Currently, medium to large sewage treatment works break down sewage using activated sludge (Martin, 2022). This involves bubbling air through wastewater to support the growth of bacteria. This method has been used for over 100 years and has played a huge global role in improving public health (Martin, 2022). However, it takes a lot of energy to pump air through the wastewater, 30-40% of a water company’s carbon emissions (Martin, 2022). This is because of the release of nitrous oxide, a greenhouse gas with a global warming potential 265 times greater than that of carbon dioxide (IPCC, 2013).
Nitrous oxide (N2O) is
produced as a byproduct of the microbial nitrogen transformation processes in a
wastewater treatment plant (Law et al., 2012) [Figure 1]. For instance, it is a
known obligatory intermediate in the heterotrophic denitrification pathway and
is also produced by autotrophic nitrifying bacteria, mainly ammonia-oxidizing
bacteria (AOB) as a by-product (Kampschreur et al., 2008) (Law et al., 2012).
 |
| Figure 1: Microbial Nitrogen Transformation Processes (Martin, 2022) |
Utilities know N2O is being emitted but they do not know how much is being emitted so they do not know the magnitude of the issue (Martin, 2022).
One
idea to decrease the N2O is to not give the microorganisms oxygen
anymore. Anaerobic treatment uses bacteria that do not nitrify so nitrous oxide
is not produced. Instead, byproducts include methane (which can be recovered
for energy production) and ammonia (in a recoverable form). While it does
require additional processes for solids and nutrient removal, compared to
activated sludge, this technique could reduce emissions by 98% and energy
consumption by 80% (Martin, 2022).
References
IPCC,
G. Myhre, D. Shindell, Anthropogenic and natural radiative forcing, T.F.
Stocker, D. Qin, G.-K. Plattner, M.M.B. Tignor, S.K. Allen, J. Boschung, A.
Nauels, Y. Xia, V. Bex, P.M. Midgley (Eds.), Climate Change 2013: the Physical
Science Basis, Contribution of Working Group I to the Fifth Assessment Report
of the Intergovernmental Panel on Climate Change, Cambridge University Press
(2013) (Chapter 8)
Law
Y, Ye L, Pan Y, Yuan Z. Nitrous oxide emissions from wastewater treatment
processes. Philos Trans R Soc Lond B Biol Sci. 2012 May 5;367(1593):1265-77.
doi: 10.1098/rstb.2011.0317. PMID: 22451112; PMCID: PMC3306625.
Martin,
B. 2022. Pioneering cold anaerobic digestion: decarbonising wastewater by
reducing nitrous oxide emissions, Birmingham: Utility Week Live 2022.
Kampschreur
M. J., Tan N. C. G., Kleerebezem R., Picioreanu C., Jetten M. S. M., Van
Loosdrecht M. C. M. 2008. Effect of dynamic process conditions on nitrogen
oxide emission from a nitrifying culture. Environ. Sci. Technol. 42, 429–435
10.1021/es071667p
