Balast Water Treatment Systems ( BWTS)
More than 90 percent of global trade is transported by sea, and each year transfers of up to 12 billion tonnes of ballast water take place around the world because of this. Ballast water is used to maintain the stability and trim of vessels, and to ensure their structural integrity. It is typically pumped in as cargo is unloaded and discharged as cargo is being placed on board.
Ballast water that is taken on in one ecological zone and discharged into another can introduce invasive (i.e.: non-native) aquatic organisms that can have a big detrimental impact on the local biodiversity, economy and even the health of local communities.
Bio invasion is one of the four greatest threats facing the world’s oceans today, alongside land-based sources of marine pollution, the overexploitation of living marine resources and the physical alteration and destruction of marine habitats.
The proper treatment of ballast water, as required by the IMO and the relevant authorities in the USA, actively removes, kills or neutralises organisms prior to discharge. Ballast water treatment differs from the older conventional process of ballast water exchange, which involves completely flushing the ballast water tanks while underway.
IMO Regulation｜ D-2 Ballast Water Performance Standard
- ▶ Over 50 ㎛ : Less than 10inds/㎥
- ▶ 10-50 ㎛ : Less than 10inds/㎖
- ▶ Bacteria
> Escherichia Coli : Less than 250cfu/100㎖
> Intestinal Enterococci : Less than 100cfu/100㎖
> Vibrio Cholerea : Less than 1cfu/100㎖
Choose the correct type of BWTS
Ballast water is usually filtered in two stages. After passing through the initial large filter, which is similar for all ballast water treatment systems, ocean water that is taken on board for ballast needs to proceed through a second filtration system to get rid of smaller, even microscopically small organisms and substances.
Eight different types of filtration processes are in use around the world.
Physical Separation or Filtration Systems
These systems remove microscopic and other sea life as well as solids either from the sediment or from the surface of the water. The backwash containing the unwanted material is then either discharged into the ocean from where the water was taken, or it is held and treated on board for use as ballast in the tanks, from where it can be discharged (without any marine life left in it) in a different location when the need arises.
Ultraviolet purification systems combine physical filtration and UV technology. In such a system, the ballast water flows through a chamber that is surrounded by UV lights. The UV radiation “sterilizes” the marine organisms. They become harmless and unable to reproduce.
Special oxidizing and non-oxidizing biocides of the types that that have been proven effective in combating marine organisms are added to the ballast water. These biocides are chosen for their effectiveness and also for their ability to biodegrade or to be removed easily to prevent the ballast water from becoming contaminated by toxins.
Oxidizing biocides include chlorine, bromine and iodine. These disinfectants destroy the cell membrane or nucleic acids of the microorganisms.
Non-oxidizing biocides counteract the reproduction and the neural or metabolic functions of the organisms.
Nitrogen or other inert gas is injected into the space above the water level in the ballast tanks. This causes the oxygen in the water to be removed. Without oxygen, the marine organisms become asphyxiated and die. The process takes two to four days. For the deoxygenation to succeed, the ballast tanks need to be perfectly airtight.
The ballast water is heated to a temperature that’s high enough to kill the organisms. A dedicated heating system can be installed to heat the water in the tank. Alternatively, the ballast water could be piped to cool the vessel’s engine. The cooling water then becomes heated by the engine’s heat, and it kills the suspended organisms. This is a slow process, which could also cause tanks to corrode.
Electric Pulse and Pulse Plasma Treatments
In this system, which is still in development, short blasts of energy are produced in the ballast water to kill the organisms. The treatment is done in one of two forms:
In the pulse electric field technology, two metal electrodes generate the energy pulse at very high power density and pressure.
In electric plasma technology, a plasma arc is generated to kill the organisms.
Ultrasonic or Cavitation Treatment
In this treatment, ultrasonic energy produces high energy ultrasound in the ballast water, the impact kills the cells of the suspended organisms. Ultrasonic or cavitation techniques are generally used together with other systems.
Magnetic Field Treatment
This is a type of flocculation treatment. Magnetic powder is mixed with coagulants and added to the ballast water in the tank. Magnetic flocs or flakes are formed. These can contain large quantities of marine organisms. Magnetic discs are used to lift the flocks from the water and the contents are disposed of safely.
Which system is the most suitable depends on various factors, including the following:
flow ballast water (m³/hr)
total amount of ballast water to be cleaned
new build or retrofit
company’s policy about waste disposal (chemical)
sailing USA water or not (US Coast Guard regulations).
Ballast Water Treatment Implementation Schedule
- 1. Ships constructed on or after 8 September 2017 shall comply with the D-2 standard for ships delivery (Constructed = Keel Laid).
- 2. In the case where a previous IOPP renewal survey had been completed between 8 September 2014 and 7 September 2017, the ships shall comply with the D-2 standard by the first IOPP renewal survey after entry into the force of the Convention.
- 3. In the case where a previous IOPP renewal survey had been completed between 8 September 2012 and 7 September 2014, the ships shall comply with the D-2 standard by the second IOPP renewal survey after entry into the force of the Convention.
- 4. Ships not subject to the IOPP renewal survey (normally less than 400GT) shall comply with the D-2 standard by 8 September 2024.