Reduced iron induced nitric oxide and nitrous oxide emission

Formation of the greenhouse gas nitrous oxide in water treatment systems is predominantly studied as a biological phenomenon. There are indications that also chemical processes contribute to these emissions. Here we studied the formation of nitric oxide (NO) and nitrous oxide (N₂O) due to chemical nitrite reduction by ferrous iron (Fe(II)). Reduction of nitrite and NO coupled to Fe(II) oxidation was studied in laboratory-scale chemical experiments at different pH, nitrite and iron concentrations. The continuous measurement of both NO and N₂O emission showed that nitrite reduction and NO reduction have different kinetics. Nitrite reduction shows a linear dependency on the nitrite concentration, implying first order kinetics in nitrite. The nitrite reduction seems to be an equilibrium based reaction, leading to a constant NO concentration in the liquid. The NO reduction rate is suggested to be most dependent on reactive surface availability and the sorption of Fe(II) to the reactive surface. The importance of emission of NO and N₂O coupled to iron oxidation is exemplified by iron reduction experiments and several examples of environments where this pathway can play a role.     

Nitrous oxide emission during wastewater treatment

Nitrous oxide (N₂O), a potent greenhouse gas, can be emitted during wastewater treatment, significantly contributing to the greenhouse gas footprint. Measurements at lab-scale and full-scale wastewater treatment plants (WWTPs) have demonstrated that N₂O can be emitted in substantial amounts during nitrogen removal in WWTPs, however, a large variation in reported emission values exists. Analysis of literature data enabled the identification of the most important operational parameters leading to N₂O emission in WWTPs: (i) low dissolved oxygen concentration in the nitrification and denitrification stages, (ii) increased nitrite concentrations in both nitrification and denitrification stages, and (iii) low COD/N ratio in the denitrification stage. From the literature it remains unclear whether nitrifying or denitrifying microorganisms are the main source of N₂O emissions. Operational strategies to prevent N₂O emission from WWTPs are discussed and areas in which further research is urgently required are identified.     

Predominance of ammonia-oxidizing archaea on granular activated carbon used in a full-scale advanced drinking water treatment plant

Ozonation followed by granular activated carbon (GAC) is one of the advanced drinking water treatments. During GAC treatment, ammonia can be oxidized by ammonia-oxidizing microorganisms associated with GAC. However, there is little information on the abundance and diversity of ammonia-oxidizing microorganisms on GAC. In this study, the nitrification activity of GAC and the settlement of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in GAC were monitored at a new full-scale advanced drinking water treatment plant in Japan for 1 year after plant start-up. Prechlorination was implemented at the receiving well for the first 10 months of operation to treat ammonia in raw water. During this prechlorination period, levels of both AOA and AOB associated with GAC were below the quantification limit. After prechlorination was stopped, 10⁵ copies g-dry⁻¹ of AOA amoA genes were detected within 3 weeks and the quantities ultimately reached 10⁶-10⁷ copies g-dry⁻¹, while levels of AOB amoA genes still remained below the quantification limit. This observation indicates that AOA can settle in GAC rapidly without prechlorination. The nitrification activity of GAC increased concurrently with the settlement of AOA after prechlorination was stopped. Estimation of in situ cell-specific ammonia-oxidation activity for AOA on the assumption that only AOA and AOB determined can contribute to nitrification suggests that AOA may account for most of the ammonia-oxidation. However, further validation on AOB contribution is required.     

The effect of pH on N2O production under aerobic conditions in a partial nitritation system

Ammonia-oxidising bacteria (AOB) are a major contributor to nitrous oxide (N₂O) emissions during nitrogen transformation. N₂O production was observed under both anoxic and aerobic conditions in a lab-scale partial nitritation system operated as a sequencing batch reactor (SBR). The system achieved 55 ± 5% conversion of the 1 g NH₄⁺-N/L contained in a synthetic anaerobic digester liquor to nitrite. The N₂O emission factor was 1.0 ± 0.1% of the ammonium converted. pH was shown to have a major impact on the N₂O production rate of the AOB enriched culture. In the investigated pH range of 6.0-8.5, the specific N₂O production was the lowest between pH 6.0 and 7.0 at a rate of 0.15 ± 0.01 mg N₂O-N/h/g VSS, but increased with pH to a maximum of 0.53 ± 0.04 mg N₂O-N/h/g VSS at pH 8.0. The same trend was also observed for the specific ammonium oxidation rate (AOR) with the maximum AOR reached at pH 8.0. A linear relationship between the N₂O production rate and AOR was observed suggesting that increased ammonium oxidation activity may have promoted N₂O production. The N₂O production rate was constant across free ammonia (FA) and free nitrous acid (FNA) concentrations of 5-78 mg NH₃-N/L and 0.15-4.6 mg HNO₂-N/L, respectively, indicating that the observed pH effect was not due to changes in FA or FNA concentrations.     

Long term partial nitritation of anaerobically treated black water and the emission of nitrous oxide

Black water (toilet water) contains half the load of organic material and the major fraction of the nutrients nitrogen and phosphorus in a household and is 25 times more concentrated, when collected with a vacuum toilet, than the total wastewater stream from a Dutch household. This research focuses on the partial nitritation of anaerobically treated black water to produce an effluent suitable to feed to the anammox process. Successful partial nitritation was achieved at 34 °C and 25 °C and for a long period (almost 400 days in the second period at 25 °C) without strict process control a stable effluent at a ratio of 1.3 NO₂-N/NH₄-N was produced which is suitable to feed to the anammox process. Nitrite oxidizers were successfully outcompeted due to inhibition by free ammonia and nitrous acid and due to fluctuating conditions in SRT (1.0-17 days) and pH (from 6.3 to 7.7) in the reactor. Microbial analysis of the sludge confirmed the presence of mainly ammonium oxidizers. The emission of nitrous oxide (N₂O) is of growing concern and it corresponded to 0.6-2.6% (average 1.9%) of the total nitrogen load.