Doctoral defence: Rauno Lust “Bioelectrochemical systems for enhanced removal of nitrate from water with a low electron donor concentration”

On 31 August at 10:15 Rauno Lust will defend his doctoral thesis “Bioelectrochemical systems for enhanced removal of nitrate from water with a low electron donor concentration” for obtaining the degree of Doctor of Philosophy (in Environmental Technology).

Supervisors:
Professor Ülo Mander, University of Tartu
Associate Professor Kuno Kasak, University of Tartu
Associate Professor Jaak Nerut, University of Tartu

Opponent:
Senior Researcher Carlos Alberto Arias, Aarhus University (Denmark)

Summary
High nitrate (NO3) concentration in surface water and groundwater is related mainly to intensive agricultural practices. A high level of NO3 in drinking water can be harmful to humans, causing methemoglobinemia or digestive tract cancer. Therefore, it is necessary to find solutions to reduce NO3 pollution. One process by which NO3 can be removed is denitrification. In this process, microorganisms use NO3 as an electron acceptor, just as humans use oxygen for breathing. However, denitrification is hindered in the ground- or surface water as the concentration of electron donors, i.e., organic compounds, is low. The main goal of the dissertation was to evaluate if the microbial electrosynthesis reactor (MESR) can be used to enhance NO3 removal at low electron donor concentrations. An electric current is applied to the MESR electrodes to make the electrons available to the microorganisms and thereby support denitrification. For experiments, a two-chamber MESR was constructed and operated for 38 days. During the investigation, the MESR was able to increase the NO3 removal rate twice to 1.4 mgN-NO3/(L×day) while lowering the greenhouse gas emissions. However, the two-chamber MESR caused pH to shift, so an experiment with single chamber MESR was conducted. Different working electrode potentials were tested during the second experiment, which lasted for 612 days. The MESR was able to reduce 3.8±1.2 mgN-NO3/(L×day), and the pH of the solution also remained neutral on the optimal potential. While trying different working electrode potentials, it was found that the MESR can also be used to enforce other nitrogen-cycle-related processes, i.e., dissimilatory nitrate reduction to ammonium or nitrification. The results show that the MESR is a promising technology to support different microbial processes and the field of application of MESR is diverse.