Regulation of key N2O production mechanisms during biological water treatment

Nitrous oxide (N 2 O) emissions account for the majority of the carbon footprint of wastewater treatment plants (WWTPs). Many N 2 O mitigation strategies have since been developed while a holistic view is still missing. This article reviews the state-of-the-art of N 2 O mitigation studies in wastewater treatment. Through analyzing existing studies, this article presents the essential knowledge to guide N 2 O mitigations, and the logics behind mitigation strategies. In practice, mitigations are mainly carried out by aeration control, feed scheme optimization, and process optimization. Despite increasingly more studies, real implementation remains rare, which is a combined result of unclear climate change policies/incentives, as well as technical challenges. Five critical technical challenges, as well as opportunities, of N 2 O mitigations were identified. It is proposed that (i) quantification methods for overall N 2 O emissions and pathway contributions need improvement; (ii) a reliable while straightforward mathematical model is required to quantify benefits and compare mitigation strategies; (iii) tailored risk assessment needs to be conducted for WWTPs, in which more longterm full-scale trials of N 2 O mitigation are urgently needed to enable robust assessments of the resulting operational costs and impact on nutrient removal performance; (iv) current mitigation strategies focus on centralized WWTPs, more investigations are warranted for decentralised systems, especially decentralized activated sludge WWTPs; and (v) N 2 O may be mitigated by adopting novel strategies promoting N 2 O reduction denitrification or microorganisms that emit less N 2 O. Overall, we conclude N 2 O mitigation research is reaching a maturity while challenges still exist for a wider implementation, especially in relation to the reliability of N 2 O mitigation strategies and potential risks to nutrient removal performances of WWTPs.

Section: Development and Current Status Of N2o Mitigation Strategiesmentioning

confidence: 99%

“…In WWTPs, different N 2 O production pathways often occur simultaneously in situ, and each pathway is regulated differently by environmental factors . Knowledge on the exact regulation mechanism on each production pathways guides the design of N 2 O mitigation strategies.…”

Section: Challenges and Opportunities For N2o Mitigation At Wwtpsmentioning

confidence: 99%

Insights into Nitrous Oxide Mitigation Strategies in Wastewater Treatment and Challenges for Wider Implementation

Duan

Zhao

Koch

et al. 2021

“…Sewage treatment was estimated to emit 0.22 Tg N 2 O–N year –1 in 1990, accounting for approximately 3.2% of the global anthropogenic N 2 O emissions . N 2 O is a byproduct formed during nitrification produced by ammonium oxidizing bacteria (AOB) through incomplete oxidation of hydroxylamine (NH 2 OH) or through nitrification–denitrification and it is a necessary intermediate during denitrification. − Intermediates such as NH 2 OH and nitric oxide (NO) produced by AOB can also lead to abiotic N 2 O production especially under low pH conditions . Given the complexity of N 2 O production mechanisms, N 2 O emissions can vary significantly between biological nutrient removal (BNR) systems of different configuration and operating parameters …”

Section: The Limitations Of Ammonium Removal In Urban Water Managementmentioning

confidence: 99%

Mainstream Ammonium Recovery to Advance Sustainable Urban Wastewater Management

Cruz

Law

Guest

et al. 2019

163

Throughout the 20th century, the prevailing approach toward nitrogen management in municipal wastewater treatment was to remove ammonium by transforming it into dinitrogen (N 2 ) using biological processes such as conventional activated sludge. While this has been a very successful strategy for safeguarding human health and protecting aquatic ecosystems, the conversion of ammonium into its elemental form is incompatible with the developing circular economy of the 21st century. Equally important, the activated sludge process and other emerging ammonium removal pathways have several environmental and technological limitations. Here, we assess that the theoretical energy embedded in ammonium in domestic wastewater represents roughly 38−48% of the embedded chemical energy available in the whole of the discharged bodily waste. The current routes for ammonium removal not only neglect the energy embedded in ammonium, but they can also produce N 2 O, a very strong greenhouse gas, with such emissions comprising the equivalent of 14−26% of the overall carbon footprint of wastewater treatment plants. N 2 O emissions often exceed the carbon emissions related to the electricity consumption for the process requirements of WWTPs. Considering these limitations, there is a need to develop alternative ammonium management approaches that center around recovery of ammonium from domestic wastewater rather than deal with its "destruction" into elemental dinitrogen. Current ammonium recovery techniques are applicable only at orders of magnitude above domestic wastewater strength, and so new techniques based on physicochemical adsorption are of particular interest. A new pathway is proposed that allows for mainstream ammonium recovery from wastewater based on physicochemical adsorption through development of polymer-based adsorbents. Provided adequate adsorbents corresponding to characteristics outlined in this paper are designed and brought to industrial production, this adsorption-based approach opens perspectives for mainstream continuous adsorption coupled with side-stream recovery of ammonium with minimal chemical requirements. This proposed pathway can bring forward an effective resource-oriented approach to upgrade the fate of ammonium in urban water management without generating hidden externalized environmental costs.

“…Nitrite (NO 2 – ), nitric oxide (NO), and nitrous oxide (N 2 O) are obligate intermediates during nitrate (NO 3 – ) reduction to N 2 ; NO 2 – and NO can be toxic for cell metabolism . The four-step reduction is carried out by enzymes encoded by the nar (and nap ), nir , nor , and nos genes; NAR is a membrane-bound enzyme, while NOR, NIR, and NOS are located in the periplasm. , Although inorganic compounds, such as hydrogen, can be used as electron donors by chemolithoauthotrophs, the majority of electron donors in treatment processes are organic compounds, used by chemoorganoheterotrophs.…”

Section: Introductionmentioning

confidence: 99%

Modeling Denitrification as an Electric Circuit Accurately Captures Electron Competition between Individual Reductive Steps: The Activated Sludge Model–Electron Competition Model

Domingo-Félez

Smets

2020

Self Cite

Heterotrophic denitrification consists of the four-step sequential reduction of nitrate to dinitrogen gas over nitrite, nitric oxide and nitrous oxide. Oxidation processes, commonly of organic compounds, provide the electrons needed for the sequential reaction steps. The intracellular electron distribution is a competitive process among the four reduction steps. In this study, a model describing organic carbon oxidation and fourstep denitrification through electron competition is proposed (Activated Sludge Model -Electron Competition, ASM-EC). The model describes denitrification rates as an analogy to how current intensity varies through a parallel set of resistors in electric circuits. 1 The ASM-EC model was calibrated with data from batch experiments with heterotrophic denitrifying communities where reduction of mixtures of nitrogen oxides was monitored while different carbon sources were supplied in excess. The carbon sources included methanol, ethanol, acetate, and their ternary mixture. The electron distribution preference and electron uptake rates varied between the carbon sources and were captured by the model structure for most of the experiments. The ASM-EC model uses fewer parameters compared to existing state-of-the-art denitrification models and performed equally well in the tested scenarios. We advocate the use of this model for denitrification in the activated sludge model, which can easily be integrated in existing model structures, as it provides a parsimonious description of electron competition during denitrification.