If you check the How it works bit on the PSA website, it explains quite
clearly the workings of the particle filter. What they empty out is most
certainly ash. The regeneration forces the system to heat up the exhaust to
a point where the soot which is clogging the filter is burned off. This
rarely has to be triggered manually as it is normally an automatic
proceedure. Eventually, after 50 to 80 k miles, depending on the age of the
vehicle, the filter has to be cleaned of the ash build up. This is done by
way of a service exchange. (Or, in some cases the dealer will try to get the
poor unfortunate owner to shell out for a new one).
You should not get a build up of the cerine fluid in the filter unless the
system is over supplying same to the fuel tank. (which does sometimes
happen).