One of the most serious issues confronting the shipping industry is the presence of invasive aquatic species in ship ballast water. These aquatic species, which pose a significant threat to the marine ecosystem, have resulted in an alarming increase in bio-invasion.
The implementation of a ballast water management plan and ballast water treatment system on board ships has thus become critical under the IMO’s “International Convention for the Control and Management of Ship’s Ballast Water and Sediments.”
Several shipping companies have begun implementing ballast water treatment systems on their ships in order to ensure that their ships comply with the IMO’s rules and regulations regarding ballast water management.
There are numerous technologies available on the market for treating ballast water on ships. However, constraints such as available space, implementation cost, and level of environmental friendliness all play a role in the use of a specific type of ballast water treatment system.
When selecting a ballast water treatment system for a ship, several factors must be considered. Some of the most important factors considered are:
- Efficacy on ballast water organisms
- Crew safety
- Installation and operation simplicity
- On-board space is limited
The following are the primary types of ballast water treatment technologies on the market:
- Filtration Methods (physical)
- Disinfection Using Chemicals ( oxidizing and non-oxidizing biocides)
- Treatment with ultraviolet light
- Treatment with deoxygenation
- Heat (thermal treatment)
- Acoustic (cavitation treatment)
- Pulse plasma/electric pulse systems
- Treatment with a Magnetic Field
Onboard ships, a typical ballast water treatment system combines two or more technologies to ensure that the treated ballast water meets IMO standards.
Physical Separation/ Filtration Systems Ballast Water Treatments
Sedimentation or surface filtration systems are used to separate marine organisms and suspended solids from ballast water.
The suspended/filtered solids and waste (backwashing) water from the filtration process are either discharged in the area where the ballast is taken or treated onboard ships before being discharged.
For ballast water filtration, the following equipment is commonly used:
- Screens/Discs: Screens/Discs: With automatic backwashing, screens (fixed or movable) or discs are used to effectively remove suspended solid particles from ballast water. These are extremely environmentally friendly because no toxic chemicals are used in the ballast water. Screen filtration is effective for removing larger suspended solid particles but ineffective for removing smaller particles and organisms.
Note: It has been discovered that, while screens are highly effective at removing the majority of suspended solid particles and organisms from ballast water, they are insufficient on their own to treat the ballast water in accordance with IMO standards.
- Hydrocyclone: Hydrocyclone: A hydrocyclone is an effective piece of equipment for separating suspended solids from ballast water. To separate solids, water is rotated at high speeds using centrifugal force. Because a hydrocyclone has no moving parts, it is simple to install, operate, and maintain onboard ships.
Note: It has been discovered that because hydrocyclone operation is heavily dependent on particle mass and density, they are ineffective at removing smaller organisms from ballast water.
- Coagulation: Because most physical filtration methods cannot remove smaller solid particles, coagulation is used prior to the filtration process to join smaller particles together to increase their size. The efficiency of the aforementioned filtration processes increases as particle size increases. The treatment of coagulation of smaller particles into small flocs is known as flocculation. The flocs settle more quickly and are easier to remove.
Note: Some ballast water treatment systems that use coagulation and flocculation use ancillary powder (sand, magnetite, etc.) or coarse filters to produce flocs. For this process, an additional tank for treating ballast water is required, necessitating additional space onboard ships.
- Media Filters: Physical ballast water treatment systems equipped with media filters can also be used to filter out smaller particles. Compressible media filters (Crumb rubber) have been found to be more suitable for shipboard use due to their compact size and lower density when compared to conventional granular filtration systems.
Magnetic Field Treatment
Coagulation technology is used in magnetic field treatment. Magnetic powder is combined with coagulants and mixed into ballast water. This results in the formation of magnetic flocs containing marine organisms. These magnetic flocks are separated from the water using magnetic discs.
Ballast Water Treatments with Chemical Disinfection (Oxidizing and Non-Oxidizing Biocides)
Biocides (both oxidizing and non-oxidizing) are disinfectants that have been tested for their ability to remove potentially invasive organisms from ballast water.
Biocides are substances that kill or inactivate marine organisms in ballast water. To prevent discharge water from becoming toxic in nature, biocides used for ballast water disinfection must be effective on marine organisms as well as readily degradable or removable.
Biocides are categorized into two types based on their functions:
Oxidizing biocides are general disinfectants like chlorine, bromine, and iodine that are used to kill organisms in ballast water. This type of disinfectant works by destroying organic microorganism structures like cell membranes and nucleic acids.
Non-oxidizing biocides are disinfectants that interfere with the reproductive, neural, or metabolic functions of organisms.
Onboard ships, the following processes use oxidizing biocides:
- Chlorination entails diluting chlorine in water in order to kill microorganisms.
- Ozonation is the process by which ozone gas is bubbled into ballast water using an ozone generator. The ozone gas decomposes and reacts with other chemicals in the water to kill organisms.
Other oxidizing biocides used to kill organisms in ballast water include chlorine dioxide, peracetic acid, and hydrogen peroxide.
Though there are several non-oxidizing biocides on the market, only a few, such as Menadione/Vitamin K, are used in ballast water treatment systems due to toxic by-products.
Much research is being conducted in this field in order to make more non-oxidizing biocides suitable for use in ballast treatment plants.
Ultra-Violet Treatment Method
UV lamps surround a chamber through which ballast water is allowed to pass in the ultraviolet ballast water treatment method.
UV lamps (Amalgam lamps) emit ultraviolet rays that act on the DNA of organisms, rendering them harmless and preventing reproduction. This method has been used successfully around the world for water filtration and is effective against a wide variety of organisms.
The deoxygenation ballast treatment method involves removing oxygen from the ballast water tanks, causing the organisms to become asphyxiated. In most cases, this is accomplished by injecting nitrogen or another inert gas into the space above the water level in the ballast tanks.
Note: It usually takes 2-4 days for the Inert Gas System to asphyxiate the organisms. As a result, this method is typically unsuitable for ships with short transit times. Furthermore, such systems can be used on ships with completely sealed ballast tanks. If a ship already has an Inert Gas System, a deoxygenation system will not take up any additional space onboard.
The ballast water is heated to a temperature that kills the organisms in this treatment.
A separate heating system can be used 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 engine heat. However, such treatment could take a long time before the organisms become inactive, and it would also exacerbate corrosion in the tanks.
Cavitation or Ultrasonic Treatment
Ultrasonic energy is used to generate high-energy ultrasound in order to kill the cells of organisms in ballast water. These high-pressure ballast water cavitation techniques are typically used in conjunction with other systems.
Electric Pulse / Plasma Treatment
The electric pulse/plasma for ballast water treatment is still in the early stages of development. Short bursts of energy are used in this system to kill the organisms in ballast water.
Two metal electrodes are used in pulse electric field technology to generate an energy pulse in the ballast water at very high power density and pressure. The organisms in the water are killed by this energy.
A high energy pulse is delivered to a mechanism placed in the ballast water, resulting in a plasma arc and the death of the organisms.
Both methods are said to have nearly identical effects on organisms.