Understanding Disinfection Processes and Mechanisms
Disinfection is the process of partially destroying disease-causing organisms, different from sterilization where all organisms are destroyed. Various mechanisms of disinfectants include damaging cell walls, altering permeability, and inhibiting enzyme activity. Chemical and physical agents such as chlorine, UV light, and heat are commonly used for disinfection, especially in wastewater treatment. Ultraviolet (UV) radiation is an effective method for disinfection.
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DISINFECTION PROCESSES Prepared by: 21424861 Gizem Sultan
DISINFECTION Disinfection refers to the partial destruction of disease-causing organisms.All the organisms are not distroyed during the process.The fact that all of the organisms are not destroyed differentiates disinfection from sterilization,which is the destruction of all organisms. Table :Indicative levels of pathogens commonly found in secondary treated wastewater. Actual numbers will vary depending on the treatment process.
MECHANISMS OF DISINFECTANTS: The five principal mechanisms that have been proposed to explain the action of disinfectans are: Damage to the cell wall Alteration of cell permeability Alteration of colloidal nature of the protoplasm Alteration of the organism DNA or RNA Inhibitation of enzym activity 1. 2. 3. 4. 5.
DISINFECTANTS USED FOR WASTEWATER : Chemical agents: Chemical agents that have been used as disinfectants include (1) chlorine and its compounds (2)Bromine (3) odine (4)Ozone (5)Phenol and phenolic compunds (6)Alcohols (7)Hydrogen peroxide Of these, the most commen disinfectants are the oxidizing chemical and chlorine is the one used most universially.
Physical Agents: - Ultraviolet light (UV) - Electronic radiation - Gamma rays - Sounds - Heat Physical agents that can be used are heat,light and sound waves.Heating water to the boling point,for example,will distroy the major disease- producing non-spore-forming bacteria.Heat is commonly used in the beverage and diary industry,but it s not a feasible means of disinfecting large quantities of wastewater because of the high cost. Sunlight is also good disinfectant,due primarily to the ultravolet(UV) radiation portion of the electromagnetic spectrum. The decay of microorganisms observed in oxidation ponds is due,in part, to their exposure to the UV component of sunlight.Special lamps developed to emit ultraviolet rays have been used successfully to disinfect water and wastewater.The efficiency of the process depends on the penetration of the rays into water. The contact geometry is extremely important because suspended matter, dissolved organic molecules,and water itself will absorb the radiation, in addition to the microorganisms.
ULTRAVIOLET (UV) RADIATION DISINFECTION The portion of the electromagnetic spectrum in which UV radiation occurs is between 100 and 400 nm. Long wave (UV-A)~near ultraviolet radiation Middle wave (UV-B) Short wave (UV-C)~far ultraviolet radiation
Process Description A UV disinfection system transfers electromagnetic energy from a mercury arc lamp to an organism s genetic material (DNA and RNA). When UV radiation penetrates the cell wall of an organism, it destroys the cell s ability to reproduce. The effectiveness of a UV disinfection characteristics of the wastewater, the intensity of UV radiation, the time the microorganisms are exposed to the radiation, and the reactor configuration. For any one treatment plant, disinfection success is directly related to the concentration of colloidal and particulate constituents in the wastewater. system depends on the The main components of a UV disinfection system are mercury arc lamps, a reactor, and ballasts. The source of UV radiation is either the low-pressure or medium-pressure mercury arc lamp with low or high intensities
UV DISINFECTION SYSTEM COMPONENTS AND CONFIGURATIONS: The principals components of a UV disinfection system consist of (1) The UV lamp (2) The quartz sleeves in which the UV lamp is placed. (3) The supporting structure for the Uv lamps and the quartz sleeves. (4) The ballasts used to supply regulated power to the UV lamps. (5) The power supply which is used to power the ballasts. BALLASTS: Three types of ballasts are used: (1) standard (core coil), (2) energy- efficient(core coil),and electronic(solid state). Ballasts are used to limit the current to a lamp.Because UV lamps are arc-discharge devices,the more current in the arc,the lower the resistance becomes.
Without a ballast to limit current, the lamp would destroy itself. Thus, matching the lamp and the ballast is of critical importance in the design of UV disinfection systems.
Source Of UV Radiation To produce UV radiation,lamps that contain mercury vapor are charged by striking an electric arc.The energy generated by the excitation of the mercury vapor contained in the lamp results in the emission of UV light. In general , UV disinfection systems fall into three categories based on the internal operating parameters of the UV lamp: low pressure low intensity low pressure high intensity medium pressure high intensity systems UV disinfection systems may also be classified as : Open channel Closed channel based on their hydraulic characteristics
The UV Lamp There are three main types of UV lamps that will be found in most UV systems Low-pressure standard UV lamps These lamps are most often used in applications where the flow rates are lower (such as in a residential home), and exposure times can be longer. These lamps are lower cost to replace, and generally the initial equipment cost is much lower as well. Low-pressure high output (HO) lamps HO lamps are used in applications where higher dosages or flow rates are required, but still have a smaller footprint. They treat water with a broader tolerance to temperature (temperature can affect lamp performance). These lamps are often used in larger flow applications or light commercial systems. Low-pressure amalgam lamps These lamps use mercury amalgam mix to control vapor pressure. They use a slightly different process to yield up to three times the UV-C output of a standard low- pressure lamp of the same length. Predominantly, these lamps are used in more commercial-type applications or for regulations requirements, depending on the type of microbiological contamination being treated.
The optimum wavelength to effectively inactivate microorganisms is in the range of 250 to 270 nm. The intensity of the radiation emitted by the lamp dissipates as the distance from the lamp increases. Low- pressure lamps emit essentially monochromatic light at a wavelength of 253.7 nm. Standard lengths of the low-pressure lamps are 0.75 and 1.5 meters with diameters of 1.5 to 2.0 cm. The ideal lamp wall temperature is between 95 and 122 F. Medium-pressure lamps are generally used for large facilities. They have approximately 15 to 20 times the germicidal UV intensity of low-pressure lamps. The medium-pressure lamp disinfects faster and has greater penetration capability because of its higher intensity. However, these lamps operate at higher temperatures with a higher energy consumption.
THE QUARTZ SLEEVE The quartz sleeve is a long, cylindrical tube of quartz glass intended to protect the UV lamp which is powered by electricity from the flow of water. The lamp is inserted into the tube, and transmits the light through the tube into the water. Sleeves can foul with minerals and other contaminants over time, and should be cleaned whenever the lamp is changed. It s a relatively simple component, but it s very necessary to the efficient
The Reactor Chamber The reactor chamber, also known as just a chamber or a reactor, is the part of the system that physically houses the UV lamp and sleeve, as well as controlling the flow of water through the system.
OPENCHANNEL The design flowrate is usually divided equally among a number of open channels.Each channel typically contains two or more banks of UV lamps in series,and each bank is comprised of a specified number of modules(or racks of uv lamps).It is important to not that a standby bank or channel should be provided for system reliability. Each module contains a specified number of uv lamps encased in quartz sleeves. A weighted flap gate, an extended sharp-crested weir or automatic level controller is used to control the depth of flow through each disinfection channel.To overcome the effect of fouling, which reduces the intensity of light in the liquid medium,the lamps must be removed occasionally from the flow channel and cleaned.Mechanically cleaned systems are used with low pressure high intensity systems to avoid fouling of the quartz sleeves. CLOSED CHANNEL A number of low and medium pressure high intensity UV disinfection systems are designed to operate in closed channels.In most design configurations the direction of flow is perpendicular to the placement of the lamps.There are ,however,design configurations in which the direction of flow is parallel to the UV lamps.
OPEN CHANNEL DISINFECTION SYSTEMS: a)Trojan horizontal lamp open channel unit. 3 modules installed in SS channel. b) Trojan open channel unit during construction. Only half of the lamp modules installed.
ADVANTAGES OF ULTRA VIOLET RADIATION The process only requires a minimal amount of electricity. It is also very environmental friendly. This is because it does not emit any by-products such as sodium into the environment. Therefore, there are no chemicals handled. The UV system is very cost effective. The process also destroys each and every microorganism that is in the water. The water flow in the whole system is just pure.
GERMICIDALEFFECTIVENESSOFUV : Definition of UV dose- the effectiveness of uv disinfection is based on the UV dose to which the microorganisms are exposed. The UV dose is defined as follows: D=I*t where D, uv dose , mw.s/cm^2 I,intensity , Mw/cm^2 t,exposure time ,s The UV dose can be varied by changing either the intensity or the exposure time.Because the UV intensity is attenuated with distance from the quartz sleeve, the average UV intensity within a UV disinfection system is often computed mathematically.
Impact of system characteristics: In practice,field-scale UV disinfection reactors have dose distributions resulting from both the internal intensity profiles and exposure time distribution.The internal intensity profiles are a reflection of the nonhomogeneous placement of lamps within the system,lack of ideal radial mixing within the system,the scattering/absorbing effects of particulate material,and the absorbance medium. The distribution associated with exposure time is a reflection of nonideal hydraulics leading to longitudinal mixing. Nonideal approach hydraulics: one of the most serious problems encountered with UV disinfection systems in open channels is achieving a uniform velocity field in the approach and exit channel.Achieving a uniform velocity field is especially difficult when UV systems are retrofitted into existing open channels , a practice is not recommended if the performance of the UV disinfection system is to be optimized.
CHARACTERISTICS OF THE MICROORGANISM The effectiveness of the UV disinfection process depends on the characteristics of the microorganisms. Before infectivity studies were conducted , it was thought that UV radiation at reasonable dosage values was not effective for the inactivation of cryptosporidium parvum and giardia lambia.However, based on infectivity studies,it has been found that both of these protozoan are inactivated with extremely low UV dosage values .
