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Mitigation of N2O Emissions from Wastewater Biofilms

Microsensors confirm that counter-diffusion biofilms have lower N2O emissions than co-diffusion biofilms.

Introduction

N2O is a very potent greenhouse gas (GHG) and accounts for up to 90% of the GHG emissions from wastewater treatment plants (WWTPs). N2O is an intermediate product in biological treatment processes at WWTPs. In this study, Professor Akihiko Terada and his research group at Tokyo University of Agriculture and Technology have investigated mitigation of N2O emissions in a membrane-aerated biofilm reactor (MABR).

In a conventional biofilm reactor (CBR), the oxygen and electron donors (organic carbon and NH4+) are supplied from the top of the biofilm from the liquid phase (co-diffusion). In an MABR, oxygen is supplied from the bottom of the biofilm through a gas-permeable membrane whereas the electron donors are supplied from the top of the biofilm (counter-diffusion). With this geometry, there will be a part in the middle of the MABR biofilm where electron acceptors co-exist with an electron donor, and this allows for simultaneous nitrification/denitrification, which could facilitate N2O mitigation.

Biofilm

Laboratory setup

The Unisense MicroProfiling System was used to complete high resolution concentration profiles throughout the depth of the biofilms in co-diffusion and counter-diffusion biofilm reactors (Figure 1). The biofilms were approximately 1,500 µm thick. The researchers used an N2O microsensor with a tip diameter of 25 µm (N2O-25) and an O2 microsensor with a tip diameter of 50 µm (OX-50) to make depth profiles throughout the biofilm inside of the biofilm reactor.

Microsensor1

Results and Conclusion

The oxygen microprofiles in the counter-diffusion biofilm showed an oxygen penetration depth of 400 µm into the biofilm from the bottom and the O2 concentration was highest at the biofilm-membrane interface (0 µm) where the air is supplied (Figure 2). The N2O concentration decreased just after O2 depletion. The N2O concentration at the biofilm-liquid interface was approximately 130 times lower in the MABR compared to the CBR.

From the concentration profiles, using the Fick’s second law of diffusion, the researchers could calculate the N2O production/consumption rates at the different depths in the biofilms (data not shown here). The authors found adjacent N2O production/consumption hot spots. The positions of these most likely explained the increased N2O consumption in the MABR biofilm.

The researchers could conclude that there was far less N2O emission from the MABR compared to the conventional CBR and that the MABR is a promising technology for mitigation of N2O emissions from WWTPs.

You can read more in the article by Kinh et al. “Counter-diffusion biofilms have lower N2O emissions than co-diffusion biofilms during simultaneous nitrification and denitrification: Insights from depth-profile analysis”, Water Research 124 (2017) 363-371.

Microsensor2

Comment from Prof. Akihiko Terada

“Unisense O2 and N2O microsensors allow fast, accurate, and reliable activity measurements of microorganisms in suspensions and biofilms. They have provided our research group with opportunities to lead to exciting discoveries of bacteria and biofilm hotspots responsible for N2O consumption. Staffs are always kind and listen to our requests to improve/retrofit their products. We are sure to enjoy a scientific journey with Unisense microsensors as buddies”.

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