TYPES OF AERATORS USED IN WASTEWATER TREATMENT PLANTS

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1 TYPES OF AERATORS USED IN WASTEWATER TREATMENT PLANTS Bianca-Ștefania ZĂBAVĂ 1, Gheorghe VOICU, Victor-Viorel SAFTA, Nicoleta UNGUREANU, Mirela DINCĂ, Mariana IONESCU, Mariana MUNTEANU University POLITEHNICA of Bucharest, Faculty of Biotehnical Systems Engineering ABSTRACT Water is vital for sustaining all forms of life on earth. The growth of human population has led to industry development which means, in addition to producing necessary goods, the consumption of natural resources, including water. Wastewater is the liquid end product or by-product of an municipal, domestic or industrial activity. The most efficient way to control and limit pollution is wastewater treatment before discharge into the receiving water course. Wastewater contains constituents that, if present in excess, will affect the quality of groundwater and other nearby water sources, and in the same time, will affect human life. The purpose of wastewater treatment is to improve water quality, so that it can be discharged into the environment without harming environmental factors. This paper presents the types of aeration systems used in the biological stage of a wastewater treatment plant. 1. INTRODUCTION Pollution is one of the major problems of mankind nowadays. It is obvious that the natural environment deteriorates gradually and that ecosystems cannot adapt to the pressure of human activities. Ecosphere self-regulation is no longer possible. The growth of human population has led to industry development which means, in addition to producing necessary goods, the consumption of natural resources, including water [1]. Water pollution is mostly due to the industrial development, population growth and urban discharge into rivers and lakes of wastewater, more or less treated. Wastewater is the liquid end product or by-product of an municipal, domestic or industrial activity. In the modern view, the protection of water quality means that the industry deals with issues of water resources quality []. Currently, a determinant key of water quality is wastewater, which is seen as today s only practical means of combating water pollution [3]. Council Directive 91/71/00 is the legal basis on legislation related to wastewater. This Directive, transposed by H.G. 188/00, defines water treatment as the removal from wastewater of toxic substances, microorganisms, etc., aiming to protect the environment, the envoy first, and also soil and air [4]. The presence of biodegradable organic compounds reduces oxygen levels in lakes and rivers, resulting in fish death and bad odours. Other organic materials, such as pesticides, detergents, fat, oil and grease, and solvents create toxic effects and esthetic inconvenience and they bioaccumulate in the food chain. Due to the problems mentioned above, the treatment of domestic wastewater becomes necessary []. A wastewater treatment plant separates solids from the liquid and consists of two basic stages: primary treatment and secondary treatment. In the primary treatment stage, larger solids are removed from wastewater by settling. Secondary treatment is a large biological process for further removal of the remaining suspended and dissolved solids. Secondary treatment removes up to 85% of the remaining organic material through a biological process of cultivating and adding sewage microorganisms to the wastewater. This process is accomplished in a trickling filter or an aeration tank. Aeration is one of the methods which helps in the removal of various contaminants found in the wastewater. 1 Splaiul Independentei 313, Sector 6, Bucharest, , bianca.dragoiu@yahoo.com 455

2 . THEORETICAL ELEMENTS In waste water treatment, for certain specific treatments, the transfer processes of gaseous components in/off water are important (introduction of atmospheric oxygen in the water, removing carbon dioxide and hydrogen sulphide from the water, introduction of chlorine or ozone in the water). Among these, the most representative treatment and widely used in practice is the introduction of gaseous oxygen in the effluent, in order to remove organic impurities under the action of a biomass of aerobic bacteria, for chemical oxidation of hardly biodegradable or mineral compounds, or to eliminate gaseous compounds for improving water taste [5]. The oxygen comes most often from the atmosphere, and in this case the process is called water aeration. Wastewater aeration process is performed by secondary sewage treatment plants, also called biological treatment. Usually, the wastewater doesn t contain gaseous oxygen due to the organic pollution load. Dissolved oxygen is naturally found only in clean natural waters (i.e., unpolluted) [6]. The process of water aeration is quantified by the oxygenation capacity. There are two methods for determining the oxygenation capacity of a system: unsteady state method and steady state method. The steady state method is based on the implementation of a biological process in an aeration tank (bioreactor) which contains activated sludge, in which all state parameters of the process remain constant and a certain amount of oxygen is introduced continuously to maintain a constant level of dissolved oxygen (between 1- mg/l). This method is more accurate and more difficult to apply in practice. The unsteady state method is much simpler to perform, and it is equally accurate and reliable. The principle of this method is the following: a gas forming part of an atmosphere in contact with a liquid in which it is soluble, will pass into a solution up to the point at which gas concentration in the liquid is in equilibrium with the concentration of the gas in the atmosphere. The equilibrium point is determined by the coefficient of absorption Ks (Eq. 1), which is the value of gas concentration in the liquid when the atmosphere is composed entirely of the considered gas: ln Dt1 ln Dt,303 log Dt1 log Dt K s (1) t t t t 1 where: Ks coefficient of absorption, which in this type of experiments, is conveniently expressed in [h -1 ]; Dt1, Dt [mg/l] two selected readings of oxygen deficit concentration in fluid; t1, t [h] corresponding values of time to achieve adequate oxygen deficits Dt1, Dt [6]. The oxygenation capacity R of a system is defined as the rate of absorption of oxygen during the aeration process of completely de-oxygenated water. The oxygenation capacity is determined by the following relation: 1 R K s F Cs( 10) V () where: R [g/h] - water oxygenation capacity; Ks [h -1 ] - coefficient of absorption; F temperature correction factor; Cs(10) [mg/l] - oxygen saturation concentration at 10ºC; V [l] the volume of liquid in the system [6]. Figure 1 showns how the variation of oxygen available in the aeration basin depends on the degree of water purification. Perfect knowledge of the processes and factors influencing the aeration process, as well as that of their best correlations, leads to biological treatment with maximum efficiency and to neutralizing the organic substances. 456

Cost analysis for wastewater treatment stations repeatedly show that the aeration is from 50% to 70% of the general budget of the station officials.

3 Figure indicates the electricity requirements for primary production in an activated sludge plant. Aeration is based bioscrubbers wastewater treatment and control the performance and operating economy of the entire wastewater treatment plant. Cost analysis for wastewater treatment stations repeatedly show that the aeration is from 50% to 70% of the general budget of the station officials. Without the right aeration technology and a properly designed aeration system can not be obtained cleansing guaranteed performance and operating costs [7,8]. Figure 1: The way of change of the oxygen required for aeration in the tank depending on the degree of purification of water [7] Figure 3 presents the technological process of the biological stage of a wastewater treatment plant. After settling, the wastewater falls into a pool - aeration basin - where is put into contact with sludge flocs, infused with oxygen and nutrients, for biochemical processes of degradation of organic substances. Neext, newly synthesized cellular material must be separated in the secondary clarifier. Part of the sludge is recycled, and the other part is removed as excess sludge from the settling sludge in the household. Figure : The level of energy used in a Figure 3: Schematic diagram typical station for wastewater treatment [7] of a biological treatment plant [9] Aerators are typically used for wastewater applications like sewage and effluent treatment. Typical applications for floating aerators include oxygenation of harbours, rivers, canals, lakes and reservoirs, but they can be equally used well for sewage and effluent treatment in balancing tanks, in concrete aeration channels or in low cost lagoon type aeration systems. 457

4 3. TYPES OF AERATION TANKS USED IN THE BIOLOGICAL STAGE OF WASTEWATER TREATMENT PLANTS If activated sludge wastewater treatment has an important role, aeration tanks form and maintain the activated sludge. Aeration tanks are building whose plane shape may be radial, rectangular or square. In terms of construction, an aeration tank has the form of a rectangular concrete pool, where biological treatment takes place in the presence of a mixture of activated sludge and wastewater. To ensure continuous contact of the two components of the mixture, constant stirring is required to ensure their breathalyser constantly the oxygen needed by aerobic colony existing in the activated sludge in the form of flakes (Figure 4). Besides stirring and aerating, the mixture in the tank aims to maintain a quasi-constant concentration of activated sludge as a result recirculating a part of sludge settled in the secondary clarifier [10]. Generally, the equipment operates in the water and under the influence of atmospheric oxygen. For this reason, corrosion-resistant materials are preferred, such as stainless steel, cast iron, bronze and other non-ferrous materials like ceramics, plastics etc. Figure 4: Schematic diagram of a surface aeration tank [10] The biological process requires oxygen taken from the atmosphere, which is introduced in reaction basin through three categories: a) pneumatic methods - the atmospheric air is compressed and introduced by means of the reaction tank equipment in the form of: fine bubbles, medium bubbles and large bubbles. b) mechanical methods - when are put in contact with activated sludge basin content and ambient air through an intense mechanical mixing. c) mixed methods - which use both air insufflation devices and mechanical agitators [11]. Generally, the functioning scheme of the pneumatic equipment for wastewater oxygenation comprises four distinct phases: the air supply; purifying the feed air; the distribution of the purified air; dispersing air into the water table. If the pneumatic aeration tank requires it, oxygen is introduced with the air. Thus, compressed air is introduced through a series of underwater pipes provided with nozzles or porous plates. Aeration tanks are characterized by the introduction of air bubbles into the water. Depending on their size, the bubbles can be: fine bubbles (with diameter between mm) (Figure 5); medium bubbles (diameter mm); large bubbles (diameter over 3 mm) (Figure 6). Fine bubbles are obtained by porous diffusers or air distribution through membranes with very fine holes. Medium bubbles resulting from the air distribution tubes provided with orifices whose diameter is between 1-5 mm and is spaced less than 5 mm from each other. Large bubbles are the result of air distribution through pipes or plates drilled with holes opening over 5 m [1]. 458

Figure 5: Fine bubble aeration systems[13] Figure 6: Large bubble aeration systems [14] If mechanical adhesion achieves

Considering the way in which the air suctionis is made, mechanical aeration systems are the following: aspirated

Mechanical aeration tanks with aspiration are rarely used.

Venturi effect and water + air mixture is pushed to the bottom.

above are used which achieves the dispersion of air into the water table (pneumatic) and forcing convection by

Figure 8: Mixed aeration tank [15] Regardless the aeration system, there must be taken into consideration the amount of

5 Figure 5: Fine bubble aeration systems[13] Figure 6: Large bubble aeration systems [14] If mechanical adhesion achieves strong water agitation in the tank, it produces intense mixing between water, mud and air. Considering the way in which the air suctionis is made, mechanical aeration systems are the following: aspirated mechanical aerators; mechanical rotor aerators; mechanical aerators with blades or brushes. Mechanical aeration tanks with aspiration are rarely used. These aerators are actually vertical tubes through which the air is sucked with water through pipes, knowk as the Venturi effect and water + air mixture is pushed to the bottom. Figure 7: Mechanical aerator surface [15] In the case of mixed aeration methods (Figure 8), the two schemes mentioned above are used which achieves the dispersion of air into the water table (pneumatic) and forcing convection by mechanical system (system recommended in waters heavily organic loaded). Figure 8: Mixed aeration tank [15] Regardless the aeration system, there must be taken into consideration the amount of air circulated and how the transfer of oxygen from air to water is achieved. Using oxygen from the air for biological wastewater treatment depends on the variation of physical quantities (pressure, temperature, etc.) and the characteristics of wastewater (biodegradability of organic materials, their amount in the water, the oxygen content initially). 459

6 4. CONCLUSIONS In wastewater treatment, for certain specific treatments, the processes of transfer in/of water of gaseous components are important. Among these, the most representative treatment, widely used in practice is the introduction of gaseous oxygen in the effluent, in order to remove organic impurities under the action of a biomass of aerobic bacteria. The oxygen comes most often from the atmosphere, and in this case the process is called water aeration. To protect the environment, and especially the emissary, the soil and the air, the process of wastewater treatment should provide favorable conditions for further use of treated water in domestic, industrial or agricultural activities. Untreated wastewater discharged into rivers has a devastating impact. References [1] Hossein J. M., Amir H. M., Ahmad J. Jafari, Khanjani N., Evaluation of dairy industry wastewater treatment and simultaneous bioelectricity generation in a catalyst-less and mediator-less membrane microbial fuel cell [] Jency, N., Devu, M., Keerthana, D., Meenu, R., Riya R.P.P., A Study on Effect of Aeration on Domestic Wastewater, International Journal of Interdisciplinary Research and Innovations ISSN , 3(): 10-15, 015. [3] Rojanschi, V., Ognean, T., Book of operator in wastewater treatment plants, TEHNICA Publishing, Bucharest, [4] Zăbavă, B.Ș., Voicu, Gh., Ungureanu, N., Dincă, M., Safta V.V., Basic equipment for the mechanical treatment of wastewater, International Symposium ISB-INMA TEH, Agricultural and Mechanical Engineering, pp , ISSN , Bucharest, 9 oct.-31 oct [5] Radu M. F., Panaitescu V. N., Technology for waste water treatment with porous tube aerators and elastic perforated membrane tubes of the biological waste water treatment [6] Safta, V.V., Toma, M.L., Ungureanu, N., Experiments in water treatment domain, PRINTECH Publishing, 01; [7] Optimization of Diffuser Systems, Technical Bulletin 153, Environmental Dynamics International, 01. [8] Steven A. Bolles Modeling wastewater aeration systems to discover energy savings opportunities, Process Energy Services, LLC [9] ADF Health Manual,Vol 0, part8, chp. [10] Rao, A., R., Bimlesh, K., Civil Engineering Department, Indian Institute of Science, Bangalore 56001, India. [11] [1] RSELOR%0DE%0APA.pdf [13] [14] [15] 460

Marginal Water Treatment by Combination of Two Stages of Stepped Cascade Weir and Spray Aerator

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