During Water Week 2017, the Water Quality Assn. (WQA)...
Cottage country” drinking water applications require much more attention than rural groundwater applications. This is due to the fact that in cottage country, most water supplies are pulled from a surface water source, such as a lake, river or stream. While groundwater is filtered naturally through the earth, surface water is exposed to the elements and picks up something from everything it touches.
Attention to the disinfection of surface water is vital not only for the removal of the most common waterborne microorganisms, but also due to the possible presence of protozoan cysts. Protozoan cysts, such as Cryptosporidium parvum and Giardia lamblia (Beaver Fever), are pathogenic microorganisms, that can cause illness, specifically, severe gastroenteritis.
Traditional methods of disinfection, one being chlorination, are not readily capable of treating these types of surface water microorganism, as they cannot penetrate the organism’s hard outer shell. One of the most effective and accepted means of treating surface water is ultraviolet (UV) light, which can easily inactivate both Cryptosporidium and Giardia, as well the most common microorganisms. It is important to remember that UV light can only be effective if absorbed by microorganisms; therefore, pretreatment of surface water is crucial to effective disinfection.
Much more consideration is demanded for efficient pretreatment of surface water. The main water quality issues for surface water tend to be color and turbidity. Both can have a tremendous effect on how efficiently a UV system performs, as both compounds absorb and block UV light. Removal of tannins can be accomplished through anion exchange, whereas turbidity removal can be done through simple filtration.
Color in water can be attributed to tannins, which are fulvic and humic acids. Tannins derive from the decomposition of organic material. If you consider lake water, and what it is composed of—weeds, fish, tree branches, organic debris, etc.—you can create a more accurate picture of how tannin concentrations arise. Different lakes will have different levels of tannins, depending on their location, boat traffic and makeup.
If the tannin level is very high, you will be able to detect color simply by filling a glass with the lake water. Lower levels are more difficult to detect, and you may not see the color unless you were to fill a white bathtub. Keep in mind that just because water is visibly clear, that does not mean it is clean. The most important aspect to consider when applying UV as a disinfectant is the UV transmittance (UVT) of the water. Depending on the tannin concentration, tannins in surface water will absorb UV light. If the water cannot transmit UV light, then a UV disinfection system cannot do its job, which is to disinfect the water.
When UV manufacturers test for UVT, they are looking to find the highest transmittance percentage possible (85 to 100%). If a low UVT is found (i.e. <70%), manufacturers will then test for tannins, as tannins will decrease water’s UVT. Therefore, if a monitored UV system were to be installed on lake water, which has a high concentration of tannins, the system would very shortly be in alarm, as the sensor would not be able to read the UV light coming off of the lamp.
The removal of tannins is not an easy or inexpensive task. Tannins can be described as a colloidal suspension, meaning it will not fall out of solution like turbidity or suspended solids; they will need to be removed via ion exchange. If you are planning to install a UV system on a surface water supply, please remember to always test the water first. There is nothing worse than having to go back to your customer and explain that the reason their UV is going into alarm is because you did not check the tannin concentration first, and now they need to install additional costly equipment.
Turbidity is the amount of solid suspended matter present in the water supply. If you fill a glass with water from a faucet and notice tiny white particulate matter floating down to the bottom of the glass, you have turbidity. Turbidity is considered an aesthetic compound, meaning it is not harmful to drink water with turbidity; however, turbidity can cause problems with disinfection, regardless of whether you are using a chemical (chlorine) or physical (UV) disinfectant. If you consider that you can actually see turbidity with the naked eye, and you will never see, say, E. coli, without a microscope, think about how many E. coli could attach themselves to one piece of turbidity. When each piece of turbidity passes by a UV lamp, it will act as a shield for those E. coli and other microorganisms, causing bacteria breakthrough.
Removal of turbidity is very important when using UV disinfection. The majority of UV manufacturers will always recommend you use at least a 5-micron prefilter preceding all UV systems. Keep in mind that you should inquire about the actual turbidity level in the water supply you are planning to treat. Very high levels of turbidity will require various levels of stepped filtration in order to ensure there is no breakthrough, which can ultimately lead to bacteria plate counts. Surface water will always have higher turbidity levels than groundwater, as it does not have the natural filtration that occurs in groundwater applications.
UV disinfection is an ideal solution for cottage country, as it is effective against both Cryptosporidium cysts and/or Giardia at very low UV dose levels. Without the proper pretreatment, however, especially when considering surface water, efficient disinfection is not possible, and ultimately, contamination will occur, leaving water unsafe for house-hold use and especially consumption. Discovering the water quality is the most difficult step in water treatment—after that, the solution is simple.