Huijun Xie

Impact of COD/N ratio on nitrous oxide emission from microcosm wetlands and their performance in removing nitrogen from wastewater

Constructed wetlands (CWs) are considered to be important sources of nitrous oxide (N(2)O). In order to investigate the effect of influent COD/N ratio on N(2)O emission and control excess emission from nitrogen removal, free water surface microcosm wetlands were used and fed with different influent. In addition, the transformation of nitrogen was examined for better understanding of the mechanism of N(2)O production under different operating COD/N ratios. It was found that N(2)O emission and the performance of microcosm wetlands were significantly affected by COD/N ratio of wastewater influent. Strong relationships exist between N(2)O production rate and nitrite (r=0.421, p<0.01). During denitrification process, DO concentration crucially influences N(2)O production rate. An optimal influent COD/N ratio was obtained by adjusting external carbon sources for most effective N(2)O emission control and best performance of the CWs in nitrogen removal from wastewater. It is concluded that under the operating condition of COD/N ratio=5, total N(2)O emission is minimum and the microcosm wetland is most effective in wastewater nitrogen removal.

Methane emissions from a full-scale A/A/O wastewater treatment plant

Methane (CH(4)) emissions from a full-scale anaerobic/anoxic/oxic (A/A/O) wastewater treatment plant (WWTP) (Jinan, China) were investigated during spring and summer of 2010. Results showed that the major emission sources of CH(4) performed the following descending order: anaerobic tanks, oxic tanks, aerated grit chambers and sludge concentration tanks. The total annual fluxes of CH(4) emissions from the Jinan WWTP were 1.69 × 10(4)kg yr(-1), with the emission factors of per capita emissions of 11.3g CH(4) person(-1)yr(-1) and flow-based emissions of 1.55 × 10(-4)g CH(4) (L of wastewater)(-1). The estimated source strength of methane for all WWTPs in China was 6.2 Gg yr(-1) (1 Gg=10(9)g). The most significant factors influencing methane emissions were dissolved oxygen concentration in aerated grit chamber and oxic tank and water temperature in high density settler tanks.

Effect of anoxic/aerobic phase fraction on N2O emission in a sequencing batch reactor under low temperature.

Laboratory scale anoxic/aerobic sequencing batch reactor (A/O SBR) was operated around 15°C to evaluate the effect of anoxic/aerobic phase fraction (PF) on N(2)O emission. The ammonia removal exhibited a decrease trend with the increase of PF, while the highest total nitrogen removal was achieved at PF=0.5. Almost all the N(2)O was emitted during the aerobic phase, despite of the PF value. However, the net emission of N(2)O was affected by PF. Under the premise of completely aerobic nitrification, the lowest N(2)O emission was achieved at PF=0.5, with a N(2)O-N conversion rate of 9.8%. At lower PF (PF=0.2), N(2)O emission was stimulated by residual nitrite caused by uncompleted denitrification during the anoxic phase. On the other hand, the exhaustion of the easily degradable carbon was the major cause for the high N(2)O emission at higher PF (PF=0.5). The N(2)O emission increased with the decreasing temperature. The time-weighted N(2)O emission quantity at 15°C was 2.9 times higher than that at 25°C.

Effect of aeration rate on the emission of N2O in anoxic-aerobic sequencing batch reactors (A/O SBRs).

Nitrous oxide (N(2)O) is a significant greenhouse gas, and biological nitrogen removal systems have been shown to be a significant N(2)O source. To evaluate the control parameters for N(2)O emission in the wastewater treatment process, N(2)O emissions were compared in the activated sludge from anoxic-aerobic sequencing batch reactors (A/O SBRs) acclimated under different aeration rates, and fed with synthetic wastewater. Results showed that a higher aeration rate led to a smaller N(2)O emission, while reactors acclimated under mild aeration performed the best in terms of nitrogen removal efficiency. Most of the N(2)O was produced during the aerobic phase, regardless of the aeration rate. Trace studies showed that incomplete denitrification appeared to be the major process responsible for high N(2)O emission at a low aeration rate (Run 1), while incomplete nitrification was the reason for N(2)O emission at a higher aeration rate (Run 2 and Run 3). For enhancing the efficiency of nitrogen removal while lowering energy consumption and reducing N(2)O emission, the optimal aeration rate would be 2.7 L(air)/(L(reactor) . h), in terms of the synthetic wastewater used.

Aerobic granulation for nitrogen removal via nitrite in a sequencing batch reactor and the emission of nitrous oxide.

In this study, the granulation of nitrifying-denitrifying via nitrite process in a sequencing batch reactor (SBR) as well as N(2)O emission patterns was investigated. After 60 days of operation, 0.8 mm granules were obtained, and partial nitrification was achieved after NH(4)(+)-N was raised to 350 mg/L. Fluorescence In-Situ Hybridization (FISH) analysis indicated that a fairly large proportion of ammonia-oxidizing bacteria (AOB) was close to the surface but nitrite-oxidizing bacteria (NOB) were rarely found. Batch experiments showed that 64.0% of NH(4)(+)-N in influent was transformed into NO(2)(-)-N, which showed the granules had excellent partial nitrification ability. Inhibition of free ammonia (FA) and limited DO diffusion within granules may contribute to the development and stabilization of partial nitrification. This process did not simultaneously lead to increased N(2)O production. N(2)O emissions at the anoxic and aerobic phases were 0.06 and 13.13 mg N(2)O/cycle, respectively.

Effect of PHB and oxygen uptake rate on nitrous oxide emission during simultaneous nitrification denitrification process

Simultaneous nitrification denitrification (SND) process was achieved in a SBR system to evaluate the impacts of intracellular carbon source PHB and oxygen uptake rate (OUR) on nitrous oxide (N(2)O) emission. Compared with the sequential nitrification and denitrification (SQND) process, SND process significantly improved the nitrogen removal. N(2)O emission during SND process was much higher than the SQND process. The amount of N(2)O emission was 26.85 mg N per cycle in SND process, which was almost four times higher than that in SQND process. About 7.05% of the removed nitrogen during SND process was converted to N(2)O-N. N(2)O emission had great relations with the OUR and the OUR could reflect the N(2)O emission trend more exactly than the DO concentration. At the aerobic stage of SND, the simultaneous denitrification could carried out using PHB as the carbon source and N(2)O emission increased because of the slow degradation of PHB.

Bacterial community variation and microbial mechanism of triclosan (TCS) removal by constructed wetlands with different types of plants.

Triclosan (TCS) is a broad-spectrum synthetic antimicrobial agent that is toxic to microbes and other aquatic organisms. Constructed wetlands (CWs) are now popular in TCS removal. However, knowledge on the effects of TCS on the bacterial community and microbial removal mechanism in CWs is lacking. The effects of TCS (60 μg L(-1)) on bacterial communities in batch-loaded CWs with emergent (Typha angustifolia), submerged (Hydrilla verticillata), and floating plant (Salvinia natans) were analyzed by 454 pyrosequencing technology. After six periods of experiment, the TCS removal efficiencies were over 90% in CWs, and negative effects of TCS on bacterial community richness and diversity were observed. Moreover, plant species effect existed. Bacterial strains that contributed to TCS biodegradation in CWs were successfully identified. In TCS-treated T. angustifolia and H. verticillata CWs, beta-Proteobacteria increased by 16.63% and 18.20%, respectively. In TCS-treated S. natans CWs, delta- and gamma-Proteobacteria and Sphingobacteria increased by 9.36%, 19.49%, and 31.37%, respectively, and could relate to TCS biodegradation. TCS affected the development of certain bacteria, and eventually, the bacterial community structures in CWs. This research provided ecologically relevant information on bacterial community and microbial removal mechanism in CWs under TCS treatment.

Effects of pH on nitrogen transformations in media-based aquaponics.

To investigate the effects of pH on performance and nitrogen transformations in aquaponics, media-based aquaponics operated at pH 6.0, 7.5 and 9.0 were systematically examined and compared in this study. Results showed that nitrogen utilization efficiency (NUE) reached its maximum of 50.9% at pH 6.0, followed by 47.3% at pH 7.5 and 44.7% at pH 9.0. Concentrations of nitrogen compounds (i.e., TAN, NO2(-)-N and NO3(-)-N) in three pH systems were all under tolerable levels. pH had significant effect on N2O emission and N2O conversion ratio decreased from 2.0% to 0.6% when pH increased from 6.0 to 9.0, mainly because acid environment would inhibit denitrifiers and lead to higher N2O emission. 75.2-78.5% of N2O emission from aquaponics was attributed to denitrification. In general, aquaponics was suggested to maintain pH at 6.0 for high NUE, and further investigations on N2O mitigation strategy are needed.

N2O emission in a partial nitrification system: Dynamic emission characteristics and the ammonium-oxidizing bacteria community

This study attempts to elucidate the dynamics of nitrous oxide (N(2)O) emission and investigate the evolution of the ammonium-oxidizing bacteria (AOB) community in a partial nitrification system producing an influent suitable for the anammox process. Based on long-term monitoring, (0.80 ± 0.19, n = 7)% of the incoming nitrogen load was emitted as N(2)O. During the partial nitrification process, the N(2)O emission rate reached a maximum at the beginning of the aerobic period and stabilized at a low level after an initial peak. Moreover, the quantity of N(2)O emission increased quickly at the beginning of the cycle operation and then production slowed after 30 min. According to polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) analysis, the dominant AOB causing the N(2)O emission from the partial nitrification system were Nitrosomonas sp. Both Nitrosomonas sp. Nm33 and Nitrosomonas sp. Nm58 were enriched at high ammonia concentrations.

Identifying sources of nitrous oxide emission in anoxic/aerobic sequencing batch reactors (A/O SBRs) acclimated in different aeration rates

Both long term and batch experiments were carried out to identify the sources of the N(2)O emission in anoxic/aerobic sequencing batch reactors (A/O SBRs) under different aeration rates. The obtained results showed that aeration rate has an important effect on the N(2)O emission of A/O SBR and most of the N(2)O was emitted during the aerobic phase. During the anoxic phase, nitrate ammonification was the major source of N(2)O emission while denitrification performed as a sink of N(2)O, in all three bioreactors. The N(2)O emission mechanisms during the aerobic phase differed with the aeration rate. At low and high aeration rates (Run 1 and Run 3), both coupled-denitrification and nitrifier denitrification were ascribed to be the source of N(2)O emission. At mild aeration rate (Run 2), nitrifier denitrification by Nitrosomonas-like ammonia oxidizing-bacterial (AOB) was responsible for N(2)O emission while coupled-denitrification turned out to be a sink of N(2)O because of the presence of inner anaerobic region in sludge flocs.