Ultraviolet (UV) Water Treatment Systems have become an established method to sterilise water due to its effective ability to kill or inactivate most species of disease-causing microorganisms. Ultraviolet water disinfection systems are effective on bacteria, protozoan parasites e.g. Giardia, Cryptosporidium, and are also effective for most viruses, providing sufficiently high UV dosage rates are used.
UV disinfection is an efficient, low cost and environmentally friendly process. UV light kills pathogenic micro-organisms quickly without leaving any residues, harmful by-products or affecting the smell or taste of the water.
Our UV units use UV lamps which emit UVC radiation at 254nm which disrupts the DNA in the micro-organisms, so they are either killed or their ability to replicate is destroyed.
The kill rate depends on the UV dose received by the micro-organisms, i.e. the time that a micro-organism is exposed to a certain intensity of UV radiation (Wm2). A UV dose of 400 J/m2, is recognized internationally as a suitable dose to ensure safe disinfection of drinking water.
The disinfection performance of a UV system is determined by:
- Intensity of the UV light,
- Water flow rate,
- Optical transmission of the water at 254nm
Geometry of the reactor.
The sizing of the UV system should be based on these parameters.
As there are no disinfection substances added to water by UV radiation, there are no residual effects once the water has passed through the UV Reactor.
This article describes how to properly size your Ultraviolet water filtration system.
UV water disinfection / treatment systems are suitable for residential and commercial applications such as:
- Agriculture: Livestock, Irrigation, Dairy Industry, etc
- Domestic drinking water, residential use
- Domestic drinking water, municipal use
- Food and Beverage Industry
- Breweries, Wineries, Distilleries
Determining the maximum flow rate
The maximum flow rate of a system occurs when water is drawn from multiple fittings and fixtures simultaneously. In general, a typical home with a 20mm (¾inch) service line will have a maximum flow rate of 40 Litres/pm. A home with a 25mm (1 inch) service line may encounter peak flow rates as high as 60 Litres/pm or more.
The effectiveness of a UV system in eradicating microbiological contamination is dependent on the chemical, physical, and micro-biological qualities of the incoming water. The key limiting water quality parameters for effective UV disinfection include:
- Total Dissolved Solids (TDS) – prevents the penetration of light through the water. TDS is only a surrogate measurement for inorganic matter and potential inorganic foulants. Some UV manufacturers suggest TDS should be less than 800 to 1,000 mg/L.
- Suspended Solids/Turbidity – shields microbes from the UV light and disease-causing micro-organisms will pass through the UV unit without being inactivated. Total Suspended Solids should be below 10 mg/L and Turbidity should be below 1.0 nephelometric turbidity units (NTU).
- Iron/Manganese – causes staining on the lamps or quartz sleeves. Iron affects the sleeves at levels as low as 0.1 mg/L of iron; ideally, iron should not exceed 0.3 mg/L and no iron bacteria should be present. Manganese concentrations should be below 0.05 mg/L.
- Hydrogen sulphide – impairs lamps at concentrations > 0.2 mg/L; ideally, hydrogen sulphide odour should not be detected.
- Calcium/Magnesium – combine to produce hardness and scale formation on the lamp or quartz sleeve at levels greater than 120 mg/L as CaCO3.
- Coliform Bacteria – UV disinfection is recommended to be limited to treating water with a maximum concentration of Total Coliforms of less than 1,000 counts/100mL.
- UVT – UV transmittance is a measure of the percentage of transmittance of UV light, and is therefore an indicator of the potential ability of the UV lamps to be effective. EPA rates water for UV disinfection as follows: UVT > 95% is Excellent; UVT > 85% is Good; UVT > 75% is Fair. Most UV manufacturers suggest UVT should be >75%. Some manufacturers mention that Tannins should be less that 0.1 mg/L as they can reduce UVT. Therefore, the UVT measurement is a more useful measurement and is simpler and less costly than doing a laboratory analysis for Tannins.
Factors affecting UV performance
- Dissolved organics and inorganics
- Clumping of micro-organisms
- Colour (tannins)
UV Lamp Replacement
The output of a UV lamp diminishes with time. Two factors that affect the lamps performance are solarization (which is the effect UV radiation has on the UV lamp causing it to become opaque) and electrode degradation occurring every time the lamp is cycled on and off. Frequent lamp cycling will lead to premature lamp aging. Average service life expectancy for low pressure lamps is approximately 8,800 hours or one year.
PROBLEMS WITH OVERSIZING OF UV LAMPS IN WATER FILTERS WITH UV WATER TREATMENT SYSTEMS
Water Filters in combination with Ultra Violet will reduce water flow through the UV reactor. Water filter cartridges come in varies sizes and micron ratings. Sediment Filters and Carbon Filters are an important factor in estimating water output volume.
For example: A 80-litres/minute UV System in combination with Water Filters may in fact supply only 40-litres/minute output.
Mains Pressure and Filter Types and Sizes are important factors in pressure drop. Over-sizing a UV lamp would cause the temperature in the UV Reactor to raise, resulting in a disinfection rate far below the intended safety expectations.
Drinking Water UV Systems are specifically designed for cold water,- when the water within the UV Reactor becomes warm, the System becomes ineffective.