A Tool for Carbon Dosage Control
N2O as a control parameter for carbon dosage
You can use N2O as a control parameter for carbon dosage in the denitrification process, when you monitor the N2O concentration in the liquid.
Nitrogen removal in wastewater is achieved through a combination of nitrification and denitrification as well as anammox (anaerobic ammonium oxidation). N2O is an obligate intermediate formed during denitrification. However, N2O is also formed during nitrification which is the main source of N2O emissions from WWTPs (Fig. 1).
Measuring N2O yields important insights about all key substances
N2O formation is linked to the ammonium (NH4+) loading, and N4+O formation increases as the NH4+ turnover increases. If high NH4+ oxidation rates lead to NO2- accumulation, N2O will build up in the liquid as a warning.
NO2- is the key substrate, besides NH4+, for anammox, but it is difficult to measure NO2- with present sensors. The measurement will be indirect as NO2- is formed and consumed inside biofilm or granules. Unlike NO2-, N2O is not consumed by the anammox bacteria. Therefore, online N2O monitoring will provide a more precise measure of the substrate balance. Thus, N2O is tightly linked to NO2- concentration through both nitrification and denitrification. This means that you can use the N2O sensor as a proxy for NO2-.
Carbon dosage can prevent excessive N2O emissions
Applied research has shown that a low COD/N ratio can lead to increased N2O production and that a COD/N ratio below 3.5 can lead to significant N2O emissions. Fig. 2 demonstrates the relationship between COD/N and N2O. The bell-shaped relationship between N2O and COD/N makes it possible for you to avoid excessive N2O emission by aiming for either a high (>3.5) or a low (<1) COD/N ratio (Fig. 2).
From a process control standpoint, the slope of the N2O curve in response to the different COD/N-ratios can be used as an indicator for the dominating sludge process. If you aim for complete denitrification to N2 to avoid N2O, you can apply the N2O sensor to monitor N2O concentration and increase the carbon load when the N2O concentration increases to ensure a COD/N ratio above 3.5.
Anammox as an Alternative
Anammox is an alternative nitrogen removal process where N2 is formed from NH4+ and NO2- (Fig. 1). Controlling the substrate availability is important for the successful implementation of anammox. Two substrate control strategies that have been suggested are PNA (Partial Nitrification/Anammox) and PdNA (Partial deNitrification/Anammox). The PNA process aims at partial nitrification by inhibiting NOB (nitrite-oxidizing bacteria) to increase the NO2- available for anammox. However, the out-selection of NOB has proven operationally difficult. On the other hand, PdNA aims at partial denitrification to increase NO2- availability (Fig. 1).
While it might appear as a detour, anammox is operationally easier to achieve. The process has been demonstrated at temperatures down to 20°C, which makes it relevant in many parts of the world during summer. In the PdNA case, the relationship between COD/N and N2O can be used for controlling the partial denitrification process to accumulate nitrite for anammox and keep the COD/N ratio below 1. The N2O sensor can monitor the N2O formation and decrease the carbon loading to achieve partial denitrification to NO2-, which can be used for anammox. The carbon can then be harvested and used for biogas production.