Chemistry of Water
It is important to consider the chemistry of the water you wish to treat as anything in the water can have an effect on the act of disinfection. This is true for any type of disinfection, whether it be chemical or physical. This does not need to be difficult as only the simplest of compounds are required to be known before going ahead with the installation of a disinfection system, specifically a UV disinfection system.
There are 9 different compounds that need to be considered when trying to determine the appropriate water treatment equipment. Unfortunately getting rid of microorganisms is not as simple as installing a UV system. In most cases the water must be treated for the removal of other compounds before the UV equipment can be effective. The compounds to be considered are pH, hardness, iron, manganese, the presence of any gases, turbidity, dissolved solids, tannins and UV transmittance.
pH is not a major concern as most water supplies that are being used for drinking water have a fairly neutral pH level. pH is the presence of hydrogen ions in the water. The pH scale goes from 0 to 14 with the higher the pH level the more basic the water and the lower the pH level the more acidic it is. The most optimum pH level is one that is neutral which would be 7 - 8. Keep in mind that if you test your water one year and the pH level is found to be 8 and then the next year the result you receive is 9 you will need to be relatively concerned as each jump in pH represents a 10x increase or decrease in pH. Therefore your water is now 10x more basic than it was the year before. You will need to ask the question why? Is something getting into my water supply? Is my water treatment equipment functioning as it should? Basically pH is an indicator that something is changing chemically in your water supply.
When it comes to the function of your UV system we need to be concerned with the pH level as low pH's can cause damage to the stainless steel reactor of the system. Water with a pH falling below 6 can actually begin to cause pitting of the steel which will eventually cause the reactor to leak. pH can also come into play when considering other forms of water treatment like water softening or tannin removal.
Hardness is something to be concerned with when you are looking to treat groundwater (well water). Let's look at where hardness comes from and what it is.
As rain falls from the sky it passes through the atmosphere absorbing gases as it falls, specifically carbon dioxide. This causes the rain water to become slightly acidic. As the water reaches the earth it will run into the ground falling deeper into the earth. As the acidic water is being absorbed into the ground it is picking up something from everything it touches so when the water comes across limestone in the earth it will absorb some of this limestone causing the water to contain dissolved solids of calcium and magnesium bicarbonates. This water reaches our wells and is drawn into our homes via pump. When the water comes in contact with heat (say in a water heater and/or UV system) a chemical reaction occurs causing the limestone to fall out of solution causing scale to form on all surfaces. This scaling is a cloudy white colour. To remove hardness from water an ion exchange system needs to be used to remove the calcium and magnesium from the water and replace those ions with sodium. This is how a water softener works.
Hardness is a great concern when considering a UV system as water within a UV reactor will become heated when the water is not being used in the home (which is 70% of the time in most homes). When the hard water heats up the limestone will begin to scale on the walls of the stainless steel reactor as well as on the quartz sleeve which surrounds the lamp. This layer of scaling will block the UV from the water therefore being ineffective against pathogenic micro-organisms.
UV manufacturers recommend that hardness be removed from water at levels greater than 7 gpg (grains per gallon) or 120 mg/L (ppm).
Iron & Manganese Concentration
Iron and manganese are absorbed into water the same as hardness. They are both dissolved away by acidic rain running into the ground and then travel through the ground as a dissolved solid. This water reaches our wells and is pulled into our homes for use causing the water to be exposed to air. Iron and Manganese can also be found in soft water, but it is more prevalent in hard water. As soon as dissolved iron and manganese are exposed to oxygen they will precipitate out of solution causing scaling. Iron scaling is an orange colour whereas manganese scaling is a blackish grey colour. Both will cause discoloration of the quartz sleeve in a UV reactor blocking the light from reaching the water.
UV manufacturers recommend that iron be removed from water at levels greater than 0.3 mg/L (ppm) and manganese at levels greater than 0.05 mg/L (ppm). The higher the concentrations of either of these compounds are the more severe and rapid the scaling will be.
Presence of Gases
All water will have some level of gases in it but at exceeded concentrations gases can cause sensor problems with UV systems. Different gases will require different levels to see an effect but if you do have a water supply where you know there is a presence of things such as radon gas for example, it would be wise to discuss this with the manufacturer.
Hydrogen sulphide is a gas which causes a strong rotten egg odour and at exceeded levels can cause black staining of fixtures and UV components (quartz sleeve). Gas concentrations will change over the course of the year as the barometric pressure does change causing fluctuations in how much gas is being forced out of the ground.
Have you ever filled a glass with water from the faucet and saw tiny white flakes floating in your cup? That's turbidity. Turbidity is the small solid particles that are present in water that can be removed by filtration. Turbidity can cause problems in both chemical and physical types of disinfection as these small particles can interfere with either the chemicals being used or the ability of the physical disinfection to be effective.
Consider physical disinfection using a UV system. Now think about if you've ever actually seen a floating particle in water. Chances are that you have. Have you ever seen an E.coli swimming around? The answer would be no unless you were looking through a high powered microscope. If you consider both of the above questions just imagine how much E.coli can attach themselves to one tiny piece of turbidity which will help shield them from the UV light. This is why is it so important that customers always install a 5 micron filter prior to the UV system to ensure that no microorganisms can leak through the system and contaminant the water supply.
Tannins are natural organic matter that are the byproducts of nature's fermentation process. They are created as water flows through large quantities of decaying vegetation or peaty soil. This process will cause the water to have a faint yellow “tea-like” colour and can cause staining on fabrics, fixtures, china and laundry. Tannins may give a tangy or tart aftertaste to water and may cause the water to have an earthy or musty smell. Tannins, also known as fulvic or humic acids are more common in surface waters and shallow wells than in deep water wells. Waters in marshy, low-lying, or coastal areas is also more susceptible to tannins.
Tannins are considered aesthetic problems in drinking water as although they do discolour the water, they present no health hazard. However, when using ultraviolet technologies, tannins can present a major issue to the functionality of a UV system.
Tannins create a light yellow to dark brown discolouration in the water. Although a local lab can test for the actual quantity of tannins in the water, an easy homeowner test is to draw a glass of water in a clear glass and let it stand overnight. If the colour settles to the bottom of the glass, the discolouration is likely caused by iron and/or manganese in the water whereas if the water remains discoloured, the likely cause is tannins in the water. Tannin levels should be less than 0.1 mg/L (0.1 ppm) when using an ultraviolet system.
Tannins can be removed from the water supply using an organic scavenging anion exchange resin. As the anion resin is sensitive to hardness, it is recommended to include a water softener as a pretreatment step prior to the tannin reduction system.
UV transmittance or simply UVT describes the measurement of a fluids ability to transmit UV light. Typically, municipal water supplies have UVT levels of greater than 95%, whereas deep water wells typically have UVT levels around 85%. The UVT of surface waters can vary greatly depending on the source and surrounding geography. UVT can be easily tested with a spectrophotometer which most laboratories or UV manufacturers have. Waters with lower UVT's will require more UV energy to deliver a similar UV dose than waters with a higher UVT. LUMINOR systems are tested at a 95% UVT level and their rated flow is based on the waters having a minimum UVT level of 95%. If your water has a lower UVT, the system will deliver a lower UVT dose, or the flow rate through the system will need to be decreased to achieve the same UV dose. Dosage curves are available for all LUMINOR systems to aid in this selection. As a precaution, it is recommended that all water treated by a UV system have a minimum UVT level of 75%.