There are many factors that go into the decision about which ballast water treatment system is right for your vessel. One of the most critical factors to your long term satisfaction with a system is often the factor that is the hardest to understand: “How much will a BWTS slow my ballast flow?”
This is a tricky question, and one that is hard to get a straight answer on. But at Hyde Marine we know you deserve the right answers, explained in a way that makes sense. So let’s start at the beginning
What is TRC?
The Treatment Rated Capacity (TRC) of a ballast water treatment system is the maximum amount of water that can be treated through that system. It is often a factor of two components: the separation stage and the disinfection stage.
The separation stage is the simpler one: this is typically the ballast water filter. Each filter has a certain maximum capacity to allow flow based on the surface area of the filter mesh. This maximum flow rate does not change and is not impacted by the quality of the water. Easy – just make sure your filter is designed to match the pump capacity of the ballast pumps hooked up to it.
The disinfection stage is more challenging. This rating for a disinfection stage is typically the maximum amount of water that can be treated by the stage. For every system, there is some type of limiting factor that impacts TRC for the stage. For a UV-based system that limiting factor is the amount of UV light that passes through the ballast water. Some systems measure this using a UV Transmissivity (UVT) sensor, but others simply use a light sensor and they measure the UV Intensity (UVI) at a strategic point in the chamber.
This UVI is compared to the minimum UVI that was observed during the type approval testing of the unit and consequently many systems have a system design limitation (SDL) based on Minimum UVI.
What Does Minimum UVI Mean?
As the UVT of the water lowers, a smaller percentage of UV light penetrates through the water to give proper treatment. For example, a UVT of 70 means that only 70% of the UV light penetrates through 1 cm of water. For a UV Chamber with 4 cm of distance between the lamp and the outer wall, this effect is cumulative, so for every 1 cm the light diminishes by 70%. At 4 cm therefore, the light has been reduced to 24 % of the original intensity (1 x 0.7 x 0.7 x 0.7 x 0.7 = .2401).
Hyde Marine has done extensive research into various ports around the world and some of their typical UVT levels.
|Chesapeak Bay, Baltimore, MD||78.1|
|Inner Passage, Alaska||97.37|
|Lake Arreso, Denmark||47|
|Lake Erie, Erie, PA||87|
|Lake Michigan, Sturgeon Bay, WI||81.1|
|Lake Superior, Houghton, MI||95.1|
|Mississippi River, Amelia, LA||42.0|
|Mouth of Thames, UK||88.4|
|Pacific Coast, Washington||90.5|
|Pittsburgh City Tap Water||95.7|
|Port of Hamburg, Germany||76.7|
|Port of Hong Kong, PRC||97.5|
|Port of Juneau, Alaska||98.43|
|Port of Pusan, South Korea||96.9|
|Port of San Juan, Puerto Rico||75.2|
|Port of San Pedro, California||95.9|
|Port of Tacoma, Washington||96.3|
|Port of Zebruggee, Belguim||84.5|
|Portage Bay, Alaska||96.6|
|St. Johns River, Jacksonville, FL||47.8|
|Surf City, North Carolina||77|
|Thames River, London||63.4|
|Vltava River, Prague||72.8|
|Yangtze River, Shanghai, China||48.8|
This number is also impacted by the turbidity of the water which leads to the UV light being scattered and not directed at the UV sensor.
Depending on what type of water a manufacturer used during Type Approval testing, this lowest level of UVI allowed may vary greatly. While is is impossible to know for sure how the individual UVI values listed for a manufacturer may impact your vessel’s operation, it is critical to know that unless you know what the minimum UVT that the system was tested in, you cannot guarantee operation of your ship in the various ports of the world you may choose to trade in.
What Happens Below that Number?
When a ballast water treatment system violates one of the system design limitations, the system is considered to be outside of the Type Approval Certificate. That means it is no longer properly meeting treatment in accordance with the Convention and your ship is out of compliance.
At Hyde Marine, we have elected a different path for our system and we use a dose-methodology. Using this dose methodology, coupled with testing down to extremely low UVT (39) water, we enable the Hyde GUARDIAN to throttle flow from 100% down to about 20% in order to maintain the minimum required dose above the system design limitations and still treat some of the most challenging water in the world.
This methodology also helps the GUARDIAN address turbidity issues in addition to UVT issues as the scatter is measured similarly by our UVI sensor.
Although it is intuitive, it is not obvious that relying on the measured intensity does not allow for flow to be restricted in challenging water. Many UV systems have developed a one- or two-stage throttling program where below a certain UVI value flow drops to 50%, but this improperly restricts flow much faster than would be necessary if the system is simply looking at dose. To see how this looks, we have mapped a few different UV-based systems based on the observed UVI and the permitted flow as a percentage of TRC:
* UVI Values based on published U.S.C.G. Type Approval Certificates
Having the confidence to know that your GUARDIAN can treat the most challenging waters while providing the maximum amount flow based on the quality of the water is why so many Ship Owners demand a Hyde GUARDIAN.
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