I forget the name of the liquid, but it's damm expensive.
It also has a nasty side effect, well certainly on my previous car.
Some cars don't need the additive - the engine gets hot enough to start the regen. Some small engines don;t, so the catalyst is needed to kick-start the process.
Certainly on my previous volvo c30 - the additive leaves a layer of ash which can't be burnt away, this builds up and the complete DPF requires repacement at about 75000 miles.
Nice little £1000 bill, + the extra £100 or so every 3 years for the liquid top up, that I wasn't told about when I bought the car. It was one of the main reasons I sold it and went back to Mini - though in the 4 years I had it the DPF worked perfectly, but then again, I didn't do short journeys.