THE BIOLOGICAL TREATMENT OF ACID-MINE DRAINAGE UNDER CONTINUOUS-FLOW CONDITIONS IN A REACTOR

The use of the sulphate-reducing bacteria (SRB) process to treat a continuous flow of acid mine drainage (AMD) was investigated. The experiments were done in a 40 l reactor designed to simulate certain conditions found at the floor of a holding pit in which AMD is being accumulated. The basic considerations in the design of the reactor were: (1) the reactor contain a sediment; (2) the design allows for the water to pass over the sediment continuously and without drainage of the column; and (3) water samples can be easily withdrawn from different locations in the reactor. The hydrodynamics of the reactor with respect to the movement of sulphate was examined in the absence of SRB. The movement and distribution of sulphate in the reactor was found to be influenced primarily by the density of the sulphate rather than by the flow rate of water through the reactor. Consequently, sulphate accumulated readily at the bottom of the reactor. The movement of sulphate to the bottom of the reactor will, therefore, not be a limiting factor in the activity of SRB. A gravel bed was introduced into the reactor and seeded with a mixed population of SRB. Once the SRB were established, a continuous flow of AMD supplemented with nutrients was implemented. The response of the SRB indicated that for AMD with a pH of 4.8, a flow rate of 1.85 ml/min (HRT = 14 d) was adequate for treatment of the water. Good metal removal (> 90%) could be attained and the pH of the water could be elevated to pH 5 and above. The results demonstrated that the maximum flow rate or, minimum hydraulic retention time (HRT) required, is related to the pH of the water and that the treatment process would work best under conditions where retention times are longest. If the acidity of the water is increased, the flow rate must be decreased or if possible a batch or semi-continuous mode should be used. For example, if the pH is less than 4.8 then a HRT of 14 days was inadequate for treatment of water.