Characterisation and improvement of a microbiological anaerobic bioreactor to remediate acidic and metalliferous titanium processing leachates

Wastewaters and leachates from mining and mineral processing are often characterised by low pH and high metal and sulfate concentrations. These can affect water catchment ecosystems and impact food webs deleteriously. In northwest Tasmania a titanium-processing plant operated between 1948 and 1996 on the Blythe River catchment. The sulfate extraction process was used to manufacture titanium dioxide pigment. This produced large quantities of acidic (pH 3 - 5), metalliferous (Fe 800 - 1200mg L -1 , Mn 15 - 45mg L-1 ) and sulfurous (SO42- 500 - 1700mg L-1 ) wastewater. The wastewater was pumped into sludge dams, which leaked into the local catchment, and eventually Bass Strait. This resulted in elevated metal concentrations and a highly visible red plume along the coastline, locally suppressing the marine benthic biota and altering biodiversity. A novel in situ bioremediation system was built in 2001 and managed subsequently by ESD (Environmental Services and Design) Pty. Ltd. The system comprised anaerobic processing sections that incorporated waste agricultural products; including potatoes, spent mushroom compost and straw, to remove the metals and increase the pH. A subsequent artificial wetland system was utilised to reduce effluent BOD (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) to drinking water quality levels. Using 16S rRNA gene based approaches microbial diversity and community structure was determined over 18 months from different stages of the treatment system. This included untreated sludge dam leachate, pre-treated effluent from an anoxic potato-containing section intended to increase alkalinity and prevent iron precipitation; and effluent from a series of mushroom compost and straw-based iron-reduction cells. 16S rRNA gene clone libraries revealed a community shift from a mixed iron- and sulfide-oxidising and iron- sulfate-reducing community in the sludge dam leachate to a community dominated by Acidithiobacillus spp. and anaerobic fermenters (related to the genera Bacteroides and Paludibacter) in the potato cell effluent. The reduction cell effluents proved to have higher microbial diversity and greater heterogeneity, including iron-and sulfate reducers, ironoxidisers, anaerobic fermenters and in one sampled effluent a high proportion of clones clustering with previously uncultured organisms of candidate division 0P3. Multivariate statistical analysis of 16S rRNA gene-based TRFLP (Terminal Restriction Fragment Length Polymorphism) data revealed community differences had occurred between treated/post-treated samples and untreated/pretreated samples. TRFLP analysis also indicated temporal shifts in the bacterial community composition occurred in the reduction cells. Although after 11 months of treatment, microbial communities in three of four reduction cells showed evidence of stabilisation probably due to exhaustion of an available carbon source and layered design of the system. There was no evidence of a seasonal effect on the microbial community. A series of laboratory-scale microcosm experiments were conducted to evaluate temperature, bicarbonate and various carbon amendments (ethanol, molasses and vegetable oil emulsion) for bioremediation of an acidic, metal- and sulfate-rich titanium processing leachate with the goal of optimising an existing field-based system (described above). In all microcosms pH increased from 4 to 6.5-8 for the length of the experiment due to the high organic matter input but had no effect on other geochemical processes which was similar to the field-scale reduction cells in their first year of operation. The oxidation-reduction potential decreased in all microcosms but was most stable in the oil emulsion microcosms. Alkalinity production was more substantial in the ethanol, molasses and oil emulsion microcosms (-2500mg L-I ) compared to the temperature and bicarbonate - microcosms (-600 - 1800mg L-1 ). The addition of bicarbonate did not increase pH or alkalinity. Iron and sulfate were initially removed but the effect could not be sustained in the unamended and bicarbonate microcosms. Liquid amendments such as ethanol, molasses and vegetable oil emulsion were found to support greater iron removal. However, sulfate removal only reached a maximum of 80% removal and was found to have a lag phase of approximately 80 days, and hence an acclimatisation stage may be needed for enhanced sulfate removal in fieldscale bioremediation systems. 16S rRNA gene sequence-based TRFLP profiles revealed all the microcosms had similar bacterial communities but the amended microcosms were more successful in promoting the growth of a select bacterial consortia needed for enhanced sulfate and iron removal. This included a combination of anaerobic fermenters, iron- and sulfate-reducers as well as iron/sulfur/sulfide oxidisers. Based on the experimental data and the literature a number of recommendations were developed to improve the operational efficiency and longevity of the field-based leachate remediation system.