Coal extraction in the Lorraine coal field ended in 2004, after 150 years of mining. Stopping of mine drainage pumping caused the flooding of 180 million m3 of mine cavities. After around 2 to 5 years of filling, pumping became necessary to keep pace with rising levels of iron‐containing water. The elevated levels of iron mineralisation in the mine water are such that the water cannot be discharged directly into the natural environment, making treatment a necessity. In the Lorraine coal field, the complete treatment system will comprise three treatment plants with a maximum cumulative flow rate estimated at 1,000 m3/h. The plants are of the passive type with the mine water flowing over aeration cascades into settling tanks and lagoons planted with reeds, before being discharged into the natural environment. Two plants are currently operating, one since 2009 the other since 2012. The La Houve plant, in operation since 2009, has provided experience over three years and this feedback enables a comparison with initial predictions. The first results show that the model for decrease in the iron concentration of the mine water used initially does not fit with the La Houve case. We have formulated a hypothesis allowing adjustment of the model to observations; other phenomena remain to be studied to explain the behaviour of the mine reservoir. Where treatment plant efficiency is concerned, 98 per cent of iron is removed on average and there is a seasonal pattern.