After the adoption of the International convention for control and management of ship’s ballast water and sediment on 13th Feb 2004 by IMO, different ballast water treatment methods have been introduced.
One such method is the installation of Ozone generator system onboard ship with ballast system.
What is Ozone?
Ozone is a colourless oxidizing biocide with a pungent smell and is formed naturally in the earth’s atmosphere. It has very unstable properties and when injected in water decomposes rapidly.
It has been found that ozone is one of the most powerful and fastest oxidizers which oxidizes bacteria present in water like moulds, yeasts, organic material and viruses etc.
An ozone generator is a machine that generates ozone gas by using oxygen from the atmosphere.
The principle of the generator is as follows:
Energy from an electric discharge field is used along with ambient air which flows through the ozone cells responsible for converting and increasing the amount of ozone gas.
The ozone gas is then injected into the ballast water by an injector unit.
This ozone gas dissolves, decomposes and reacts with other chemical and kills organisms present in water and thus makes ballast safer to discharge in any ocean water.
Working of Ozone Generator
Ozone Generator produces ozone taking air or pure oxygen as the feed-gas source; by breaking apart O2 molecules into single atoms which further attach with O2 to form O3 (ozone).
Ozone generators generally use two methods to produce Ozone:
● Silent Corona Discharge: These machines use the electric discharge to split diatomic oxygen atoms into single atoms.
● Ultra-Violet Radiation: This process is similar to the process in which the sun’s UV rays split oxygen atoms into two. However, this method is considered less efficient than Silent Corona Discharge.
Classification of Ozone Generators:
Ozone Generator is classified based on:
- The Control Mechanism- Voltage or Frequency Unit
- The Cooling Mechanism- Water, Oil or Water+Oil
- The Physical Arrangement of the Dielectrics- Horizontal or Vertical
However, Ozone Generators manufactured by different companies have unique characteristics too.
Mechanism Involved in Using Ozone as a Disinfectant in Water Treatment System:
1) As ozone is a strong oxidant and virucide, it furnishes direct oxidation and destruction of the cell wall resulting in leakage of cellular constituents outside the cell wall.
2) Reactions with radical-by products of ozone competition.
3) Carbon-Nitrogen bond is broken which leads to depolymerization
4) Damage to constituents of nucleic acids- purines and pyrimidines
General Lay Out of the Ballast Water Treatment System
The basic layout of the system comprises an ozone generator, injector unit, line filter, and neutralizer.
A line filter is fitted before the ozone generator and the injector unit, whereas the neutralizer is fitted on the discharge line of the ballast system. The neutralizer comes in use when the water is de-ballasted from the ship.
Neutralization is done because ozone is a toxic gas and can harm humans. The gases that are not dissolved thus need to be removed by neutralizer before discharging back to sea
The Ozone generator system is more effective for the removal of larger organisms when combined with other treatment methods like electrolysis or UV methods.
The figure shows the working of the Ozone generator based Ballast system
Working of Ozone Water Ballast System
Ozone Generator generally works on applying voltage and sometimes voltage regulator is installed to maintain a specific range.
Ozone is fed into a down-flow contact chamber containing the wastewater to be disinfected after it is generated through Ozone Generator.
The main purpose of the contactor which is generally made up of stainless steel is to transfer ozone from the gas bubble into the bulk liquid while providing sufficient contact time for disinfection.
The commonly used contactor types diffused bubble (co-current and counter-current) are positive pressure injection, negative pressure (Venturi), mechanically agitated, and packed tower.
Ozone must be contacted uniformly in a near plug flow contactor as ozone is consumed quickly.
The off-gases released succeedingly in the contact chamber must be treated to destroy any remaining ozone before releasing into the atmosphere. Thus, it is essential to maintain the optimum amount of ozone for better efficiency.
When pure-oxygen is used as the feed gas it generates a gas stream that contains 3-12% of ozone by weight whereas the gas stream generated from air contains about 0.5 to 3.0% ozone by weight.
When pure oxygen is used as the feed gas, the ozone off-gases that are not used are either destructed or sent for recycling.
The key parameter that controls Ozone disinfection are as follows:
- Contact time
An ozone disinfection system aims for the maximum solubility of ozone in wastewater and disinfection entirely depends on the transfer of ozone to the wastewater.
The amount of ozone that will dissolve in wastewater at a constant temperature is a function of the partial pressure of the gaseous ozone above the water or in the gas feed stream.
Advantages of Using Ozone in BALLAST System:
● More Efficient than chlorine in destroying viruses and bacteria
● Water Treatment through Ozonation require a short time frame (10 to 30 minutes)
Disadvantages of Using Ozone in Ballast System:
● As ozone is a dangerous substance, the room where the ozone generator is installed should be equipped with an ozone detector.
● Ozonation is a more complex system that requires complicated equipment and a contacting system.
● Requires corrosion-resistant material such as Stainless Steel because it is highly corrosive and reactive.
Environmental Benefits of Using Ozone as a Disinfectant:
As the ozone is produced in situ and without the usage of any chemicals, it has a few environmental benefits which include:
● No formation of residuals or pollutants as ozone decomposes rapidly.
● Waste produced is safe to enter and waterways
● Wastewater can be used for other greywater applications
● No chemical is required as ozone is generated on-site which immediately eliminates the need and risk of transporting chemical drums.
● Further, it also eliminates the risk associated with the storage of chemicals and delivery complications.
You may also like to read:
- Reverse Osmosis: Modern way of Shipboard Water Production
- Ballast Water Exchange and Management Plan: Everything You Wanted to Know
- 10 Important Points to Comply With Ballast Water Convention
- Different Technologies For Ballast Water Treatment
- What is Ballast Water Management Plan?
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