Bacterial mitigation of pollutants in acid drainage using decomposable plant material and sludge

Abstract

Effects of a clay-lime spiked sewage sludge and fresh decomposable ryegrass on the mitigation of an acid drainage were studied in the laboratory. Treatments (dry ameliorant weight/leachate ratio) were: (1) sludge (air-dried) at rates of 0, 8, 16 and 24%, (2) ryegrass at 0, 1, 1.5 and 2% (dry weight), (3) sludge (at the above-mentioned rates) and 1.5% ryegrass mixture. Measurements of mitigation (according to the criteria of changes in pH, Fe, S, Al and heavy metals) made every 10th day for 100 days showed ryegrass/sludge combination the most effective while sustaining mitigation longest, with or without the influence of sulphate-reducing bacteria (SRB). Sulphate and Fe in the acid drainage decreased in the order: sludge+ryegrass>sludge>ryegrass by 180, 40, 19; and 96, 83 and 54% respectively, compared with controls. An 11-fold decrease in soluble Al was caused by the highest rate of the combined sludge-ryegrass treatment but Al was doubled by the sludge-only treatment, and only minimally affected (2% reduction) by the ryegrass-only treatment. For the sludge plus ryegrass treatments at the highest rate of application, pH levels increased significantly, from 2.3 to >7 units and within 20 days of SRB activation, the concentration of Co, Cu, Mn, Ni and Zn decreased 3-, 15-, 90-, 3- and 50-fold respectively.