Evaluating longevity of vertical flow bioreactors through water quality analysis

Abstract

Past studies have shown the importance of carbon sources and microbial inocula on the effectiveness of microbially-based passive systems for acid mine drainage treatment. It is clear that a mixture of labile and recalcitrant carbon sources is necessary to have efficient, long-term metal removal and alkalinity generation in these systems. However, almost all studies have been performed in columns or batch experiments using known mixtures of fresh substrates, rather than comparing field-scale passive treatment systems operating for different periods of time. For this study, a batch experiment using substrate cores from existing passive treatment systems of different ages was completed. Passive treatment systems included Mayer Ranch, OK; Hartshorne, OK; Red Oak, OK and SR109, PA. Batch experiments were set up under controlled conditions and the experiment run for 168 days with a weekly change of influent synthetic AMD. In terms of pH, it was determined that the reactors received AMD with 5.5±0.4 initially. Influent dissolved concentrations were 181±14 mg/L Ca; 0.09±0.03 mg/L Cd; 67.8±8.3 mg/L Fe; 65.3±4.7 mg/L Mg; 7.3±1.0 mg/L Mn; 44.8±5.5mg/L Na; 0.26±0.08mg/L Pb; 3.9±0.6mg/L Zn; and 817±80 mg/L SO4. The oldest substrate (>10 years) had a decrease in pH to 4.9±0.3. However, pH increased with younger substrate age reaching 7.3±0.2 in the youngest substrate. Similar results were observed with alkalinity, where the older system had an alkalinity of 6.0±2.9 mg CaCO3/L eq. and the youngest provided 830±615 mg CaCO3/L eq. Younger systems had higher sulfate reduction rates when compared to older systems. However, the rate decreased over time of the experiment. Results suggest that the performance of reactors declines with age of substrate. Chemical characterization of the organic substrates will provide insight into processes that account for observed differences in water quality. Copyright © (2010) by the American Society of Mining & Reclamation.