Ammonium (NH₄⁺) is a pollutant present in all kinds of water bodies, leading to multiple environmental and health issues. There are several commercial ammonium-removing technologies available, but all of them are high-energy-consuming and, subsequently, very costly. Microbial electrochemical technologies (METs), which use electroactive microorganisms, have emerged as a low-energy-consuming biotechnological alternative to convert ammonium into harmless nitrogen gas (N₂). However, further research and industrial development is still needed to achieve competitive results at full scale. The doctoral thesis “Bioelectrochemical systems for ammonium removal in contaminated water” by Miguel Osset Álvarez aims to contribute to the industrial development of the two main ammonium-removing METs: bioelectrochemical nitrification and the combination of aerobic nitrification with bioelectrochemical denitrification.

Bioelectrochemical nitrification is a very promising technology to remove NH₄⁺. However, the mechanisms behind this process have not been fully unveiled. In this thesis, a new nitrifying bioelectrochemical system (niBES) was built, operated and studied for 550 days. A nitrifier bacteria (Achromobacter sp.) was found to be the dominant microorganism in the niBES, while hydroxylamine (NH₂OH) and nitrite (NO₂^–), two nitrification intermediates, were revealed as electroactive compounds. Overall, these results suggest that ammonium was converted into dinitrogen gas (N₂) by a process combining bioelectrochemical NH₄⁺ oxidation, denitrification and, to a lesser extent, anaerobic ammonium oxidation (anammox).

On the other hand, a new ammonium-removing bioelectrochemical system (BES), the e-biofilter, was built to integrate bioelectrochemical denitrification into a biotrickling filter. E-biofilters were used to transform the NH₄⁺ in synthetic aquaculture wastewater into nitrate (NO₃^–) and N₂, enabling the reuse of this water for hydroponic culture. Moreover, the e-biofilters were capable of removing most of the ammonium, organic matter and suspended solids present in the secondary effluent of an urban wastewater treatment plant (WWTP), showing that e-biofilters can provide holistic wastewater treatment.