We developed this eBook to give you the information you need to know about UV treatment for municipal wastewater. It’s chock-full of insight and answers many frequently asked questions, such as:
- Why does wastewater need to be treated?
- What is ultraviolet (UV) light?
- What happens to microorganisms when they are exposed to certain wavelengths of UV light?
- How does UV light treat wastewater?
- How long has UV been used as a treatment method?
- What are the distinct types of UV lamps?
- What are the operating advantages of UV?
- What are the cost advantages of UV?
- How does UV treatment fit within an existing water treatment regulatory framework?
Are you currently using a different method to treat wastewater and are wondering why you’d ever convert to UV? Well, then this eBook is for you.
Are you already treating wastewater with UV and want to learn more about how it works? This eBook is perfect for you too!
Even if you’re just looking for some general information about UV and wastewater treatment, you’re in the right place.
WHY DOES WASTEWATER NEED TO BE TREATED?
The final step in municipal wastewater treatment is the inactivation process which is required to reduce microorganism populations in the wastewater before discharge into the receiving body of water. These microorganisms are typically microbes that may cause disease in humans, which must be treated before the wastewater is discharged to a lake or river. There are often recreational activities such as swimming or fishing where the public can come into contact with the local bodies of water. By reducing the concentration of microorganisms in the water, treatment is able to benefit the public.
Growing awareness of potential long-term negative impacts of chemical treatment of wastewater along with the generation of toxic byproducts has led to the adoption of treatment alternatives such as ultraviolet light (UV).
There are over 11,000 TrojanUV municipal installations throughout the world – these installations include municipal drinking water and wastewater applications and help municipalities achieve their water quality objectives and serve more than one billion people globally.
WHAT IS ULTRAVIOLET (UV) LIGHT AND HOW DOES IT WORK?
UV light is a form of light that is invisible to the human eye. It occupies the portion of the electromagnetic spectrum between X-rays and visible light. The sun emits ultraviolet light; however, much of it is absorbed by the earth’s ozone layer.
A unique characteristic of UV light is that wavelengths between 200 and 280 nanometers (billionths of a meter), are effective for inactivation of microorganisms such as E.Coli and Fecal Coliforms. This region between 200 and 280 nm is also known as the UV-C region or the inactivation zone. This capability has led to widespread adoption of UV light as a highly effective way to treat wastewater and drinking water.¹
HOW UV LIGHT TREATS WATER
A UV lamp is quite different than your standard incandescent light bulb. Yes, electricity is still passed through a filament which heats up, but that energy “excites” a very small amount of mercury vapor contained in the UV lamp. It is the mercury vapor that glows and emits the UV light.
In water treatment applications, UV light provides rapid, effective inactivation of microorganisms through a physical process. When microorganisms are exposed to certain wavelengths of UV light, they are instantaneously rendered incapable of reproducing, and if they cannot reproduce, they are unable to infect.
Microorganisms are inactivated by UV light as a result of damage to nucleic acids. The high energy associated with short wavelength UV energy, primarily at 254 nm, is absorbed by cellular RNA and DNA. This absorption of UV energy forms new bonds between adjacent nucleotides, creating double bonds or dimers. Dimerization of adjacent molecules, particularly thymine, is the most common photochemical damage. Formation of numerous thymine dimers in the DNA of microorganisms prevents replication.¹
UV light has demonstrated efficacy on microorganisms, including those responsible for cholera¹ polio typhoid,¹ hepatitis,¹ and other diseases.
Rendering of UV energy damaging a microorganism’s DNA.
CAN MICROORGANISMS BE REPAIRED?
Photochemical damage caused by UV may be repaired by some microorganisms if the UV dose is too low – this is called photo reactivation or dark repair. However, studies have shown that there is little to no potential for photo reactivation at higher doses (EPA UVDGM). In fact, it has been shown that some microorganisms, like Cryptosporidium, do not exhibit any evidence of repair under light and dark conditions following low-pressure or medium-pressure lamp irradiation at UV doses as low as 3 mJ/cm².
That’s why it’s critical that UV systems be designed with enough UV dose to ensure cellular damage cannot be repaired. Sizing of a UV system should be based on bioassay validation where a challenge organism is used to assess the performance of an UV system over a range of operating conditions (i.e. flow, water quality and UV lamp power) to quantify the degree of inactivation of a microbe for that UV system.
EFFICACY AGAINST RESISTANT MICROBES
Cryptosporidium (i.e. also known as crypto) and Giardia are resistant to chlorine treatment (Handbook of chlorination).
UV is effective for Cryptosporidium and Giardia inactivation. Protection against chlorine-resistant microorganisms is critical, as these same bodies of water may be relied on by communities as a source of drinking water and for recreational use.
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