Invasive Aquatic Species in ship’s ballast water is one of the biggest problems faced by the shipping industry. Posing a great threat to the marine ecosystem, these aquatic species has led to an increase in bio-invasion at an alarming rate. Under IMO’s “International Convention for the Control and Management of Ship’s Ballast Water and Sediments”, implementation of ballast water management plan and ballast water treatment system on board ships has thus become important.
In order to ensure their ships comply with the rules and regulations set by IMO regarding Ballast Water Management, several shipping operators have started implementing ballast water treatment systems on their ships. A variety of technologies are available in the market for treating ballast water on ships. However, constraints such as availability of space, cost of implementation, and level of environmental friendliness play an important role in usage of a particular type of ballast water treatment system.
A number of factors are taken into account for choosing a ballast water treatment system for a ship. Some of the main factors taken into consideration are –
- Effectiveness on ballast water organisms
- Safety of the crew
- Cost effectiveness
- Ease of installation and operation
- Space availability on board
- Ship Deballasting
The main types of ballast water treatment technologies available in the market are:
- Filtration Systems (physical)
- Chemical Disinfection ( oxidizing and non-oxidizing biocides)
- Ultra-violet treatment
- Deoxygenation treatment
- Heat (thermal treatment)
- Acoustic (cavitation treatment)
- Electric pulse/pulse plasma systems
- Magnetic Field Treatment
A typical ballast water treatment system on board ships use two or more technologies together to ensure that the treated ballast water is of IMO standards.
Physical Separation/ Filtration Systems Ballast Water Treatments
Physical separation or filtrations systems are used to separate marine organisms and suspended solid materials from the ballast water using sedimentation or surface filtration systems. The suspended/filtered solids and waste (backwashing) water from the filtration process is either discharged in the area from where the ballast is taken or further treated on board ships before discharging.
The following equipment is mainly used for ballast water filtration:
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.
Ballast water filters
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 remove or inactivate 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 biocides are general disinfectants such as chlorine, bromine, and iodine used to inactivate organisms in the ballast water. This type of disinfectants acts 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.
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, per acetic acid, and hydrogen peroxide are also used to kill organisms in the ballast water.
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 act 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.
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.
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 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.
Several new technologies to ballast water on board ships are entering the market every month.