3.3.2. N2O emissions under irregular aeration
Fig. 3 shows the N2O emission pattern together with the concentration
of NH4
+, NO2
− and NO3
− (Fig. 3A) during three consecutive days in
which the aeration flow and the DO levels were heavily modified
(Fig. 3B). Generally, in terms of aeration and DO, there were two differentiated
periods. During the first 36 h, the aeration flow ranged from
around 500 to 1000 m3/h and the DO was kept at levels between 0.5
and 1.5 mg/L. Then, the aeration flow increased sharply and reached
values up to 1700 m3/h. At the same time the DO also increased and
was maintained at ~2 mg/L. During these two periods, N2O emissions
presented a very low and stable profile (nearly 0 ppm v).
Additionally, there were three isolated N2O peaks ranging from
around 10 to over 40 ppm v (Fig. 3A). These were registered right
after three episodes of ~2 h aeration drop occurring along the experimental
profiling (Fig. 3B). When aeration was re-started after each
2 h-period and DO was recovered, N2O peak emissions occurred. In general
terms, both NH4
+ and NO3
− concentrations showed a decreasing
trend along this three-day period of irregular aeration and DO. However,
NH4
+ peaks of different level (2.5–12 ppm v)were observed in parallelwith
the N2O peaks mentioned before and small NO2
− concentrations
were also detected.