Disinfection Tech Info
Published on Oct 4, 2014
 

Disinfection Tech Info

Biosecurity and pathogen control are critical to the successful management of an aquaculture facility. Effective disinfection of water supply is the first and most important step in ensuring protection from disease. Although deep-aquifer ground water sources may be pathogen free, a wide range of organisms may be present in surface water sources or shallow aquifer wells, making disinfection a requirement. Disinfection technologies are also commonly incorporated into the treatment process of Recirculating and Partial Reuse Aquaculture Systems to reduce the risk of disease transmission between culture tanks as the water is recycled.

Disinfection treatments commonly used in aquaculture include ultraviolet (UV) irradiation and chemical oxidation with ozone (O3) or, less commonly, hydrogen peroxide (H2O2). The disinfection technology selected for each application will depend on capital and operating budgets, water quality, flow rate, and the specific disinfection goals of the facility.

Ultraviolet Disinfection

Ultraviolet light is a clean, safe and fish-friendly disinfection technology for aquaculture applications. At sufficient dosage levels, UV irradiation has proven to be effective at inactivating waterborne, pathogenic microorganisms such as bacteria, fungus, mould, viruses and algae. UV irradiation does not produce toxic by-products or residuals, and as such, there is no risk of overdosing fish as there is with chemical oxidants.

UV disinfection systems use mercury vapour lamps emitting UV light of a specific wavelength (254 nm) that is optimum for germicidal effects. UV irradiation disinfects by penetrating the cell wall of an organism, damaging the organism’s genetic material (RNA and DNA) and destroying its ability to reproduce.

Dosage is the amount of UV required to inactivate an organism, which varies with the type of organism to be inactivated. Each pathogen or microorganism requires a specific minimum dosage; the product of UV light intensity and exposure time. As such, the size and type of UV system is ultimately determined by the facility’s specific protection requirements.

A very important water parameter when sizing a UV system is the UV transmissivity. This is a measure of the ability of the germicidal UV light to pass through the water and it is heavily influenced by the size, type, and concentration of particles present in the water. Pre-filtration, typically in the form of media or microscreen filtration, is often required to achieve optimum transmissivity and effectiveness of treatment.

Both low-pressure and medium-pressure lamps are available, referring to the pressure of the mercury gas vapour inside the lamp when it is excited by electricity. Low-pressure lamps are very efficient at converting electrical power input to germicidal-range UV output; however, the output intensity is less than that of medium-pressure lamps and therefore more bulbs are required. Medium-pressure systems have higher output and therefore require fewer lamps, but are less energy efficient.

Ozone

Ozone is an aggressive and ecologically sound chemical oxidant that is highly effective as a disinfectant in aquaculture applications. Ozone will react non-discriminately with almost any contaminant or microorganism in the water.

Disinfection effectiveness is dependant on the ozone dosage, the length of contact time, and the presence of other materials which may consume the ozone. For each pathogen, different concentrations and/or contact times may be required. Because ozone may cause fish health problems at higher concentrations, it is important that residual ozone is not present after the treatment process. Depending on the amount of oxidative material present in the water, the ozone will decompose to simple oxygen in a matter of minutes or hours.

In addition to its disinfection abilities, ozonation may have other water quality benefits such as the control of dissolved solids, color and odour. As a result, ozone is highly effective in the optimization of water quality. Lower mortality rates in disease outbreaks and higher density tolerances in systems using ozone have been extensively reported.

O3 gas cannot be stored and must be generated onsite using either compressed air or oxygen. Introduction of ozone into the process stream is usually accomplished with Venturi induction. Sufficient contact time must be provided to allow the gas to dissolve into the process stream and react with the water constituents. Contact time is usually accomplished with a purpose-built contact pressure vessel or directly with process piping with appropriately specified length and size.

Ozone presents some difficult challenges in terms of handling and safety. Material selection, process control, installation location, and alarm systems must be considered carefully in the design of ozone systems.

Hydrogen Peroxide

Liquid hydrogen peroxide (H2O2) is a very powerful oxidant and can be a very effective disinfectant. However, it is also more stable and therefore more likely to persist in the culture water with the potential to accumulate to toxic levels. There is also relatively little information available to determine the concentration and contact time needed to kill common aquaculture pathogens and parasites. More research is required on this promising technology.

Treatment Combinations

Although each of the above mentioned disinfection methods may be used alone, they may often be used in combination for improved disinfection effectiveness or a multi-barrier approach to treatment. Ozone and UV, when used together, have been documented to provide greater protection from pathogens than either technology on its own. Also, when arranged such that it is downstream of ozonation, UV irradiation can provide protection from ozone residuals which may be destructed by UV light of sufficient dosage.

PR Aqua offers a range of high quality, proven disinfection technologies for use in aquaculture systems, including UV irradiation units, and ozone generation and injection equipment. We can help you in selection of a technology that is appropriate for your specific flow rates, water quality, and pathogen concerns. If required, our designers can also help you to develop a more complete biosecurity plan and treatment solution that will provide consistent, reliable protection for your valuable fish stocks.