On 18 December 2017, two Erasmus Mundus Joint Doctorate PhD Defences took place. Ms. Lea Chua Tan and Ms. Shrutika L. Wadgaonkar successfully presented and defended their PhD theses. Professor Piet Lens, Professor of Environmental Biotechnology at IHE Delft, was their Promotor. The co-promotors were, Dr. Esposito, department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Italy and Dr. van Hullebusch, associate professor in Biogeochemistry at the University of Paris-Est, France.
Dr. Lea Chua Tan thesis
Anaerobic treatment of mine wastewater for the removal of selenate and its co-contaminants
This research aimed at addressing the effect of wastewater characteristics (i.e. co-contaminants, heavy metals and pH) on the biological reduction of selenate (SeO42-) and evaluating process integration and configurations for selenium-laden wastewater treatment with co-contaminants.
The first part of the study focused on the effect of co-electron acceptors and low pH on the bioremediation of SeO42-. Experiments showed that the molar ratio of NO3- and SO42- to SeO42- has a controlling factor in either increasing or decreasing the selenium (Se) removal efficiency. Additionally, study on biofilm-Se interactions revealed the presence of either NO3- or SO42- influences the Se speciation, biogenic Se (Se0) levels and biomass activity. Upflow anaerobic sludge blanket (UASB) reactor operation with a gradual decrease in the influent pH from 7.0 to 5.5 showed a stable removal performance of NO3-, SO42- and SeO42-, before a 20% decrease in removal of all these components was observed at pH 5.0. Furthermore, long-term operation of the UASB reactor at pH 5.0 showed the enrichment of Geobacteraceae and Spirochaetaceae families, which were not detected at pH > 5.0.
The second part of the study explored the effectiveness of different removal techniques for the treatment of SeO42- with co-contaminants. Comparing the SeO42- removal performance in the presence of SO42- in a biotrickling filter (BTF) and UASB reactor revealed that SO42- largely influenced the attached biofilm growth and increased SeO42- removal by > 200%. On the other hand, SeO42- removal was similar in the UASB reactor irrespective of the presence or absence of SO42-. Biomass characterization revealed the formation of spherical Se0 and poly-selenium sulfide in the biomass of both bioreactors. Addition of Ni in both bioreactors led to a decrease in SO42- and SeO42- removal performance by ~20-30%. Ni removal was > 80% in both bioreactors. Ni was removed via nickel sulfide precipitation. Evaluation of integrated process system for SeO42- and SO42- removal was conducted by coupling an ion exchange column (IX) and UASB bioreactor, using IX as either a pre-treatment (IX a UASB) or post-treatment (UASB a IX) unit for the bioreactor. The pre-treatment process scheme showed a better overall removal efficiency of 99% SO42- and 94% total Se reaching < 100 mg/L SO42-, < 0.3 mg Se/L total Se and < 0.02 mg Se/L dissolved Se in the effluent over 42 days of continuous operation.
Dr. Shrutika L. Wadgaonkar thesis
Novel bioremediation processes for treatment of seleniferous soils and sediment
The aim of this Ph.D. was to develop a technology for the remediation of seleniferous soils/sediments and to explore microbial reduction of selenium oxyanions under different respiration conditions and bioreactor configurations.
Seleniferous soil collected from the wheat-grown agricultural land in Punjab (India) was characterized and its soil washing was optimized by varying parameters such as reaction time, temperature, pH and liquid to solid ratio. In order to maximize selenium removal and recovery from this soil, effect of competing ions and oxidizing agents as chemical extractants for soil washing were also studied. Although oxidizing agents showed a maximum selenium removal efficiency (39%), the presence of oxidizing agents in the leachate and the agricultural soil may increase the cost of their post-treatment. Aquatic plants, Lemna minor and Egeria densa were used to study phytoremediation of the soil leachate containing oxidizing agents. However, the selenium removal efficiency by aquatic weeds was significantly affected by the high concentrations of these oxidizing agents in the soil leachate.
Seleniferous soil flushing revealed the selenium migration pattern across the soil column. Migration of soluble selenium fraction from the upper to the lower layers and its subsequent reduction and accumulation in the lower layers of the soil column was observed during soil flushing. The selenium removal efficiency by the soil flushing method decreased with an increase in the column height. Furthermore, the soil leachate containing selenium oxyanions obtained from soil washing was treated in a UASB reactor by varying the organic feed. Effluent containing less than 5 mg L-1 selenium was achieved, which is in accordance with the USEPA guidelines for selenium wastewater discharge limit.
Moreover, ex situ bioremediation of selenium oxyanions was studied under variable conditions. An aerobic bacterium (Delftia lacustris) capable of transforming selenate and selenite to elemental selenium, but also to hitherto unknown soluble selenium ester compounds was serendipitously isolated and characterized. Alternatively, anaerobic bioreduction of selenate coupled to methane as electron donor was investigated in serum bottles and a biotrickling filter using marine sediment as inoculum. Finally, the effect of contamination of other chalcogen oxyanions in addition to selenium was studied. Simultaneous reduction of selenite and tellurite by a mixed microbial consortium along with the retention of biogenic Se and Te nanostructures in the EPS was achieved during a 120-day UASB bioreactor operation.