If you really want to do it on a td5 I can show you how to set it up after you decide where to put your tank.

all you need is a 65psi intank fuel pump, (Id also suggest a fuel preheater in the tank) and a pair of electically operated single spool change over valves plumbed to suit the fuel lines (more filters go without saying)

hook it up like any other diesel change over system but when you switch to bio from normal fuel you hit both switches at once and when you turn back you turn the supply over first, let it run for a couple of minutes (make sure you dont have a full bio tank) then switch the return over which is essentially purging the lines.

finding places to put it all is going to be the challange.