3.3.3. N2O emission during nitrification instability
Fig. 4 represents a 28 h-period in which the concentration of NH4
+
tended to accumulate in sampling Site 3 of the bioreactor, showing an
increasing trend fromaround 5–10 mg N/L in the beginning of the period
to 15 mg N/L in the end. NO3
− values were in general steady and
remained at levels between 0 and 3 mg N/Lwhile NO2
−was also present
ranging from0.1 to 0.6mgN/L along the period (Fig. 4A). N2O emissions
described a large peak (around 900 ppm v),which was simultaneous to
themoment in which NH4
+ started to accumulate and the NO2
− concentration
was at its peak. This N2O peak lasted for around 3 h before it decreased
to lower levels (~1–2 ppm v for the rest of the period). The
curve described by the aeration flow presented a valley (decreasing
from 800 to 500 m3/h) during the same time in which N2O emissions
peaked, but the DO level was regular during the whole period, always
around 2 mg/L as shown in Fig. 4B.
3.4. Dissolved CH4 and N2O concentrations along the WWTP
Grab samples for dissolved CH4 analysis were taken to determine its
presence not only in the bioreactor but also at different locations of the
plant. Results are shown in Fig. 5. Over 0.05 mg CH4/L was measured
at the inlet of the plant, in samples of influent wastewater before it entered
the system. Reject wastewater flowing from the sludge digester