With the completion of a pilot project in March to identify polluting wastewater outlets along the Yangtze River, the country's top environmental watchdog is turning full-bore to satellite remote sensing and unmanned aerial and surface vehicles as it expands the process to the river's major tributaries.

The Ministry of Ecology and Environment will undertake a comprehensive investigation of wastewater outlets covering nine tributaries, including the Minjiang, Chishui and Ganjiang rivers, over the next two years, in addition to its further investigations of the main channel of the Yangtze and Taihu Lake.

Yang Haijun, an expert in the ministry's satellite environment center, said satellite remote sensing helps in defining the area for the investigation because of its capacity to see the distribution of population and enterprises.

Before the pilot program in Taizhou, Jiangsu province, for example, satellite remote sensing and unmanned aerial vehicles equipped with infrared imagers were sent to find suspected outlets.

Guo Qingwei, an expert at the ministry's South China Institute of Environmental Sciences, said field investigations are needed to discover outlets on the surface where there is no temperature difference, and sonar can be used to find underwater ones.

"The trial found at least 10 underwater outlets, though many of them are not currently in use," Guo said.

Gong Xianjun, an official with the ministry's law enforcement bureau, said one of the key tasks for the pilot projects in Chongqing and Jiangsu is to train environmental officers.

"As new technologies emerge, our officers need to learn about these technologies and know how to handle them before they are put into wider application," he said.

After the comprehensive two-year investigation along the Yangtze and its tributaries, the ministry will monitor the outlets before it traces pollution to its sources and moves to control it.

The government has emphasized protection of the Yangtze since early 2016, when President Xi Jinping urged the 11 provincial regions along the river to concentrate on ecological restoration and protection and to avoid large-scale development.