.

Ultraviolet Light

.Ultraviolet light (UV) disinfection of water is a unique method of treatment, as it does not use chemicals for the inactivation of pathogenic microorganisms. UV radiation inactivates organisms by photochemical reaction with nucleic acids and other vital cell components essential to cell function. The optimum wavelength for disinfection is between 245 and 285 nm. Low-pressure UV lamps emit a narrow range with 85 percent of the light at 253.7 nm. Medium-pressure, high intensity lamps emit UV radiation over a wide range, primarily between 200 and 700 nm.

UV radiation is considered to be effective for inactivating bacterial and protozoan pathogens like Giardia and Cryptosporidium. Relatively higher doses of UV radiation are necessary for virus inactivation compared to other pathogens. Since UV disinfection does not provide any residual in water after treatment, it is usually followed by a secondary disinfectant to provide the residual protection. UV disinfection is a physical process and hence water quality parameters like temperature, pH, and alkalinity do not have a significant impact  on the disinfection effectiveness.

However, disinfection efficiency of UV reactors can be reduced significantly by the accumulation of solids on the surface of UV lamps. Waters having high concentrations of iron, hardness, hydrogen sulfide, and organics are more susceptible to scaling. Solids or particles can also affect disinfection efficiency by harbouring pathogens and protecting them from UV radiation. It is generally believed that higher turbidities (typically greater than 5 NTU) and suspended solids levels of water can reduce disinfection efficiency.

Often, there is a need to assess or validate the ability of commercial UV reactors to meet desired treatment goals. Such a process allows comparison of competing UV technologies with conventional systems. It also provides a level of comfort that a given UV lamp configuration will provide adequate protection of public health. This is important for UV systems because unlike chlorine residuals, ultraviolet radiation is not distributed uniformly throughout a reactor. Generally, a bioassay procedure is used to estimate the delivered dose of a reactor. The test typically involves an indicator organism like bacteriophage (MS2 phage), which is subjected to varying UV doses in the laboratory using a collimated-beam apparatus under different conditions.

.

The principle advantages of using UV systems in drinking water treatment are as follows:

• No increase in the concentration of biodegradable or assimilable organic carbon (AOC), thereby limiting the re-growth potential within the distribution system.
• No concerns with respect to interactions with pipe material.
• No known formation of disinfection by products (e.g., THMs, HAAs, aldehydes, bromate, ketoacids).
• To achieve the same log inactivation of Giardia and Cryptosporidium, it is less costly than ozone and chlorine dioxide.
• When used in conjunction with chloramines as the secondary disinfectant, there is almost no formation of chlorinated DBPs of concern.

The principle disadvantages of using UV systems in drinking water treatment are as follows:

• Higher dose is required to inactivate viruses
• No residual protection and hence the application of secondary disinfectant is necessary
• Difficult to monitor equipment performance
• Difficult to measure germicidal dose

.

Chlorine Dioxide

Chlorine dioxide is a powerful disinfectant. It is effective for inactivation of bacteria, viruses, and protozoa, including Cryptosporidium. As a disinfectant it is more effective than chlorine, but not as effective as ozone. Chlorine dioxide is also used for taste and odour control, and iron and manganese oxidation.

Chlorine dioxide in general forms fewer halogenated by-products than chlorine. The predominant end-product is chlorite (ClO2-). This has a significant impact on disinfection since chlorite is a regulated drinking water contaminant in the United States with a maximum contaminant level of 1.0 mg/L (USEPA 2003a). Based on a 50 to 70 percent conversion of chlorine dioxide to chlorite, the maximum dose is limited to 1.4 to 2.0 mg/L unless the chlorite is removed through subsequent treatment processes (USEPA 2003a).

.

The principle advantages of using chlorine dioxide in drinking water treatment are as follows:

• Effective against a wide range of pathogens in drinking water
• Does not form halogenated by-products.

.

The principle disadvantages of using chlorine dioxide in drinking water treatment are as follows:

• By-product formation of chlorite and chlorate limits the dosage of chlorine dioxide.
• Less stable than other chlorine species and hence difficult to maintain an effective residual in the distribution system for a long time.
• Disinfection efficiency is reduced significantly at low temperatures.
• Significantly higher CT requirements for effective disinfection of Cryptosporidium.
• Must be generated on-site.
• Chemical costs are high.
• Can be explosive at high temperatures and pressures.
• Decomposes on exposure to sunlight and UV radiation.
• Documented cases of unusual smells (“kerosene-like” and “cat-urine like”) in new homes due to reactions between unknown chemicals used in the preparation of carpet materials and gaseous chlorine dioxide in tap water.

.

Other Alternative Disinfectants

.

Hydrogen Peroxide

The use of hydrogen peroxide is not acceptable as a primary or secondary disinfectant in water treatment. Very few studies have been conducted with hydrogen peroxide to determine its efficacy against pathogens. Further, many of these studies (Yoshpe-Purer and Eylan 1968; Toledo et al. 1973; Lund 1963) did not document the dosage of the hydrogen peroxide applied in the water during disinfection.

.

Bromine

Bromine is highly reactive with ammonia and other amines, which may seriously limit its effectiveness under conditions typically found in water treatment. The data on the effectiveness of bromine against bacteria are complicated by the reactivity and the lack of characterization of the residual species in disinfection studies (NAS 1980). Hence, the use of bromine is not acceptable as a disinfectant for drinking water treatment.

.

Iodine

The use of iodine as a disinfectant for drinking water has not been extensive mainly because it is not cost effective. More studies are necessary to determine the consequences for human health of the long-term consumption of iodine in drinking water with special regard for more susceptible subgroups of the population (NAS 1980). The use of iodine is thus not acceptable as a disinfectant for drinking water treatment.

.

Applicability of other modes of disinfection

Throughout the history of water disinfection, mankind has tried a number of different methods for disinfecting drinking water. Some of these are potassium permanganate, silver, ferrate ionizing radiation, high pH conditions etc. Many of these have little scientific basis due to the lack of data, particularly data on the bacterial and virucidal efficacy. In some of them, the practical dosage of disinfectants necessary for effective disinfection is not available.

In others the laboratory techniques used for measuring the disinfection effectiveness are not reliable. All these factors make these methods unreliable for application in drinking water disinfection. Hence, the use of these methods for the purpose of disinfection of drinking water is not acceptable. Water systems in Manitoba, particularly small drinking water systems, should refrain from using any of these methods for the purpose of water disinfection.