Doughty Hanson has pledged to return funds to investors after raking in profits from the sale of a division of a Dutch purification technology group to a US industrial group for €503m, according to two sources familiar with the matter.

Doughty Hanson, which acquired Norit in 2007 in a deal valuing the company at about €650m, has sold Norit's Clean Process Technologies division to US industrial group Pentair, one of the sources said.