News To Me, Y.M....
I know there is a fuel/water separator that is used in some diesel engines on commercial vehicles, but they usually just have a spin-on dessicant filter that gets changed regularly, or an automatic system that removes the trace water.
Maybe it's that you are in humid, sub-tropical Southeast Asia, and we're here in a mostly non humid climate, but I have never seen, heard of, or actually had to "bleed" water out of a diesel fuel system. Whatever condensation develops is usually so minor that it's burned up in the combustion process. Aside from that, the EPA would be all over the commercial transportation industry for the amount of diesel fuel finding it's way into ground water from the millions of trucks in this country "bleeding" their fuel systems.
And black smoke is not produced by having water in diesel fuel. It's produced by poorly trained drivers who mash down too hard on the throttle when taking off from a dead stop. If your company is having black smoke problems with your buses, then you need to train your drivers not to "overfuel" the engine by using a less "aggressive" throttle technique.
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."