Ballast Treatment

Three methods of ballast water exchange have been evaluated and determined to be acceptable to IMO; viz the sequential method, the flow-through method and the dilution method. A description of these methods is as follows:

?Sequential method – A process by which a ballast tank intended for the carriage of water ballast is first emptied and then refilled with replacement ballast water to achieve at least a 95% volumetric exchange.

?Flow-through method – A process by which replacement ballast water is pumped into a ballast tank intended for the carriage of water ballast, allowing water to flow through overflow or other arrangements. At least three times the tank volume is to be pumped through the tank.

?Dilution method – A process by which replacement ballast water is filled through the top of the ballast tank intended for the carriage of water ballast with simultaneous discharge from the bottom at the same flow rate and maintaining a constant level in the tank throughout the ballast exchange operation. At least three times the tank volume is to be pumped through the tank. The flow-through method and the dilution method are often referred to as “pump-through” methods.

?Screens/Discs : Screens (fixed or movable) or discs are used to effectively remove suspended solid particles from the ballast water with automatic backwashing. These are extremely environmentally friendly as they do not require usage of toxic chemicals in the ballast water. Screen?filtration?is effective for removing suspended solid particles of larger size but is not very handy in removing particles and organisms of smaller sizes.

?Note: It has been noticed that though screens are highly effective in removing majority of suspended solid particles and organisms from ballast water, they alone are not sufficient to treat the ballast water according to IMO standards

?Hydrocyclone : Hydrocyclone is an effective equipment for separating suspended solids from the ballast water. High velocity centrifugal force is used to rotate the water to separate solids. As hydrocyclone doesn’t have a moving part, it is easy to install, operate and maintain on board ships.

Note: It has been found that as the operation of hydrocyclone heavily depends on the mass and density of the particle, they are not successful in removing smaller organisms from the ballast water.

Coagulation : As most of the physical?filtration methods are not able to remove smaller solid particles, the method of coagulation is used prior to the?filtration?process to join smaller particles together to increase their size. As the size of the particles increase, the efficiency during the above mentioned?filtration?processes increases. Such treatment involving coagulation of smaller particles into small flocs is known as flocculation. The flocs settle more quickly and can be removed easily.

Note: Some ballast water treatment systems using coagulation and flocculation utilize ancillary powder (sand, magnetite etc.) or coarse filters to produce flocs. An additional tank is required for treating ballast water for this process and thus extra space is required on board ships.

Media Filters : Physical ballast water treatment systems with media filters can also be used in order to filter out smaller sized particles. It has been found that compressible media filters (Crumb rubber) are more suited for shipboard use because of their compact size and lower density as compared to conventional granular?filtration systems.

Magnetic Field Treatment?

The magnetic field treatment uses the coagulation technology. Magnetic powder is mixed with the coagulants and added to the ballast water. This leads to the formation of magnetic flocs which includes marine organisms. Magnetic Discs are used to separate these magnetic flocks from the water. (Refer the figure above)

Chemical Disinfection (Oxidizing?and non-oxidizing biocides)?Ballast Water Treatments?

Biocides (Oxidizing and non-oxidizing) are disinfectants which have been tested to potentially remove invasive organisms from ballast water. Biocides removes or inactivates marine organisms in the ballast water. However, it is to note that the biocides used for ballast water disinfectant purpose must be effective on marine organisms and also readily degradable or removable to prevent discharge water from becoming toxic in nature.

On the basis of their functions, biocides are mainly divided into two types:

·?????Oxidizing

·?????Non-Oxidizing

Oxidizing biocides : Oxidizing biocides are general disinfectants such as chlorine, bromine, and iodine used to inactivate organisms in the ballast water. This type of disinfectants act by destroying organic structures of the microorganisms such as cell membrane or nucleic acids.

Non-oxidizing biocides : Non-oxidizing biocides are a type of disinfectants which when used interfere with reproductive, neural or metabolic functions of the organisms.

Oxidizing Biocides?

Some of the processes utilizing oxidizing biocides used on board ships are:

Chlorination – Chlorine is diluted in water to destroy the micro-organisms.

Ozonation – Ozone gas is bubbled into the ballast water using an ozone generator?. The ozone gas decomposes and reacts with other chemicals to kill organisms in the water.

Other oxidizing biocides such as chlorine dioxide, peracetic acid, and hydrogen peroxide are also used to kill organisms in the ballast water.

Non-Oxidizing Biocides

Though there are several non-oxidizing biocides available in the market, only a few such as Menadione/ Vitamin K are used in ballast water treatment system as they tend to produce toxic by-products. A lot of research is being made in this field to make more non-oxidizing biodes feasible for use in ballast treatment plant.

Ultra-Violet Treatment Method

Ultraviolet ballast water treatment method consists of UV lamps which surround a chamber through which the ballast water is allowed to pass. The UV lamps (Amalgam lamps) produce ultraviolet rays which acts on the DNA of the organisms and make them harmless and prevent their reproduction. This method has been?successfully used globally for water?filtration?purpose and is effective against a broad range of organisms.

De0xygenation

As the name suggests, the deoxygenation ballast treatment method involves purging/removing of oxygen from the ballast water tanks to make the organisms asphyxiated. This is usually done by injecting nitrogen or any other inert gas in the space above the water level in the ballast tanks.

Note: It generally takes approximately 2-4 days for the inert gas to asphyxiate the organisms. Thus, this method is usually not suitable for ships having short transit time. Moreover, such type of systems can be used on ships with perfectly sealed ballast tanks. If a ship is already installed with an inert gas system, then a deoxygenation system will not require more space on board ships.

Heat Treatment?

This treatment involves heating the ballast water to reach a temperature that will kill the organisms. A separate heating system can be utilized to heat the ballast water in the tanks or the ballast water can be used to cool the ship’s engine, thus disinfecting the organisms from the heat acquired from the engine. However, such treatment can take a lot of time before the organisms become inactive and would also increase the corrosion in the tanks.

Cavitation or Ultrasonic Treatment?

Ultrasonic energy is used to produce high energy ultrasound to kill the cells of the organisms in ballast water. Such high pressure ballast water cavitation techniques are generally used in combination with other systems.

Electric Pulse / Plasma Treatment?

The electric pulse /plasma for ballast water treatment is still in the development stage. In this system, short bursts of energy are used to kill the organisms in ballast water.

In the pulse electric field technology, two metal electrodes are used to produce energy pulse in the ballast water at very high power density and pressure. This energy kills the organisms in the water.

In electric plasma technology, high energy pulse is supplied to a mechanism placed in the ballast water, generating a plasma arc and thus killing the organisms.

Both these methods are said to have almost the same effect on the organisms.

A Typical Ballast Treatment Treatment System on Ships

Most of the ballast water treatment system use 2-3 disinfectant methods together, divided into different stages. A general ballast water treatment plant comprises of two stages with one stage using physical separation while the second stage employing some disinfectant technology. The choice of treatment system used in combination depends on a variety of factors such as type of ship, space available on the ship, and cost limitations as mentioned before.

A typical ballast water treatment system on ships looks like this:

Filter and UV

Short description

These systems filter the particulates and bigger organisms before UV disinfection. Usually, filters automatically back flush when a certain differential pressure is exceeded. The UV light kills or inactivates the organisms by disrupting their DNA, leaving them unable to perform vital cellular functions. During ballasting, the seawater is filtered and UV-treated, only to then be UV-treated a second time during de-ballasting.

These systems do not produce any harmful by-products and they are mostly independent of temperature and salinity. Different UV transmittance in the seawater will imply higher energy demand.

?Filter and electrolysis

Short description

These systems filter the particulates and the bigger organisms before active substances generated from the electrolysis are injected into the ballast water. The electrolysis can be installed inline or in a side stream, where the disinfectant breaks down the cell membranes of the organism.

The active substances are produced through oxidation of seawater in the electrolysis chamber. Electrolysis also produces hydrogen gas which shall be correctly handled for safety of the ship. During ballasting, the seawater is filtered and active substances are injected. During de-ballasting, the active substance is neutralized prior to discharge overboard.

?Ozone

Short description

These systems disinfect by injecting O3 generated from ambient air. The O3 oxidizes and neutralizes aquatic species by reacting with seawater to produce effective disinfectants. Salinity and temperature are not obvious factors affecting the efficiency of these BWMS, however longer holding time may be required.

Residual by-products are required to be neutralized prior discharge. Since ozone is toxic, additional safety measures and crew training are necessary.

?Chemical injection

Short description

These systems are often used in combination with filtration. A chemical solution is injected into the ballast water to ensure disinfection. The disinfectant may be liquid or granular and will require neutralization prior to discharge overboard.

Chemicals used are trademarked, and supply might be limited to specific ports. The chemicals must be stored on board in closed containers and may be hazardous. The use of chemicals requires implementation of strict safety provisions and crew training. These BWMS have a higher operational cost than other ballast water technologies.

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