Nitrous oxide emission mitigation from biological wastewater treatment - A review

Variable emission factors of CH4 and N2O from WWTPs: A model-based analysis on available data

Climate change and carbon emissions are increasingly becoming a global concern, and thus wastewater treatment plants (WWTPs) are also receiving extensive attention due to direct greenhouse gas (GHG) emissions of methane (CH4) and nitrous oxide (N2O). Although there have been many emission factors (EFs) of CH4 and N2O in literature, they are changeful due to different processes and boundaries, which limits their values for reference and comparison. With this study, in situ monitored CH4 and N2O data reported in literature were retrieved for recalculating their EFs. The average EFs are found to be 0.0011 g CH4/g BOD5-influent, and 0.0017 g N2O-N/g TNinfluent, based on the secondary treatment. Subsequently, the data were analyzed using multivariate linear regression and neural network. The results indicate that BOD5 is the first factor affecting the EF of CH4, revealing a negative correlation and that TN is the second factor affecting the EF of CH4, but having a positive correlation. On the other hand, the neural network is a powerfully predictive and generalizable tool for EFN2O. BOD5 is negatively correlated with EFN2O, and EFN2O reaches to its maximum value at TN = 35 mg/L. Overall, the direct GHG emission intensity is the lowest in the AAO and AO processes, or with the BOD5/TN ratio between 2.5 and 4.9. Medium-sized WWTPs and the Oceania region exhibit the highest GHG emission intensity. With this study, an approximate approach is established to estimate the EFs of CH4 and N2O, which can facilitate to account the carbon footprint of WWTPs and also to aid in optimizing their operational schemes.

Effects of aeration control strategies on nitrous oxide emissions in alternating anoxic-oxic sequencing batch reactor systems

Reducing N2O emissions is key to controlling greenhouse gases (GHG) in wastewater treatment plants (WWTPs). Although studies have examined the effects of dissolved oxygen (DO) on N2O emissions during nitrogen removal, the precise effects of aeration rate remain unclear. This study aimed to fill this research gap by investigating the influence of dynamic aeration rates on N2O emissions in an alternating anoxic-oxic sequencing batch reactor system. The emergence of DO breakthrough points indicated that the conversion of ammonia nitrogen to nitrite and the release of N2O were nearly complete. Approximately 91.73 ± 3.35% of N2O was released between the start of aeration and the DO breakthrough point. Compared to a fixed aeration rate, dynamically adjusting the aeration rates could reduce N2O production by up to 48.6%. Structural equation modeling revealed that aeration rate and total nitrogen directly or indirectly had significant effects on the N2O production. A novel regression model was developed to estimate N2O production based on energy consumption (aeration flux), water quality (total nitrogen), and GHG emissions (N2O). This study emphasizes the potential of optimizing aeration strategies in WWTPs to significantly reduce GHG and improve environmental sustainability.

Environmental impacts on algal-bacterial-based aquaponics system by different types of carbon source addition: water quality and greenhouse gas emission

Carbon source addition is an important way improving the carbon and nitrogen transformation in aquaculture system; however, its effectiveness of algal-bacterial-based aquaponics (AA) through carbon source addition is still vague. In this study, the influences of organic carbon (OC-AA system) and inorganic carbon (IC-AA system) addition and without carbon source addition (C-AA system) on the operational performance of AA system were investigated. Results showed that 10.1-19.5% increase of algal-bacterial biomass enhanced the purifying effect of ammonia nitrogen in OC-AA system and IC-AA system relative to C-AA system. Moreover, extra electron donor supply in the OC-AA system obtained the lowest NO3--N concentration. However, that was at the cost of aggravated N2O conversion ratio, which increased by more than 2.0-folds than other systems, attributing to 2.9-folds increase of nirS gene abundance. In addition, carbon source addition increased the pH and then decreased the fish biomass production of AA system. The results of this study would provide theoretical supports of carbon source addition on the performance of nutrient transformation and greenhouse gas effect in AA system.

Nitrogen removal and nitrous oxide emission in the partial nitritation/anammox process at different reflux ratios

The partial nitritation/anammox (PN/A) process has been widely used in wastewater treatment owing to its notable advantages, including a low aeration rate and the non-requirement of an additional carbon source. In practical implementation, nitrite accumulation affects the nitrogen-removal efficiency and the amount of N2O released during the PN/A process. By implementing wastewater reflux, the nitrite concentration can be decreased, thereby achieving a balance between the nitrogen-removal efficiency and N2O release. This study conducted the CANON process with varying reflux ratios of 0 to 300 % and ~300 mg/L ammonium in the influent. The highest removal efficiency of ammonium and total nitrogen (98.2 ± 0.8 and 77.8 ± 2.3 %, respectively) could be achieved at a reflux ratio of 200 %. Further, a reflux ratio of 200 % led to the lowest N2O emission factor (2.21 %), with a 31.74 % reduction in N2O emission compared to the process without refluxing. Additionally, the reactor at a reflux ratio of 200 % presented the highest relative abundance of anaerobic ammonium-oxidizing bacteria (30.98 %) and the lowest proportion of ammonium-oxidizing bacteria (9.57 %). This study aimed to elucidate the impact of the reflux ratio on the nitrogen-removal efficiency of the CANON process and to theoretically explain the influence of different reflux ratios on N2O release. These findings provide a theoretical framework for enhancing the nitrogen-removal efficiency and mitigating carbon emissions in practical applications of the CANON process.

Enhanced nitrogen removal in sewage treatment is achieved by using kitchen waste hydrolysate without a significant increase in nitrous oxide emissions

Kitchen waste hydrolysate (KWH) is an effective replacement for commonly used carbon sources such as sodium acetate (NaAc) and glucose (Glu), in wastewater treatment plants (WWTPs) to enhance the total nitrogen (TN) removal efficiency in sewage and reduce the operating cost of WWTPs. However, KWH utilization introduces complex organic matter that may lead to increased nitrous oxide (N2O) emissions, compared with that of NaAc and Glu, causing significant damage to the atmosphere. Therefore, this study aims to compare the effects of KWH, Glu, and NaAc on N2O emissions in sewage treatment. The results indicated that KWH introduction did not lead to a significant increase in N2O emissions, with a conversion rate of only 5.61 %. Compared with raw sludge, the addition of only Glu and NaAc significantly increased the abundance of the nar G gene, indicating that the readily degradable carbon sources initiated denitrification at a faster rate than KWH. When KWH was added, there was a notable increase in the abundance of genes associated with partial nitrification and denitrification (nir K, hzo, and nos Z). In contrast, Glu and NaAc did not have a significant effect on the nos Z gene. The results suggested that KWH supplementation was more effective to reduce N2O to N2. Moreover, the KWH addition significantly increased the microbial diversity in the sludge and promoted the presence of shortcut nitrification and denitrification bacteria (Comamonadaceae) and denitrification bacteria (Rhodobacteraceae), further indicating the potential of KWH for enhanced denitrification and reduced N2O emissions. Overall, to the best of our knowledge, this is the first study that demonstrated KWH, as a novel and complex organic carbon source, can be safely used in sewage treatment processes to improve the pollutant removal efficiency without causing a significant increase in N2O emissions.

Novel self-stratifying bioelectrochemical system for municipal wastewater treatment halves nitrous oxide emissions

Reducing carbon footprint and greenhouse gas emissions are prime global goals. Wastewater treatment contributes significantly, and this study developed a technology with a focus on utilisation in small-decentralised plants. Bioelectrochemical systems (BES) utilise bacteria to remove pollutants while generating power and a range of experiments were performed to investigate their suitability compared to conventional trickling filters. A lab-based trickling filter was inferior to one adapted with electrodes both in terms of organic matter (COD) and phosphate reduction, but the BES did not generate electrical output due to inferior cathode configuration. An enhanced, novel, dual-BES system was developed with improved cathode positioning and operated as a cascade. This demonstrated improved COD (79 %) and total nitrogen (102 %) removal over the trickling filter. Concomitantly it emitted 47 % less N2O and generated an electrical output of 0.62 mA at 311 mV. Further work is needed to optimise BES but these results are encouraging in the development of sustainable biotechnologies.

Modelling greenhouse gas emissions from biological wastewater treatment by GPS-X: The full-scale case study of Corleone (Italy)

Greenhouse gas (GHG) emissions from wastewater treatment plants (WWTPs) can affect climate change and must be measured and reduced. Mathematical modelling is an attractive solution to get a tool for GHG mitigation. However, although many efforts have been made to create reliable tools that can simulate "sustainable" full-scale WWTP operation, these studies are not considered complete enough to include GHG emissions and energy consumption of biological processes under long-term dynamic conditions. In this study, activated sludge model no. 1 (ASM1) was modified to model nitrous oxide (N2O) emissions with a plant-wide modelling approach. The model is novel compared to the state of the art since it includes three steps denitrification, all N2O production pathways and its stripping in an ASM1. The model has been calibrated and validated through long-term water quality and short-term N2O emissions data collected from Corleone (Italy) WWTP. Different dissolved oxygen (DO) concentrations and return sludge (RAS) ratios were tested with dynamic simulations to optimise the full-scale WWTP. The scenarios have been compared synergistically with effluent quality, direct GHG emissions, and energy footprint by the water-energy‑carbon coupling index (WECCI). This modelling study is novel as it fully covers long-term calibration/validation of the model with N2O measurements and tests the dynamic optimisation. Decision-makers and operators can use this new model to optimise GHG emissions and treatment costs.

Engineering-scale application of sulfur-driven autotrophic denitrification wetland for advanced treatment of municipal tailwater

An engineering-scale sulfur driven autotrophic denitrification vertical-flow constructed wetland (SADN-VFCW) was established to treat low C/N ratio tailwater from municipal wastewater treatment plants (MWTPs). One-year stable operation results indicated that the addition of sulfur prominently enhanced TN, NO₃^--N and TP removal with efficiencies higher than 68.9%, 69.2% and 45.5%, respectively. Higher nitrogen and phosphorus removal rates were achieved in summer than that in other seasons. Furthermore, the microbial analysis revealed the structure of the microbial community changed significantly after sulfur addition, which proved that sulfur promoted the enrichment of autotrophic (Thiobacillus, Sulfurimonas, Ferritrophicum) and heterotrophic (Denitratisoma, Anaerolineaae, Simplicispira) functional bacteria, thus facilitating pollutants removal. Function prediction analysis results also indicated the abundance of nitrate removal/sulfur metabolism functions was significantly strengthened. This study achieved reliable engineering-scale application of SADN-VFCW and offered great potential for simultaneous in-depth treatment of N and P in municipal tailwater by SADN system.

Nitrogen removal from mature landfill leachate through enhanced Partial Nitrification-Anammox process in an innovative multi-stage fixed biofilm reactor

In the current integrated PN/A method/process for mature landfill leachate treatment, microbial inhibition and low nitrogen removal capacity are the big barriers due to high ammonia concentration and low C/N. This study aimed to evaluate the performance of a high-rate nitrogen removal lab-scale reactor, which combines pre-denitrification and Partial Nitrification-Anammox (PN/A) in a multi-stage fixed biofilm reactor (MFBR), for mature landfill leachate treatment. A nitrogen removal efficiency (NRE) of 90.43 % and an average nitrogen removal rate (NRR) of 0.94 kg/m³·d were observed at an influent NH+ 4-N concentration of 2274.39 mg/L during the last operational phase. The nitrogen mass balance showed that the nitrogen concentration gradually decreases along the course, and nitrogen was mainly removed in the aerobic chambers, in which Anammox contributed to 86.4 % of the removed nitrogen, while the front anoxic chamber is mainly used to remove NO- 3-N from the recirculation. Redundancy analysis showed that the variation in NH+ 4-N concentration along the course was the main factor affecting microbial community succession, which shows that the reactor configuration enables efficient cooperation and distribution of different microorganisms. Moreover, economic analysis of MFBR process showed that the energy consumption and carbon addition were reduced by 58.9 % and 100 %, respectively. Therefore, the MFBR established in this study, with its new configuration, achieves efficient treatment of landfill leachate in a single reactor and is environmentally friendly, and could be considered as a reference for full-scale landfill leachate treatment.