Everything You Need to Know about Ultraviolet Water Purification

A number of factors combine to make ultraviolet radiation a superior means of water purification for rainwater harvesting systems. Ultraviolet radiation is capable of destroying [UV does not actually destroy, but inactivate bacteria, see note] all types of bacteria. Additionally, ultraviolet radiation disinfects rapidly without the use of heat or chemical additives which may undesirably alter the composition of water.

 

The most familiar part of the spectrum is a narrow band of wavelengths visible to the human eye. Another band with wavelengths shorter than those of visible light, and not visible to the eye, is the ultraviolet part of the spectrum. Ultraviolet radiation can cause changes in living matter. The sun’s rays cause sunburn. Rays from a welder’s torch burn the unprotected eyes of an observer. The ultraviolet spectrum includes wavelengths from 2000 to 3900 Angstrom units (Å). One unit is one ten billionth of a meter. The 2000 to 3900 Å range may be divided into three segments:

Long-wave ultraviolet – The wavelength range is 3250 to 3900 Å. These rays occur naturally in sunlight. They have little germicidal value.

Middle-wave ultraviolet – The wavelength range is 2950 to 3250 Å, also found in sunlight. Middle-wave UV is best known for its sun-tanning effect; it provides some germicidal action, with sufficient exposure.

Short-wave ultraviolet – The wavelength range is 2000 to 2950 Å. This segment possesses by far the greatest germicidal effectiveness of all ultraviolet wavelengths. It is employed extensively to destroy bacteria, virus, mold, spores, etc., both air- and water-borne.

Short-wave ultraviolet does not occur naturally at the earth’s surfaces, because the atmosphere screens out sunlight radiation below 2950 Å. In order to take practical advantage of the germ-killing potential of short-wave ultraviolet, it is necessary to produce this form of energy through the conversion of electrical energy. The conversion of electrical energy to short-wave radiant ultraviolet is accomplished in a mercury vapor lamp.

Mercury Vapor or Germicidal Lamps

The low-pressure variety of mercury vapor lamp, which can be referred to as a germicidal lamp, provides the most cost-effective and efficient source of short-wave ultraviolet energy. Germicidal lamps are made of special quartz glass that will allow 70 to 90 percent of the short ultraviolet rays to pass. Ordinary glass is not transparent to wavelengths below 3200 Å. The low pressure mercury vapor lamp emits radiation that is predominately at 2537 Å. This is in the region of maximum germicidal effectiveness.

The germicidal lamp works on the following principle: An electric arc is struck through an inert gas carrier, in a sealed special glass tube. Heat from the arc causes vaporization of the small amount of mercury contained in the sealed tube. The mercury, when vaporized, becomes ionized and in the electric arc gives off UV radiation.

Required Germicidal Energy

Bacteria withstand considerably more ultraviolet irradiation in water than in dry air. E.coli, for example, (common in the water of unprotected catchment systems), requires more UV exposure for their destruction in water than in dry air. In either case, the germicidal radiation must strike a microorganism to destroy it. This requires that the water be clear enough to allow transmission of an adequate quantity of UV energy. The degree of microbial destruction is a function of both the time and intensity of the radiation to which a given microorganism is exposed. A short exposure time at high intensity is as effective as a long exposure time at low intensity, provided the product of the time and intensity remains the same.

Any turbidity in the water reduces the range of transmission to UV radiation. Water that is naturally turbid, or that has become turbid from corrosion products formed during storage in steel tanks and liners, should be filtered before UV purification. A 5-micron filter prior to the UV lamp is recommended.

Purifier Design

Several design features are combined to determine the dosage delivered:

1. Wavelength output of the lamp.
2. Length of the lamp – when the lamp is mounted parallel to the direction of water flow, the exposure time is proportional to the length of the lamp.
3. Design water flow rate – exposure time is inversely related to the linear flow rate.
4. Diameter of the purification chamber – since the water itself absorbs UV energy, the delivered dosage diminishes logarithmically with the distance from the lamp.

In a typical operation, water enters the inlet of a UV lamp and flows through the annular space between the quartz sleeve (which contains the germicidal lamp) and the outside chamber wall. The irradiated water leaves through the outlet nozzle.

Features to look for:

1. Expandable system – parts should be as uniform and as interchangeable as possible to permit easy expansion later.
2. Sight port – enables visual monitoring of lamp operation; also permits later adaptation to electronic monitor device using the same port.
3. Single lamp per chamber – provides greater safety through more accurate monitoring than does a multi-lamp /single-chamber system.
4. Quartz protection sleeve – cold water moving past an unshielded lamp will reduce the lamp temperature and the radiation yield. A protective quartz sleeve will allow the higher lamp temperature required for optimum output of 2537 Å radiation.
5. Mechanical wiper – for cleaning the sleeve surface without shutdown or disassembly of the unit.
6. Optional accessories – Flow controls, UV light bulb monitor, electronic water shut-off valves and alarms, should be available to provide fail-safe operation without operator attendance.

A single lamp purifier can be designed to handle any flow rate up to approximately 2400 gallons per hour (gph). By multiplying purifier units, in series and in parallel, higher flow rates can be achieved.

The versatility of UV purification includes:

1. UV purification produces germ-free potable water for home, institutional and municipal use.
· Application to water wells: bacterial contamination of wells is unpredictable and may occur from seepage of surface water or sewage.
· Installation on outlet side of rainwater harvesting cisterns, most cisterns foster the proliferation of bacteria in untreated water.
2. It provides bacteria-free food process water without the use of germicides, oxidants, algaecides or chemical precipitants; particularly applicable where chlorine adversely affects flavor.

Chlorine Versus Ultraviolet Purification

As a tertiary treatment for water, chlorination offers the advantage of continued disinfection after initial treatment, since some chlorine remains in the water with residual germ fighting action. The ultraviolet method, however, has none of the following disadvantages of chlorine:

1. Chlorine treatment requires operation attention.
2. In small installations, when chlorine gas is liberated from a chlorine cylinder or moistened crystals or pellets, the fumes are extremely dangerous and may even be lethal.
3. Chlorine itself is a highly corrosive and toxic chemical.
4. Chlorine is an additive material which may impart an undesirable taste to the water and a decrease in pH.
5. Chlorine is chemically active and can react with foreign ingredients (e.g., as found in industrial waste-waters) to form toxic compounds, a matter of increasing concern to the Federal Government and to many states and municipalities.
· It may combine with ammonia to form “chloramine” which is acutely toxic to fish even at low concentration.
· It may combine with phenol to form “clorophenols”, another dangerously toxic compound.

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Complete Guide to Home Water Filtration

Whole house water systems use a wide variety of water purification technologies to treat  contamination in tap and well water. The most common methods for removing pollutants from the water include carbon adsorption, sediment filtration, ion-exchange, catalytic conversion, oxidation reduction and UV radiation.

Carbon Adsorption
In the water industry, activated carbon adsorption is the most commonly used filtration methods for both home and commercial general water purification applications. Activated carbon is created from a variety of carbon-based materials in a high-temperature process that creates a matrix of millions of microscopic pores and crevices. The pores trap microscopic particles and large organic molecules, while the activated surface areas attract and adsorb, small organic molecules. Contaminants that are removed include chlorine, chloramines, chemicals, gases and odors. Through the process of adsorption, these compounds are subsequently diffused into the carbon matrix and are adsorbed or locked into the internal pore structure removing it from the water. Many people are now installing whole house activated carbon systems in their homes to protect themselves from exposure to these unhealthy chemicals which can be absorbed or inhaled during showers or baths.

Sediment Filtration
Whole house sediment filters can remove suspended contaminants such as sand, dirt, silt, scale and organic matter from the water. These materials are most commonly found in well and natural water sources, and they may cause turbidity which makes the water cloudy and unclean. Basic 20” sediment filters use cylindrical cartridges made of paper, polypropylene, string and cellulose to trap suspended water particulates on the surface or within the material of the filter. For heavy-duty residential or commercial sediment removal, a larger backwashing mineral tank system is used. Most modern systems use a natural blend of zeolite which treats turbidity through mechanical straining, sedimentation, ion-exchange and physical & electrostatic absorption. The adsorption properties of zeolite in particular give it an advantage over traditional sediment filters that use sand, sediment and anthracite. Whole house sediment filters will improve the look, feel and taste of your water, and they make excellent pre-filters for carbon and UV filtration systems.

Ion Exchange
The ion exchange process is another whole house water purification method that is widely used in the home. This term is used to describe the processes of purification, separation and decontamination of ions in water through the use of ion exchange beads. These spherical resin beads are treated with special solutions which allow them exchange positively and/or negatively charged ions with minerals and metals in the water. Water softeners are a popular type of whole house ion exchange filter that removes hard water ions in the form of calcium and magnesium by exchanging with sodium or potassium ions. The benefits of removing these “hardness ions” from the water throughout your home include scale removal protection, softer skin and hair, and cleaner brighter clothes.

Catalytic Conversion
Catalytic conversion is a new advancement in water filtration technology that is used to treat water hardness and scale without the use of salt. Unlike traditional water softeners which use ion-exchange to remove calcium and magnesium, salt-free catalytic conversion systems treat the water by changing the properties of these hardness minerals without removing them.  When hard water comes into contact with the surface of the catalytic media, calcium and magnesium become attached to the ceramic granules and are converted into a scale resistant crystalline form which will not attach to hard surfaces. Hard water scaling properties are effectively neutralized without the need for sodium, regeneration, or backwashing. These systems are more eco-friendly because they do not require rinse water and electricity, are easier to maintain, and they are 100% salt free making them a healthier alternative to traditional water softening techniques.

Oxidation Reduction
Oxidation reduction (Redox) is a water purification method that works based on the principles of electrochemical oxidation, reduction and adsorption actions to remove unwanted contaminants from the water.  Redox includes all chemical reactions in which atoms have their oxidation state changed through a transfer of electrons. Iron water filters are a good example of a redox water purification process that removes iron and manganese through the oxidation process. These filters use manganese dioxide which acts as an oxidizer that removes electrons and precipitates iron, hydrogen sulfide and manganese out of the water making it easy to filter them out downstream with a sediment filter. Whole house iron filters can be used to remove iron red stains and rust, manganese brown stains and rotten-egg odors caused by hydrogen sulfide.
UV Sterilization
Ultraviolet (UV) sterilization is a water disinfection method that uses ultraviolet light to deactivate and eliminate bacteria, viruses and microorganisms. This germicidal irradiation is accomplished by using short-wavelength UV to disrupt and attack microbial DNA eliminating their ability to function and reproduce. Water is purified by running it through a 360 degree stainless steel cylindrical chamber that contains an UV lamp, and microbes in the water are exposed to lethal doses of germicidal UV energy from all sides. This process is extremely effective at eliminating a wide range of bacterial and viral contaminants including e.coli, cryptosporidium, giardia, cysts, coliform bacteria and much more. UV sterilization is also the most natural form of water disinfection as it does not add any chemicals, odors or taste to the water like chlorination does. A whole house UV filter will effectively remove harmful pathogens from your tap and well water to deliver microbially safe water to every faucet in your home.

Whole House Total Solution       
The reason there are so many different types of whole house water filters is due to the fact that water quality varies greatly from home to home and no single system can treat all water contaminants. People who want a more comprehensive water filtration solution for their home may need to consider using more than one system to purify their water. A common type of setup would include installing both a whole house filter and a water softener to treat chemicals and hardness in the water. If iron is present, a combination of carbon, manganese dioxide and sediment filtration may be required. Additional UV light systems can be added to treat microbial contaminants in well or tap water.

For drinking water, the process is a bit different as it does not require whole house treatment. Instead, a small undercounter reverse osmosis (RO) system installed under a kitchen sink would be the ideal solution to deliver ultra-pure and safe drinking water. RO systems are a great complement to whole house filters and are often used together to provide a “Total Solution”.
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