In-depth | OPINION
It must also be mentioned that when
ozone is used, if the treated ballast water is pumped out in fresh water, the ozone will remain active for up to 30 minutes, thus having the potential to harm aquatic life in the area. Tere are also some potential problems
with BWTS that operate with an inert gas/nitrogen blanket. This will present the same issues experienced with cargo tanks using an inert gas blanket. The tank gauging system must be capable of operating properly over the full range of inert gas pressures including the fault conditions of high pressure and under pressure. An incorrect level reading may cause an overflow when filling, the pumping of air (and possible pump damage) when discharging or the improper balance of a vessel during cargo loading / discharge operations. As with cargo tanks, you have to the
monitor tank pressure so you
can monitor over and under pressure conditions. Even if the system works with very small pressures, the system is closed and when you discharge and a relief valve doesn’t open, the tank can collapse. You most likely can’t use a standard
air-purge system in this BWT solution. Air purge systems typically pump in air
(with oxygen) while inert gas / nitrogen BWT systems try to eliminate oxygen in the ballast water. Do you need to change your system on a retrofit or can you modify it to be suitable? Air-purge is mentioned, as it is currently the most commonly used solution for level gauging in ballast tanks.
Can some ballast water treatment systems create more problems than they solve?
You will have to make the tanks gas
free to add oxygen if you need to perform service in the tank. Obviously, it will be best if all service can be performed from outside the tank. Haraldsson highlights that you can
have an approved BWM solution and an approved tank level gauging system, but they may not work together. Tese two systems are connected and must be
Table: Maximum Ballast Water Effluent Limits for Residual Biocides Biocide or Residual
Chlorine Dioxide
Chlorine (expressed as Total Residual Oxidizers (TRO as TRC))
Ozone (expressed as Total Residual Oxidizers (TRO as TRC))
Peracetic Acid Hydrogen Peroxide (for systems using Peracetic Acid) 200 µg/l 100 µg/l 100 µg/l 500 µg/l 1,000 µg/l
Limit (instantaneous maximum)
viewed together. What will happen if a vessel comes to a port and is told that it can’t discharge its ballast water due to too many live organisms in the water, while at the same time, the owner can prove that he has approved BWT equipment and that it is working? Both chlorine and ozone are heavier
than air gases and this means that any ballast tank or hold that has been treated with these chemicals should be uprated, in the ship enclosed space management plans, to dangerous until they are completely ventilated and tested. Natural ventilation will be inadequate for these spaces and forced ventilation that reaches to the bottom of these tanks will be necessary for complete ventilation. No one should enter tanks that have
contained chlorine-or ozone-treated ballast water until the tanks can be verified as being clear of the gases. Tis could be problematic for vessels that need to have tanks surveyed or examined in the port of discharge and without adequate forced ventilation equipment, delays could be experienced. All ships that use chlorine or ozone
treatments should carry ozone and chlorine gas detectors. Where vessels are using ozone or
chlorine ballast water treatment, mud or sediment build-up must be avoided and the bottoms of ballast tanks should be water-blasted periodically to clear the mud or sediment from the bottom. NA
Chemical-free options are available for owners
30 The Naval Architect September 2012
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