Over the last 20 years, the Cryptosporidium parasite has become one of the most common causes of waterborne diseases affecting humans globally.
The parasite lives in the intestines of infected humans or animals and is released into the environment via bowel movements. The presence of Cryptosporidium in both surface and groundwater potable water supplies is of particular concern because it spreads rapidly and is resistant to chlorine and other traditional chemical disinfectants. A person becomes infected by swallowing Cryptosporidium parasites, typically from water or ingesting something that has been in contact with infected water, such as food washed in contaminated water. Common symptoms of Cryptosporidiosis include dehydration, stomach cramps, fever, nausea and vomiting. In extreme cases, dehydration can escalate into death.
U.K., U.S. Regulations Pertaining to Cryptosporidium
In December 2007, the Water Supply (Water Quality) Regulations 2000 (Amendment) Regulations 2007 introduced new legislation in the United Kingdom regarding Cryptosporidium. The new regulations put the onus on water suppliers, making the detection of harmful levels of Cryptosporidium in their potable water supplies a criminal offense. Water companies are now required to carry out a risk assessment by sampling their raw water sources for several parameters. Should harmful organisms be found, the water supplier is required to implement the necessary treatment steps. The responsibility is on the water company to justify and validate the treatment steps and how they can be monitored to prove efficacy.
U.K. Water Industry Research (UKWIR) is currently working on a project to determine the efficacy of ultraviolet (UV) disinfection and make recommendations on the operational guidelines for UV disinfection in the United Kingdom. The UKWIR work has drawn heavily on experiences in other countries where UV technology is a far more prevalent treatment method, particularly in the United States. In 2006, the U.S. Environmental Protection Agency released the Long Term 2 Enhanced Surface Water Treatment Rule (LT2), Ultraviolet Disinfection Guidance Manual (UVDGM), which provides guidelines and requirements for U.S. public water systems to monitor, and, if required, install UV disinfection equipment to treat for Cryptosporidium and other bacteria. The UVDGM provides equipment manufacturers with bioassay based validation criteria to assure that the equipment will furnish sufficient disinfection to handle Cryptosporidium and other waterborne pathogens. Once fully implemented, all equipment manufacturers supplying to public water systems in the United States must have their equipment validated by a third party in accordance with the UVDGM.
Use of UV Disinfection Increasing
Cryptosporidium has been found to be resistant to traditional disinfectants such as chlorine. The proven treatments for the parasite are microfiltration, ozonation and UV disinfection. Microfiltration is typically accomplished with membranes. The initial capital expense, combined with the electrical costs associated with pumping the water through the membrane, can place a high burden on the water supplier.
Ozone, the cost of which has come down significantly in the last 15 years, is a powerful oxidizer that breaks down the naturally occurring organic matter present in water, causing it to become a nutrient source for bacteria. While ozone inactivates Cryptosporidium, it can also stimulate the growth of harmful bacteria in the distribution system. Ozone also has the potential to introduce bromate ions into the water in sufficient quantities to exceed the regulatory limits for bromate.
As of the year 2000, more than 400 facilities worldwide were treating drinking water with UV disinfection, and the number of public water systems using UV is expected to increase significantly over the next decade. UV has the ability to inactivate pathogenic microorganisms without forming disinfection byproducts or affecting the taste and odor of the treated water. UV is a safe and proven disinfection treatment method that does not pose any of the safety concerns for management and handling when compared to traditional disinfectants such as chlorine.
When selecting a UV reactor for potable water, the reactor must be sized to provide a UV dose capable of inactivating the target pathogens. Significant research has been done to determine the log inactivation (i.e., the percent removal) of various pathogens (see table).
Target Pathogens |
Log inactivation |
0.5 |
1.0 |
1.5 |
2.0 |
2.5 |
3.0 |
3.5 |
4.0 |
Cryptosporidium |
1.6 |
2.5 |
3.9 |
5.8 |
8.5 |
12 |
15 |
22 |
Giardia |
1.5 |
2.1 |
3.0 |
5.2 |
7.7 |
11 |
15 |
22 |
Virus |
39 |
58 |
79 |
100 |
121 |
143 |
163 |
186 |
UV Dose Requirements (mj/cm2)
The UV dose delivered is inversely proportional to the flow rate and the UV transmittance (UVT) of the water. Transmittance measures the amount of UV light which can pass through the water, and is measured as a percent of UV. Combining UV disinfection with a form of pretreatment such as sand filters or deep bed filtration is an economical means of removing Cryptosporidium and can yield water quality with a better UVT value.
Operating costs for a typical drinking water UV system include power consumption, parts and maintenance. Traditionally, UV reactors use either low pressure high output (LPHO) lamps or medium pressure (MP) lamps. LPHO lamps draw 100 to 300 watts per lamp, operating at a supplied power efficiency of around 30 percent. Medium pressure lamps draw 1,000 to 3,000 watts per lamp and operate at a wall power efficiency of around 15 percent. Both types of lamps fade as their electrodes wear out over time, which leads to systems that are oversized when first installed. Therefore, power consumption is a primary concern, especially for larger drinking water facilities, which utilize hundreds of lamps to maintain proper disinfection.
Maintenance costs are an additional concern for drinking water plant operators. LPHO lamps typically last 9,000 to 12,000 hours (12-16 months), with a prorated warranty if they fail prior to meeting their projected life. Therefore, if a lamp fails within the first six months, the user will only receive 50% credit for the failed lamp from the UV equipment manufacturer. Each LPHO lamp can cost up to $391 U.S. each. Medium pressure lamps are warranted for even less time, usually only 4,000 hours, and can cost more than $1,956 U.S. per lamp. In addition to replacement costs for lamps and related items, plant operators also must account for the time it takes to replace defective lamps as well as any equipment down time.
An alternative to traditional UV disinfection systems are microwave-powered UV systems, which offer the benefits of traditional systems - no handling safety concerns, no disinfection byproducts, no taste and odor issues – with improved reliability and lamp life, resulting in significant operational cost savings compared to traditional UV technology.
For more information, e-mail info@severntrentservices.com.
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