The International Organization for Standardization has approved a set of China-led global guidelines for testing and classifying sewage sludge for use on land. It's the first time an international sludge standard has been led by China.

Sludge, often called biosolids, is the organic mud left over after wastewater is treated. While it is packed with nutrients such as nitrogen and phosphorus that help plants grow, it can also hide dangerous pollutants. These include heavy metals, toxic chemicals, and germs that can harm the environment and human health if not handled properly.

The standard, approved in December, creates a clear, two-step safety check to ensure sludge can be reused without causing harm. In the first step, scientists analyze the sludge for high levels of heavy metals and harmful organic chemicals. If these pollutants exceed specific limits, the sludge is rejected and disposed of in an environmentally friendly manner. If the chemicals pass the first test, the sludge undergoes a second step involving living organisms. This includes "seed germination tests" to see if plants can still sprout in the soil and "reproduction tests" on earthworms to ensure the material does not interfere with their life cycles.

It is the first international standard to include "forever chemicals", known scientifically as per — and polyfluoroalkyl substances. These are man-made chemicals that do not break down easily in nature. The guidelines set a strict total limit on two specific types of these chemicals: PFOA, which is often used in nonstick pan coatings and firefighting foam, and PFOS, which is commonly found in waterproof and stain-resistant products.

To make the guidelines easy to follow, the standard sorts sludge into a three-grade classification system. This system determines where the material can be safely spread, ranging from public areas where people are likely to be present, such as parks and gardens, to remote landscaping areas that are inaccessible to the general public.

Xu Guoren, a professor at the University of Chinese Academy of Sciences and lead author of the standard, said the rules are the result of 20 years of research. Experts from countries such as China, Canada, Italy and Norway spent three years compiling and contributing data.

Xu said that the standard received a 90 percent approval rate from ISO member nations, which he said demonstrates a global recognition of China's solutions for recycling waste. He added that the core value of the rules is providing a creative technical route that covers everything from the initial assessment of the waste to its final disposal.