A bit more trap info:

Technically all Diesel exhaust stream pollution control systems are "aftertreatment systems" as they are not technically catalysts. The NoX brick has to be regenerated as it holds the NoX on the surface of the brick and otherwise will eventually plug. There are two "in use" technologies in use in the USA, one using urea and one using additional fuel. The chemistry differs but the result is the same. BTW - I'm named (WAY in the back) on the patent for the additional fuel method.

I say "in use in the USA" as there's another system requiring that the vehicle be returned to a service center and have the NoX brick cleaned there (I don't know what that technology looks like). The EPA decided aginst this as here in the States we can't even get people to bring their cars in for an oil change never mind an emissions service.

As of when I retired, VW chose the added fuel method for their lower HP engines and urea for their higher HP engines. I don't know their reasoning for this.

Dan