Impact of N 2 O Emissions on Nitritation in Two Sequencing Batch Reactors: Activated Sludge Reactor and Biofilm System

Abstract

The objective of this study was to evaluate nitrous oxide (N₂O) emissions on nitritation in activated sludge sequencing batch reactor (ASSBR) and sequencing batch biofilm reactor (SBBR) treating ammonium-rich wastewater at two different influent ammonium concentrations. During stable operation, there was no significant difference of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} removal efficiencies between the two reactors. At the end of nitrification, both nitrite accumulation rates were above 85%. However, SBBR had a better total nitrogen (TN) removal efficiency (81.7% ± 1.5%) than that of ASSBR (66.5% ± 5.7%). According to typical cycles, when influent \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} concentrations were 120 mg/L and 240 mg/L, N₂O emissions during nitritation in SBBR accounted for 2.3% ± 0.4% and 6.5% ± 0.9% of the TN removed, respectively, which were 1/3-1/2 of that in ASSBR (6.6% ± 0.6%, 14.1% ± 1.6%). In addition, N₂O production increased with increase of feed ammonia concentration in both reactors. These results contribute to further understanding of N₂O emission in partial nitrifying system.

Introduction

Nitrogen removal via nitrite process is regarded as one of the cost-effective and sustainable biological nitrogen removal processes. The energy and carbon savings of partial nitrifying system are by virtue of restricting ammonia oxidation to nitrite rather than to nitrate (Wei et al., 2014). Therefore, it has great potential especially for ammonium-rich waste water treatment, in which organic matter is a limiting substrate to anoxic denitrification (Wei et al., 2013).

So far, most of the influencing factors including dissolved oxygen (DO), solid retention time, temperature, free ammonia (FA), and free nitrous acid (FNA) were investigated to achieve the reduction of the activity of nitrite oxidizing bacteria (NOB), without affecting ammonia oxidizing bacteria (AOB) (Guo et al., 2010). It is believed that DO is one of the critical parameters among those factors for achieving and maintaining nitritation, since oxygen affinity constant of AOB (0.3 mg/L) is lower than that of NOB (1.1 mg/L) (Ciudad et al., 2005).

Simultaneous nitrification denitrification (SND) via nitrite process is also a promising process, due to its high nutrient removal efficiency and low energy consumption (Zeng et al., 2004). The sequencing batch biofilm reactor (SBBR) can create a beneficial anoxic micro environment under aerobic conditions and has better performance of simultaneous nitrification and denitrification (Rong et al., 2008).

However, nitrite accumulation in either suspended activated sludge or biofilm reactor is commonly considered as an important factor causing nitrous oxide (N₂O) production (Kong et al., 2013). N₂O, a potent greenhouse gas and a major sink of stratospheric ozone (IPCC, 2007), can be produced by AOB via two main pathways: (1) the reduction of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_{2}^ -$$ \end{document} as terminal electron acceptor to N₂O via nitric oxide (NO), known as AOB denitrification (Kim et al., 2010) and (2) N₂O as a side product during incomplete oxidation of hydroxylamine (NH₂OH) to \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_{2}^ -$$ \end{document} , which is one of the key steps involved in ammonia oxidation (Law et al., 2012). It also would produce more N₂O in the denitrification using intracellular storage compounds as carbon source when treating low C/N ratio wastewater (Jia et al., 2012).

In recent years, many studies focused on the investigation of N₂O production usually associated with nitritation process. It has been found that N₂O production during nitrogen removal via nitrite was 1.5 times as much as that during nitrogen removal via nitrate (Yang et al., 2009). Zhang et al. (2009) compared the N₂O emission during nitritation in activated sludge system and SND in SBBR filled with carbon fiber by pulse feeding (ethanol was added as a carbon source every 2 h), and implied that the N₂O production during SND accounted for 1/3 of that under nitritation in activated sludge system.

However, compared to SBBR with pulse feeding, N₂O production and nutrient removal are quite different in SBBR performed with no addition of external carbon source. Sufficient carbon source inside the reactor led to low \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ - { {\hbox {-}}} { \rm N}$$ \end{document} accumulation (an important factor affecting the N₂O production) in SBBR with pulse feeding. Thus, the N₂O production in SBBR with pulse feeding was relatively low. It is desirable to explore the difference of N₂O emissions associated with activated sludge system and SBBR operated under nitritation mode without external carbon source. But little information is available regarding this.

Based on the above-mentioned discussion, the objective of this study was to evaluate the characteristics of nitrogen removal and N₂O emissions using activated sludge sequencing batch reactor (ASSBR) and SBBR. It is expected that the obtained results could contribute to further understanding of N₂O emission in partial nitrifying system.

Materials and Methods

Reactor setup and operation

Two SBRs (ASSBR and SBBR) each with a working volume of 5 L and 13 L were used to conduct this study (Fig. 1). The biomass in ASSBR and SBBR originated from the Wastewater Treatment Plant of Chang'an District of Xi'an, China. The initial concentration of the mixed liquor suspended solids (MLSS) in both reactors was 3000 mg/L. The percentage of VSS in TSS for ASSBR was 80% ± 2%. SBBR was a fixed bed sequencing batch reactor filled with plastic fiber. The fill ratio was 12%. The volumetric exchange ratios of both reactors were 36% with a cycle of 12 h. A constant airflow for aeration was introduced through a fine air diffuser at the bottom of the reactor and the aeration rate was kept at 20 L/h for all of the tests (DO concentrations were 0.10–0.25 mg/L in ASSBR and 1.50–2.00 mg/L in SBBR). A time controller was used to achieve the two SBRs running automatically. The temperature of the two SBRs was maintained at 30°C ± 2°C using a thermostatic heater in water bath. The solids retention time was kept at 15 days by discharging excess sludge.

FIG. 1.

Schematic descriptions of experimental systems: (a) is ASSBR and (b) is SBBR. ASSBR, activated sludge sequencing batch reactor; SBBR, sequencing batch biofilm reactor.

Both reactors were operated sequentially in 12 h-cycle under alternating anaerobic/aerobic/anoxic conditions. Each cycle of ASSBR and SBBR consisted of feeding (10 min), anaerobic reaction (50 min), aerobic and anoxic reaction (the length of the aerobic phase was determined by the change of DO and pH: air was still provided for another 0.5 h after the appearance of the “ammonia valley” point on the DO and pH curve (Fig. 2), then the anoxic reaction phase began), settling (40 min), and decanting (10 min).

FIG. 2.

Typical variations of nitrogen compounds and control parameters with low ammonia nitrogen (120 mg/L) in two SBR cycles: (a1, b1, c1) are COD, N-compounds, pH, DO and N₂O in ASSBR; (a2, b2, c2) are COD, N-compounds, pH, DO and N₂O in SBBR. DO, dissolved oxygen; FA, free ammonia.

In a preanoxic system, the anoxic phase is placed before the aerobic phase and mixed liquor containing nitrite is recirculated from the aerobic stage to the anoxic stage and in this kind of process complete removal of total nitrogen (TN) is almost unattainable. On the contrary, in a postanoxic process the anoxic phase is placed after the aerobic phase and mixed liquor recirculation is not necessary. Under anoxic condition, the internal carbon sources stored during the former reaction could be used for denitrification. In addition, the anaerobic/aerobic/anoxic process adopted here was a new process for recovery of nitrous oxide, which was reported by Zhao et al. (2015). In this article, the anaerobic/aerobic phases of two anaerobic/aerobic/anoxic processes were investigated, one was in a sequencing batch activated sludge reactor and the other was in a sequencing batch biofilm reactor, to compare the characteristics of N₂O emissions during nitritation of the two reactors.

After running for over 3 months, nitritation in the aeration process in two reactors were successfully achieved. The N₂O production during nitrification was measured more than three times. At the same time, liquid samples were also taken to measure the water quality ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N} , { \rm NO}_2^ - { {\hbox {-}}} { \rm N} , { \rm NO}_3^ - { {\hbox {-}}} { \rm N}}$$ \end{document} , and COD).

The \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ -$$ \end{document} concentration (<5 mg/L) was low at the end of a cycle (anaerobic/aerobic/anoxic) in SBBR; consequently, less \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ -$$ \end{document} participated in the next reaction phase. However, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ -$$ \end{document} concentration (>20 mg/L) at the end of a complete cycle in ASSBR was significantly higher than that in SBBR. Before the typical cycle (comparison test), the higher \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ -$$ \end{document} that remained in the ASSBR at the end of a cycle was washed out using distilled water to avoid the influence of remaining high \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ -$$ \end{document} on next cycle's N₂O production, so that the two reactors could be compared under the similar initial conditions.

Ammonium-rich wastewater

Composition of synthetic ammonium-rich waste water was as follows (mg/L): COD (as glucose), 500 mg/L; \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} (as ammonium bicarbonate), 120 and 240 mg/L; KH₂PO₄, 330 mg/L; CaCl₂, 16 mg/L; NaHCO₃, 1500 mg/L; MgSO₄.7H₂O, 50 mg/L, and trace element solution 1 mL/L (Lovley and Phillips, 1988).

Analytical methods

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}} , { \rm NO}_2^ - { {\hbox {-}} { \rm N} , { \rm NO}_3^ - { {\hbox {-}} { \rm N}}}$$ \end{document} , COD, VSS, TSS, and MLSS were measured according to the Chinese State Environmental Protection Agency (SEPA) Standard Methods (Chinese SEPA, 2002). TN was based on the sum of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}} , { \rm NO}_2^ - { {\hbox {-}} { \rm N} , { \rm NO}_3^ - { {\hbox {-}} { \rm N}}}$$ \end{document} rather than an independent TN test. On-line data were collected by probes for pH value, DO concentration, and temperature.

FA concentration was calculated according to Anthonisen et al. (1976). Dissolved N₂O concentration was measured using N₂O microsensor (Unisense, Denmark). Nitrite accumulation ratio (NAR) was calculated according to Zeng et al. (2014).

N₂O production and emission

N₂O generation [r_g; mg N/(L·s); Eq. (1)], accumulation [r_a; mg N/(L·s); Eq. (2)], and emission [r_e; mg N/(L·s); Eq. (3)] rates were calculated using balances based on on-line N₂O liquid concentration measurements (J. Gabarró et al., 2014). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}r_g = r_e + r_a \tag{1}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}r_a = dc_{N_{2}O} / dt \tag{2}\end{align*} \end{document}

When N₂O diffused from water to air, the N₂O emission rate can be calculated by Equation (3) (Hu et al., 2014). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}r_e = dc_{N_{2}O} / dt = - K_{LaN_{2}O} * ( C_{N_{2}O} -C_{S}) \tag{3}\end{align*} \end{document}

where \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $$K_{LaN_{2}O}$$ \end{document} is the volumetric mass transfer coefficient of N₂O from water to air (s⁻¹); \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $$C_{N_{2}O}$$ \end{document} is the N₂O concentration in liquid (mg N/L); C_S is N₂O saturation concentration in the liquid in equilibrium with air (mg N/L; assumed as 0); t is time (sec). Equation (3) was rewritten as \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}r_e = dc_{N_{2}O} / dt = - K_{LaN_{2}O} * C_{N_{2}O} \tag{4}\end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $$K_{LaN_{2}O}$$ \end{document} was determined by the following experiment: first, reactor that was at the end of settling phase was diluted using distilled water several times to remove the background components; then, moderate N₂O gas was ingested into the liquid of the reactor and the decrease of N₂O concentrations were measured. The r_e under certain aeration conditions was found by calculation of the N₂O reduction per second; finally, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $$K_{LaN_{2}O}$$ \end{document} was obtained by a linear fitting of r_e and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $$C_{N_{2}O}$$ \end{document} . The total N₂O production was obtained by the integral calculation of producing rate during reaction time.

Results and Discussion

Pollutant removal performance

Table 1 showed the contaminant removal performance of the two SBRs. The COD removal rates in ASSBR and SBBR were above 91%. In both reactors nearly all the \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} in the influent was removed completely with the removal rate of 98–99%. However, there was an obvious difference in TN removal efficiencies of the two reactors with TN removal efficiencies being 66.02–66.50% and 81.72–82.06% for ASSBR and SBBR, respectively. Obvious nitrite accumulations with above 85% of NAR were achieved at the end of the aerobic phases of the two reactors. The \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ - { {\hbox {-}} { \rm N}}$$ \end{document} concentrations in ASSBR were significantly higher than that in SBBR under the same influent load.

Table 1.

Mean Contaminant Concentrations with Standard Deviations (in Parentheses) and Removal Efficiencies (%) in Each System

	Influent (1)	Influent (2)	ASSBR (1)	ASSBR (2)	SBBR (1)	SBBR (2)
COD (mg/L)	500.58 (20.35)	471.35 (12.99)	40.08 (6.78)	21.42 (2.69)	38.78 (1.44)	27.71 (6.80)
COD removal (%)			91.74	95.44	91.99	94.12
\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ +$$ \end{document} (mg/L)	120.71 (11.16)	240.05 (13.36)	0.20 (0.34)	1.21 (2.10)	0.12 (0.17)	0.41 (0.64)
\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ +$$ \end{document} removal (%)			98.84	99.49	99.74	99.82
TN (mg/L)	121.51 (10.71)	242.05 (13.36)	37.98 (7.30)	79.45 (16.27)	21.11 (3.13)	50.58 (15.42)
TN removal (%)			66.50	66.02	81.72	82.06
\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} (mg/L)	0.65 (0.64)	1.25 (0.54)	1.45 (1.96)	3.81 (1.63)	2.28 (1.00)	7.01 (4.93)
\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ -$$ \end{document} (mg/L)	0.14 (0.12)	0.55 (0.40)	36.13 (5.88)	74.43 (14.08)	18.52 (3.28)	44.50 (17.11)
NAR (%)			92.03	95	90.65	85

TN, total nitrogen; NAR, nitrite accumulation ratio; ASSBR, activated sludge sequencing batch reactor; SBBR, sequencing batch biofilm reactor.

Typical circles with low nitrogen load

Pollutant removal characteristics with low nitrogen load

Figure 2a1, a2, b1, and b2 showed the time profiles of COD, TN, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}} , { \rm NO}_3^ - { {\hbox {-}} { \rm N}} , { \rm NO}_2^ - { {\hbox {-}} { \rm N}}$$ \end{document} , FA, and DO concentrations and pH value in a typical cycle for both reactors with influent COD and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} being 500 and 120 mg/L respectively. In the anaerobic period, the \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} concentrations in ASSBR remained invariant after injection. While at the 0–10 min after feeding in SBBR, TN decreased rapidly as the dilution and ammonia oxidation. COD in both reactors decreased rapidly and reached a minimum at the end of the anaerobic phase.

At the initial aerobic stage of ASSBR and SBBR, pH increased till 140 min and 90 min (Fig. 2b1, b2), respectively. It was one reason that CO₂ accumulated in the anaerobic phase released out of liquid. In addition, the degradation of the organic acid by aerobic bacteria was another reason. Then, the pH in the ASSBR and SBBR reactions decreased with a higher \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} decrease and rapid \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ - { {\hbox {-}} { \rm N}}$$ \end{document} accumulation in nitritation. COD and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ - { {\hbox {-}} { \rm N}}$$ \end{document} had been maintained at a low level. The DO concentrations were maintained at 0.12–0.23 mg/L in ASSBR and 1.69–1.94 mg/L in SBBR. The different phenomenon of two reactors was that the pH value in ASSBR decreased significantly but not in SBBR. The more effective SND in SBBR resulted in a relatively stable pH value in SBBR.

At 520 min in ASSBR and 200 min in SBBR of oxic phase, the typical bending point, ammonia valley occurred, which indicates the end of nitritation. Complete conversion of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} into \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ - { {\hbox {-}} { \rm N}}$$ \end{document} occurred at the end of the oxic phase, along with the maximum concentration of nitrite (42.49 mg/L and 22.12 mg/L, respectively). The FA concentrations were above the threshold concentration of NOB inhibition (0.1–1.0 mg/L), but below the threshold concentration of AOB inhibition (10–150 mg/L), which explained the \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ - { {\hbox {-}} { \rm N}}$$ \end{document} accumulation from the initial stage of the oxic phase in both reactors (Anthonisen et al., 1976).

N₂O emission characteristics with low nitrogen load

Figure 2c1 and c2 showed the track study of N₂O production with low nitrogen load during the two SBR cycles. During the anaerobic phase and the former aerobic phase (180 min in ASSBR and 100 min in SBBR), both the reactors expressed relatively low N₂O concentrations. N₂O production obviously increased when nitrite concentration accumulated to 5–6 mg/L. Nitrite accumulated as soon as no more degradable organic matter was available. In this study, both the reactors were supplied with high \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} sewage under low COD/N ratio. Kampschreur et al. (2009) reported that COD limitation and high \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} concentration could lead to \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ - { {\hbox {-}} { \rm N}}$$ \end{document} accumulation during nitrification phase. It is generally accepted that \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ - { {\hbox {-}} { \rm N}}$$ \end{document} is a key factor that could be used as a terminal electron acceptor to produce N₂O by AOB (Jia et al., 2013b). N₂O emissions in both reactors were correlated with nitrite accumulation in this article, which may be the main reason for N₂O emissions according to previous literature (Jia et al., 2013b).

Typical circles with higher nitrogen load

Pollutant removal characteristics with higher nitrogen load

Figure 3 (a1, a2, b1, b2) showed the time courses of parameters in a typical cycle with influent COD and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} were 500 mg/L and 240 mg/L. The changed trend of COD, TN, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}} , { \rm NO}_3^ - { {\hbox {-}} { \rm N}} , { \rm NO}_2^ - { {\hbox {-}} { \rm N}}$$ \end{document} , FA, and DO concentrations and pH value were consistent with that in low nitrogen load. It illustrated that the changed rule of all parameters above had repeatability in different nitrogen loads.

FIG. 3.

Variations of nitrogen compounds and control parameters with higher ammonia nitrogen (240 mg/L) in two SBR cycles: (a1, b1 and c1) are COD, N-compounds, pH, DO and N₂O in ASSBR; (a2, b2 and c2) are COD, N-compounds, pH, DO and N₂O in SBBR. ASSBR, activated sludge sequencing batch reactor.

N₂O emission characteristics with higher nitrogen load

Figure 3c1, c2 showed the track study of N₂O production with higher nitrogen load during the two SBR cycles. Compared with Fig. 2c1 and c2, the biggest difference was that N₂O in SBBR with high nitrogen load started to significantly rise at the beginning of the aeration as initial \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_2^ - { {\hbox {-}} { \rm N}}$$ \end{document} concentration was 16.64 mg/L (from the previous cycle or carried by sludge). This result also was consistent with Li et al. (2012) and Rassamee et al. (2011) who found that the high nitrite concentration was the main cause for N₂O generation. Meanwhile, the N₂O production (21.37 mg/L during anaerobic/aerobic phase) in ASSBR with high \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} load were much higher than that with low \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} load (6.25 mg/L during anaerobic/aerobic phase). Similar phenomenon was also found in SBBR. N₂O production increased with the increase of the feed ammonia load, which indicated that N₂O production was directly related to the feed ammonia load. This result was consistent with Yang et al. (2009).

Comparison of N₂O production in two reactors

Table 2 showed the TN removal efficiency and N₂O conversion rates under different operating conditions treating wastewater in recent literatures. The N₂O conversion rates in this study were 6.6% ± 0.6% in ASSBR and 2.3% ± 0.4% in SBBR when input \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} concentration was 120 mg/L. While influent \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} concentration was 240 mg/L, the N₂O conversion rates were 14.1% ± 1.6% in ASSBR and 6.5% ± 0.9% in SBBR respectively. The obtained results showed that the N₂O conversion rate in SBBR was 1/3-1/2 of that in ASSBR with the same ammonia load. It was mainly caused by the following reasons.

Table 2.

Summary of Operation Parameters and N₂O Emission in Each Reactor

Reactor	Operation condition	TN removal efficiency (%)	N₂O-N conversion rate (%) ^a	References
ASSBR	Anaerobic–aerobic (low DO [0.12–0.23 mg/L]; input COD = 500 mg/L; \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} N}$$ \end{document} = 120 mg/L)	66.5 ± 5.7	6.6 ± 0.6	This study
	Anaerobic–aerobic (low DO; input COD = 500 mg/L; \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} N}$$ \end{document} = 240 mg/L)		14.1 ± 1.6
SBBR	Anaerobic–aerobic, DO >1.8 mg/L (input COD = 500 mg/L; \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} N}$$ \end{document} = 120 mg/L)	81.7 ± 1.5	2.3 ± 0.4
	Anaerobic–aerobic, DO >1.8 mg/L (input COD = 500 mg/L; \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} N}$$ \end{document} = 240 mg/L)		6.5 ± 0.9
SBR	Anaerobic–aerobic (low DO [0.35–0.80 mg/L])	92.5	7.7	Jia et al. (2013a)
SBR	Anaerobic–aerobic, DO >2 mg/L	66.3	2.1	Jia et al. (2012)
SBR	Anaerobic–aerobic (low DO [0.15–0.5 mg/L]), acetate as carbon source	65.3	13.5–43.2	Zhu and Chen (2011)
SBR	Aerobic–anaerobic, methanol addition, DO = 1.5–2.0 mg/L	76.0	2.7	Liu et al. (2008)

N₂O conversion rate = (total N₂O-N production)/(total nitrogen removed) × 100%.

ASSBR, activated sludge sequencing batch reactor; DO, dissolved oxygen.

(1) The higher nitrite accumulation in ASSBR repressed the N₂O reduction more obviously, which led to more N₂O production in ASSBR. Park et al. (2000) also found that the production of N₂O has a good correlation with the presence of nitrite. The diffusion limitations of DO concentration could bring an anoxic zone in SBBR and a part of nitrite and N₂O in SBBR were reduced by the heterotrophic denitrification that occurred in the interior of biofilm during aerobic conditions.

(2) Although the aeration rates of two reactors were same, the DO concentrations were maintained 0.12–0.23 mg/L in ASSBR and 1.69–1.94 mg/L in SBBR. Most of researches reported that nitrifier denitrification was attributed as the dominant source of N₂O production in nitrification (Jia et al., 2013a and Wunderlin et al., 2012). Denitrification of AOB would be more favorable at low-oxygen condition. Oxygen stress is important for nitrifier denitrification. Colliver and Stephenson (2000) found that the N₂O yield under oxygen limiting conditions by Nitrosomonas europaea, which was probably the most representative nitrifier, was 3–5 times higher than that at high DO conditions. This is also a reason for the different N₂O production in two systems.

(3) The cycle of nitritation in ASSBR was significantly longer than that in SBBR at the same influent \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} load, which means the ammonia oxidation rate of ASSBR was relatively lower than that of SBBR. More intermediate substance produced by incomplete ammonia oxidation process led to more N₂O production in ASSBR. (4) Naoki et al. (2003) reported that some aerobic denitrifiers produce low levels of N₂O under aerobic conditions. Such bacteria with low N₂O production were likely to be enriched in biofilm system owing to the bacteria in biofilm system that were more complicated than those in activated sludge system.

TN removal efficiency during anaerobic-aerobic process in other literatures was 51.0–92.5%. In this study, the TN removal efficiency of ASSBR was 66.5% ± 5.7%, which was similar to Jia et al. (2012); Zhu and Chen (2011). Compared with the three researches above, it was found that N₂O emissions (6.6–43.2%) under low DO were much higher than that (2.1%) under high DO, which implied the low-oxygen stimulated the N₂O emission. The TN removal efficiencies were relatively high in SBBR (81.7% ± 1.5%) of this study, which was similar to Jia et al. (2013a) and Liu et al. (2008). Meanwhile the N₂O emission rates (2.3–7.7%) in the process with high TN removal efficiency were relatively less than that in the process with lower TN removal efficiency, which proposed that improvement of TN removal efficiency might reduce the production of N₂O.

Generally, the N₂O emissions in this study were different from the average 1.9% of the TN load from a lab-scale nitritation system (De Graaff et al., 2010) and 1.7% of the nitrogen load from a full-scale nitritation system for reject water treatment (Kampschreur et al., 2008). The different emissions can be contributed to different experimental conditions, wastewaters, type of reactors, and measurement methods used to estimate N₂O emission. For example, the N₂O emission rate observed in both ASSBR and SBBR were much higher than that (1.5%) presented in the article by Kong et al. (2013).

The different method of calculation was one possible reason for that. N₂O conversion rate in Kong's article was the ratio of N₂O production and the incoming nitrogen load. Furthermore, different modes of operation of the reactor may be one of the reasons. An intermittently aerated SBBR was used in the research by Kong et al. (2013). The operational mode had a great influence on N₂O production. Cycle configurations with intermittent aeration (aerated phases up to 20–30 min followed by short anoxic phases) were proven to effectively reduce N₂O emissions (Rodriguez et al., 2015). Zhang et al. (2015) investigated the effects of operational modes (fully aerobic mode and anoxic–aerobic mode) on N₂O production in SND process and found that the amount of N₂O emitted per cycle in the fully aerobic SND was 21.9% ± 7.1% of the removed TN, which was three times higher than 7.0% ± 1.6% observed in the anoxic–aerobic SND. In this study, low DO (0.12–0.23 mg/L) and high nitrite accumulation in ASSBR stimulated high N₂O production, even up to 14.1%.

Conclusions

An ASSBR and a SBBR were set up to evaluate the N₂O emissions under input \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} being 120 mg/L and 240 mg/L. Results implied that both the two reactors achieved similar \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NH}_4^ + { {\hbox {-}} { \rm N}}$$ \end{document} removal efficiencies, but TN removal efficiencies of SBBR (81.7% ± 1.5%) was higher than that of ASSBR (66.5% ± 5.7%). According to typical cycles, the N₂O production increased with the increase of the feed ammonia concentration in both reactors. Meanwhile, the N₂O production amount during nitritation of SBBR was lower than that of ASSBR with the same nitrogen load input.

Footnotes

Acknowledgments

This work was supported by the Fundamental Research Funds for the Central Universities (Grant No. 2013G3292017) and the Shaanxi Province Science and Technology Development Program (Grant No. 2014K15-03-02).

Author Disclosure Statement

No competing financial interests exist.

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Impact of N 2 O Emissions on Nitritation in Two Sequencing Batch Reactors: Activated Sludge Reactor and Biofilm System

Abstract

Abstract

Introduction

Materials and Methods

Reactor setup and operation

Ammonium-rich wastewater

Analytical methods

N2O production and emission

Results and Discussion

Pollutant removal performance

Typical circles with low nitrogen load

Pollutant removal characteristics with low nitrogen load

N2O emission characteristics with low nitrogen load

Typical circles with higher nitrogen load

Pollutant removal characteristics with higher nitrogen load

N2O emission characteristics with higher nitrogen load

Comparison of N2O production in two reactors

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

N₂O production and emission

N₂O emission characteristics with low nitrogen load

N₂O emission characteristics with higher nitrogen load

Comparison of N₂O production in two reactors