April 22 (Reuters) - An Indonesian court is due to give a verdict on Tuesday in a pollution trial involving Newmont Mining Corp <NEM.N>. that is seen as a test of attitudes toward foreign firms and environmental protection.

The U.S. miner and the chief of its local unit, Richard Ness, are charged with dumping toxic substances into a bay near a now defunct gold mine on Sulawesi island, making villagers sick. They have denied the charges.

Here are some key facts on Submarine Tailings Disposal (STD), the process of disposing of mine waste into the sea through submerged pipes.

HOW DOES IT WORK?:

- Tailings are treated to remove the most harmful chemicals. They are then de-aerated and diluted with seawater in a mixing tank on the seashore. Pumped into the sea through a submerged pipe, the heavier-than-water tailings settle on the sea floor.

WHY IS IT CONTROVERSIAL?: - Mine tailings commonly contain sulfides as well as mercury, cadmium, nickel, chromium and arsenic. The sulfides oxidise when exposed to air; and if oxidised tailings then come into contact with water they produce toxic sulfuric acid.

- Critics say tailings on the seafloor can be spread by currents, storms and upwelling -- where deep ocean water moves to the surface. Tailings are also abrasive and pipes are vulnerable to breaks and leaks.

- The toxic elements can potentially contaminate water, damage coral, affect fish and marine organisms and through them, enter the human food chain.

HOW MUCH WASTE IS INVOLVED?:

- Open pit mining has a very high waste-to-product ratio - producing roughly 99 tonnes of waste for each tonne of copper, and far more in gold mining.

- Newmont's now defunct Minahasa Raya mine, for example, discharged more than 2,000 tonnes of waste a day into the ocean between 1996 and 2004 at a depth of 82 metres.

WHERE IS IT USED, AND NOT USED?:

- The submarine tailings process has been effectively banned in the United States and Canada by regulations protecting water and fish. It is not used in Australia because it's seen as geologically unsuitable.

- A 2002 Mining Watch report said it is being used in mines in Chile, Turkey, Britain, Indonesia, Papua New Guinea and the Philippines.

WHY IS IT USED?:

- The method is cheaper than land-based disposal, with no need for dam construction or long-term management of tailings.

- A 1993 U.S. Department of the Interior study estimated it resulted in an average 17 percent reduction in capital costs. Sources: Mineral Policy Institute Australia, (www.mpi.org.au/campaigns/waste/submarine_tailings_disposal) Mining Watch (www.miningwatch.ca/updir/01.STDtoolkit.intr.pdf)

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