As we all know, 90% of the world trade is carried out by the sea. It is estimated that vessels transport a whopping 10 billion tons of ballast water worldwide each year. Ballast water is used to maintain the stability of the vessel especially when the ship is carrying very little or no cargo. We pump in the ballast water when the cargo is discharged at a port (source port), carry this ballast water throughout the voyage till the ship reaches the port when it has to be loaded then discharge the ballast water after loading (destination port).
This ballast water may contain microorganisms, bacteria and fungus that are harmful to the local marine environment. To protect the marine environment regulation D2 of the ballast water convention was implemented. This convention made it mandatory for all the ocean-going vessels to have a ballast water management system that complies with the D2 standard. Hence the treatment of ballast water before discharging is important.
The ozone generator is one of the ballast water treatment methods used onboard. Let’s discuss this system and understand how it works.
What is ozone?
Ozone is just a tri atomic state of oxygen. The normal oxygen which is present in the air is in a stable state. When high energy hits the oxygen molecule the atoms break off and as a result, combine with other O2 molecules to form a triatomic molecule of oxygen O3 called ozone. This O3 molecule carries an extra atom thus it is always ready to give off the extra atom to come back to the stable state. It can give its extra oxygen atom to almost any and every substance. Ozone is: highly unstable i.e. it cannot be stored, highly reactive, and highly bondable. It is said that there is no better oxidizing agent than ozone on this planet.
Ozone as a disinfectant
Ozone is commonly used to disinfect water the process is termed ozonation. In this process, we induce ozone in the water to be treated to disinfect it i.e. we kill all the germs and bacteria which are present in the water. As we discussed earlier, ozone is highly unstable and reactive, it desperately searches for something to react with and this is what we do we make this highly reactive ozone reacts with the cells of microorganisms and bacteria which in turn totally distorts their cell structure neutralizing it hence it acts as a germicide.
How is ozone generated?
As ozone cannot be stored for a long time, we have to generate ozone near the ballast water inflow also it needs supply to the ballast water as soon as it is generated. There are mainly two types of ozone generators onboard.
1) Ultraviolet type ozone generators – The radiation from ultraviolet light can break molecules as well as distort DNA structure. It carries a lot of energy enough to break the O2 bonds. Then the nascent oxygen atom combines with bi-atomic O2 to form ozone. This happens only at a particular wavelength which is 185 nano meters.
2) Corona Discharge type ozone generators – As the ultraviolet type uses radiations to break the oxygen atoms, Corona Discharge type uses an electrical discharge. An electrical discharge is produced by ionizing the air and passing it over a conductor carrying a current of high voltage hence the oxygen breaks its bonds and combines with other O2 atoms to form ozone.
Ozone Generator Ballast Water Treatment System
The ozone generator ballast water treatment system is so designed that it can fit easily into the engine room of any ship type and size. The 4 main subsystems of this ballast water treatment method along with its components are listed below
1) Oxygen generator
- Air compressor
- Oxygen generator
- Oxygen Receiver tanks
2) Ozone generator
- Ozone generator
- Ozone destroyer
3) Ozone injector
- Circulating pump
- Jet nozzle
4) Monitoring and Control
- Ozone Concentration analyzer
- Total Residual oxidants or TRO monitor
1. When the system turns on, the air compressor starts and pumps air to the oxygen generator.
2. Oxygen the generator generates oxygen and stores it in the oxygen receiver tank.
3. This oxygen is then supplying to the ozone generator and the generator produces ozone.
4. The ozone destructor removes any excess ozone produced allowing a correct amount of ozone inducing in the ballast water.
5. Chiller helps to keep the system at subzero temperatures.
6. Ballast the water pumps turns on and some amount of ballast water moves into the side-stream injector with the help of a circulating pump where it infuses with ozone which injects from the injector.
7. This side stream feeds back to the mainstream by a jet nozzle
8. The microorganisms and bacteria get killed or neutralized by the ozone.
9. Monitoring and control system has an Ozone Concentration analyzer that controls the ozone dosage into the system and a total residual oxidant monitor which enables us to keep the residual oxidants within safe limits.
10. When ballasting is completed the system is turned off and purging is carried out by seawater.
Benefits of using Ozone as a disinfectant
· More efficient than chlorine disinfectant
· Requires a very short time frame for disinfection
· No chemicals are required hence eliminating risk associates with storage and transportation of chemicals
· No residual is formed and ozone is eco-friendly.