Nitrous Oxide Emissions from Trickling Filters
These measurements were performed with Unisense laboratory equipment, which allows for a customized signal compensation to measure in air. See www.unisense.com for more details.
In the UK, trickling filters account for 60-70% of the biological wastewater treatment units. However, information regarding N2O emissions from trickling filters is limited, partly caused by the difficulties in capturing off-gases. Projects have provided an estimate of N2O emissions by implementing a hood for gas collection and analysis. Unisense N2O sensors present an opportunity for cost-effective monitoring of off-gas N2O concentrations. The sensors have mainly been used for N2O analysis in liquid, but have also been applied for off-gas measurements (Marques et al. 2016). The N2O Wastewater Sensor can be implemented for gas phase analysis in a hood as well as for liquid monitoring of the effluent to quantify the emissions from trickling filters (see Fig. 1).
The effect of temperature on N2O emission
One of very few studies is Wang et al. (2014) who investigated the effect of temperature on N2O emission from a trickling filter treating domestic wastewater. The N2O emission was monitored during a year and it was observed that the emission was higher during the summer compared to winter. In trickling filters, where air is supplied through natural ventilation, the ventilation is driven by temperature differences. With limited temperature differences between air and water during summer, temperature becomes the governing factor for N2O release since low air flow and oxygen limitation leads to incomplete nitrification and N2O release. A low COD/N ratio has been shown to lead to N2O formation during denitrification, but as nitrification is the dominant process in the trickling filter, it is not a significant factor for N2O release. Søvik and Kløve (2007) also found that the N2O release from a trickling filter was related to nitrification.
Further Monitoring of N2O Emissions Needed
The air flow used for calculating N2O emission was calculated according to AR = ε ∙ us ∙ f, where AR is the airflow (m3∙s-1), ε the surface fluid velocity (m∙s-1) and f the area of the trickling filter (m2). They estimate an emission of 20.5-554 g N2O/(m3∙year), corresponding to 0.1%-0.8% of the oxidized ammonia released as N2O-N. Studies are limited, but Søvik and Kløve (2007) and references therein report that 0.004-8% of the nitrogen load was released as N2O-N. Wang et al. (2014) suggest that a solution to limiting N2O emission could be to control the O2 supply to the trickling filter biofilm by relying on controlled ventilation instead of natural ventilation.
In conclusion, the lack of data and high reported emissions emphasize the need for further monitoring N2O emissions from trickling filters. To implement N2O monitoring, it is important to further develop a method for implementing N2O measurements and constructing an N2O emission model for this type of system. Monitoring the N2O emission using the N2O Wastewater Sensor will drive a deeper understanding of the N2O release triggers in trickling filters and mitigate the N2O emission.