How does storage affect the quality and quantity of organic carbon in sewage for use in the bioremediation of acidic mine waters?

Pit lakes (abandoned flooded mine pits) represent a potentially valuable water resource. However, acid mine drainage (AMD) generation due to mining activities often results in pit lake waters with low pH, high sulphate and dissolved metal concentrations. Sulphate reduction-based bioremediation offers tremendous scope for removal of acidity and metals from pit lake water. In this study, the effect of storing sewage on its carbon quality for bioremediation of acidic pit lake water was studied. In addition, the effectiveness of labile organic carbon (lactic acid and ethanol) on SRB activity was tested. Bioremediation experiments were performed in controlled and replicated microcosms with acidic (pH 2.2) water from a pit lake by addition of stored (3 years at 4°C) sewage for stimulation of sulphate reducing bacteria (SRB) activity. This sewage had been previously used successfully in remediating to pH 7 water from this pit lake. The initial aim was to test the sewage at lower doses (18 and 28g/L) and in a pulsed addition (over 5 weeks). Bioremediation efficacy was evaluated by measuring pit lake water pH increase, redox potential decrease, and acidity and sulphate removal. Though the stored sewage had retained a very similar high total organic carbon (TOC) equivalent to prior to storage, it failed to increase dissolved organic carbon (DOC) levels in pit lake water. Microcosms amended with doubled doses of sewage and an extended remediation time still failed to demonstrate any substantial improvement in water quality, other than a small amount of sulphate reduction and direct neutralisation by the sewage. In order to determine if low DOC concentrations in sewage were the cause of the bioremediation failure, labile organic carbon (LOC), consisting of 50:50 (w/w) lactic acid and ethanol, was added to all microcosm treatments at concentrations of 3000, 6000 and 9000mg/L. After LOC addition, water quality improved with effective removal of acidity, sulphate and metals in the lowest carbon concentration (3000mg/L). However, 6000 and 9000mg/L LOC concentrations showed a delay in response due to the increased acidity associated with the lactic acid addition. The experiments showed that pulsed dosing of carbon simply slowed the commencement of remediation but it was ultimately able to reach the same effectiveness as the equivalent quantity added all at once. Prolonged storage of sewage leads to loss of LOC. In situ pit lake remediations which aim to make use of sewage as the main carbon source will need to factor in the storage time required to obtain sufficient sewage for the treatment into the design. Pulsing may help reduce issues with storage or supplementation with LOC may need to be considered. Results highlight that LOC is a more useful indicator of material effectiveness compared to a simple measures of TOC.