Reagent-free Polishing Filtration of Mine Wastewater with High Fine Suspended-Solid Content on Granular Media Filters

This paper presents the results of exploratory and experimental investigations into reagent-free polishing filtration of mine wastewater with a high content of fine suspended solids (1000–1200 mg/L; particle size predominantly <15 μm) on rapid granular media filters operating in both downflow and upflow modes. The pilot apparatus comprised transparent acrylic (plexiglass) filter columns 3.6 m tall with a 100 × 100 mm cross-section. Real process water from a gold-mining operation, pre-clarified through a 50 mm hydrocyclone, was used throughout. The effects of media grain size (d = 1.0, 1.2 and 1.45 mm), filtration velocity (5, 10 and 15 m/h), initial suspended-solid concentration (400, 800 and 1200 mg/L) and flow direction on headloss dynamics, dirt-holding capacity and ultimate pore saturation were studied systematically. Headloss increase was found to follow a parabolic (non-linear) pattern, in contrast to the linear behaviour observed during filtration of dilute coagulated suspensions. The optimum grain size under the studied conditions was d = 1.2 mm at a filtration velocity of 10 m/h. Upflow filtration with a graded, stepwise-decreasing grain-size profile reduced the headloss build-up rate by 47