Article written by: Jason Danaher Ph.D.
Disease can have serious impacts on fish growth and health; thus, maintaining a disease-free aquatic system is desired for the productivity and profitability of aquaculture operations. Four major categories of disease-causing organisms include: bacteria, viruses, fungi and parasites. For a disease to infect the target species (host) it must be present in the system with the proper environment for the disease to proliferate (Figure 1).
Figure 1. Venn-Diagram illustrating the relationship between a pathogen, host, and aquatic environment in an aquaculture system.
If an aquaculture disease organism is identified in the system or begins to affect the well-being of the fish it can be treated with chemical methods. Chemical treatments include pesticides and antibiotics and should be administered only after an aquaculture pest has been properly identified and the control method has been determined to be safe for the fish, applicator, and consumer of the harvested product.
Non-chemical methods use low risk management strategies to maintain biosecurity, eliminate vectors of disease or pests, provide optimal water quality parameters, regularly monitor for disease, and keep accurate fish health records. Besides biosecurity and proactive management strategies, another non-chemical method of controlling disease organisms is ultraviolet light (UV) technology incorporated into the process flow of the production system.
UV technology is a proven, energy efficient, chemical-free water treatment that can be easily integrated into freshwater and saltwater systems. UV disinfection is non-selective and all organisms or viruses passing through the UV unit can be inactivated depending on the applied UV radiation dose. An UV sterilizer's main components consist of a reaction chamber or housing, a quartz sleeve, a lamp and a ballast (Figure 2). The lamp is encased in the quartz sleeve and the housing encloses the quartz sleeve and bulb. The ballast supplies power to the unit.
Figure 2. Main components of a basic ultraviolet (UV) light sterilizer.
UV light is a type of electromagnetic radiation that has shorter wavelengths than visible light and is invisible to the human eye (Figure 3). The most effective germicidal wavelengths, ideal for disruption of viruses and other pathogens, are in the range of 254 - 275 nanonmeters (nm). UV light has wavelengths between 10 and 400 nm, while visible light is between 400 to 800 nm.
Figure 3. Electromagnetic spectrum illustrating the ultraviolet wavelength range necessary for germicidal control in an aquatic environment.
UV light treatment in aquaculture can be a disruptor and inactivate the reproduction process of bacteria, viruses, fungi and protozoans. DNA and RNA absorb electromagnetic energy in the wavelength range of 200 – 280 nm and if enough damage occurs, the biological repair process of the organism or virus cannot repair the damaged DNA or RNA. Under this circumstance reproduction of the organism or virus discontinues.
You may encounter discussions on UV intensity and UV dose when reading about UV technology and its use in aquaculture. The UV intensity is directly related to the UV light available for disinfection over a specific area and the power output of the UV lamp. When UV intensity is combined with the exposure time a UV dose is created. A UV dose describes the UV energy the microorganism will receive when passing through the UV treatment area. The UV dose is measured in microwatts per square centimeter (µW/cm2) or millijoules per square centimeter (mJ/cm2). The amount of UV energy required for the inactivation of DNA and RNA is different based on the targeted microorganism (Figure 4). The UV doses typically range from 30,000 to 180,000 µW/cm2 or 30 to 180 mJ/cm2.
Figure 4. Known dosage rates of ultraviolet light required to inactivate reproduction processes of a few specific fungi, algae, bacteria, nematodes and protozoans encountered in aquatic systems.
It is important to remember UV light can only treat bacteria, viruses, algae and fungi suspended in the water column and passing through the UV unit. The UV cannot treat organisms or viruses attached to the inside of PVC pipes, on the tank walls, or on the fishes’ bodies. As a result, the fish will not be harmed by the UV radiation as they are not in direct or indirect contact with the UV source in a well-designed application. The UV dose is dependent on the system’s water quality and the flow rate. Turbidity can drastically decrease the intended UV dose emitted by the bulb. As total suspended solids concentration increases, the intensity of irradiation generated by the UV light decreases because the suspended particles scatter the light. For this reason, it is best to place the UV unit after the aquatic system’s mechanical filtration that has in theory reduced the total suspended solids concentration.
The system operator needs to consider the aquatic system’s required flow rate to maximize the UV dose the microorganism will receive. In general, UV systems with higher intensities can deliver the required UV dose under shorter exposure times, which makes them suitable for applications with high flow rates. However, multiple UV units, operated simultaneously and in series, may be required or can be utilized to properly dose the volume of water. In addition, if the UV sterilizer is not already equipped with a flow sensor it is highly recommended a flow sensor be installed on the inlet side of the unit to de-energize the bulb and prevent potential fire, shock hazards or damage to the UV housing from overheating.
UV Transmittance (UVT) is an important parameter to consider in selecting the correct sized UV unit. UV transmittance is a measurement of the amount of ultraviolet light at 254 nm able to pass through 10 millimeters of water and is expressed as a percentage (% UVT). The amount of light that gets through the water sample is an indicator of the general water quality. An operator may have a targeted organism, associated dosage rate and known flow rate to size a UV unit; however, if the %UVT is not accounted for then the unit could be undersized and prove to be ineffective.
Additional factors an operator needs to consider with UV technology are UV lamps age over time and the intensity of UV light emitted by the lamp can decrease by one-half its original strength within six-months of normal operation. This decrease in efficiency should be accounted for in the initial selection process and may vary from one UV manufacturer to another. Nonetheless, it is standard operating procedure to replace standard UV lamps every 9,000 hours or basically after 1-year of operation when the unit is used 24 hours a day, 7 days a week. Amalgam UV lamps offer many advantages over standard UV lamps; one of which includes extended service life (12,000 to 16,000 hours) or nearly 1.5 years when the unit is operated 24 hours a day, 7 days a week.
Regardless of the UV lamp style used, the quartz sleeve should be removed, inspected and cleaned periodically to remove any mineral deposits that may have formed on it. The operator should wear nitrile gloves so fingerprints are not left on the sleeve and a weak acid or denatured alcohol can be used to remove any deposits before reinstalling the sleeve into the UV sterilizer. Furthermore, the operator needs to consider the amount of vertical or horizontal space or clearance requirements to properly change UV lamps and quartz sleeves for standard maintenance. Generally, additional space considerations require an additional 1.5 times the length of the UV unit to remove and replace the quartz sleeve and lamp regardless if the unit is mounted in a vertical or horizontal position.
This brief talk discusses important concepts and considerations that should go into sizing a UV unit and knowing where to place the UV unit into the system’s process flow. Proper maintenance is also essential once the UV unit is installed to provide a reliable disinfection process. The staff at Aquatic Equipment & Design, Inc. would be happy to discuss UV technology and its potential application in your future or currently operating aquatic system. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com.
Before calling, the following parameters would help to expedite the sizing of a UV unit:
1) System type (pond, recirculating system, flow through)
2) Targeted disease organism to control
3) % UV Transmittance of system water
4) System flow rate (gallons/minute or liters/minute) to determine exposure time and UV light dose
5) Available electrical connection (115V/230V; 50Hz/60Hz)