Fast start-up and controlled operation during a long-term period of a high-rate partial nitrification activated sludge system

Partial nitrification of a high-strength ammonium wastewater (1150 +/- 150 mgN-NH4+ L-1), mimicking reject water, was achieved in an activated sludge pilot plant with a configuration of three continuous reactors in series plus a settler. Stable and robust partial nitrification was maintained during 800 days of operation at 30 degrees C with a sludge retention time (SRT) of 8 +/- 3 days. A high volumetric ammonium oxidation rate (2.0gNL(-1) d(-1)) was obtained with a [N-NO2-]/[N-NOx-] ratio of 1, i.e. full nitritation. The start-up of the partial nitrification system was quickly and successfully performed with an on-line control system using municipal wastewater treatment plant (WWTP) sludge as inoculum. An ammonia-oxidizing bacteria (AOB) fraction of 72 +/- 10% was obtained after only 30 days of start-up. The applied SRT of 7-10 days with the combination of free ammonia inhibition and dissolved oxygen limitation provided the selective washout of nitrite-oxidizing bacteria (NOB) and an active nitrifying population with high ammonium oxidizing rates.