The Effects of Sulfate on the Physical and Chemical Properties of Actively Treated Acid Mine Drainage Floc

It is important to consider floc properties when designing acid mine drainage treatment (AMD) systems. Relatively few studies have evaluated the effects of neutralizing base, neutralization pH, and sulfate in solution on floc properties in active treatment systems. We used NaOH and NH₄OH as neutralizing bases, 0:1, 2.5:1, and 5:1 SO₄:Fe molar ratios, and neutralization pH of 7, 8, and 9 in laboratory studies. Neutralizing cation, sulfate content, and neutralization pH had significant effects on floc mass and volume, but SO₄:Fe ratio was the most important parameter. Settled floc volumes were slightly larger in the sodium system. Floc mass and volume both decreased with increasing pH. Floc generated in the presence of sulfate required significantly more time to reach a total suspended solids discharge limit of 70 mg L⁻¹, had slower initial settling rates, and smaller settled volumes than floc generated without sulfate. The systems we studied were less complicated than actual AMD, but understanding the effects of sulfate, neutralizing cation, and neutralization pH on floc properties may help to design more efficient treatment systems. Choosing the appropriate treatment chemical and designing adquate pond sizes will ultimately increase treatment efficiency and improve stream water quality.