Ships take up water in ballast tanks for stability when they are not carrying cargo. When cargo is loaded onto the ships, the ballast water is discharged. Prior to discharge, the ballast water has to be treated by chlorination, ozonation, or UV irradiation to prevent introduction of invasive species. IMO Ballast Water Management (BWM) Convention requires that all ships constructed prior to Sept, 2017 have to install BWM Systems by Sept, 2024.
In the case of UV treatment, the system is designed for a UV transmittivity (UVT) of the water. UVT is a measure of water quality and is the ability of UV light to penetrate through water. UVT is measured at the same wavelength (255 nm) used for UV treatment. Measuring the incoming UVT of water in a ballast water treatment system is important since the UVT or water quality varies significantly from port to port as seen in Figure 1, below. If the system is designed for very low UVT, then more UV lamps are needed, increasing the space & power requirements. Typically, most systems are designed for an intermediate value of UVT to avoid system overdesign. Deploying a UVT sensor to monitor the quality of the ballast water and control UV treatment helps to maintain compliance and at the same time, save energy whenever possible.
For example, if the ship has low UVT water in its ballast, the UVT sensor would measure a lower value and automatically increase the power to the UV lamps during the treatment process or alternatively slow the flow of water into the ballast water treatment system. This ensures that the UV dose is correct all the time and the vessel is in compliance with the IMO rules. At the same time, if the ballast water UVT is higher than the design UVT, then the sensor would reduce the power to the lamps and the ship saves on fuel. Thus, UVT sensors have an important role to play in ballast water treatment systems. For the actual measurement of UVT, 255 nm LEDs are increasingly being considered as light sources instead of mercury lamps at similar wavelengths. This is on account of the long lifetime and duty cycle operation of LEDs, which enables longer, maintenance free operation of the sensor.
Table 1: UVT Transmittance of Various Ports Around the World
|Hong Kong, China||80%|
|New Orleans, USA||53%|
Source: Trojan Marinex