Bioremediation of acid mine drainage using decomposable plant material in a constant flow bioreactor

Under stagnant conditions, the ability of 15 earth materials (non-lime) including various inorganic 2:1 and 1:1 layer silicates, an amorphous oxide, and two 'whole' soils were tested for their pH-buffering efficiency in an acid mine drainage (AMD) water. The purpose was to decrease AMD acidity to a level where sulfate-reducing bacteria (SRB) placed in it may be activated. Of all materials, a whole soil (a high cation-exchange capacity clayey mollisol containing 40% clay, and 4% soil organic matter) caused the greatest pH increases from 2.5 up to 5.5 units after 10 days in the AMD water. Influent AMD was then ameliorated at various speeds through an SRB driven bioreactor using a 50/50 weight over weight (w/w) combination of the mollisol and ryegrass (MR) as the pH buffer substrate. This substrate combination decreased the SRB acclimatisation period (from 50 days in a previous experiment utilising sludge + ryegrass) to <10 days in the present experiment. After causing pH increases from 2.8 to >6 units in 5 days, the buffer reduced the hydraulic retention time (HRT) of the constant-flow reactor from 12 days at flow speeds of 100 ml/day to 2 days at 25 ml/day, respectively. After 10 days, soluble Fe, Al and sulfate were all decreased >1,800-, >40- and 3-fold, respectively. This was a more efficient performance than the no-flow bioreactor of a previous experiment using sludge + ryegrass. This method of AMD rehabilitation is an alternative for localities that lack cheap sources of calcium compounds for chemical treatment, but have a similar soil type and copious quantities of fresh decomposable plant wastes.