It is presumed that foaming, which only occurred with anaerobic sludge, leads to a higher exchange surface and hence enhanced NH4-N stripping rates. The NH4-N removal rates (Fig. 5b) achieved within 4 h were 86% (flue gas) and 47% (biogas). An additional parameter, the carbonate buffer (CO22+, HCO3− and dissolved CO2) in the stripped sludge, was measured during these experiments (Fig. 5d). These data underline the CO2 concentration’s impact in the strip gas on the CO2 stripping efficiency.