A minewater treatment plant. The flood prevention system installed when operations ceased at the Creutzwald mine combines a pumping station, water pipes and a passive minewater treatment system made up of a series of settling tanks and lagoons. The photograph shows a settling pond at the disused Creutzwald mine (Moselle, NE France, 2012).© Laurent Mignaux - MEDDE

The Vouters Pit: new mine water treatment plant ready to go !

08.10.2016
The third of its kind in the Lorraine coal basin, the mine water treatment plant at the Vouters site in Moselle is an improved version of an environmentally compatible passive treatment system. This is a typical example of the work carried out by the BRGM to address new environmental surveillance issues.

For many years, the Vouters mine, which opened in 1855 in the  municipality of Freyming-Merlebach in Moselle (E France), was one of the main coalmines in the  Lorraine coal basin.  The Vouters pit, the deepest in France at 1 327 metres, was active  from 1962 to 2003.  After its closure, safety work was completed in
2006 and the mine company continued to operate the  plant pumping mine water from the  central sector.

The Lower Triassic sandstone aquifer underlies the  entire Lorraine basin,  reaching down as far as the  impermeable Permian formation beneath. The carboniferous formation from which the  coal was extracted is older than the Permian layer, which was fractured during mining operations, causing water from the aquifer to filter through into the mine workings. The water was pumped continuously to the surface so that mining could continue. When  the mine closed, pumping also ceased, leaving the water to flood the underground chambers.

The settling ponds where iron oxides and suspended matter sink to the bottom. © BRGM

The settling ponds where iron oxides and suspended matter sink to the bottom. © BRGM

An improved environmentally compatible passive treatment system

To maintain the  quality of the  groundwater, it therefore became necessary to infiltrate water "artificially"  into  the  mine reservoir to prevent the formation of mineralised plumes in the  water  and to control its rise to the  surface, which  is endangering derelict facilities around the mine. The Vouters treatment plant is the third of its kind in the Lorraine coal basin.  Its purpose is to treat the  mine waters to reduce their iron and manganese concentrations before they are released into  the environment. The plant uses  an environmentally compatible passive treatment system.

A pumping station.© BRGM

A pumping station. © BRGM

Cascade aeration, settling ponds and lagoons

The mine water pumped out of the  reservoir is characterised by high concentrations of dissol- ved  iron  and  manganese leaching out from iron pyrites. Iron and manganese are both water-soluble and  precipitate after aeration, clouding the  water with typically orange-coloured particles of iron hydroxide.

The treatment plant reduces these iron and manganese concentrations to the permitted statutory levels (1 mg/l for manganese and  2 mg/l for iron). The treatment process at Vouters is referred to as "passive", since the oxidised particles settle naturally as the water moves through the different stages.

The mine water  is first channelled down  a series  of four aeration cascades, which allow for flexibility in the operations.Studies by  the  post-mining unit for  Eastern France  (UTAM Est) have produced innovations in this  area. 

For example, the  cascades have hollow steps to improve aeration and  reduce turbulence. The water then flows into  settling tanks where the iron  hydroxide and other suspended matter fall to the  bottom. The mine water, with  a much reduced iron concentration, is then channelled into lagoons. These are planted with reeds  whose  extensive root systems complete the passive treatment process by filtration and  oxidisation. By using broad nozzles, the  water is evenly sprayed across the  surface of the tanks and lagoons, which  improves the settling process. After about two days of passive treatment, the  purified water can  be released into the  environment, after  tests for quality and quantity at the lagoon outlets.

Cascades with hollow steps that improve aeration and the precipitation of iron particles. © BRGM

Cascades with hollow steps that improve aeration and the precipitation of iron particles. © BRGM

Results  are very promising and the plant is now treating the mine water at a rate of 200 m3/h, which will increase to 500 m3/h in future. Treatment efficiency is fully in line with expectations. New features are in place to guarantee safety during the periodic sampling and maintenance operations undertaken by the plant managers. 

BRGM - 3 avenue Claude-Guillemin - BP 36009 45060 Orléans Cedex 2 - France Tel.: +33 (0)2 38 64 34 34