BEIJING, March 27 (Xinhua) -- China has made rapid improvements in its sanitary services in recent years by building wastewater treatment plants. Yet its 4,000 sewage treatment plants could pose a risk of secondary contamination if they are unable to dispose excessive sludge properly, according to a Chinese scientist Tuesday.

Sludge accumulates as a result of the treatment of domestic sewage and industrial wastewater. Once created, sludge can be highly toxic and needs to be treated before disposal.

"Over 90 percent of municipal sewage and over 50 percent of industrial wastewater are treated using an activated sludge (AS) process in China. But challenges such as high energy-consumption, frequent sludge bulking and foaming still remain, causing water contamination," said Qiu Dongru, a researcher at the Institute of Hydrobiology of Chinese Academy of Sciences.

The AS process was developed in England in 1914, and involves the production of an activated mass of microorganisms capable of aerobically stabilizing the organic content of waste water.

As a main wastewater treatment technology, the AS process has been adopted by most countries around the world, yet the key factor in the technology had been unclear.

To unveil this mystery, a research group led by Qiu studied the microbiome of AS and found that bacterial floc formation plays a central role in the process, allowing the sludge-and-effluent separation and recycling of AS.

Their findings also provided the first direct experimental evidence that PEP-CTERM proteins are required for bacterial floc formation. The study was published on the "Environmental Microbiology" website Monday.

Sludge can be reused if treated properly, which can be both environmentally beneficial and cost effective, according to Qiu.

"Our findings provided insights into the innovation of activated sludge-based technology and the efficient disposal and utilization of excessive sludge," Qiu said.