Autotrophic nitrogen removal from anaerobic supernatant of Florence's WWTP digesters

In municipal WWTP with anaerobic sludge digestion, 10–20% of total nitrogen load comes from the return supernatant produced by the final sludge dewatering. In recent years a completely autotrophic nitrogen removal process based on Anammox biomass has been tested in a few European countries, in order to treat anaerobic supernatant and to increase the COD/N ratio in municipal wastewater. This work reports the experimental results of the SHARON-ANAMMOX process application to anaerobic supernatant taken from the urban Florentine area wastewater treatment plant (S. Colombano WWTP). A nitritation lab-scale chemostat (7.4 L) has been started-up seeded with the S. Colombano WWTP nitrifying activated sludge. During the experimental period, nitrite oxidising bacteria wash-out was steadily achieved with a retention time ranging from 1 to 1.5 d at 35 °C. The Anammox inoculum sludge was taken from a pilot plant at EAWAG (Zurich). Anammox biomass has been enriched at 33°C with anaerobic supernatant diluted with sodium nitrite solution until reaching a maximum specific nitrogen removal rate of 0.065 kgN kg⁻¹ VSS d⁻¹, which was 11 times higher than the one found in inoculum sludge (0.005 kgN kg⁻¹VSS d⁻¹). In a lab-scale SBR reactor (4 L), coupled with nitritation bioreactor, specific nitrogen removal rate (doubling time equal to 26 d at 35 °C and at nitrite-limiting condition) reached the value of 0.22 kgN kg⁻¹ VSS d⁻¹, which was approximately 44 times larger than the rate measured in the inoculum Anammox sludge.