Piloting carbon-lean nitrogen removal for energy-autonomous sewage treatment

Energy-autonomous sewage treatment can be achieved if nitrogen removal does not rely on organics (~chemical oxygen demand, COD), so that a maximum of the COD can be redirected for energy recovery. Shortcut nitrogen removal technologies such as partial nitritation/anammox and nitritation/denitritation are therefore essential, enabling carbon- and energy-lean nitrogen removal. A 13 m³ pilot-scale reactor was operated on pre-treated sewage with a COD/N ratio of 3.2±1.2 at 12-24°C. Selective suppression of unwanted nitrite-oxidizing bacteria over aerobic ammonium-oxidizing bacteria was achieved with the innovative aeration control in combination with strict flocculent sludge retention time (SRT) management, resulting in a minimal nitrate production ratio of 17±10%. Additionally, activity of anoxic ammonium-oxidizing bacteria (AnAOB) could be retained in the reactor for at least 150 days due to the successes of the novel AnAOB nursery tank and differential SRT control. Consequently, shortcut nitrogen removal was achieved with a low COD/N removal ratio of 2.3±0.7, almost three times lower that the currently applied technology. The effluent total nitrogen concentration of 17±3 mg TN/L (at 21±1°C) and 17±6 mg TN/L (at 15±1°C) were however too high to be applicable at sewage treatment plant Nieuwveer. Corresponding nitrogen removal efficiencies were 52±12% and 37±21%, respectively. Further optimization of this technology or retaining a fraction of the currently applied effluent recirculation should therefore be explored. This study showed the development of the Mainstream Anammox System (MAS) technology, paving the way to resource efficient and off-the-grid sewage treatment.