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The toxic sludge that poured through several communities in Hungary has reached the Danube River, an environmental official says.

The red sludge reached the western branch of the Danube early Thursday and was flowing into its broad main stretch by noon. By evening, it was moving southward toward Serbia and Romania.

The caustic red slurry was released Monday in Ajka, about 160 kilometres southwest of Budapest, after a reservoir burst at an alumina refining plant owned by Magyar Aluminium Termelo es Kereskedelmi (the Hungarian Aluminum Production and Trade Company), also known as Mal.

Canada's alumina refinery

The Hungarian site is a refinery that turns bauxite ore into an aluminum-based product called alumina. Rio Tinto Alcan operates the only alumina refinery in Canada, in Jonquière, Que.

A spokesman for the facility tells CBC News that the site uses a completely different process to store and safely dispose of the byproducts.

Unlike the wet sludge in Ajka, up to 90 per cent of the Quebec site's waste products are dried before being stored, making a flood such as the one that released one million cubic metres of waste in Hungary extremely unlikely, the company says.

The facility has withstood a flood and an earthquake over the past 25 years without incident, the company notes.

Alumina refineries typically produce up to two tonnes of red mud for every tonne of alumina created. Even after the spill, more than 30 million tonnes of red mud are still stored around the Hungarian site.

The effect of water makes a direct comparison between the Hungarian refinery and Quebec's refinery impossible, but the Quebec site produced 1.37 million tonnes of alumina in 2008.

The flood left four people dead, injured more than 100 and damaged several communities.

Rescue crews rushed to the scene to try to help the victims and contain the slurry before it reached the Danube, a major European waterway.

But emergency services officials said Thursday that alkalinity was elevated in the Raba River, which flows into the Danube, and that some pollution had been detected in the Mosoni branch.

Emergency spokesman Tibor Dobson said crews are trying to reduce the alkaline content of the slurry but he did not address concerns that there might be toxic metals in the sludge, The Associated Press reported.

"The alkaline content of the river is much lower than it was originally," Budapest-based freelance journalist Henk Hirs told CBC News on Thursday.

"By the time the pollution will have reached the proper river Danube, likely in a few days time, it will have been diluted even more," he said.

Hirs said that environmental officials believe that the lower concentration of sludge should limit the environmental damage to communities downriver.

But environmentalists have expressed concerns, particularly about the communities surrounding the reservoir.

Gabor Figeczky, acting CEO of World Wildlife Fund Hungary, said in a statement that it's impossible to tell how badly the environment was damaged by the sludge.

"It came with a high pressure because trees and fences have been knocked out," Figeczky said after a visit to the hard-hit town of Kolontar.

"I have come from a house in which the red sludge is waist-high. Everybody is wearing masks and gloves as they are shovelling the red sludge. The air is poisoned as well. It is very irritating to breathe in."

tp-sludge-soldiers-cp-95455

Hungarian soldiers work to shovel out a yard flooded by toxic mud in Devecser on Thursday. ((Darko Bandic/Associated Press))

Meanwhile, on Thursday, Hungarian Prime Minister Viktor Orban visited Kolontar.

He said he sees "no point" in rebuilding some of the houses in the same location made uninhabitable by the flood.

"It's a serious ecological catastrophe, but we don't know the size of it," Orban told reporters.

Local officials said 34 houses in the village of about 800 were so badly damaged by the caustic slurry that they cannot be refurbished.

Police in Hungary have opened a criminal inquiry into the flood.

Mal officials have insisted the sludge is not considered hazardous waste according to EU standards. The company has also rejected criticism that it should have taken more precautions at the reservoir.

With files from The Associated Press