This is my experience with the PD fuel tanks and pumps. (I'll try to make this short.)
1st - 2004 Jetta, BEW engine, with "bad" lift-pump (leaving out a lot of the story). I took the pump out of the fuel tank. Yes, it was loaded with a black coating (tank, sending unit, and pump). The black stuff is soot. Yes, it may have some algae. However, algae is suspended in the fuel and doesn't coat anything. (I've seen the very fine in-tank strainer in the MK1 diesels totally plugged with slimy black algae (totally different from soot). Algae will not coat the tank.
Anyway, I cleaned up the lift-pump motor and got it working. I had to use a lot of carb cleaner, WD-40, Bolt Buster, etc., to get it working (reversed the current to help break it loose). However, the owner wanted a new pump (leaving out more of the story). IIRC, the lift-pump assembly unit was close to $300.00. Someone on this forum gave me a link for just the pump ($89.XX shipped). Installed and it worked great.
Here is a link to the above incident, short with one photo>
http://forums.tdiclub.com/showthread.php?t=460746
2nd - about two weeks ago, I had to pull the lift-pump unit in another 2004 Jetta, BEW engine, because the guy had put in a couple of gallons of RUG (gasoline). The lift-unit and inside of the tank had the same soot junk. From the engine, with compressed air, I blew out both fuel lines back to the tank. Then, I pumped all the fuel out (left about 1/2 pint) (zero water in this tank). So, I put it all back together, dumped in 5 gallons of diesel fuel, cycled the pump about 5 times, cranked the engine for 15 to 20 seconds and it began to come to life. Remember, the tandem pump (fuel pump and vacuum pump, thus tandem) and everything down stream to the injectors had a gasoline/diesel mix. But, after a few seconds it ran just fine.
Someone on this forum told me about the soot. Apparently, some soot (obviously very, very small amount) gets kicked back from the Injector Tips and flows back with the return fuel. And, I'm sure the filter catches a lot this soot as the fuel is pumped back up again and again! Thus, it probably could be far worse.
I'll try to find my pics and the Thread I started on the first experience and edit. (No Thread on the second experience but I do have pics of the tank.)
Hope this doesn't cause an uproar.