Ultraviolet (UV) irradiation is a common method for the disinfection of private supplies. Chlorination may be more suitable for larger supplies where it is necessary to maintain a residual disinfectant during storage and distribution.
Installation of a UV unit
There are many UV units on the market and care must be taken when selecting a unit for a private water supply. Units must be designed for drinking water treatment. Where possible validated units are to be recommended. Validated units are tested to ensure that the stated level of disinfection is achieved under normal operating conditions. UV units for ponds and fish tanks are not suitable for use on supplies for human consumption.
Special lamps are used to generate UV radiation. The lamps are enclosed in a reaction chamber made of stainless steel or plastics. Low pressure mercury lamps are most commonly used. These lamps are similar in design, construction and operation to fluorescent tubes except that they are constructed of UV transparent quartz instead of phosphor coated glass. The optimum operating temperature of the lamp is around 40°C so the lamp is normally separated from the water by a sleeve to prevent cooling by the water.
The usual UV reactor configuration comprises a quartz sleeved low pressure mercury lamp in direct contact with the water. Water enters the unit and flows along the annular space between the quartz sleeve and the wall of the chamber. Other configurations include lamps separated from the water, for example, those were the lamps are surrounded by bundles of PTFE tubes through which the water flows.
Disinfection will only be effective provided that a sufficient dose of UV is applied. The dose of UV radiation is expressed as an energy flux, in units of mW.s/cm2 (milliwatt seconds per square centimetre), or more commonly as millijoules per centimeter squared (mJ/cm2) which is the product of the intensity given out by the lamp and the residence time of water in the reactor. The minimum dose required for disinfection depends on several factors, including the susceptibility of microorganisms but is generally taken to be around 40mJ/cm2.
The clarity of water is usually expressed in terms of the amount of UV light that can pass – its ‘Ultraviolet Transmittance’ (UVT). Minimum UVT values, typically greater than 90-95%, are commonly specified by UV equipment manufacturers/suppliers.
It is important, to ensure effective disinfection, that both residence time and UV intensity are adequate. Manufacturers’ recommendations must be followed regarding installation, operation and maintenance. In particular, the maximum design flow rate should not be exceeded, lamps should be allowed to reach their operating temperatures before water is passed through the unit and lamps should be cleaned and replaced as recommended. A continuous UV monitor and an alarm or failsafe device is strongly recommended and although not usually fitted as standard on point-of-use units, they are usually available as an extra. Some units incorporate a manual wiper for cleaning whilst others incorporate automatic mechanical cleaning.
Testing a UV unit
UV disinfection efficiency is particularly affected by water quality and flow rate. For UV disinfection be effective the water must first be of good quality and in particular low in turbidity. Unlike chlorination, UV is effective in inactivating Cryptosporidium provided that a sufficient UV dose is applied. However, where Cryptosporidium is likely to be present and its removal is required then pre-filtration capable of achieving a turbidity of less than 1NTU is required prior to UV disinfection. Pre-filtration provides an additional barrier to passage of oocysts into the treated water removes particles that shield microorganisms from the UV light and helps to reduce fouling of the UV lamp. Colour can be removed by activated carbon cartridges or membrane filters to around 20°H or lower. Iron and/or manganese can be removed by oxidation and filtration in proprietary units to around 200 μg/l or 50 μg/l or lower, respectively.
The usual measure for the suitability of treatment by UV disinfection is UV254 absorbance, which may be measured with online monitors or sampled and analysed at a laboratory. UV254 can often vary with levels of colour and organic matter in the water and manufacturers of UV disinfection equipment will be able to advise on the suitability for particular waters.
Maintenance of UV units
UV irradiation equipment is compact and simple to operate. Maintenance requirements are often modest (when compared to the alternatives), although specific systematic maintenance is essential. Other advantages include short contact time and the absence of any known by-products of significance to health. The principal disadvantage is the absence of any residual effect, necessitating careful attention to hygiene in the storage and distribution system.
The build-up of scale on the sleeves of the lamps will eventually reduce their transmittance and they must be cleaned or replaced regularly. Some units have UV intensity monitors and alarms which provide a continuous check on performance and these are strongly recommended. These devices may prevent the flow of water if the required intensity of UV radiation is not achieved, for example when the lamps are warming up or because of scale formation. UV intensity monitors may not be available on smaller units and it is therefore essential that the manufacturer’s instructions regarding lamp warmup, cleaning and replacement are followed to ensure optimal performance.
Additional monitoring and control features are available, including UV intensity monitor, lamp run time, and automatic water shut off in the event of lamp or power failure. Such features are desirable, and should be considered where practicable as a way of providing some basic safeguards against consumption of disinfected water.
In the event of power or lamp failure, if the flow of water is not automatically interrupted, the drinking water produced will not be disinfected.
Similarly, validated systems are recognised as offering additional safeguards regarding confirmation of applied dose. It is recognised however that few validated systems are available in the sizes typically used in private supplies. In all cases approved products designed for drinking water treatment should be used.
The intensity of UV radiation emitted decreases with lamp age. Typical lamp life is about 10 to 12 months, after which the output is about 70% of that of a new lamp. Manufacturers will advise on lamp maintenance requirements. It is worth noting that visually a lamp will still appear the same, even when output has decreased, and so the fact that a lamp is emitting light is not necessarily a guarantee that it is delivering the required dose. Lamp replacement is usually a simple operation but may involve some downtime of the system. This difficulty may be overcome by use of multiple units or by having a treated water storage tank capable of maintaining supply whilst maintenance is carried out.