HOUSTON, Sept. 1 (Xinhua) -- At least 12 sewage overflows and toxic chemical spilling from about 36 industrial facilities in Houston area have been reported, according to authorities on Friday.

At least one more container of volatile organic peroxides exploded Friday afternoon at the Arkema chemical plant, which is about 40 km from downtown Houston.

The plant was knocked out of power by floodwaters on Monday. It needs electricity to keep volatile chemicals stored at the facility cool. The first explosion was reported on Thursday.

The chemicals at risk -- organic peroxides -- are stored in nine 18-wheeler box vans with 36,000 pounds each. Once they lose refrigeration, they generate heat that causes them to degrade and burn.

Authorities cautioned nearby residents to stay inside, turn off their air conditioning, and close their windows and doors. All employees at the plant had been evacuated by late Tuesday, as were residents nearby.

About 36 industrial facilities have reported toxic chemical spills to the National Response, and 13 of these incidents saw spills into adjacent waterways. An unknown amount of polychlorinated biphenyls (PCBs) was involved in the spills.

Meanwhile, according to Environment Texas, a statewide nonprofit advocacy group, up to 2 million gallons (about 760 million liters) of sewage have been released in previous storms with only 10 inches (about 25 cm) of rain or less, so Harvey's higher rain totals could cause an overflow of millions of gallons of sewage.

Tropical cyclones in general are very efficient rain producers, because they draw large quantities of water vapor into the atmosphere from a warm ocean. Tropical Storm Harvey has set a record for rainfall with almost 1.4 meters in the continental United States, according to the National Weather Service.

Harvey made landfall as a Category 4 hurricane on Friday, but has since been downgraded to a tropical storm and moved towards east on Tuesday.