Influence of gas flow-induced shear stress on the operation of the Anammox process in a SBR

The start up and performance of the Anammox process were tested in sequencing batch reactors with two different configurations: a bubble column (SBR-B) and a gas-lift reactor (SBR-G). Different off-gas upflow velocities were tested (3.53-12.3 cm min^-1) in order to expose the biomass to different shear conditions and to study their effects on both efficiency and physical properties of the Anammox granular biomass. For the SBR-B the minimum gas upflow velocity needed to achieve biomass suspension inside the reactor was 12.3 cm min^-1. Such velocity made impossible the stable operation of the process. The fluidization of biomass for the SBR-G was reached at a gas upflow velocity of 3.52 cm min^-1. This system maintained an efficiency of nitrite removal around 98% at values up to 5.29 cm min^-1 but when the gas upflow velocity was increased from 5.29 to 9.70 cm min^-1 a significant decrease of the specific Anammox activity of the biomass from 0.35 to 0.05 g N g^-1 VSS d^-1 was measured. The system lost 85% of its nitrogen removal efficiency which was not restored in spite of returning the gas upflow velocity to its initial value.