation capacity is 24 gro 2 /m 3 water.hr. With increasing the depth, the oxygenation capacity is increased to about 120 gro 2 /m 3 water.hr at the 5.0 m submerges. The advantages of LDO sensors for dissolved oxygen concentration measurements provides a new alternative. In comparison with the electrochemical methods, optical methods offer users considerable advantages in terms of the quality of the measured values and the optimum maintenance. Electrochemical sensors advantages: It can adapt to changing oxygen concentration fast, with a purposive response time, approximately 60 seconds for a change from zero to 98% of saturation. Electrochemical sensor disadvantages: They can easily get ineffective due to fouling of the membrane. When fouling appears medium contents and parts of cells attaches to the membrane and stop oxygen from passing the membrane, making the measurement more insensitive. The sensor is also sensitive to a fluctuating pressure or temperature and has to be recalibrated if any of the two parameters changes. LDO sensor advantages: The sensor doesen t consume any oxygen and therefore it has no effect on the measuring environment and doesen t need continuous liquid circulation. Possibility to long

9 494 Monitoring, Controlling and Architecture of Cyber Physical Systems storage time. The technology has a good measuring precision and high sensitivity in small oxygen changes. LDO sensor disadvantages: Even if the response time is sufficient it differs when concentration increase and when it decrease. In the paper this results proved to be very useful for further optimization efforts during full operation, as was shown in the case study. Acknowledgement: This paper is supported by the Sectoral Operational Programme Human Resources Development POSDRU/159/1.5/S/ financed from the European Social Fund and by the Romanian Government. References [1] Casey, T.J., Diffused Aeration Systems for the Activated Sludge Process, Aquavarra Research Publications Water Engineering Papers, Ireland, 2009, pp Retrieved from: [2] Olsson, G., Nielsen, M., Yuan, Z., Lynggaard-Jensen, A., Steyer, J.P., Instrumentation, Control and Automation in Wastewater Systems, IWA Publishing, London, UK, 2005, pp [3] American Water Works Association, Water Quality and Treatment, A Handbook of Community Water Supplies, McGraw-Hill Handbooks, USA, 5 th edition, Chapter 4, 1999, pp [4] Fändriks, I., Alternative Methods for Evaluation of Oxygen Transfer Performance in Clean Water, Printed at the Department of Earth Sciences, Uppsala, 2011, pp [5] M.Bayramoğlu, A.Çakıçı, and T.Tekin, Modelling of oxygen transfer rate in diffused-air aeration tanks, Process Safety and Env.Protection;Trans.IChemE., Part B, 78, 2000, pp [6] Groves, K.P., Daigger, G.T., Simpkin, T.J., Redmon, D.T., Ewing, L., Evaluation of Oxygen Transfer Efficiency and Alpha Factor on a Variety of Diffused Aeration Systems, Water Environmental Research, 64(5), 2009, pp [7] Glazer, B.T., Marsh, A.G., Stierhoff, K. and Luther, G.W. The dynamic response of optical oxygen sensors and voltammetric electrodes to temporal changes in dissolved oxygen concentrations. Analytica Chimica Acta., Vol. 518, 2004, pp [8] Chachuat, B., Roche, N., & Latifi, M. A., Optimal aeration control of industrial alenating activated sludge palnts. Biochemical Engineering Journal, 23, (2005),

10 Monitoring, Controlling and Architecture of Cyber Physical Systems / Analysis of Oxygen Transfer and Dissolved Oxygen Concentration Measurement Tests in a Wastewater Treatment Plant /