Factors Affecting UV Water System Performance December 11 2013

UV water treatment technology is a very robust water treatment solution that can be used effectively in many conditions.  But like all technologies, it has it's limitations.  For UV, it is the incoming water quality that has the greatest influence on system effectiveness.  Identifying possible water quality challenges is the easiest and best way to make sure your UV system will deliver the performance you need it to.

In this article we go over the most important water quality parameters to consider.  As you're reading this, please keep in mind that the vast majority of waterborne disease-causing organisms are actually very easy to destroy with UV.  Most UV systems deliver at least 2 or 3 times the UV dose necessary to destroy these organisms.  So, even a UV system that is only half-working (not ideal!) will still afford a great deal of protection.  It's also important to distinguish between UV system manufacturers whose systems are designed to deliver the bare-minimum UV dose of 16mJ/cm² and the systems sold by aQuatell which deliver a minimum 30mJ/cm² dose.  This difference in UV dose allows aQuatell UV systems to perform well even when water conditions are not ideal - making them a perfect choice for the myriad of applications in South Africa and beyond.   Now, let's look at water quality and how it impacts system performance:

 

Water Quality Considerations

There are three main elements of water quality that can negatively impact UV system performance:  Iron, Water Hardness, and UV Transmittance.  There are two mechanisms by which a UV system can be impaired.  Some minerals when dissolved in water absorb the germicidal wavelength of UV light leaving less UV energy available to disinfect the water.  Other dissolved materials in the water can act as foulants where they deposit on the UV system quartz sleeve and prevent the light created by the lamp from entering the water column.

 

Iron
Iron is found in most rock, sand, and soil, so its not surprising that the vast majority of groundwater sources contain some amount of iron.  Iron is a two-fold problem for UV systems - it acts both as a UV absorber in its dissolved state, and it readily deposits on surfaces making it a problematic foulant of the quartz sleeve in a UV system.  For these reasons, Iron levels should be no higher than 0.3 ppm (same as mg/L) for the incoming water.  If iron levels are higher than this can be addresses in one of two ways.  When iron levels are marginally higher than 0.3 ppm, the issues caused by the iron can be addresses by oversizing the UV system.  An example would be using a 100 litre per minute system for a flow rate of 60 litres per minute.  This provides more exposure time between the UV lamp and the water and compensates for the impairment caused by the iron.  The other solution is to increase the cleaning regimen for the UV system.  The UV systems offered by aQuatell are renowned for their ease-of-maintenance.  Quartz sleeves and UV sensors (for systems that have them) should be removed and cleaned with an acid-based de-scaler.

 

Hardness
Water hardness is a measure of the amount of calcium and magnesium salts dissolved in water.  Total water hardness is expressed usually as ppm, mg/l, or grains per gallon (gpg).  Water hardness is a foulant to UV systems.  If water hardness is greater than 7 grains per gallon (120ppm) it will very quickly deposit a white/grey film on the quartz sleeve and/or UV sesnor.  As it is with Iron, ground water usually has a higher concentration of water hardness than other sources.  If water hardness is higher than 7 gpg (120ppm) the water can be pre-treated with a water softener or, the frequency of cleaning of the quartz sleeve and UV system can be increased.

 

UV Transmittance

UV Transmittance, often abbreviated to UVT is the measure of how easily germicidal wavelengths of UV light can pass through the water.  UVT has a major impact on the effectiveness of a UV system.  There are a number of different substances that can affect the UVT of water:

    • Particulate matter suspended in the water can lower UVT values.  This is why all UV systems must be preceded by a 5-micron rated sediment filter
    • Organic material dissolved in the water (primarily Tannins and Humic Acids) can give many surface waters a characteristic brown tint.  This tint can absorb a large amount of UV light, making it unavailable for disinfection
    • Dissolved minerals, namely Iron, can also act as UV absorbers in the same way as Tannins.
It can be difficult to make accurate predictions about UVT but there are some rules of thumb that can be applied.  Typically the most problematic water sources are surface waters.  Many lakes, ponds, streams, and rivers are feb by catchment areas where decaying vegetation creates concentrations of Tannins and Humic Acids.
Ground water sources tend to have very low concentrations of these substances and the resulting UVT is very high (often over 90%).  Rain water that is harvested from roof tops also usually has a high UVT.  Any time a UV system is being considered for a sensitive application (public drinking water, as an example) it is a good idea to have the UVT tested by an environmental lab.  Have the lab use a spectrophotometer to measure the UV absorbance for the 254nm wavelength for a 1cm path length.  In situations that are not as sensitive it may be more practical to simply oversize a UV system when surface water with a high tint is being used as the source.