* Heatwaves expected to return to China over next 10 days

* Manufacturing provinces vulnerable, power grid to be tested

* Red alerts issued by some cities in Zhejiang province

BEIJING, July 22

China will suffer the return of more heatwaves over the next 10 days from east to west, with some cities already issuing the highest level warning on Friday.

A sharp temperature spike is expected on Saturday, before building up into heatwaves, defined as periods of atypically hot weather of three days or more. This Saturday is the day of the 'big heat' in the Chinese Almanac based on the lunar calendar.

The hot spell is expected to be similar in scope as heatwaves from July 5-17, but more regions could be hit by temperatures of 40 degrees Celsius (104 degrees Fahrenheit) or higher, Fu Jiaolan, chief forecaster at the National Meteorological Centre, told state media.

Some cities in Zhejiang province, home to many factories and exporters, on Friday issued red alerts, the highest in a three-tier warning system, forecasting temperatures of at least 40C (104F) in the next 24 hours.

The load on the national power grid could reach a new high this summer, with safe operation facing 'severe tests', the Ministry of Emergency Management warned on Friday.

'For all of the factories in China and in Shanghai we have regulations that need to be followed,' said Leo Zhang, president of chemical product maker Sika China.

'Every year we do things to make the work more comfortable, for example giving workers ice-creams when it gets too hot.'

Zhejiang, as well as parts of Fujian, Guangdong, Hunan, Jiangxi and the city of Chongqing, also stand at risk of forest fires in the near term, the ministry said.

The heat in China this summer has been described as extreme, boosting demand for air-conditioning by homes, offices and factories and pushing the load on power grids in more than a dozen provinces to records.

From June 1 to July 20, the Yellow River and Yangtze River basins - major centres of industry and commerce - were hit by at least 10 high-temperature days more than the norm.

Since June, heatwaves have also scorched other parts of East Asia, Western Europe, North Africa and North America, sparking wildfires in many countries.

Scientists caution that climate change will only make heatwaves hotter and more frequent.

The highest-ever recorded temperature in China is a matter of debate.

According to Chinese media, the hottest period in the last 300 years was in July 1743 during the Qing dynasty, with a French missionary in Beijing said to have recorded an all-time high of 44.4C.

In 2015, a news portal in Xinjiang reported 50.3C at a weather station near Ayding, a dry lake in the Turpan Depression, one of the hottest places in China during summer.