The invention relates to an active pressure intensifier which comprises an axial piston pump having a housing in which a drum which is driven via a drive shaft for rotation and has at least two piston chambers is disposed, the piston chambers respectively having a liquid inlet and a liquid outlet and, in the piston chambers respectively, a piston with at least one piston rod being disposed. Likewise, a reverse osmosis plant which has this active pressure intensifier is provided according to the invention. Likewise, a method for changing the concentration of dissolved components in liquid solutions by means of reverse osmosis is provided according to the invention. The subject according to the invention is used in particular in sea- and brackish water desalination, in waste water treatment, in the foodstuff industry, in the chemical industry and in mining.