Ultraviolet Disinfection: A Study of Various Technologies

Overview

Studies have shown that ultraviolet light can penetrate cells and debilitate DNA, thereby destroying an organism’s ability to reproduce

Such a discovery has paved the way for the development of ultraviolet disinfection technology

Contaminants removed with use of UV-system: microbial; nevertheless, UV systems may be coupled with a pre-filter to extricate larger organisms and particulates with attached bacteria

UV units are used as individual treatment systems or as parts of other purification processes

Ultraviolet Light: Background

Ultraviolet light is characterized by a wavelength shorter than 400 nanometers

It emanates from the sun and is almost wholly filtered out through the ozone layer upon entering the earth’s atmosphere

Nevertheless, some UVA and the UVB (280-400nm) does enter the atmosphere causing damage to the DNA of the cells it penetrates

Ultraviolet light engenders photoproducts which alter the DNA structure of harmful microorganisms, thereby inhibiting their replication

UV Disinfection Systems: Orthodox Design

An orthodox package constitutes mercury arc lamps (low or high intensity), a reactor, and ballasts

Contact types Non-contact types a series of mercury UV lamps suspended outside a lamps, enclosed in quart transparent conduit, which carries sleeves to decrease cooling wastewater to be purified (not as effects of wastewater, submerged common as contact reactor) in water A ballast, or control box, engenders starting voltage for the lamps and maintains

continuous current Flap gates, or weirs, control level of water being treated Water runs perpendicular or parallel to lamps Note: spare parts- operators in remote locations should maintain and house a set of

spare parts on site More advanced systems: mechanical cleansers, ultrasonic cleaners, other self-

cleaners, and alarm systems that indicate minor and major failures

General UV Disinfection Systems: Diagram

Two UV reactors with submerged lamps placed parallel and perpendicular to the direction of water flow (contact types)

General Operation and Maintenance

Ideal operating temperature: 40 degrees Celsius Because UV treatment does not leave any residual in water, suggested installment at a location

proximate to final distribution system and use as a final step of purification UV units should be implemented on cold water line before any branch lines Lamp changes: once every year or if light transmission efficiency has decreased to 70 percent Filter changes: times vary according to water quality; ordinarily every 3-6 months All surfaces between UV radiation and water should be clean; chemical treatment: use of citric

acid, mild vinegar solutions, or sodium hydrosulfite (ideal for noncontact reactor systems) Quart sleeves should be wiped down every 6 months with soapy solution; if residue remains after

cleaning, necessary non-abrasive cleaner (does not scratch surface and created to remove iron and scale buildup); replaced every 5 to 8 years

There should be no fingerprints left on glass Note: use of UV system and a pump on same electrical line can mar the life of UV lamp and

ballast Ballast must be compatible with lamps and should be ventilated to protect it from excessive

heating; usually replaced every 10 years All UV units must be pilot tested upon implementation to ensure applicability At a minimum, drinking water systems should implement two UV units (design ensures

continuous disinfection when one unit is being cleaned and operation during low-flow demand periods)

General Operation and Maintenance: Personnel and Technology

Operators: Operators must ensure continuous dose measurement (example: accurate intensity

and flow-rate measurement) and proper maintenance (cleaning as well as lamp and sleeve replacement regimes)

Operators should follow a maintenance schedule which envelops inspecting site periodically, changing lamps, inspecting and cleaning surfaces and UV chamber interior (once every six months), and inspecting and replacing ballasts, O-rings, valves, and switches

Operators should monitor water turbidity (dissolved minerals such as calcium especially harmful) and color as they constitute natural barriers to UV light transmission

Operators should make it a point to reduce on/off cycles of lamps, since lamps lose efficacy as a result of repeated cycles

Technology: Many advanced systems include mechanical cleaners, ultrasonic cleaners, other

types of self cleaners, and alarm systems that assert a minor or major problem Gravity systems: should be designed to automatically stop water flow or provide

alternative means of disinfection during power outages

A Specific Technology: UV-Tube

All UV-tube designs entail a germicidal bulb suspended over water in a horizontal tube or covered trough

The water passes through one end by means of an inlet in the top of the tube, courses along the bottom (underneath germicidal bulb), and exits through an outlet

The height of the outlet determines the depth of the water in the tube and governs the hydraulic detention time

There are three UV-tube designs: materials- steel lined PVC, concrete, or

pottery

bulb sizes- 36”, 18”, or 15”

UV Tube Design: Diagram

Topics To Consider: Theory of UV Technology

Fluence (Dose) Intensity Absorption Coefficient Flow Rate Turbidity Water Depth (Weir Height) Bulb Types Re-activation of bacteria Materials inherent to wastewater/how affect disinfection

Fluence

Fluence (dose) is the amount of UV light exuded from the germicidal bulb Fluence = product of light intensity and exposure time

depends upon: bulb strength & geometry & hydrodynamics

geometry of of reactor

reactor

Standard doses used by a UV systems: between 15 and 50 mW-sec/cm2

Note: the EPA has not established an official rule concerning dose requirement

Intensity (further information)

Decreases because of attenuation and dissipation In other words distance from the source = strength of light (intensity)

because the light is spread across a larger area (as a result of dissipation) and because the light interacts with molecules inherent to the water (attenuation)

Absorption Coefficient

The absorption coefficient delineates how much light is lost as it passes through a medium (measured in inverse centimeters)

Absorption coefficients are calculated experimentally While the absorption coefficient of pure distilled water is near zero; natural

organic matter, iron, nitrate, and manganese imbibe UVC light and consequently increase the absorption coefficient of a water sample

Absorption coefficients in drinking water should be approximately 0.01 to 0.2 Another type of absorption coefficient is the naperian value (using base e) Note: The EPA has not finalized a standard for levels of UV absorbing

compounds in drinking water

Flow Rate

Flow rate = detention time = dose acquired by water

These variances must be taken into consideration when determining an appropriate flow rate

Turbidity

Turbidity is the calculation of the amount of particulates in a solution (measured in NTU)

Research has shown that turbidity inhibits ultraviolet disinfection only when organisms are lodged within the particles or when the particles themselves are UV-absorbers. Otherwise turbidity is not a hindrance to disinfection.

These lodged organisms can be extricated from water supply through the use of a pre-filter

Water Depth (Weir Height)

Water depth determines both the residence time and water thickness of a water sample in question. Consequently, calculating an appropriate water depth encompasses a trade-off between the two entities:

water height = volume of water = average residence time (positive quality)

Nevertheless, water height = water thickness = attenuation of light = dose reaching water at bottom tube (negative quality)

• Because attenuation is proportional to the absorption coefficient, the ideal water height will rely upon the absorption coefficient

Bulb Types: Comparison of Two Models

There are two main models considered in regards to use in a UV-tube: the low pressure bulb and the medium pressure bulb

Small-scale ventures make use of low pressure mercury vapor arc lamps (opportune lamp wall temperature: 95-122 degrees F)

the peak output of low pressure lamps (253.7 nm) is adequate to address the maximum UV absorbance of DNA (260-265 nm)

Medium pressure lamps are used for large facilities; 15-20 times germicidal intensity of low pressure lamps

Characteristic: Low Pressure/ Low Intensity

Medium Pressure/ Medium Intensity

Typical Energy Use

60 W 5,000 W

Percentage Output at 253.7 nm

88% 44%

Ozone Production

None Possibly

Susceptibility to Cooling

Yes No

Susceptibility to Cooling

Good Poor

Benefits Efficiency (lower energy requirements)

Smaller, less maintenance, use with poor quality water

Reactivation of Bacteria

Because the UV-tube does not offer residual disinfection, some bacteria can repair its DNA and re-activate after a few days of exposure to visible light

Reactivation, when occurs, on the order of a 1-log increase in post-treatment concentration

Related to UV dosage; one study asserted that water dosed with 130,000 uW-s/cm2 showed no reactivation after 24 hours

Materials Within Wastewater

Cost: A Comparison of Various Disinfection Techniques

In relation to the other kinds of water disinfection systems outlined , the UV-tube (as a form of UV disinfection technology) is an affordable option

Although the use of chlorine as a disinfectant is less expensive, the UV tube supplies many more advantages than chlorine. Chlorine requires an inconvenient contact time; is less successful at incapacitating protozoa, helminthes, and some viruses; and is difficult to dose.

In general, the cost of UV disinfection systems depends on the manufacturer, the site, the capacity of the plant, and the make-up of the incoming water

Options Start-Up Cost ($)

Monthly Cost ($)

Average Monthly Cost for first year ($)

UV-tube $41 $1 $4

Chlorine Addition

$1 $0 <<$1

Boil Water (using LPJ)

$20-30 $10 $11

Purchase Bottled Water

$3 $12 $11

commercial UV System

$300 $1 $26

Disadvantages

While UV technology is becoming an increasingly viable option for water purification, its design is still undergoing the process of refinement.

Because UV light is harmful to microorganisms, it is also dangerous to humans. In this respect, UV light may cause skin irritation and severe eye damage if direct exposure takes place

Providing a power source for UV treatment may be difficult to attain (lack of electricity faculties in community)

conventional UV systems are not effective against cysts like Cryptosporidium; must be filtered out of water before UV disinfection (nevertheless, some advanced UV systems address this problem by using a stainless steel screen with 2- um openings to capture the cysts)

Lack of oxidation capability; some locations require an oxidant for attaining purification standards

Lack of chemical residual produced; nevertheless, the use of chlorine or chloramines may be applied to address this inadequacy

Disinfection not appropriate for treatment of water with high levels of suspended solids, turbidity, color, or soluble organic matter (substances can react with UV radiation, decrease purification efficacy)

Advantages

The UV technology (especially UV tube) is opportune for water purification in developing nations as it may incorporate local resources in its construction (concrete, pottery). In this way, the UV-tube design can be disseminated efficiently through community workshops initiated by local, non-governmental groups or sold by small-scale business people

The UV-tube does not require water pressure to operate and therefore can be attached directly to a faucet or filled with a funnel and bucket

UV-tube models are becoming an increasingly viable option for communities as research is being done to both refine these models on a physical and cultural level

UV technology offers a simplicity of application, appropriate for small systems

UV disinfection is a physical process and not a chemical one; eliminates the necessity of generating, handling, moving, or storing toxic chemicals

Rapid disinfection (12 seconds), low electricity use, low maintenance (every 6 months), high flow rate (15 l/min), and ability to work with un-pressurized water

References

“Basics of Ultraviolet Disinfection Technology”. http://socrates.berkeley.edu/%7

Erael/uvtube/uvdisinfection.htm

Cotruvo, Joseph A, et al. “Providing Safe Drinking Water In Small Systems

Technology, Operations, and Economics”. Lewis Publishers, Washington D.C. 1998

“The UV-tube Project”. http://socrates.berkeley.edu/%7Erael/uvtubeproject.htm

“Ultraviolet Disinfection”: Tech Brief. September 2000

“UV Disinfection-General”. http://www.triangularwave.com/a3a.htm

“Wastewater Technology Fact Sheet: Ultraviolet Disinfection”: United States

Environmental Protection Agency. September 1999