Sterilization using UV light

Ultraviolet (UV) rays are part of the light emitted by the sun. The UV spectrum has a higher frequency than architectural lighting and a lower frequency than X-rays. This also means that common UV has a longer wavelength than X-rays and a shorter wavelength than architectural lighting ; The energy levels from low to high are visible light, UV rays, X-rays. As a water treatment technique, UV rays are known to be effective disinfectants thanks to their strong bactericidal (inactivating) ability; Ultraviolet rays have enough energy (ionizing radiation) to break chemical bonds and kill bacteria. Ultraviolet light disinfects water containing bacteria and viruses, and can be effective against protozoa such as Giardia lamblia cysts or Cryptosporidium cysts. UV has been used commercially for many years in the pharmaceutical, cosmetics, beverage and electronics industries, especially in Europe. In the US, it was used to disinfect drinking water in the early 1900s but was abandoned due to high operating costs, unreliable equipment, and the growing popularity of chlorination.

Due to safety issues associated with chlorination and improvements in UV technology, UV disinfection has become increasingly accepted in both municipal and household systems. There are only a few large-scale UV water treatment plants in the United States, although there are more than 2,000 such plants in Europe. There are two types of sterilization systems, certified by NSF and classified according to Standard 55 – Class A and Class B units.

Class A – These UV water treatment systems must have an intensity and saturation level of at least 40,000 µsec/cm2. Type A point-of-entry and point-of-use systems covered by this standard are designed to inactivate (destroy) or remove from contaminated water microorganisms, including bacteria, viruses, Cryptosporidium cysts and Giardia cysts.. Systems covered by this standard are not intended to treat obviously contaminated water or intentional sources such as raw wastewater, nor are they intended to transfer Convert wastewater into drinking water. These systems are designed for installation on transparent water surfaces.

Class A – These UV water treatment systems must have an intensity and saturation level of at least 40,000 µsec/cm2. Type A point-of-entry and point-of-use systems covered by this standard are designed to inactivate (destroy) or remove from contaminated water microorganisms, including bacteria, viruses, Cryptosporidium cysts and Giardia cysts. Systems covered by this standard are not intended to treat obviously contaminated water or intentional sources such as raw wastewater, nor are they intended to transfer Convert wastewater into drinking water. These systems are designed for installation on transparent water surfaces.

Therefore, the type of device depends on your condition, water source and water quality. The transmitted UV dose is influenced by water clarity. Water treatment equipment depends on raw water quality. When turbidity is 5 NTU or more and/or total suspended solids is greater than 10 ppm, water should be pre-filtered. Typically, a 5 to 20 micron filter should be installed before the UV disinfection system.

Principle of ultraviolet sterilization

Ultraviolet radiation has three wavelength regions: UV-A, UV-B and UV-C, and it is this last region that short-wave UV-C has germicidal properties for disinfection. Low-pressure mercury arc lamps like fluorescent lamps produce UV rays in the range of 254 nanometers (nm). One nm is one billionth of a meter (10^-9 meters). These lamps contain elemental mercury and an inert gas, such as argon, in an ultraviolet transmitting tube, which is usually quartz (which, unlike glass, is transparent to ultraviolet light). Traditionally, most mercury arc UV lamps have been called “low pressure” types, because they operate at a relatively low mercury partial pressure, with a low overall vapor pressure (about 2 mbar). , low external temperature (50-100°C) and low power. These lamps emit nearly monochromatic UV radiation at a wavelength of 254 nm, which is within the optimal range for absorption of UV energy by nucleic acids (about 240-280 nm); Ultraviolet rays break bonds in nucleic acids, killing microorganisms.

In recent years, medium pressure UV lamps operating at much higher pressures, temperatures and power levels and emitting a broad spectrum of higher UV energy between 200 and 320 nm have become widely available. market. However, for UV disinfection of drinking water at the household level, low pressure lamps and systems are perfectly suitable and even preferred over medium pressure lamps and systems. This is because they operate at lower power, lower temperatures and lower costs while being highly effective in disinfecting enough water for daily household use. The essential requirement for UV disinfection using a lamp system is an available and reliable power source. Although the energy requirements of low-pressure mercury UV lamp disinfection systems are modest, they are essential for the operation of the lamp to disinfect water. 

Since most microorganisms are affected by radiation around 260 nm, UV radiation is within the appropriate range for bactericidal activity. There are UV lamps that produce radiation in the 185 nm range that is effective against microorganisms and will also reduce the total organic carbon (TOC) content of the water. For a typical UV system, about 95 percent of the radiation passes through the quartz tube and enters the untreated water. Water flows like a thin film on the lamp. The quartz tube is designed to keep the lamp at an ideal temperature of approximately 104°F.

UV Radiation (How It Works)

UV radiation affects microorganisms by changing the DNA in cells and interfering with reproduction. UV treatment does not remove organisms from the water but only inactivates (kills) them. The effectiveness of this process is related to exposure time and light intensity as well as general water quality parameters. Exposure times are reported as “microwatt·seconds per square centimeter” (µWatt·sec/cm²) and the US Department of Health and Human Services has established a minimum exposure level of 16,000 µWatt·sec/cm² for UV disinfection systems. Most manufacturers provide lamp intensities between 30,000-50,000 µWatt·sec/cm². In general, for example, coliform bacteria are destroyed at a rate of 7,000 µWatt·sec/cm². Because lamp intensity decreases over time, lamp replacement and appropriate pretreatment are keys to the success of UV disinfection. In addition, the UV light system needs to be equipped with a warning device to warn the homeowner when the light intensity drops below the germicidal threshold. The following gives the irradiation time required to completely inactivate various microorganisms at a dose of 30,000 µWatt·sec/cm² at a UV wavelength of 254 nm.

When used alone, UV radiation does not improve the taste, odor or clarity of water. UV lights are very effective disinfectants, although disinfection can only occur inside the device. Unlike chlorination, there is no residual disinfectant in the water to inactivate bacteria that may exist or may enter after the water passes through the UV source. The rate of microbial destruction depends on UV intensity, exposure time, raw water quality, and proper equipment maintenance. If material accumulates on the quartz tube or the particle load is high, the UV intensity and treatment efficiency will decrease. At sufficiently high doses, all enteric pathogens in water are inactivated by ultraviolet radiation. 

The general order of microbial resistance (from least to most) and the corresponding UV dose for widespread inactivation (>99.9%) is: vegetative bacteria and protozoan parasites Cryptosporidium parvum and Giardia lamblia at low doses (1-10 mJ/cm²) and enteric bacteria viruses and bacterial spores at high doses (30-150 mJ/cm²). Most low-pressure mercury lamp UV disinfection systems can easily achieve UV radiation doses of 50-150 mJ/cm² in high-quality water and thus effectively disinfect essentially all germs. domestic disease. Note: In this paragraph, the unit has been changed from µW·sec/cm2 to mJ/cm². Both are units of measurement for radiation (power/area); One watt (W) is one joule/second; 1000 µ (micro) = 1 m (milli). 1000 µW·sec/cm2 = 1 mJ/cm².

However, dissolved organic matter, such as natural organic matter, some inorganic contaminants, such as iron, sulfites and nitrites, and suspended matter (particulates or turbidity) will absorb radiation. UV radiation or shielding bacteria from UV radiation, resulting in a lower delivered UV dose and reduced disinfection ability of microorganisms. Another concern about sterilization of bacteria with lower doses of UV radiation is the ability of bacteria and other cellular microbes to repair UV-induced damage and restore infectivity, a phenomenon known as reactivation.

Ultraviolet light inactivates bacteria primarily by chemically altering nucleic acids. However, UV-induced chemical damage can be repaired by cellular enzymatic mechanisms, some of which are light independent (dark repair) and others that require visible light. ant (optical correction or optical activation). Optimal UV water disinfection therefore requires the delivery of sufficient UV doses to cause greater levels of nucleic acid damage and thereby overcome or overwhelm DNA repair mechanisms.

Table 1 | Estimated irradiation time to inactivate microorganisms at a dose of 30,000 µW·sec/cm² ultraviolet light at 254 nm wavelength

Name	Lethal doses that kill 100%	Name	Lethal doses that kill 100%
	(seconds)		(seconds)
Bacteria
Dysentery bacillus	0,15	Candidus bacteria	0,4 – 1,53
Leptospira SPP	0,2	Salmonella bacteria	0,41
Legionella pneumophila	0,2	Mycobacterium tuberculosis	0,41
Corynebacterial diphtheria	0,25	Hemolytic Streptococcus	0,45
Shigella dysentery	0,28	intestinal bacteria Salmonella	0,51
Anthrax bacillus	0,3	Salmonella typhimurium	0,53
Clostridium tetani	0,33	Vibrio cholera	0,64
Escherichia coli (E. coli)	0,36	Clostridium tetani	0,8
Pseudomonas aeruginosa	0,37	Staphylococcus albus	1,23
Virus
Virus Coxsackie A9	0,08	Echovirus 1	0,73
Adenovirus 3	0,1	Hepatitis B virus	0,73
Bacteriophage	0,2	Echovirus 11	0,75
Flu	0,23	Polio virus 1	0,8
Rotavirus SA 11	0,52	Tobacco mosaic	16
Mold spores
Mucus	0,23 – 4,67	Penicillium roquefortine	0,87 – 2,93
Oospora lactis	0,33	Penicillium chrysogenum	2,0 – 3,33
Aspergillus amstelodami	0,73 – 8,80	Aspergillus niger	6,67
Digital Penicillium	0,87	Dung fungus	number 8
Algae
Common Chlorella	0,93	Protozoa	4 – 6,70
Green algae	1,22	Paramecium	7.3
Nematode eggs	3,4	Green Algea	10 – 40

Inactivating dose for Giardia and Cryptosporidium

UV dose is the product of UV intensity (radiation) and exposure time in seconds (IT), expressed in units: mWs/cm2 or mJ/cm2. IT is similar to chemical dose or CT (concentration x time). Bacteria show a wide range of UV sensitivity as demonstrated by UV data. Cryptosporidium and Giardia are more sensitive to ultraviolet light than bacteria and viruses. Similar results are achieved using low-pressure, medium-pressure and pulsed UV irradiation – Look for a Class A UV disinfection system. UV dose required to neutralize 4 The log of selected pathogens in water is presented below:

Table 2 | UV dose 4 log Inactivated 

Pathogens	UV dose mJ/cm/2 4log is inactive
Oocyte Cryptosporidium parvum	<10
Cyst Giardia lamblia	<10
Vibrio cholera	2.9
Salmonella typhi	8.2
Shigella sonnies	8.2
Hepatitis A virus	30
Polio virus type 1	30
Rotavirus SA11	36

UV irradiation pretreatment

Sediment filtration or activated carbon filtration must take place before the water passes through the UV unit. Particulate matter, color and turbidity affect the transmission of UV light to microorganisms and must therefore be removed for successful disinfection.

Table 3 | Maximum recommended contamination level in water entering UV treatment equipment.

Parameter	Unit/Range
Turbidity	5 FTU or 5 NTU
Suspended solids (5 to 10 micron pre-filtration recommended)	< 10 mg/L
Color	None
Iron	< 0,3 mg/L
Mangan	< 0,05 mg/L
pH	6,5-9,5

UV is often the final device in a treatment chain (a series of treatment devices), following reverse osmosis, water softening or filtration. UV equipment should be located as close to the point of use as possible because any part of the plumbing system can become contaminated with bacteria. It is recommended that all plumbing systems be disinfected with chlorine before using the UV system for the first time. ‍

Types of equipment

A typical UV treatment device consists of a cylindrical chamber containing a UV lamp along its central axis. A quartz tube encloses the bulb; The water flows parallel to the light bulb, so a power source is needed. The flow control device prevents water from passing too quickly through the bulb, ensuring adequate radiation exposure time to the flowing water. It has been reported that turbulent (disturbed) water exposes organisms more completely to UV radiation.

The UV system housing must be stainless steel to protect all electronic components from corrosion. To ensure they are free of contaminants, all welds in the system must be plasma-fused and purged with argon gas. The main differences between UV treatment units are in capacity and optional features. Some are equipped with UV emission detectors to alert users when the device needs cleaning or when the light source is damaged. This feature is extremely important to ensure a safe water supply. A detector that sounds or turns off the water flow is preferred to a warning light, especially if the system can be located where the warning light cannot be immediately noticed. ‍

System maintenance

Because UV radiation must reach bacteria to inactivate them, the light source housing must be kept clean. Commercial products are available for washing equipment to remove any film on the UV source. Overnight cleaning with 0.15% sodium hydrosulfite solution or citric acid will effectively remove such films. Some units have wipers to aid in the cleaning process.

UV systems are designed to operate continuously and should only be turned off if treatment is not required for several days. It takes a few minutes to warm up the lamp before using the system again after turning it off. In addition, the house’s plumbing system needs to be thoroughly rinsed after a period of non-use. Whenever the system is serviced, the entire plumbing system should be disinfected with a chemical such as chlorine before using the UV system for disinfection.

UV lamps gradually lose their effectiveness when used; Lamps should be cleaned regularly and replaced at least once a year. It is not uncommon for a new lamp to lose 20% of its intensity within the first 100 hours of operation, although that level of intensity is maintained for the next several thousand hours. As stated previously, devices equipped with properly calibrated UV emission detectors will alert the owner when light intensity drops below a certain level..

Treated water must be monitored for coliform and heterotrophic bacteria monthly for at least the first 6 months of equipment use. If these organisms are present in the treated water, the light intensity should be checked and the entire plumbing system should be disinfected with chemicals such as chlorine.

Quick facts about UV water treatment

1 | UV disinfection does not add chemicals to the water.

2 | Ultraviolet rays are effective against bacteria and viruses; and can be effective against Giardia lamblia or Cryptosporidium if the system is custom designed to meet these disinfection requirements.

3 | UV sterilization has no residual sterilization.

4 | A minimum lamp intensity of 16,000 µwatt·sec/cm2 is recommended..

5 | UV is usually the last device in the water treatment chain.

6 | UV equipment must have an audible UV emission detector to notify the user when lamp intensity is insufficient.

7 | Regular maintenance and replacement of lights is essential.

Ultraviolet disinfection system capacity

Ultraviolet is a point-of-entry treatment system that treats all water used in the home. Capacity ranges from 0.5 gallons per minute (gpm) to several hundred gpm. Because bacteria can be shielded by particles in the water, pretreatment may be necessary to remove turbidity. There are also limits to the amount of bacteria that can be treated. The upper limit for UV disinfection is 1,000 total coliforms/100 mL of water or 100 fecal coliforms/100 mL..

Special considerations

Pre-filtration is necessary to remove color, turbidity and particles that shield microorganisms from the UV source. Water containing high mineral content can stick to the light tube and reduce treatment efficiency. Therefore, pretreatment with a water softener or phosphate spray may be necessary to prevent mineral buildup on the lamp. Table 3 lists the maximum levels of certain contaminants allowed for effective UV remediation.

General recommendations

The installation of a UV treatment system or any other water disinfection system is not a substitute for proper well design and construction. If you have a dug well as your source of supply, replacing the well is probably a better long-term option. If a dug well or stream is your only supply option, consider all treatment options before deciding what to do. Make sure you get advice from an expert! Recommended treatment options:

1 | Collect information about your water source.

2 | Test your water – At least annually.

3 | Identify any problems related to infrastructure deficiencies, such as cracked casing, no well cover, improper sealing, poor surface drainage, etc. Make necessary repairs and improvements to the system.

4 | Install necessary water treatment systems. I have provided some online links about water treatment systems, but I always recommend a preliminary water test.

See more: Did you know: Effect of Ozone Gas on Human Life

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