But such an effort would be costly and some experts question whether the central government would be prepared to commit sufficient resources to the funding. They also wonder if replacement is the most efficient solution.

Water safety on the mainland has come under scrutiny following a tap water scare in Lanzhou, Gansu province, last week. Benzene, which can cause cancer, was discovered in tap water at levels 20 times higher than national limits. An investigation continues, but lax supervision, the proximity of petrochemical facilities and corroded supply pipes have all been identified as possible causes.

Professor Guan Yuntao, associated with a laboratory in Shenzhen under the auspices of Tsinghua University, has been carrying out government-funded research on micro-organisms in urban water networks for several decades. The organisms, together with organic compounds and heavy metals, are the main contributors to unsafe drinking water. In some cities, membranes consisting of a variety of species can form on the inside surface of pipes – much like plaque build-up in human arteries – “in a matter of days”.

Parts of these colonies occasionally break off and interrupt the flow of water, but often stay put and “chew” on the pipeline from the inside out.

The organisms release corrosive waste materials that significantly accelerate the ageing process of the pipe network, Guan said. Leaks are a common problem and in some cities up to half the supply is wasted due to breaches in the distribution network.

Scientists have also expressed concern about the presence of pathogens, a class of organisms that cause disease. More than a dozen officially backed research projects in recent years have discovered a range of pathogens including Legionella, a bacteria that can cause deadly outbreaks of pneumonia, Guan said.

The professor was not permitted to reveal a full list of “sensitive micro-organisms” that had been uncovered, due to the risk of creating panic among the public. The testing methods employed also faced constraints because they could identify only a relatively small number of the organisms present in the pipelines. Other pathogens could have gone unnoticed, Guan said.

The rising living standard on the mainland had raised public expectations for water hygiene, he said, and the government felt pressure to raise standards to be on a par with developed neighbours such as South Korea and Japan.

New national standards for drinking water quality were issued in July 2012 and are expected to be enforced next year; they include more than 100 items from the WHO guidelines concerning organic substances, microbes and purification levels. Requirements for indicators such as arsenic, chromium and lead were also stricter, said Zhang Lan, deputy director of the water office under the China Centre for Disease Control and Prevention.

Another option to improve quality is to replace urban drinking water networks, researchers say.

“The water research community and related industries are all excited,” Lan said. “Judging by the progress so far, a national pipeline replacement campaign will probably begin in the next five to 10 years, which will cost more than 10,000 yuan [HK$12,600] per metre.” For Beijing alone, with more than 9,000 kilometres of pipeline, the cost of replacement could exceed 90 billion yuan.

The capital consumed 3.6 billion cubic metres of water last year, according to municipal authorities.

Most pipelines on the mainland are made of cast iron, which makes them vulnerable to chemical erosion resulting from the micro-organisms, among other agents. The new pipelines would be made of either steel or plastic with high-tech coatings that reduce the speed and amount of organism growth.

Professor Liu Jingqing, who specialises in urban water supply at Zhejiang University and who leads a government research project on pipeline bio-film, said tackling the issue was difficult given the many questions that remained unanswered.

“Pathogens have been found in pipelines in every city [on the mainland], but we are still struggling with uncertainties – how to identify them, their distribution, where they come from and at what level do should they become a public health crisis. Our research is only a few years old,” he said. “We have carried out simulations in the laboratory, but we find the situation in real pipelines is far more advanced and serious than what is estimated using theoretical models.”

A popular method worldwide to suppress the growth of organisms is by adding germicidal chemicals such as chlorine to the water. But the amount needed to be effective on the mainland would be so great that consumers would be scared off by the strong odour.

Liu was sceptical that a massive effort to replace pipelines would solve the problem. “Germs stay and proliferate in even the most advanced pipelines. So far there is no technology that can create a completely germ-free environment,” he said.

“Some countries are using pipelines that are more than a century old, but still provide clean water. Even if all the pipelines in China are replaced, the germs will return sooner or later. So the real problem is not the germs, but us.”

Professor Yu Xin, a water safety expert at the Chinese Academy of Sciences’ Institute of Urban Environment in Xiamen, Fujian province, said micro-organisms in mainland pipelines thrived because of the “abundant food”.

The presence of assimilable organic carbon, the favourite food of micro-organisms, was higher in China’s urban water system than in Western countries and contributed to the rapid growth of colonies in pipelines.

Large amounts of organic carbon are present in sources of natural drinking water and can squeeze through most filters at water treatment plants.

Though treatment plants in large mainland cities such as Beijing and Shanghai used some of the most advanced technology from the West and had reduced bacterial counts to very low levels, no technology could effectively remove the enormous amount of organic carbon.

Agricultural run-off and residential waste in landfills near drinking water sources often go undetected, but government inspectors are overwhelmingly concerned about industrial polluters. For the government, the most cost-effective solution would be to protect drinking water sources from pollution, Yu said.

“Replacing pipelines is extremely costly and very disruptive to urban life with all the digging and street closures,” he said. “Providing Western-quality drinking water directly from the tap will require an enormous investment and advanced technology.

“Even if all the obstacles are overcome and tap water becomes safe to drink, few Chinese citizens will trust it, and water will still be boiled before drinking.”