The deal values the entire unit at $99m.
Severn Trent said its share will be settled by a cash payment of $20.1m and the rest via the settlement of net intercompany debt.
The transaction is expected to be completed by the end of June.
The water purification business delivers disinfection and filtration technology solutions for the worldwide onshore and offshore energy, marine and municipal markets.
The gross assets of the business were £103.6m as at 30 September 2014. A loss before interest and tax of £39.2m was reported in 2014 including a goodwill impairment charge of £24.6m.
Severn Trent chief executive officer Liv Garfield said: "This sale will allow us to focus on our core water and waste water service businesses in the UK and abroad.
"We remain committed to our Business Services portfolio, which is focused on delivering growth in water and waste water services in the UK and internationally."
Industrie De Nora chief executive officer Paolo Dellachà said: "Water treatment has always been in the DNA of our company, and a substantial portion of our turnover is generated by sales of products addressing water issues.
"This acquisition certainly enhances the value creation for all customers in need of sustainable solutions for their water treatment requirements and for all our other stakeholders."
De Nora and the water purification business are currently implementing various joint projects across several key market requirements like ballast water treatment, re-use of water used in fracking processes, and denitrification of wells for drinking water.
