Most common ballast water treatment systems used for Ballast Water Management
Oceangoing cargo ships load seawater in their double bottom or side tanks to increase draught, and to achieve a positive trim. Keeping ballast is essential for a vessel to maintain positive stability during all stages of a voyage. However, the ballast water being loaded at a place can contain thousands of aquatic or marine microbes, plants, and animals, transferred to other geographical locations. Untreated ballast water released at the ship's destination could potentially introduce a new invasive marine species. Hundreds of such invasions have already taken place, sometimes with devastating consequences for the local ecosystem.
The International Convention for the Control and Management of Ships' Ballast Water and Sediments (BWM Convention) was adopted in 2004 to introduce global regulations to control the transfer of potentially invasive species. With the treaty now in force, ships need to manage their ballast water.
Ballast Water Treatment Systems must now comply with the BWM Convention and can be categorized by technology, biology, capacity, costing, size, regions, and regulations. The system can use a combination of mechanical, physical, and chemical/biocide methods to comply with the BWM Convention. A ship's size
determines the capacity of its ballast system hence the requirement of ballast water treatment requirement.
Ballast water treatment system
Active Substances used during chemical/ biocide treatments must be readily degradable or removed from the ballast water to limit their effect on the environment.
There are three fundamental treatment technologies which may be combined into one system.
Research is still being conducted into the above methods. However, it is agreed that any treatment must be safe, environmentally acceptable, cost-effective, and must meet the standards set out by the IMO or the USCG.
Two Stage Treatment
Most ballast water treatment systems use a two-stage approach.
Stage 1 :Typical components:
Large Fine mesh filter with automatic backflushing downstream of the Ballast pumps
Back flushing system for Filters
Cyclonic separation has proven to be less effective than filtration, most treatment options now feature a filter as the mechanical separation component.
Ballast water treatment two stages
Stage 2 :Physical
UV is suitable for vessels with large spare generating capacity or low capacity ballast systems. UV systems can become impractical at larger flow rates due to the large footprint of the equipment.
Many systems use lamps containing mercury, these should only be handled by trained and qualified personnel. Typical components include:
cleaning in place systems
measuring and metering system.
Chemical Injection is suitable for larger vessels or vessels with less spare generating capacity. Typical components include:
chemical mixing and dosing system
measuring and metering system.
Example of Ballast Water Treatment System
This ballast water treatment system shown in diagram uses a filter and UV reactor to treat ballast water. The system comprise of mechanical (filter) and physical (UV reactor).
Ballast water treatment system components
Such systems are mostly self-contained once installed and have the following characteristics:
Only small amounts of cleaning chemicals are required.
Filters, UV Reactors and Ultrasound Treatment units need to be fitted directly into the ballast pipework and treat the full ballast flow.
Inline components can be significant and need space to be installed. It must allow for the large radius pipe bends.
Contain Cleaning In Place (CIP) units to clean the UV lamp's glass cage to maintain UV intensity and reactor performance. Some systems use the flow of water through the chamber to clean.
Power is maintained from a Lamp Drive Cabinet or a similar large power cabinet, which is usually situated in the same compartment as the reactor.
Chemical Treatment and Biocides
These treatments use Active Substances which are defined by the IMO as:
"substances or organisms, including a virus or a fungus, that have a general or specific action on or against harmful aquatic organisms and pathogens."
Regulation D-3 requires that the IMO shall approve ballast water management systems which make use of Active Substances under the "Procedure for approval of ballast water management systems that make use of Active Substances (G9)".
This procedure applies a two tier evaluation criteria to determine the risk to the:
ship and crew
Chemical Treatment and Biocides:
Some systems that use chemical treatment methods produce hazardous gasses such as hydrogen which must be removed via gas separators or ventilation system.
System components are generally smaller than that of UV or Electrolytic systems, except for filtration equipment.
Chemical dosing systems generate the dosing chemical from seawater. Neutralising chemicals are stored onboard.
Active substances must degrade to a level acceptable to the discharge standards or be removed from the ballast water using a neutralizing agent. The by-products of neutralization are also subject to the discharge standards.
For effective treatment using active substances the water must be dosed at the correct rate for the right length of time to ensure treatment is effective.
Other Factors to be Considered
When choosing a treatment system, other factors need to be considered in addition to those that comply with the BWM Convention.
Congested pump rooms or deep-well ballast pumps may require equipment to be mounted on deck or in a cofferdam, and this can increase pressure drops in piping and decrease pump performance.
Suitable access for sampling points must be maintained so that required testing of the ballast water can be conducted.
Not all components in many systems are suitable for use in hazardous environments such as cargo oil pump rooms where an explosive atmosphere may be present.
Will the vessel operate in US territorial waters? Remember, the USCG apply their own Type Approval requirements.
Is the vessel required to ballast in fresh water? This may limit options for equipment selection.
Before compliance testing, the IMO recommends that an indicative analysis of ballast water should be undertaken to establish if a treatment system is potentially compliant or non-compliant.
The results of the analysis cannot be used as a definitive measure of compliance with the BWM Convention. Still, they can identify grounds for the investigation of potential non-compliance and immediate mitigation action.
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