Acidic mine water drainage (AMD) containing heavy metals from abandoned mine and spoil heaps is a major concern as it may generate significant surface water pollution. Research performed across the main Romanian closed mining perimeters revealed a high variability of chemico-physical characteristics of the AMD flows. Their chemical matrices show various ratios between specific chemical species. The efficiency of the AMD treatment is influenced by these ratios. The priority for the AMD treatment is the pH adjustment and heavy metals removal (most often done by oxidation and precipitation with alkaline reagents). One of the inherent issues, regardless of the treatment method, is represented by the high volumes of amorphous precipitates that are formed. After the gravitational separation – settling, these sludges can represent up to 35% (typically 10–25%) reported to the reaction tank outfow fow-rate. The solids content of the sludge is between 0.5 and 3.0% dry solids. As the generation of acidic mine water will continue for decades or even longer times, the improvement of the treatment methods is a must. Results of sludge densifcation tests using conventional high density sludge (HDS) method are discussed here, in correlation with the AMD source and the effects on the treated water quality.