Su arıtımında ozonlama prosesinin incelenmesi

Abstract

Bu çalışmada su arıtımında ozon, konusu incelenmiştir. İlk bölümde, çalışmanın önemi, amaç ve kapsamı açıklanmıştır. İkinci bölümde, ozonla arıtımın tarihsel gelişimi ve ozon tesisleri kullanan ülkeler hakkında bilgiler sunulmuştur. Üçüncü bölümde, ozonun çevre mühendisliğinde kullanılma amaçları, ayrıca ozonla arıtımın avantaj ve dezavantajları ifade edilmiştir. Dördüncü bölümde ozon kimyası hakkında ana bilgiler verilmiştir. Bu bölüm; sudaki ozon reaksiyonları, ozon transferi, ozon ayrışması ve ileri oksidasyon prosesleri gibi konuları içermektedir. Beşinci bölümde, bir tesiste ozon sistemi kurmak için gerekli adım ve prosesler anlatılmıştır. Altıncı bölümde, ozonun çeşitli maksatlar için uygulamaları örneklerle açıklanmaya çalışılmıştır. Yedinci bölümde, bir suyun arıtma karakteristiklerini belirlemek için kullanılan antılabilirlik çalışmaları hakkında genel bilgiler verilmiştir. Sekizinci bölümde ise, ozon reaktörlerinde organik kirleticilerin giderilmeleri ile ilgili bir modelleme incelenmiştir. Bu modellemenin baz alınmasıyla, reaktiviteleri ve uçuculukları farklı üç kirletici madde seçilerek ozonla giderilme verimlilikleri tahmin edilmeye çalışılmış, ayrıca reaktördeki proses parametrelerinin, giderim ve ozon absorpsiyon verimliliklerine etkileri incelenmiştir.

In this thesis, ozone in water treatment has been studied. In the first section, the importance of this study and its aim have been explained. In the second section, the historical development of treatment with ozone has been told. In the third section, primarily application fields of ozone in environmental engineering have been told. These are; -Removal of inorganics (Fe++, Mn++), -Removal of micropollutants (SOCs), -Improvement of taste, odour and colour of water, -Algae control in plant, -Disinfection, -Control of disinfection by-products -Biological stabilization. -Particulate removal. Ozone is a powerful oxidant and has higher reactivity and oxidation power than the other chemicals used for water treatment. Because of its high oxidation potential, ozone requires short contact times and dosages for disinfection and oxidative purposes. Its disinfectant and sterilizing effect is independent on presence of NH3 and pH of water unlike chlorine. Ozone reacts favourably with substances with which chlorine fails to enter into reaction or reacts unfavourably. Ozone is "purest" and "most friendly to environment" oxidant. Because it does not give foreign materials into water, directly. But, ozonation can produce some toxic, mutagenic and/or carcinogenic compounds. The quick decomposition of ozone in water creates adverse effect, because no residual ozone content can be maintained unlike chlorine. Although the capital costs of ozonation systems are high, their operating costs are moderate. In the fourth section, fundemental informations about ozone chemistry have been given. The solubility of ozone in water obeys Henry's law. This means that Cs values are proportional to partikal pressure of ozone Py at a given temperature. Diffusion of ozone in water obeys Fick's law for molecular diffusion. Ozone transport is much faster in the gas phase than in the liquid phase. The liquid phase determines the overall transport rate. Reactions of ozone in water can be categorized by speed as follows: XVI -Very slow reactions, -Slow reactions, -Fast reactions. Generally, ozone acts on various compounds in the following two ways; -Direct reaction with molecular ozone, -indirect reaction with radicals that formed when ozone is decomposed in water (Free radikal reaction). Molecular ozone reactions are extremely selective and limited to un saturated aromatic and aliphatic compounds as well as to specific functional groups. The stability of dissolved ozone is readily affected by pH, UV Ugh, ozone concentration and the concentration of radical scavengers. The decomposition rate, measured in the presence of excess radical scavengers which prevent secondary reactions, is expressed by a pseudo first-order kinetic equation: ('"waL k = Pseudo first-order rate constant for a given PH value. This evalution reflects the fact that the ozone decomposition rate is first-order, with respect to both ozone and hydroxide ions, resulting in an overall equation of the following form; d [03] 1 k' = k[03][OH-], and *« - it *-i~".i~» J ' "*" [OH'] The presence of scavengers such as bicarbonates in the water may inhibit the free radical reaction chain, hence slowing down decomposition of ozone. Hydrogen peroxide (H202) and UV radiation are among the factors likely to induce decomposition of ozone in water, generating highly reactive hydroxyl radicals. These two entities are used to activate ozone in a neutral pH water and when combined with ozone, provide advanced oxidizing treatment techniques (AOPs). m the fifth section, the steps and processes that are necessary to establish an ozonation system in a plant have been explained. The instability of ozone generally necessitates its generation at the time and on the site of its application. For most applications, ozone is generated by applying a high voltage to generate an electrical field under the influence of which the oxygen undergoes partial dissociation into radicals. To generate ozone, it can be also used the other methods such as photochemical, electrolytic, radiochemical. There are several feed gases to be evaluated for an ozonation facility. These include air, high-purity oxygen, recycled high-purity oyygen and oxygen-enriched air. Dried, filtered air is the most commonly used gas in ozonation systems throughout the world. The quality of feed gas is critical to XVII the performance of any ozone generation system. For successful long-term operation with minimum maintenance, the feed gas, whether air or oxygen, must be extremely dry; free of particulates, hydrocarbons and other contaminants; and relatively cool. The most important factor affecting quality is moisture content. Excessive moisture will adversely impact ozone production. Removal of moisture from the gas can be accomplished through compression, cooling, and desiccant drying. Gas compression, followed by an appropriate aftercooler, will remove some moisture, since the moisture holding capacity is reduced at increased pressures. Likewise, moisture holding capacity is reduced at lower temperatures, and direct gas cooling will remove additioal moisture. İn most systems, several steps are used to reduce the moisture content, but only desiccant dryers are effective in ultimately achieving the very low dewpoints necessary for an ozone feed gas, and are always the last gas drying step prior to the generators. Ozone generators may be described on the basis of a number of parameters. These parameters include type of dielectric, frequency, and mode of generator cooling. Dielectrics incorporated in commercial scale ozone generators include glass plates, metallized glasstubes, and ceramic plates, in addition to even more proprietary concepts. Ozone generators frequency has been generally grouped into three broad categories.. Low frequency: 50 or 60 Hz. Medium frequency: 60 to 1000 Hz. High frequency: >1000 Hz The mode of generator cooling is mainly water but may also be air as well as other media such as oil and freon. Ozone generators types such as the low-frequency horizontal-tube water-cooled, the medium-frequency horizontal-tube water-cooled, and Van der Made are the most commonly used. To carry out the desired function in water treatment, ozone must be transferred from gas phase into liquid phase. For this purpose, ozone contacting units are used. A number of techniques are available for dissolution of ozone in the liquid to be treated. These are; 1. Conventional fine bubble diffision, 2. Turbine mixers, 3. Injectors, 4. Packed columns, 5. Spray chambers, 6. Deep U tube, 7. Submerged static radial turbine contactor. The fine bubble diffuser contactor is the most widely used of the available ozone transfer systems because it is operated without the further addition of energy in addition to that initial gas compression. The xvm technology is not new; in addition to its use in ozonation systems, fine bubble diffusers have been used for many years as a means of oxygen transfer in both water treatment and wastewater treatment aeration basins. In the sixth section, the applications of ozone in water treatment have been told by means of examples. The standart oxidation-reduction potential and reaction rate of ozone are such that it can readily oxidize iron and manganese in groundwater and water of low organic content. The oxidation of manganese with ozone is less dependent on pH than for other oxidants. Oxidation with chlorine and chlorine dioxide can lead to satisfactory color abatement, but ozone remains the most efficient oxidizer, and ozonation is the treatment most often mentioned in the literature for oxidative color removal. It has been shown that ozone can be effective at treating water for taste and odour problems, especially when the water is relatively free from radical scavengers. İt has also been observed that ozone in combination with other downstream treatment processes, especially GAC filtration, can greatly increase taste and odor treatment efficiency and reliability. Ozone or advanced ozonation, processes can remove many micropollutans. This removal leads to the chemical transformation of these molecules into toxic or nontoxic by-products. Some of these oxidation by-products are easily removed in subsequent stages of treatment, especially when GAC is used. Removals through ozonation of easily oxidized micropollutants take place primarily as the result of direct reaction. The contribution of indirect radical reactions for these compounds in small, owing to the presence of free radical scavengers as carbonates and bicarbonates. For oxidation of micropollutants, a general second-order rate law has been proposed: -d[M]/dt= k.[03].[M] Ozone is capable of destroying some volatile compounds, in particular alkenes and aromatics,under the conditions of treatment applied to drinking water. The destruction of alkanes is very slow except if a radical mechanism is encouraged through increasing the pH (pH>9). Ozone, like any other oxidant such as chlorine or chlorine dioxide, has a lethal effect on some algae or limits their growth. Ozone is effective for disinfection (the elimination of inactivation of bacteria, viral particles, and parasites). Its efficiency increases with increasement of temperature. The U.S EPA has specified the aC.t" concept to assure adequate disinfection, where C is the concentration of dissolved disinfectant (in miligrams per liter) and t is the nominal contact time (in minutes). The use of ozone may cause changes in the concentration of disinfection by-products (DBPs) in water. These changes may come about because ozone can either: (1) immediately destroy or form by-products, (2) XIX immediately destroy or create new by-product precursors, (3) alter the nature of the water so that subsequent processes are better or worse at removing the by-products, (4) alter the nature of the water so that subsequent processes are better or worse at removing the by-product precursors, or (5) permit use of lower doses of other disinfectants or changes in their point of addition. The primary by-products of ozonation are oxygen-containing derivatives of the original organic materials, mostly aldehydes, ketones, alcohols and carboxylic acids. Ozone, howewer, produces toxic oxidation products from a few organic compound. THM precursors can be reduced by ozonation. TOXFP and DCANFP are reduced by ozonation, DCAAFP appears to be relatively uneffected by preozonation. In the seventh section, the general informations about treatability studies with respect to ozone have been given. Treatability studies are used to determine the treatment characteristcs of a spesific water. They can range from determining the feasibility of a single treatment process to the optimization of the entire treatment train. The scale of the studies can include bench-scale, pilot-scale, or even demonstration-scale evaluations. In the eighth section, A model with respect to the removal of organic pollutants in ozone reactors has been studied. The objectives of this model are to estimate organics removal and ozone asbsorption efficiencies in bubble reactors and to analyze the model for its sensitivity to the process parameters. In a ozone reactor, the major mechanisms contributing to the removal of organic pollutants can be identified as physical stripping (volatilization) and chemical oxidation by ozone molecules (direct oxidation) and by free radicals (indirect oxidation). Thus, the following factors are expected to determine the overall removal efficiency: (1) The chemical reactivity of the compound, as measured by the rate constants of direct (k) and indirect (k1) reactions; (2) The volatility of the compound, as measured by the Henry's law constant (H), (3) the aqueous ozone concentration ([03]); (4) the contact opportunity between the water and the gas phase, as measured by the mass transfer coefficient in the reactor (KLa); (5) the chemical composition of the water. Based on their k ve H constants, organic compounds can be categorized as follows: Group 1: Highly reactive and nonvolatile compounds (ex: phenol, cresol) XX Group 2: Less reactive and volatile compounds (ex: Benzene) Group 3: Moderately reactive and volatile compounds (ex: xylene) The mass transfer coefficient, which determines the rate of ozone absorption as well as the rate of volatilization of organic compounds, is a function of reactor geometry and of the operatig conditions in the reactor. Furthermore, KLa can be effected by the chemical composition of the water. The rate of chemical oxidation of reactive organic pollutants is directly proportional to the ozone concentration in the water, which is determined by the operational conditions of the reactor, such as the flow rates of the gas (Qg) and water (Ql), the concentration of ozone in the applied gas stream (03)^, the water detention time (t), KLa, and the chemical composition of the water. The water's chemical composition may be a significant factor in determining the overall performance of a reactor, because various organic or inorganic constituents can compete for ozone, accelerate or hinder self-decomposition of the ozone, scavenge free radicals, ana affect the mass transfer characteristcs. İn addition, the rate of self-decomposition of the ozone is a strong function of the pH of the water. A study has been carried out by considering this model developed by Mirat D. Gürol. For this purpose, three different organic pollutants in terms of volatility and reactivity have been chosen. Organic Pollutants Chosen For This Study -Group 1: NAPTHALENE-ko3= 3000 L/mol.s. H= 0.02 (Dimensionless) -Group 2: TRİCHLOROETHYLENE- ko3= 17 L/mol.s H= 0.42 (Dimensionless) -Group 3: 1.2.4 TRİMETHYLBENZENE- ko3= 400 L/mol.s, H= 0.24 (Dimensionless) Formulas Used For This Studv: -For estimating the aqueous ozone concentration, [03]. 103]= (03)inf /|(wt+iy(QG/QL). [l-exp(-ea33*H03) (8.29) - To estimate the removal efficiencies [Mil/IMj] = l/Moks+Av0l) (8.17) Aox = kT. t. [03] (8.18) Avol= Hj. (Qg/Ql). [l-expf-Gj)] (0.1<ei5) (g20) XXI = (KLa)j.t (0^0.1) (8.21) For determining ozone absorption efficiency; (wt+1) FOA = - {(wt+l; + Ho3.(Qg/Ql) 1 [1-exp (-0oJ] J (8.30) ©03= (KLa)o3.t/H^. (Qq/Ql) <8.28) Figures 8.2-8.8 have been constructed by simultaneously solving Eqs 17-21 and Eqs 28-30 to estimate the efficiency of organics removal, [03] and FOA for various operational conditions. İn these figures, the sensitivity of the model to each parameter have been tested by varying each parameter over a range of values while the other parameters are held constant at the following base values: -Qg/Ql=2 _(03)inf= 6.5 mg/L (0.5 percent by weight in air) - (KLa)03 = 60 h"1 -t= 15 min. -w= 0.5 min1 According to the model and this study, removal efficiencies of organic pollutants in ozone reactors are determined mainly by the reactivity and volatility of the pollutants. The chemical composition of water, the mass transfer characteristcs of the reactor, and the operational parameters Qg/Ql> (C^inf» an<^ * were identified as the process parameters that control the performance of the reactors. The Model has success in predicting aqueous ozone concentrations, [03], and ozone absorption efficiencies, FOA. İn order to use the model, the raw water matrix must be characterized in terms of its w value. Then the H and kTotai constants of organic pollutants must be determined, exactly. Once the water and the target organic pollutants are characterized, the model equations will aid the optimal selection of the operating conditions to achieve a desired removal efficiency. The model also can be used to make cost and performance comparisons between ozonation and other alternative treatment methods, such as activated carbon adsorption and air stripping. </ei