Once upon a time this river was filled with fish; now, nothing swims in it any more,” says Wat Chak Daeng temple’s abbot, Phra Mahapranom Dhammalangkaro, as he looks out over Bangkok’s Chao Praya River.

As a novice monk in the 1980s, he remembers seeing children playing in the river and people scooping up handfuls of water to drink. But when he became abbot of Wat Chak Daeng more than 25 years later, those bucolic images were a thing of the past. Instead, when he arrived at the 240-year-old temple, he was saddened by the sight of the dirty river and the rubbish-strewn grounds surrounding it.

Dhammalangkaro knew that if nothing was done, the situation would only get worse. He built a recycling centre in the temple grounds, which evolved from collecting a handful of bottles to upcycling 300 tonnes of plastic a year. His biggest problem was that he was unable clean the river itself.

But then he met Tom Peacock-Nazil, chief executive of Seven Clean Seas, an organisation that finds solutions for plastic pollution. Last week the two men launched the Hippo, a solar-powered boat, which aims to remove 1.4m kilos of plastic a year from Bangkok’s busiest waterway.