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94Ballast Water ManagementA pressure/vacuum (P/V) valve helps isolate the tank atmosphere from the external atmosphere, yet allows for small changes as per pressure variations inside the tank.Figure 14.7 Pressure/vacuum valveExamples of the commercial use of deoxygenation/supersaturation technology within a ballast water treatment are OceanSaver and Venturi Oxygen Stripping (VOS). Both systems are particularly cost effective for vessels with larger ballast capacities and many orders have been taken for installation onboard large tankers. The Coldharbour BWTS was specifically designed for use with large tankers and LNG/LPG vessels. It should be noted that the upgraded OceanSaver Mark II system offers supersaturation only as an option to reduce corrosion.14.2.4 OceanSaver Ballast Water Management SystemThe OceanSaver BWMS is manufactured by OceanSaver AS. While the original system (Mark I) combines several treatments to disinfect ballast water: filtration, cavitation, deoxygenation and production of active substance onboard (in the C2E™ unit), the updated and streamlined Mark II system uses an improved filtration unit as well as the C2E™ disinfection unit, and is only operated during ballast water intake. The N2 supersaturation process present in the Mark I system is now an optional feature in the Mark II, installed to reduce corrosion. Due to the improvements in the filtration unit, the energy demanding cavitation units have become unnecessary and are not part of the Mark II offering.The Mark I OS BWTS was Type Approved in April 2009 and during 2010 achieved a particularly high order number for OceanSaver BW systems onboard large tankers. The Mark II upgraded system requires new Type Approval (although the IMO Final Approval for the Mark I system covers the Mark II), the system has completed and passed all tests and TA is expected in October 2011. As well as servicing larger vessels, the more compact Mark II system is also suitable for medium sized vessels and retrofit situations.Figure 14.8 The OceanSaver BWMS was installed onboard the MV ‘Höegh Trooper ’ during dry-docking in 200514.2.5 Venturi Oxygen Stripping™ (VOS) BWMSThe VOS system has been designed and is manufactured, sold and supported by the US company N.E.I. Treatment Systems. The system was granted Type Approval in September 2008. It is licensed for manufacture, sales and service in Japan by Mitsubishi Kakoki Kaisha and called the Mitsubishi-VOS system. In Korea the VOS system is called the Samgong-VOS and licensed by Samgong Co.The VOS system is a single unit treatment that has a combination of effects on the ballast water: deoxygenation (O2 concentration reduced to 0.2 mg/l to 1 mg/l), reduced pH (between 5.5 and 6) and cavitation in the Venturi injector. The Stripping Gas Generator (SGG) produces the inert gas when necessary which, when combined with the mechanical effects of the Venturi Injectors, leads to the removal of 95% of the dissolved O2 in less than ten seconds.