UV Treatment Purifies Ballast Water
Brent D. Johnson
Every hour, 2 million gallons of ballast water are released into US waterways, contaminating the ecosystem with a host of bacteria, viruses and other undesirable organisms. These intrusive creatures are delivered by high-speed container ships that are penetrating biological barriers once isolated by the high seas.
Ballast water released along coastlines and in port systems is a prime source of marine-species transport throughout the world. This UV radiation system is being used onboard ships to destroy harmful microorganisms.
The US and Australia are pushing for systems that would stem the tide of these parasitic stowaways by implementing ballast water filters. At a convention planned for 2003, the International Maritime Organisation will make the final decision about implementing the technology. However, the use of the devices is already meeting resistance from members of the shipping industry.
In an attempt to mollify the concerned ship owners, OptiMarin AS and Heraeus Optics Inc. have developed an unobtrusive UV system that can be installed with a ship's ballasting system without disrupting normal ballast procedures. UV radiation from 215 to 315 nm causes irreparable damage to DNA in microorganisms.
The ideal wavelength for damaging DNA is 253.7 nm.
Halvor Nilsen, president of OptiMarin, said that the heart of the system is a series of low-pressure, 200-W mercury lamps from Heraeus that can treat 200 to 250 cubic meters of water per hour under harsh conditions. The narrowband emission of the lamps at 254 nm has a UVC efficiency of 30 percent and 58 W of output. The company also is developing systems that use medium-pressure lamps that allow them to treat 800 to 1000 cubic meters per hour.
The MicroKill UV system has an arrangement of 12, 14 or 16 UV lamps. As water runs through the UV chamber in both the ballasting and deballasting processes, it is exposed to intense UV radiation at a path length no greater than 1 cm. The energy required to sterilize the offending organisms varies. Shigella paradysenteriae, which causes dysentery, can be inactivated by a UV dosage of 4260 µW/cm2. Hardier organisms that carry tuberculosis or typhoid require as much as 15,000 µW/cm2. Medium-pressure lamp intensity can be regulated from 70 to 100 percent, depending on water quality.
The MicroKill system has been installed on Princess Cruises' Regal Princess, and the company has ordered two more systems for other ships in its fleet.
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