Nitrogen removal over nitrite by aeration control in aerobic granular sludge sequencing batch reactors

Abstract

This study investigated the potential of aeration control for the achievement of N-removal over nitrite with aerobic granular sludge in sequencing batch reactors. N-removal over nitrite requires less COD, which is particularly interesting if COD is the limiting parameter for nutrient removal. The nutrient removal performances for COD, N and P have been analyzed as well as the concentration of nitrite-oxidizing bacteria in the granular sludge. Aeration phase length control combined with intermittent aeration or alternate high-low DO, has proven to be an efficient way to reduce the nitrite-oxidizing bacteria population and hence achieve N-removal over nitrite. N-removal efficiencies of up to 95% were achieved for an influent wastewater with COD:N:P ratios of 20:2.5:1. The total N-removal rate was 0.18 kgN·m-3·d-1. With N-removal over nitrate the N-removal was only 74%. At 20 °C, the nitrite-oxidizing bacteria concentration decreased by over 95% in 60 days and it was possible to switch from N-removal over nitrite to N-removal over nitrate and back again. At 15 °C, the nitrite-oxidizing bacteria concentration decreased too but less, and nitrite oxidation could not be completely suppressed. However, the combination of aeration phase length control and high-low DO was also at 15 °C successful to maintain the nitrite pathway despite the fact that the maximum growth rate of nitrite-oxidizing bacteria at temperatures below 20 °C is in general higher than the one of ammonium-oxidizing bacteria.

Figure 1

Schematic representation of the aeration strategies during the five experimental phases.

Figure 2

Illustration of a DO profile during a SBR cycle with alternate high-low DO periods with full aeration during the high DO periods. The slope change of DO towards the end of nitrification occurring during the third high DO period is indicated.

Figure 3

Nutrient removal performances of parent reactor between days 1 and 155. (A) Nutrient removal performances. (B) Concentrations of N compounds in the effluent and (C) Nitrospira 16S rRNA gene concentrations measured by qPCR. At day 96 (start of Phase II), the oxygen supply was increased, but automatically stopped upon completion of ammonium oxidation.

Figure 4

Concentrations of N and P compounds during one SBR cycle (at day 98) operated (A) with high-low DO strategy and (B) in a test cycle with uncontrolled full aeration. Concentrations at time 0 were calculated based on the effluent concentrations of the previous cycle and the influent concentrations. Aeration started after 60 min of plug-flow feeding (vertical dashed line). The dotted lines show schematically the aeration strategy.

Figure 5

Nitrification batch test with granular sludge taken at the end of the starvation phase without COD supply (at day 118).

Figure 6

Nutrient removal performances at 20 °C (reactor A) from days 156 to 261. (A) Nutrient removal performances. (B) Concentrations of N compounds in the effluent and (C) Nitrospira 16S rRNA gene concentrations measured by qPCR. Until day 186, the reactor was operated with alternate high-low DO and aeration phase length control (Phase III), from days 187 to 220 the reactor was fully aerated during 2 h (Phase IV), and finally, from days 221 to 261 (Phase V), the reactor was operated with intermittent aeration and aeration phase length control.

Figure 7

Nutrient removal performances at 15 °C (reactor B) from days 156 to 261. (A) Nutrient removal performances. (B) Concentrations of N compounds in the effluent and (C) Nitrospira 16S rRNA gene concentrations measured by qPCR. Until day 186, the reactor was operated with alternate high-low DO and aeration phase length control, from days 187 to 220 the reactor was fully aerated during 2 h, and finally, from days 221 to 261, the reactor was operated with intermittent aeration and aeration phase length control.

Figure 8

Typical profiles of nitrous oxide concentrations in the effluent gas and the bulk liquid during a SBR cycle. The dashed lines show schematically the aeration strategy: (A) N-removal over nitrite with intermittent aeration, (B) N-removal over nitrate with intermittent aeration, (C) N-removal over nitrite with constant DO of 30% and (D) N-removal over nitrate with constant DO of 30%. The shown profiles have been measured with 400 mgCOD·L⁻¹ influent concentration.