Mechanical dewatering of wet mined peat produced peat mining process water (PMPW) with low pH (5.49) and high total suspended solids (TSS 432 mg L−1), Al (1.39 mg L−1), Fe (4.36 mg L−1), Hg (37.1 ng L−1), MeHg (0.485 ng L−1), Zn (55 μg L−1), total nitrogen (TN 7.92 mg L−1), total phosphorus (TP 303 μg L−1) and true colour (532 TCU). High removal efficiencies were calculated for acrotelm peat filters (mesocosms) used to treat PMPW (TSS 45–83%; particulate organic carbon (POC) 47–89%; metals 53–100%; TN 85%; TP 81%), though final mesocosm leachate concentrations did not all achieve Canadian Water Quality Guidelines. In one study, mesocosm leachate had higher colour and dissolved organic carbon (DOC) concentrations than PMPW. Our mesocosm studies suggest an initial reduction in PMPW solids be considered by industry before peatlands are used as treatment systems.