UV Işınlarıın koliform bakteri üzerindeki inaktivasyon ve fotoreaktivasyon etkisi
Başlık çevirisi mevcut değil.
- Tez No: 55928
- Danışmanlar: PROF.DR. DİNÇER TOPACIK
- Tez Türü: Yüksek Lisans
- Konular: Çevre Mühendisliği, Environmental Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1996
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 129
Özet
ÖZET Ülkemizde yeni ve pahalı bir arıtma teknolojisi olan Ultraviole (UV) ışınları ile dezenfeksiyon metodu, şimdilerde ancak içme sularının mikrobiyolojik olarak dezenfeksiyonu maksadı ile kullanılmaktadır. Ayrıca bu, mikrobiyolojik olarak tek fiziksel yöntemler ile dezenfeksiyon metodudur. Bu konuda daha çok veri elde edilmesi, teknolojinin mümkün olduğunca ekonomik bir sekile dönüştürülmesi ve kullanım alanlarının yaygınlaştırılması için halen araştırmalar devam etmekte ve bir çok deneysel çalışmalar yapılmaktadır. Bu deneysel çalışma ile de içme sularına bulaşabilecek toplam koliform bakterinin UV ışınları ile dezenfeksiyonu ve uygulanan UV dozunun toplam koliform bakteri üzerindeki inaktivasyon ve fotoreaktivasyon etkisi incelenmiş, etkin UV doz aralığı belirlenmiştir. Çalışmanın ilk bölümünde UV ışınlan ile dezenfeksiyon konusuna giriş yapılmış, konunun önemi üzerinde durularak, çalışmanın amacı ve kapsamı belirtilmiştir. İkinci bölümde, UV ışınlan ile dezenfeksiyon sisteminin mekanizmasından bahsedilmiştir. UV radyasyonunun tanımı yapılarak, kaynakları ve kullanım alanları belirtilmiş, kimyasal dezenfeksiyon metodlarına göre avantajları ve dezavantajları belirlenmiştir. UV sistemlerinin dezenfeksiyon işlemindeki potansiyel faydaları, verimliliği ve işletme güvenirliği açısından dikkat edilmesi gereken koşullar üzerinde durulmuştur. Daha sonra UV ışınlarının germisidal (mikrop öldürücü) etkilerinden bahsedilerek, bunun mikroorganizmanın DNA yapısı üzerindeki fotokimyasal harabiyeti yani inaktivasyon etkisi ile DNA yapısında meydana gelen değişiklikler ve bu fotokimyasal harabiyetin fotoreaktivasyon etkisi ile nasıl düzeltildiği anlatılmıştır. Üçüncü bölümde, UV ışınları ile dezenfeksiyon prosesinin dizayn esaslarından bahsedilmiştir. Bu üç kategoride ele alınmış, ilk olarak dezenfekte edilecek suyun kalitesinin dezenfeksiyon işleminin etkinliği açısından önemi belirtilmiştir. İkinci olarak, UV ışın şiddetini hem bioassay prosedürüne göre belirleme, hem de nokta kaynak toplama (PSS) metoduna göre hesaplama esasları anlatılmış, dağılma ve absorbsiyon mekanizmaları ile ışın şiddetinde meydana gelen azalma ve lamba konfıgürasyonunun bunun üzerindeki etkisi açıklanmıştır. Üçüncü olarak ise, UV reaktörünün hidrolik davranışının üzerinde durularak, hidrolik şartlar belirlenmiştir. Ayrıca, fotoreaktivasyon esnasında, mikroorganizmayı fotoreaktive edici UV dozunu hesaplama yönteminden de kısaca bahsedilmiştir. Dördüncü bölümde, UV ışınlan ile dezenfeksiyon konusunda günümüze kadar yapılmış olan deneysel çalışmalar hakkında literatürden örnekler verilmiştir. Beşinci bölümde, yapılan deneysel çalışmadan, kullanılan yöntem ve düzeneklerden, UV reaktörlerinin dizaynlarından ve lamba konfigürasyonlarından ayrıntılı olarak bahsedilmiştir. Bu çalışmada, laboratuvar koşullarında, kanalizasyon suyundan izole edilen koliform bakteri ilavesi ile sentetik olarak hazırlanan musluk suyu kullanılmış, deneyler sonunda canlı kalan koliform bakteri sayısı Membran Filtre Tekniği' ne göre tespit edilmiştir. Deneyler 18 seri olarak, maksimum, ortalama ve minimum debiler için, 1, 2 ve 3 cm su derinliklerinde yapılmış ve bu 1, 2 ve 3 dakikalık UV ışınına maruz kalma zamanlarında her seri için tekrar edilmiştir. Altıncı bölümde ise, deneysel çalışma sonunda elde edilen veriler değerlendirilerek, bu çalışmada kullanılan UV reaktörleri ile yapılacak olan bir sonraki dezenfeksiyon çalışmalarına temel teşkil edecek sonuçlar ve öneriler verilmiştir. XV
Özet (Çeviri)
SUMMARY THE EFFECT OF THE UV RADIATION ON THE IN ACTIVATION AND PHOTOREACTIVATION OF THE COLIFORM BACTERIA Recent evidence has suggested that chlorine residuals are toxic to aquatic organisms and that chlorination may cause the formation of carcinogenic compounds. This has generated interest in disinfection alternatives to chlorination. Disinfection with UV radiation which is a physical process, appears to be a potential alternative to chlorine. Ultraviolet radiation is generally defined as those radiations with wavelengths greater than the longest X-ray and less than the shortest wavelength visible to man. The UV source mostly used today for water disinfection is low pressure mercury lamp. The primary reason for its acceptance is that approximately 85 percent of its energy output is nearly monochromatic at the wavelength of 254 nanometers (nm), which is within the optimum wavelength range of 250 to 270 nm for germicidal effects. The use of UV radiation for drinking water disinfection in order to use of the other chemical disinfectants have a lot of advantages. Some of them are that there is no risk of the formation of toxic by-products, excellent disinfection performance with bacteria and viruses and short contact times required to inactivate bacteria and viruses. The disinfection of drinking water by UV rays have also some disadvantages that limited experience with UV technology and uncertainities regarding accuracy and reliability in measuring UV dose. UV light which comes from a source, directly effects the microorganism's cellular material and this energy cause a lethal effect on the replication ability of the microorganism and it stops the microorganism's photochemical reactions and inactivates the microorganism. The DNA molecule is considered to be the principal target of UV photons, and the primary component where significant biological effect, or damage, is incurred. UV radiation cause photochemical damages that are dimer formation, hydrate formation, denaturations of the DNA and further effects on the DNA. But the most dominant photochemical damage is the dimerization of two pyrimidine molecules. The photoreactivation mechanism is typically a photoenzymatic repair, requiring longer wavelength light in the near UV and visible spectrum. When an inactivated microorganism exposure to a visible light greater than 300 nm. wavelength, the bacterial cells which have been damaged by UV radiation can be repaired DNA damages itselves. This repairing work is termed photoreactivation. It is important to note that photoreactivating light is present in sunlight. The effects are quick, occuring within minutes after exposure to the necessary reactivating light. A second photoreactivation mechanism has been demonstrated to occur without the light requirement, called dark repair. It is a multi - enzymatic mechanism. Photoreactivation can impact the performance and design of a UV system in certain situations. The environmental factors, such as water and/or XVIwastewater quality, the long of exposure time, the type of microorganism and UV dose are also influence the degree and effect of photoreactivation. The design information of the UV disinfection process falls into three major categories: water and/or wastewater characteristics, UV intensity and the hydraulic behaviour of the reactor. The quality and the flow rate of the water will be disinfected, UV intensity and the exposure time have an effective role in the design of the UV disinfection systems. These parameters are directly related to UV dose. The overall criterion in UV disinfection is the UV dose. It is the product of UV intensity and exposure time and is expressed in ^wattsn/cm2 (microwatt-second per square centimeter). The use of a single exposure time presumes the ideal case of perfect plug flow in the reactor, with no axial dispersion. The information that would be required to effectively use the UV design model related to the characteristics of the water and/or wastewater to be disinfected, and to the physical characteristics of the reactor itself. The process flow, Q, is implicitly required to determine velocities and loadings to the system. Initial bacterial density, N0, should be determined under the average and maximum conditions anticipated for the plant. Particulate bacterial density, Np, associated with the particulate forms the minimum density level which can be achieved by the UV process. It is typically determined as a function of the suspended solids concentration The one parameter which is solely in the venue of UV disinfection is the UV demand of the water and/or wastewater. It is called UV absorbance coefficients. Specific organic and inorganic componds in the water will absorb energy at the 253.7 nm. wavelength. This absorbance will effect the intensity of the radiation within the reactor; in specific design situations, the level of absorbance will effect the sizing of a system and possibly the configuration of the lamps. There are a number of ways to expres the absorbance of a water. The easiest way of them is to measure directly absorbance of the water by a UV spectrophotometer. The transmittance of the water is also a common parameter used to describe the demand of the sensitivity of the bacteria to UV radiation. The value of the bacterial inactivation rate (K) is estimated as a function of the intensity of UV radiotion which a particular reactor can deliver. An estimation of this rate, therefore, requires knowledge of the actual intensity levels within the UV reactor. That's why, it becomes important to be able to quantify the intensity in a given system. The intensity in a reactor is a function of the UV source, the physical arrangement of the source relative to the water and the energy sinks present which will attenuate the source output before it can be utilized for disinfection purposes. Several approaches have been proposed to estimate light intensity, including chemical actinometry, biological assays, and direct calculation. The two procedures which have received the greater attention are the bioassay and direct calculation methods. The bioassay procedure has been applied in a limited fashion for a number of design specifications, primarily as a technique for quantifying the dose delivered by a specific piece of UV equipment. It can also be to implicitly derive the intensity within a system. The second method is the direct calculation of intensity. This accomplished by the point source summation (PSS) method. The steady-state bioassay method, generally using Bacillus subtilis, has been used as a specification in demonstrating the dose capacity of a given system. The dose is estimated as a function of flow rate through a scaleable pilot module. A minimum dose is then cited for the equipment specification. The bioassay procedure, although a valid and unique experimental design, has several disadvantages which, at present, detract from its use as a routine XVlltesting and evaluation procedure, tt is not standardized and results will vary from laboratuary lo laboratuary, both in the calibration and system assays. It can be costly and cumbersome to accomplish on a routine basis, although this should resolve itself with continuing direct experience. It is also wholly empirical and limited in its use for extrapolation to alternative system configurations. On the positive side, however, the bioassay can offer several advantages. It is an independent verification of system design, and implicitly of design procedures. As such it can be used effectively as a post-construction performance test or to compare the performance of competing commerical units during design and/or bid phases of a facility installation. The calculation approach is suggested as the method of choise because of its versability and flexible application to varying configurations. The technique used to calculate intensity is the point source summation (PSS) method. It presumes that the lamp is a finite series of point sources that emit energy radially in all directions. The intensity at a given point in a reactor would be the sum of intensities from each of these point sources. UV intensity will attenuate as the distance from the source increases. This occurs by two basic mechanisms: dissipation and absorption. Dissipation is simply the dilution of the energy as it moves away from the source. The area upon which the energy is being projected is increasing; thus the energy per unit area is decreasing. The second attenuation mechanism relates to the absorptive properties of the medium through which the energy is transmitted. An analysis of intensity in a submerged or teflon tube lamp battery system requires that the calculations be made at numerous receiver locations within the lamp battery. This is accomplished by dividing the cross-sectional space between lamps into equal-area grid system. The average of the receivers located in the center of equal area grid elements would then be equivalent to the average intensity within the total grid area. The model takes into lamps and enclosures (e.g. quartz or teflon) and the given UV absorption properties of the fluid. Since the low pressure mercury vapor lamps are excellent absorbers of light at the 253.7 nm. wavelength, the model calculations pressume that any energy at this wavelength entering a lamp from a neighboring lamp will be completely absorbed by that lamp. Four lamp arrays are considered: As uniform array, concentric array, staggered uniform array, tubular array. These are in common use today and, in effect, cover almost all practical configurations one would consider. The basic parameters are residence time distribution (RTD), dispersion, turbulence, head loss and effective volume for hydraulic design of a UV disinfection reactor. First the unit should be a plug flow reactor in which each element of fluid passing through the reactor resides in the reactor for the same period of time. Second, the flow motion should be turbulent radially from the direction of flow. This is to allow for each element to receive the same overall avarage intensity of radiation in the non-uniform intensity field which exist in the reactor. The trade off in this requirement is that some axial dispersion will be introduced, yielding a dispersive or non-ideal flow reactor. Third, maximum use must be made of the entire volume of the reactor; conversely, dead spaces must be minimized, such that the effective volume is very close to the actual volume available. The evaluation of a spesific reactor relies on the construction of the RTD appropiate for that reactor configuration. This can be accomplished by a number of experimental procedures; subsequent analysis of the residence time distribution curves determines the hydraulic characteristics of the unit. xvmThe ideal reactor design for UV disinfection forces the dispersion number very low, preferably less than 0.01. From its definition, this will be forced by a low dispersion coefficient, a high velocity, and/or a long dimension. An important consideration in the hydraulic design of a UV reactor is the turbulence of the fluid. By having turbulent flow, any particle has and equal probability of being at any point in the cross-section of the conduit as it travels in the direction of flow. The importance of turbulence lies in the fact that the intensity field in the reactor, regardless of the way the lamps are configured, is non-uniform. Thus, if a particle is forced to move erratically by the turbulent conditions, it will likely see all intensity levels in the non-uniform field. In the case, then, it is acceptable to use the average intensity in the reactor to evaluate dose levels microorganisms receive as they move through the reactor. If true laminar conditions existed, streamlines may move through areas of low-intensity and receive little dose relative to the streamlines moving close to the lamps. The lamp battery volume is that portion of the total system occupied by the UV lamps. By this fact, it is very important that the reactor is designed such that full use be made of the entire volume. A dose reduction factor (DRF) has been determined to quantify the effect of photoreactivation on the ultimate disinfection performance of UV light. The DRF is always less than or equal to 1. The concept of the DRF has been employed undirectly for UV disinfection systems to predict the increase in applied UV dose required to achieve a certain log. reduction when photoreactivation is taken into account. The degree of photoreactivation represents the fraction of inactivated cells that has been photoreactivated. No published work involving water and/or wastewater disinfection has thus for used this method to evaluate photoreactivation. Most studies involving photoreactivation of water and/or wastewater have employed a log increase methodology to quantify photoreactivation. In this method, the difference in the log survival before photoreactivation and after photoreactivation yields a log increase due to photoreactivation. In this study, the effect of the UV radiation on the inactivation and photoreactivation of the total coliform bacteria was investigated. Two different types of UV reactors were used. Each UV reactor preceded by an inlet box and followed by a weir box. 15 W and 30 W low pressure mercury vapor lamps were employed in UV reactor 1 and UV reactor 2, respectively. The lamps were centered upper the surface of the water in the each reactor and oriented parallel to the water flow. The reactors were operated at different flow rates, with the minimum of 0.025 l/s, the average of 0.09 l/s and the maximum of 0.125 l/s. In this study, the synthetic tap water which was prepared in the laborotuary was used. The coliform bacteria were isolated from domestic wastewater and were enjected to the tap water in the storage tank. The total coliform bacteria were enumerated in samples drawn from the influent and effluent of each of the UV reactor in series and again after exposure to photoreactivating light. The membrane filter technique was used to enumerate total coliform bacteria according to Standard Methods. Measurement of the suspended solids in the synthetic tap water indicated that this parameter can be neglected. That's why, the UV absorbance coefficient (a) was determined in unfiltered samples directly by using a spectronic BECKMAN DB-GT UV spectrophotometer at 254 nm wavelength. The average UV intensity within the reactors were determined using the point source summation (PSS) method of calculating average intensity within a reactor. XIXPhotoreactivation was initiated by exposing the samples to direct sunlight for ^Thöur at noon to allow TormaximuhTsoter radiation exposure. Air temperature and sky conditions were recorded. The results obtained from this study can be summarized as follows : The study done in two different types of the UV reactors has indicated that inactivation of the total coliform bacteria increases with increasing UV dosages. As a result, high disinfection efficiency; up to % 99.99, could be obtained. Another observation is that, disinfection efficiency increases as the distance between the UV lamp and the surface of the water in the reactor decreases. For UV reactor 1, the best inactivation of the total coliform bacteria has been obtained at the minimum flow rate of 0.025 l/s, the water depth of 1 to 3 cm and the UV dosages of 25 000 nwattsn/cm2 to 88 000 jawattsn/cm2. However, for UV reactor 2, the best inactivation of the total coliform bacteria has been obtained at the flow rate of 0.025 l/s to 0.125 l/s, the water depth of 3 cm, and the UV dosages of 56 400 |j.wattsn/cm2 to 235 800 nwattsn/cm2. It is concluded that, at UV reactor 2, UV dosage required without photoreactivation of the total coliform bacteria should be greater than 56 000 nwattsn/cm2. For UV reactor 1 this dosage was 35 000 nwattsn/cm2 to 105 000 nwattsn/cm2 at 3 cm water depth. It is also observed that inactivation rate of the total caliform bacteria increases with increasing water depth, within 60 seconds to 180 seconds of exposure times to UV. Similarly, the inactivation rate of the total coliform bacteria decreases with increasing water depth for any flow rate and for the exposure time range of 60 seconds to 180 seconds. It is concluded that, the reason for this would be photoreactivation effect. XX
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