Water starts its journey to the ballast tank by being pumped onboard. It may go immediately to the filter or have to travel a distance across the ship to get there. Once it arrives at the filter, it goes through a prefiltration section that removes large particles. The smaller particles then enter the filtration chamber where organisms and dirt are trapped by the screen. As more and more particles build up on the screen, a sensor recognizes a difference in the pressure of the water coming into the filter and going out of it. This triggers the automatic self-cleaning cycle, where the dirt on the screen is sucked up by the nozzles and is dumped overboard. The pressure returns to normal and the filtration continues. At this point the water is ready to go to the secondary treatment stage to be neutralized before filling the ballast tanks.
Sometimes the water is filtered and treated after it has already been in the tanks, before being pumped overboard.