Nitrous Oxide Emissions from Trickling Filters

Nitrous oxide (N₂O) is a greenhouse gas (GHG) which is released from wastewater treatment plants (WWTP) during nitrogen removal. Removing nitrogen is an essential function of WWTPs and there are many ways of constructing plants to achieve this. Therefore, it is important to understand the N₂O emission triggers in order to implement mitigation controls through changes of process parameters.

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.

Off-Gas Measurements

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 ∙ u_s ∙ 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.

References

Marques et al. 2016. Assessment of online monitoring strategies for measuring N₂O emissions from full-scale wastewater treatment systems. Water Research 99, 171-179.
Søvik and Kløve. 2007. Emission of N₂O and CH₄ from a constructed wetland in southeastern Norway. Sci. Tot. Environ. 380, 28–37.
Wang et al. 2014. Impact of temperature on nitrous oxide emission in a tricking filter. Zhongguo Huanjing Kexue/China Environmental Science 34, 2796-2804.