Pilot scale solids/liquid separation in hot geothermal discharge waters using dissolved air flotation

Abstract Hot geothermal discharge water, flowing through a pilot plant at 6 t/h, was continuously dosed with ferric sulphate, a flocculant and a surfactant; up to 89% of the resulting iron floc could be separated by dissolved air floation (DAF) in a total treatment time of six minutes. The aim was to coprecipitate arsenic (initial content 4.5 g/t) and inhibit silica scaling (by lowering the pH from 8 to 4), and provide a possible economic alternative treatment to re-injection. A commercial treatment plant would require an additional polishing filter for optimum operation. After filtration, 98% of the arsenic present in the water was removed by dosing with 17 g/t iron, and 93% of the arsenic was removed with 10 g/t iron. Several methods for recovering arsenic from the sludge and re-using the iron were examined; the most promising involves washing the sludge with dilute sodium hydroxide and precipitating calcium arsenate. The present pilot plant was also used for separating precipitated calcium silicate from geothermal waters. In hot water DAF appears to be a more successful solid/liquid separation system than settling (sedimentation). DAF operates faster, and produces sludges with higher solid contents, than settling.