Full-scale mitigation strategies are still at early stages but it has already been shown that sufficient and controlled aeration strategies can lower the N2O emissions. It is clear that treatment plants operated to achieve low total nitrogen effluents also have relatively low N2O emission levels.
Unisense Environment has shown that simple changes of the timing of sequential phases in an alternating activated sludge process can significantly reduce the N2O emission from the aerated phase and the accumulation in the non-aerated phase.
Unisense Environment is currently working together with collaborators on proving further reductions of N2O emissions through controlled influent flow balancing; recycle rates, sludge age, ammonium turnover rates and intrinsic COD balancing.
In overview, process parameters that can be adjusted to minimize the formation and emission of N2O:
- Oxygen/aeration controls
- COD/N ratio for denitrification and COD addition control
- Turn-over rates of NH4+, NO2- and reduction of N2O accumulation
- Load balancing influent and NH4+ from reject water
- Phase control of nitrification and denitrification
- Sludge age and biomass concentration
- Recycle rates
- Hydrolysis processes and intrinsic COD harvesting
Many researchers have demonstrated the above strategies in laboratory and pilot scale - a few have even been trailed in full scale, e.g. ethanol addition to SBR reactors or efficient oxygen supply. This proves that N2O mitigation strategies are certainly possible and many at low cost as it only involves PLC program changes. Furthermore, a steady progress in understanding the mechanisms involved in N2O emission in wastewater treatment systems will enable the development of more holistic mitigation strategies also including other process parameters, e.g. the energy balance and effluent criteria.
N2O emissions from wastewater treatment processes vary substantially between plants depending on the design, operation of the plants, characteristics of wastewater etc. and reliable real-time N2O monitoring is a prerequisite for most of the above mitigation strategies.