The economic benefits of scrubbers have shrunk, with more countries restricting the use of open-loop types, creating market challenges for users and ­manufacturers on the road to IMO 2020.

The latest regulator to give the thumbs down is China’s transport ministry, which has banned the discharge of wash water from open-loop scrubbers at the country’s seaports and two main rivers.

The decision by the world’s largest seaborne trading nation follows comparable measures in Singapore, Belgium and some US states, amid growing concerns about how the wash water — containing sulphates — could affect the ecology of coastal and inland waters.