N₂O Emission from Partial Nitrification and Full Nitrification in Domestic Wastewater Treatment Process

Abstract

Using actual domestic wastewater as the research object, nitrogen compounds and their combinations were added to different nitrification (partial nitrification, full nitrification) processes to investigate nitrous oxide (N₂O) emission and its nitrification mechanisms. The presence of influent NH₄⁺ was the driving force of N₂O emission during nitrification. Compared with full nitrification, NO₂⁻ in partial nitrification more readily generated N₂O by denitrification. Under the proportional gradient of NH₄⁺-N:NO₂⁻-N/NO₃⁻-N, 30:0, 20:10, 10:20, and 0:30, total N₂O emissions during partial nitrification were 2.81, 11.30, 65.20, and 11.67 times greater than the total N₂O emissions during full nitrification. Full nitrification was more beneficial to N₂O emission reduction. This provides a control strategy for N₂O emission reduction in wastewater treatment processes under the background of reducing the production of greenhouse gases.

1. Introduction

Nitrous oxide (N₂O) is a potent greenhouse gas whose greenhouse effect exceeds CO₂ by ~300 times [1]. N₂O is also a potential ozone depleting substance (ODS) [2]. Wastewater biological treatment is an anthropogenic source of N₂O emissions, and both nitrification and denitrification processes produce N₂O [3,4,5,6]. Full nitrification involves two steps: NH₄⁺ oxidization to NO₂⁻ via NH₂OH utilizing Ammonia Oxidizing Bacteria (AOB) followed by subsequent oxidation of NH₂OH to NO₂⁻ by AOB (the energy generation step); Secondly, NO₂⁻ is further oxidized to NO₃⁻ using Nitrite Oxidizing Bacteria (NOB). In this process, ammonia mono-plus oxidase (AMO) catalyzes the oxidation of NH₃ to NH₂OH and O₂ acts as the electron acceptor [7]. Hydroxylamine oxidase (HAO) catalyzes the oxidation of NH₂OH to NO₂⁻ and O₂ acts as the main electron acceptor [8]. In the nitrification process, there are two possible pathways for N₂O emission as a byproduct: (1) N₂O is produced during autotrophic denitrification by AOB, NO₂⁻ acts as an electron acceptor, and is converted into N₂O via NO using nitrite reductase and nitric oxide reductase [9,10]; (2) N₂O is produced by the incomplete oxidation of NH₂OH [11,12,13].

A few studies have reported some operational control factors related to N₂O emission regarding full nitrification, such as dissolved oxygen (DO), temperature, pH, Sludge Retention Time (SRT), salinity, and toxic substances [14,15,16,17,18,19,20,21]. However, fewer studies on N₂O emission during partial nitrification have been published. Most studies were conducted using culture medium and artificial wastewater, which does not accurately simulate the complex actual nitrification.

In this study, a Sequencing Batch Reactor (SBR) and specific test rules explored N₂O emission and some nitrification mechanisms using partial and full nitrification of sludge cultivated with actual domestic wastewater. These results provide a theoretical basis for the control of N₂O emission in wastewater treatment.

2. Methods

2.1. Sludge and Wastewater

Two 12L SBRs were used for cultivation of partial and full nitrification sludge and the index of domestic wastewater inflow is shown in

Table 1. The quality of real domestic wastewater.

	COD (mg/L)	NH4+-N (mg/L)	NO2−-N (mg/L)	NO3−-N (mg/L)	TN (mg/L)	pH	Alkalinity
minimum	88	39.6	0	0	56.4	6.9	262
maximum	276	91.2	2.8	1.2	98.5	7.7	343
average	182	65.4	1.4	0.6	77.4	7.3	303

. The traditional full nitrification operation mode was as follows. The average operational cycle was 420 min, which included feeding (30 min), aeration (240 min), anoxic denitrification (120 min), and settling (30 min), 3 cycles per day, with the DO was 2 mg/L and Mixed Liquid Suspended Solids (MLSS) remaining at about 3000 mg/L. SRT was 20 days, and the rate of nitrate accumulation during full nitrification was about 99%. The partial nitrification operation mode was similar to that for full nitrification, except the DO was 1 mg/L and running temperature was 30 °C, SRT was 11 days; the rate of nitrite accumulation during partial nitrification was about 98%. After anoxic denitrification, the effluent’s composition, consisting of NH₄⁺-N, NO₂⁻-N, and NO₃⁻-N were all below 1 mg/L. The full nitrification and partial nitrification sludge taken from the SBRs was aerated for 12 h, then washed 3 times with deionized water repeatedly, for batch testing. The effluent’s COD was less than 50 mg/L and could not be oxidized for longer.

2.2. Batch Test Rules

The experimental batch test reactor is shown in Figure 1. The effective volume of the reactor was 3 L. At the beginning of each batch test, 1 L of concentrated full nitrification and partial nitrification sludge was added into the reactor, followed by 2 L of wastewater, and the MLSS were controlled at 3000 mg/L. Nitrogen compounds, DO and pH levels were then adjusted for the batch set upon commencing operation (

Table 2. Batch test rules.

Batch Test Number	Sludge—Water Mixture Type	Initial pH	Reaction Time (h)	NH4+-N (mg/L)	NO2−-N (mg/L)	NO3−-N (mg/L)	DO (mg/L)
1	partial nitrification sludge + effluent	7.5	3	30	0		0.5
	partial nitrification sludge + effluent	7.5	3	20	10		0.5
	partial nitrification sludge + effluent	7.5	3	10	20		0.5
	partial nitrification sludge + effluent	7.5	3	0	30		0.5
2	full nitrification sludge + effluent	7.5	3	30		0	0.5
	full nitrification sludge + effluent	7.5	3	20		10	0.5
	full nitrification sludge + effluent	7.5	3	10		20	0.5
	full nitrification sludge + effluent	7.5	3	0		30	0.5
3	full nitrification sludge + effluent	7.5	3		20		0.5
	full nitrification sludge + effluent	7.5	3		20		1
	full nitrification sludge + effluent	7.5	3		20		2
	full nitrification sludge + effluent	7.5	3		20		3

). The running time for batch tests was 180 min.

2.3. Detection Method

COD, NH₄⁺-N, NO₂⁻-N, NO₃⁻-N were measured according to methods previously described [22]. DO, pH, T were measured by an oxygen, pH and temperature meter (WTW 340i, WTW Company, Munich, Germany). The Mixed Liquid Suspended Solids concentration was measured at the beginning and at the end of each test to obtain an average value, which was used for the calculation of the NH₄⁺-N oxidation rate, NO_x⁻-N production rate and N₂O emission rate. The total N₂O production consists of the N₂O emitted in the gaseous phase (emission-gas N₂O) and the N₂O dissolved in the mixed liquid phase (dissolved N₂O). The N₂O concentrations in gas samples were analyzed in triplicate using a gas chromatograph (Agilent 6890N, Santa Clara, CA, USA). The overhead space method was used to analyze the dissolved N₂O. Water and N₂O samples were taken at 30-min intervals.

3. Results and Discussion

3.1. N₂O Emission in Partial Nitrification Process

Figure 2 shows the variations of N₂O emissions under different NH₄⁺-N and NO₂⁻-N ratios during partial nitrification (partial nitrification’s effluent +NH4Cl/NaNO₂). As shown in Figure 2a–c, the maximum N₂O emission occurred when NH₄⁺-N was about to be oxidized completely, and its maximum value was 1.20 mg/L, 1.37 mg/L, and 1.48 mg/L. When the ratios of NH₄⁺-N to NO₂⁻-N were 30:0, 20:10, and 10:20, the time for N₂O to reach its highest level decreased, and the N₂O emission rates increased; they were 0.16, 0.30, and 0.49, respectively (in mgN/(gMLSS·L·h). This indicated that the initial emission rate of N₂O increased with added NO₂⁻ in the presence of NH₄⁺-N. This was due to the electrons provided by the NH₄⁺ oxidation process being used for autotrophic denitrification of AOB in the partial nitrification process.

When NH₄⁺ oxidation provided electrons, higher concentrations of electron acceptor NO₂⁻ led to increased autotrophic denitrification of AOB. As shown in Figure 2b,c, NO₂⁻ concentrations decreased with NH₄⁺ oxidation in the first 30 min of the reaction. However, in the absence of NH₄⁺, as shown in Figure 2d, even a NO₂⁻ concentration of 30 mgN/L did not result in N₂O emission above 0.15 mg/L. This reaction resulted in denitrification of only 3.5 mgN/L of NO₂⁻. The lack of BOD (effluent) suggested its electron source might be internal organic matter (PHB) stored in AOB sludge [23,24], hydrogen, and pyruvate [25]. In addition, under four different ratios of NH₄⁺-N and NO₂⁻-N, the total production of N₂O was 3.29, 4.52, 3.26, and 0.35 mgN/L, respectively. N₂O production was maximized when NH₄⁺-N and NO₂⁻-N ratios were 20:10. This was due to the presence of electron acceptor NO₂⁻ (10 mgN/L), and contrasted with the minimal levels observed with the ratio of 30:0 (Figure 2b). Compared with 10:20, the reaction in Figure 2b had more electron donors from NH₄⁺.

3.2. N₂O Emission in Full Nitrification Process

Figure 3 shows the variation of N₂O emission during full nitrification (full nitrification effluent +NH₄Cl/NaNO₂) under different NH₄⁺-N and NO₃⁻-N ratios. As shown in Figure 3a–c, with decreasing NH₄⁺-N and increasing NO₃⁻-N, the time it took for N₂O to maximize decreased; however, the N₂O emission rates also decreased (0.051, 0.029, and 0.005, mgN/(gMLSS·L·h)). When the ratios of NH₄⁺-N to NO₃⁻-N were 30:0, 20:10, and 10:20, the maximum yields of N₂O were 0.46, 0.18, and 0.02 mgN/L, respectively. As shown in Figure 3a–c, the maximum production of N₂O occurred as NH₄⁺-N was oxidized, which was the same as for the N₂O emissions in the AOB enrichment system. Figure 3 also shows that under the four proportional gradients, the total N₂O production was 1.17, 0.40, 0.05, and 0.03 (mgN/L), which indicated that with full nitrification (where AOB and NOB co-exist), the production of N₂O mainly depended on the initial concentration of NH₄⁺, rather than the concentration of NO₃⁻. In addition, as shown in Figure 3d, in the absence of COD, a very small amount of N₂O was still generated when NO₃⁻ was added to the system, which may be caused by the denitrification of using internal organic matter in sludge.

Figure 4 shows the variation of N₂O emissions during full nitrification (full nitrification effluent +NaNO₂ 20 mgN/L). This batch test added 20 mgN/L NaNO₂ to the nitrification system effluent and investigated the oxidation of NO₂⁻ by NOB to produce N₂O under different DO concentrations. As shown in this figure, N₂O reached its maximum value after 30 min, and its maximum production decreased with increasing DO, with values of 0.054 mg/L, 0.051 mg/L, 0.014 mg/L, and 0.007 mg/L. The NO₂⁻ oxidized to NO₃⁻ within 60 min. After 60 min, no NO₂⁻ remained in the system (data not shown), but the production of N₂O was not 0, which indicated that during full nitrification, there was still N₂O production during NO₂⁻ oxidation to NO₃⁻ by NOB. Although the amount was very small, this N₂O may come from microorganisms using endogenous substances to provide electrons for denitrification. During the reaction, the total production of N₂O was 0.14 mg/L, 0.09 mg/L, 0.04 mg/L, and 0.03 mg/L for the four different DO gradients, and the proportion of N₂O in influent NO₂⁻-N was 0.70%, 0.45%, 0.20%, and 0.15%. The percentage was smaller than N₂O production in an AOB enriched system (Figure 2d), which was 1.17% (when DO = 0.5 mg/L) and indicated that NO₂⁻ in an AOB enriched system was more readily denitrified than with full nitrification.

As shown in Figure 2 and Figure 3, under the same ratio of NH₄⁺-N to NO₂⁻-N and NO₃⁻-N (DO = 0.5 mg/L), and under four proportional gradients, in the AOB system with NO₂⁻, the total N₂O emissions were 2.81, 11.30, 65.20, and 11.67 times greater than the total N₂O emission during full nitrification, which indicated that under the same conditions of influent NH₄⁺-N, partial nitrification produced more N₂O than full nitrification. During partial nitrification, NO₂⁻ was more involved in autotrophic denitrification of AOB as a product, while during full nitrification, NO₃⁻ would not participate in autotrophic denitrification as a nitrification product. Moreover, NO₂⁻ further oxidized to NO₃⁻ by NOB as an intermediate product of full nitrification; therefore, only a small amount of NO₂⁻ was involved in autotrophic denitrification by AOB.

4. Conclusions

The N₂O emission in partial and full nitrification of actual domestic wastewater was investigated under specific conditions and yielded the following conclusions:

(1) The presence of influent NH₄⁺ was the driving force of N₂O emissions in full and partial nitrification processes.

(2) Compared with full nitrification, NO₂⁻ was more likely to participate in denitrification during partial nitrification to produce N₂O.

(3) Under four proportional gradients, the total production of N₂O during partial nitrification was 2.81, 11.30, 65.20, and 11.67 times greater than the total N₂O production during full nitrification.