The Bureau of Mines has completed 4 yr of deep-bed filtration research comparing the efficiencies of granular magnesium oxide (mgo) and conventional filter sand in single- and dual-medium filters when filtering mineral-processing and mine waters. Bench and field evaluation tests were conducted using water types ranging from synthetic suspensions of asbestos, kaolin, metal hydroxides, and milled sand to process waters from magnetite beneficiation, iron ore flotation, granite cutting, and a steel mill. Even though the turbidity reductions were similar, mgo filters were found to be advantageous when filtering water pretreated with alum because the volume throughput before breakthrough (turbidity > 1 nephelometric turbidity unit) was much more for the mgo filter than for the sand filter. Reduced head loss owing to the greater porosity of the mgo filter beds is potentially the most beneficial advantage in employing mgo as the filter medium, especially when filtering water pretreated with polymer flocculants. Granular mgo (periclase) is durable and can be backwashed like sand. It is compatible with anthracite in a dual-medium filter and apparently is not poisoned by dissolved metals in the process water. No single solid-removal mechanism could be identified for the improved filtration observed with granular mgo.