Each morning, a group of Filipinos rakes up piles of trash on the banks of one of the world's most polluted rivers, filling sacks in an endless pursuit to clean a waterway that is also a major source of ocean plastics.

These "river warriors" are a decade-old group of about 100 people who work to clear the glut of garbage floating or washed up along Manila's notorious Pasig River.

The 27km river cutting through the Philippine capital was once a vital trade...