One of the main challenges of urban wastewater treatment plants (WWTPs) is to remove nitrogen compounds in a sustainable way. Nitrogen is necessary for plants growth but when rejected in excess it causes water pollution and alters aquatic ecosystems. In conventional WWTPs of the XX century nitrogen was mostly eliminated through the combined action of two types of bacteria. One of them, the nitrifiers, utilise oxygen to oxidise ammonium oxygen into nitrite, which is in turn oxidised into nitrate. The second group of bacteria, the denitrifiers, convert these oxidised forms of nitrogen into dinitrogen gas by oxidising organic matter, which acts as electron donor. However, the demand of resources and energy of both treatments is very high: nitrification requires aeration and denitrification needs the supply of an external source of organic matter.
The discovery of Anammox bacteria in 1995 changed our understanding of the nitrogen cycle and shed some light on the way wastewater treatment plants could increase their sustainability. Unlike the aforementioned nitrifers, Anammox bacteria are able to perform the autotrophic oxidation of ammonium. The most applied configuration of Anammox process is “partial nitritation-anammox” (PNA), where nearly half influent ammonium is converted to nitrite by ammonia-oxidizing bacteria (AOB), and the remaining ammonium is oxidized by anammox bacteria (AnAOB), which uses nitrite as electron acceptor and does not need oxygen. By following this approach, anammox has been credited as a viable technology to achieve energy-neutral wastewater treatment plants and improve resource efficiency. Compared to the conventional nitrification-denitrification processes, the association partial nitritation-anammox requires less oxygen, which reduces aeration costs; diminishes sludge production; and organic matter is no longer needed to remove nitrogen.
Anammox-based processes have been already applied to treat reject water in the sidestream line of urban WWTP, with high nitrogen content and low C:N. However, in the recent years, it has increased the interest in applying anammox-based processes in the mainstream line, which accounts for the majority of nitrogen. The application of autotrophic nitrogen removal by PNA has still some technical challenges to face, such as low anammox growth rate, mainly at low temperature conditions, and the repression of nitratation process. The doctoral thesis by Tiago Vitor Akabocci, entitled “Operational strategies towards nitritation-anammox implementation for mainstream municipal wastewater treatment” has studied new operational strategies to deal with the challenges aforementioned, in order to achieve stable PNA at mainstream urban WWTP. Thus, Tiago Vitor Akabocci investigated the following parameters: (i) extremely low dissolved oxygen (DO); (ii) effects of temperature; (iv) influence of different IC:N ratios; and (iv) different reactors configurations (sequencing batch (SBR) and plug-flow reactor).
Results showed that by controlling DO at extremely low levels, average nitrogen removal of 0.37 ± 0.07 kg N·m-3·d-1 is achieved at 25 ºC. The researcher also demonstrated that low DO is not sufficient for complete nitritation repression, because nitrite-oxidizing bacteria (NOB) account for limiting nitrate production, reducing the potential to achieve higher removal efficiencies. After decreasing the temperature operation to 15 ºC, PNA processes were stable for several days, but at long-term operation anammox activity was reduced, and it limited nitrogen removal. Tiago Vitor Akabocci also studied bacterial community by high-throughput molecular techniques. The dominant bacterial groups were clustered within the phyla Planctomycetes, Proteobacteria, Chloroflexi, and Bacteroidetes.
UdG LEQUIA research group has been pioneer in developing and upscaling PNA processes for treating complex effluents. Thesis supervisors Dr Jesús Colprim, Dr Maël Ruscalleda (currently working at Createch360º) and Dr Maria Dolors Balaguer, have applied the Anammox process in urban wastewater treatment plants’ effluents and in urban landfill leachates. This doctoral thesis, funded by a scholarship awarded by the Brasilian government, goes a step further to benefit from the high potential of Anammox bacteria to adjust nitrogen content in water effluents in a sustainable way.