Equipment Guide · Ballast Water Treatment System
Ballast water keeps the ship upright and trimmed, but the water you load in one port is alive with plankton, larvae and bacteria that must not be dumped alive in the next. A ballast water treatment system (BWTS) cleans that water on the way in and on the way out so what leaves the ship meets the IMO D-2 discharge standard. Tap a step or press "Play" to follow the water from the sea chest, through the backflush filter and the UV reactor, into the ballast tanks — then treated again on discharge and out overboard. After that, work the simulator: push the pump flow, dirty the water and foul a lamp, and watch the UV dose cross the D-2 line yourself.
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A UV reactor kills by dose — the ultraviolet intensity hitting the water multiplied by the time the water spends under the lamps. That residence time is just the reactor volume divided by the flow, so the faster you pump, the less time each litre sees the light and the lower the dose. Every certified system therefore has a maximum treatment-rated flow: the pump rate at which the dose still lands on the D-2 line. Take the water it treats to a good dose at, say, 2,500 m³/h and drive the pump to 3,500 m³/h to save an hour, and the dose falls by roughly a third — straight through the line. Dirty water makes it worse: low UV transmittance and a fouled quartz sleeve both cut the intensity, so on a muddy day the rated flow is even lower. The dose is invisible; the flow gauge and the system's own dose read-out are the only things telling you whether the water is really being treated.
Set the flow, the water clarity and the lamp output, then press Start to run the treatment and watch the dose against the D-2 line.
About the simulator
The pump sets the flow; the flow sets how long the water sits under the lamps; the lamps and the water clarity set the intensity. Dose is intensity × residence time, and it must clear the D-2 minimum (150 mJ/cm²) or living organisms go overboard. Speed the pump up, dirty the water, foul a lamp — or open the emergency bypass and send raw ballast straight to sea. Keep the dose above the red line.
Dose limits, marked on the meter — D-2 minimum 150 · marginal < 172 mJ/cm². Below 150 the D-2 organism limit is breached and the discharge is illegal.
The D-2 standard caps living organisms in the discharge — fewer than 10 per m³ of the larger (≥50 µm) organisms. The UV dose sets how many survive: on the D-2 line the count sits right on 10; comfortably above it, near zero; below the line it climbs fast and the water is non-compliant even though it went through the reactor. Open the bypass and there is no treatment at all — raw ballast overboard, a serious Port State Control finding that must go in the Ballast Water Record Book.
Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; the exact standard, technology and settings vary by ship and maker. Reviewed and verified by a Master Mariner.