A polluted river in Hebei province that turned red-brown this month is undergoing treatment and further analysis, said a local environmental protection authority.

The Zhongting River, 100 kilometers south of Beijing in Shengfang town, Bazhou, was polluted by wastewater containing iron ions released by iron and steel factories, said Hao Yongjun, a town official in charge of environmental protection.

According to Hao, about a third of the river started to become red-brown earlier this month.

"It's a long-term problem," he said.

Hao said the iron ions were discharged into the river before 2005, when there were no effective sewage disposal facilities.

The town has more than 130 iron and steel companies, according to its government website.

Mud with iron ions subsided to the bottom of the riverbed, Hao said, and recently became suspended in the water because of reactions caused by rising temperatures and spring runoff, making the water look red-brown.

"The sediment is hard to clear out. We have done our best to monitor the current drainage by heavy industries," he said.

The town has invested more than 100 million yuan ($15.4 million) on controlling river pollution since 2005, and has built a comprehensive sewage treatment plant to handle industrial and domestic wastewater.

Before sending wastewater into the treatment plant, factories have to treat it with their own facilities.

"The double handling procedures help the discharged water meet water quality standards," Hao said.

"Experts at the Ministry of Environmental Protection took a sample of the water for analysis on Wednesday, " Hao said, adding that they would adjust pollution-control measures after the results are released.

Feng Haibo, head of the Hebei Research Academy of Environmental Sciences, said it would be possible to clean the river through physical, chemical or biological methods, but that would cost a lot of time and money.

"Local governments haven't paid enough attention to controlling water pollution, not as much as air pollution. It's harder than controlling air pollution," he said, adding that weather factors, such as wind, may help air quality but do nothing for water.

"Water pollution control had just begun," he said.

Fang Xin, mayor of Ba-zhou, said earlier this year that water pollution control for rivers including the Zhongting River would be the priority of water quality work this year.

"Sewage treatment facilities in companies and waste water treatment plants will be under stringent supervision from this year on," Fang said.

Contact the writers at zhangyu1@chinadaily.com.cn and wangwei@chinadaily.com.cn