Why Use a Pond Uv Light?

Algae and Your Pond

Algae Growth (Photosynthesis)

Koi ponds are usually closed recirculating aquatic systems, meaning that these systems, unlike natural ponds with streams flowing through them, lack a fresh water flow-through source. Rather, closed recirculating aquatic systems rely on filtration to purify and reoxygenate their water. Outdoor ponds that are closed systems are subject to seasonal algae blooms and attached filalmentous algae growth due to nutrient and carbon dioxide buildup. Increased levels of nutrients and carbon dioxide create the potential for a healthy algae population.

Pigments (chlorophyll, fucoxanthin and carotenoids) in the algae absorb light energy and use it to convert carbon dioxide and nutrients into new cell biomass through photosynthesis. The primary nutrients of concern are nitrogen and phosphorus. Nitrogen is a byproduct of decomposed fish waste, uneaten fish food and accumulated sludge in the bottom of the pond. You can read more about the Nitrogen Cycle inĀ  "The Science of Water " section of this website.

Planktonic Algae

Planktonic algae are waterborne single-cell algae, most commonly referred to as Green Water. When your water turn green, this is considered an algae bloom of this planktonic form of algae. An algae bloom usually occurs as a result of increased levels of nutrients and carbon dioxide in pond water, combined with the energy of sunlight. Planktonic algae can be controlled with aquatic plants, shade, ultraviolet sterilization or chemical methods, and proper biological filtration. Using aquatic plants as an algae control requires that a specific number of mature plants are added to the pond to compete with the algae for the available nutrients and carbon dioxide. One problem with this is that algae can take hold early in the season when the water is too cold for plants to grow and consume the excess nutrients. Shade is available only if foliage or a shelter inhibits the pond's exposure to sunlight. Chemical treatments can be successful, but they can be expensive, temporary, and potentially harmful to plants and fish.

One of the most effective methods of planktonic algae (green water) control is ultraviolet sterilization.

The many advantages of UV sterilization make it a very attractive option for algae control. First, it is a physical treatment, so it does not change the water chemistry of the pond. Second, it is very easy to install. Third, the treatment takes place outside the pond, away from the fish and plants.

Attached filamentous algae can be seen growing on the rocks of a waterfall or on the sides of the pond. The growth of filamentous algae results from high levels of nutrients, carbon dioxide, and the catalyst, the sun's energy. Methods of controlling filamentous algae include increased shade, reducing the frequency of fish feedings, practical water changes, and the use of chemicals or plecostomus (algae-eating fish) during the summer months.

Algae is not all bad

Algae in certain forms/types can be either beneficial or detrimental to a pond, depending on the owner's viewpoint. It provides nutrients for newly hatched fry and indirectly act as a color enhancer, and it also can beneficially add to the biological surface area for nitrifying bacteria to live. As algae grow in a pond, a population of zooplankton will also develop, on which the fish feed. These natural live feeds help develop the intense coloration desired in most Koi. Unfortunately, algae blooms prevent viewing the fish, so that sick fish can go undetected for days or even weeks.

Algae influence the water quality of the pond mainly by affecting the balance among dissolved oxygen, pH, carbon dioxide, and nutrients. During photosynthesis, algae produce oxygen, remove nutrients, and take up respired carbon dioxide from both the fish and the algae itself. In heavily stocked ponds the water becomes supersaturated with carbon dioxide. High levels of carbon dioxide can quickly depress the pH of the water to levels below seven if the operator is not careful to maintain proper alkalinity levels and adequate aeration for stripping. During active periods of photosynthesis during daylight hours, algae can quickly strip the carbon dioxide out of the water, and pH levels can rise above nine in a matter of hours. Fish not acclimated to such sharp shifts may initially show signs of stress.

At night both algae and fish consume oxygen from and exhale carbon dioxide into the system. Algae compete with the fish for available oxygen in the water. A potentially serious impact of an algae bloom is the risk of an "algae crash" triggered by temperature or barometric pressure. When an algae bloom collapses, dead algae cells settle to the bottom of the pond, adding to the decomposing sediment's oxygen demand. If the crash is severe, the pond's oxygen supply can be quickly depleted, endangering the fish unless backup aeration is available. Additionally, as the dead algae cells rupture, they can release organic nitrogen and phosphorous back into the water, adding to the system's nutrient load. The biological cycle starts again with bacteria converting the organic nutrients to inorganic elements, which are then available to be recycled, and the algae bloom continues.

Practical Algae Control

View your pond as an ecosystem, one requiring you to manage it to maintain proper balance. Fish ponds without inadequate plantings or little to no water changes are most susceptible to algae bloom problems. These ponds are usually overstocked with overfed fish. The absence of aquatic plants eliminates competition with algae for available nutrients in the pond water. Practical fish stocking densities and feeding must be managed closely, and periodic water changes should be done as well.

UV sterilization is a proven method for controlling waterborne algae, but not filamentous algae too effectively. Combining sterilization with adequate mechanical filtration and operating the two properly is most effective in eliminating algae blooms and maintaining clear water. This combination will not, however, control nitrogen or carbon dioxide levels. Practical fish stocking densities and responsible feeding, along with routine filter and UV sterilizer maintenance, play a big part in achieving a balanced system. Partial water changes (approximately ten percent of the pond volume weekly with non-chlorinated water) will aid in diluting nutrients. Filamentous algae may grow and will benefit the pond by consuming nutrients and carbon dioxide. Responsible fish feeding will encourage the fish to graze on the filamentous algae, which is good in their diet. Filamentous algae may also be harvested and used as a fertilizer in gardens; remove it by hand or with a long-bristled brush.

The Science of UV Light

Ultraviolet Sterilization is unmatched in its efficiency, simplicity, and dependability when applied as a microorganism disinfectant. UV sterilization is a proven solution to waterborne planktonic algae as well as other harmful pathogen problems. Certain critical UV performance factors greatly affect all UV sterilizers, no matter who's the manufacturer. The information contained in this outline should be considered before purchasing any UV equipment.

Factors Influencing UV Effectiveness

Whether you choose to label a UV as a clarifier or a sterilizer, the same design, performance, and operating principles apply. Successful UV operation destroys the targeted microorganism. Here are five main factors that will help determine the ability of a UV sterilizer (or clarifier) to achieve this desired effect:

1. The type of lamp used in the application (low-pressure or medium-/high-pressure)
2. The length of the lamp being used (the ARC length)
3. The physical design of the UV's water exposure chamber
4. The condition of the water being treated
5. The water flow rate through the UV's exposure chamber

The Science of UV

Let's start at the beginning. Ultraviolet light is a spectrum of light just below the range visible to the human eye (below the blue spectrum of visible light in the chart below). UV light is divided into four distinct spectral areas -- Vacuum UV (100 to 200 nanometers), UV-C (200 to 280 nanometers), UV-B (280 to 315 nanometers), and UV-A (315 to 400 nanometers). The UV-C spectrum (200 to 280 nanometers) is the most lethal range of wavelengths for microorganisms. This range, with 264 nanometers being the peak germicidal wavelength, is known as the Germicidal Spectrum.

The Targeted Microorganism

It is critical to first identify the microorganism. Each type of microorganism requires a specific UV-C radiation exposure rate to successfully complete the disinfection process. The targeted microorganism must be directly exposed to the UV-C radiation long enough for the radiation to penetrate the microorganism's cell wall. However, it takes only seconds for UV-C light rays to inactivate waterborne microorganisms by breaking through their cell walls and disrupting their DNA. This often totally destroys the organism, or at the very least impairs its ability to reproduce.

The UV Lamp-the Source of UV

UV light sources primarily come as low-pressure or medium high-pressure lamps. Low-pressure lamps produce virtually all of their UV output at a wavelength of 254 nanometers -- very close ot the peak germicidal effectiveness curve of 264 nanometers. These lamps generally convert up to 38 percent of their input watts into usable UV-C watts. This is much higher than other classes of lamps (i.e., a 150-watt low-pressure lamp will have approximately 57 watts of UV-C power). Low-pressure lamps typically run on low-input power currents of 200 to 1,500 milliamps and operate at temperatures between 100 and 200 degrees Fahrenheit. They have a useful life of 8,000 to 12,000 hours, depending on the operating current of the lamp.

Medium high-pressure lamps produce wavelengths widely ranging from 100 nanometers to greater than 700 nanometers, well into the visible light spectrum. These lamps are very poor producers of usable germicidal wavelengths; they generally convert only up to eight percent of their input watts into usable UV-C watts (i.e., a 400-watt medium-pressure lamp will have approximately 32 watts of UV-C power; the remaining 368 watts are converted into heat and visible light). Medium high-pressure lamps typically run on high-input power currents of 2,000 to 10,000 milliamps and operate at temperatures between 932 and 1,112 degrees Fahrenheit. They have a useful life of only 1,000 to 2,000 hours, depending on the lamp's operating current. As you can see from these comparisons, low-pressure lamps perform safely and efficiently. They are the better option for use in UV sterilization.

UV Lamp Length + UV-C Output + Useful Lamp Life = Lamp Value

UV lamp length is a critical performance factor that helps establish UV exposure.

Evaluating UV lamp performance based on input watts is inaccurate! The "Input vs. UV-C Output Watts" chart above demonstrates the poor germicidal value of medium-pressure lamps compared to low-pressure type UV lamps. Low-pressure UV lamps convert approximately 38 percent of their input watts into UV-C output watts while the medium-pressure UV lamps convert only eight percent. Low-pressure style UV lamps offer greater germicidal value than medium-pressure lamps for this reason.

Knowing when to replace UV lamps is critical to achieving a consistent UV disinfection dose, but not all lamps offer the same useful operating life!

Single UV Lamp Array Diagram

Multiple UV Lamp Array Diagram

Open-Channel UV Lamp Array Diagram

Design of the Water Exposure Chamber

The design of the water exposure chamber is completely overlooked by some manufacturers, but it is key to successful operation. The distance UV light energy has to travel from the surface of the lamp to the inner wall of the UV's water containment vessel determines how much UV the water will receive. This is known as the "UV dose rate." The amount of water passing through the UV filter ultimately determines the unit's actual UV dose rate, which is expressed in microwatts per second per square centimeter or (u-watts-sec/cm2). When selecting a UV Sterilizer for your application:

Make sure the UV lamp is positioned between the water inlet and outlet ports of the unit's water containment vessel. Any portion of the UV lamp(s) not located between the water ports is useless. When calculating the UV's performance data, only the ARC length located between the water ports can be applied to the calculation, reducing its capabilities if portions of the lamp are not between the ports.

Select the unit with the largest diameter water containment vessel in the wattage you are considering. A unit with a larger diameter will always have a greater contact time. (For example, a 25-watt model with a 3" diameter housing will flow more water than a 2" housing model.)

Make sure the unit you are considering uses a quartz sleeve. A quartz sleeve isolates the UV lamp from the water to avoid a short circuit path for the lamp's electrical power. It also allows the lamp to operate at its optimum temperature by acting as an insulator.

Does the manufacturer list water flow rates a the end of a lamp's life or the beginning? Most UV manufacturers give a water flow rate, but do not indicate whether it applies to a new lamp or to one that is at the end of its useful life. Try to find a manufacturer that includes the water flow rate in the unit's end-of -lamp life rating. The end-of-lamp life rating takes into account the lamp losing UV-C output due to age so it is a more realistic prediction of how the unit will perform.

Do the manufacturer's water flow rates account for the reduced effectiveness UV light has when treating green water? This information should be listed as some type of percent transmissibility rate or absorption coefficient (decimal value). Units that account for green water will have lower water flow rates.

UV Transmittance

UV transmittance is also largely overlooked, but it is one of the most critical factors in determining the ability of a UV sterilizer to treat a given volume of water. Regardless of the type of UV light source used, any body of water containing impurities will absorb UV energy. Green water, water plagued by algae and microorganisms, will absorb the UV energy emitted by our UV light source in proportion to its density (or how green the water is). The greater the amount of algae in the water, the more of a reduction in percent transmittance. Percent transmittance is the ability of a body of water to be effectively treated by a UV light source. This value indicates the quality of the water being treated. The higher the percent transmittance, the easier the UV sterilizer will be able to treat the water at a given flow rate. A lower percentage of transmittance means the UV sterilizer will be less effective in dealing with the algae problem. If the sterilizer's water flow rates have not been calculated with a reduced transmittance rate, the unit will have considerable trouble in dealing with an algae bloom.

Water flow rate through the UV's contact chamber

A sound UV sterilizer design revolves around the careful selection of lamp type, lamp length, lamp position, and body diameter. These factors, together with the intended water flow rate, percent transmittance of the water to be treated, and UV dose rate needed to kill the targeted microorganism, should be your basis for the selection of a unit for your pond. When researching which type of UV sterilizer to purchase, remember the criteria laid out in this article, read the manufacturer's literature, ask questions, and, most of all, ask yourself, "Does this information make sense to me?" If not, consider another UV manufacturer.

Green Pond Water? You Need a UV Light

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