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Article

The Suppression of Nitrite-Oxidizing Bacteria Using Free Nitrous Acid and Limited Available Dissolved Oxygen to Maintain the Stability of Toilet Wastewater Biofilm Nitritation

by
Ao Li
1,
Minghuan Lv
2,
Xue Jing
1,
Hongliang Wang
1,* and
Huijuan Feng
1,*
1
Chinese Research Academy of Environmental Sciences, Beijing 100012, China
2
China National Research Institute of Food and Fermentation Industries, Beijing 100015, China
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(23), 10165; https://doi.org/10.3390/su162310165
Submission received: 19 October 2024 / Revised: 16 November 2024 / Accepted: 18 November 2024 / Published: 21 November 2024
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Abstract

:
Researchers have found that maintaining the long-term stability of nitritation becomes challenging when relying on a single inhibitor. Currently, a feasible solution to this problem is to apply two or more inhibitors to achieve the synergistic suppression of NOB. However, studies on this solution have mainly focused on mainstream wastewater, while few have focused on non-mainstream wastewater. Moreover, most of the studies relating to non-mainstream wastewater have only focused on the spontaneous achievement of nitritation within a short operation time or have described nitritation collapse. Since toilet wastewater (TW), as non-mainstream wastewater, can endogenously produce free nitrous acid (FNA) through spontaneous nitritation, an attempt was made in this study through a series of field experiments to combine another inhibitor—a low concentration of dissolved oxygen (DO) available to NOB in the inner layer of biofilm—for biofilm nitritation. Under different levels of DO in the nitritation unit, the working effect and mechanism of high FNA–low available DO dual-factor suppression in maintaining nitritation stability were investigated. The results showed that the dual-factor suppression maintained the long-term stability of TW biofilm nitritation and triggered negative feedback regulation when the nitritation was unstable. A feasible method for establishing a low level of available DO based on a normalized FNA inhibitor when the COD/TN in the nitritation unit exceeds 0.50 is possible when the influent COD/TN of the unit is over 1.57. This study aimed to construct an endogenous and unregulated synergistic suppression strategy for stabilizing nitritation in non-mainstream wastewater to support the application of efficient and sustainable N-removal technology.

1. Introduction

Nitritation is a necessary intermediate step of efficient N-removal reactions, such as short-cut denitrification and anaerobic ammonia oxidation (ANAMMOX). Compared to the traditional N-removal process, NH3-N→NO2−-N→NO3−-N→N2(g), the ANAMMOX process is more sustainable and theoretically reduces oxygen consumption by 60% and organic carbon consumption by 100% while achieving the same total amount of nitrogen removal and sustainability. In the early research on nitritation, it was found that preventing nitrite from being successively oxidized to nitrate requires delicate regulation and is difficult to achieve. Through comprehensive research, and with the advancement of software and hardware conditions, such as the Internet of Things, automation, etc., several solutions have been discovered. However, these solutions can only suppress the activity of nitrite-oxidizing bacteria (NOB) and not that of ammonia oxidation bacteria (AOB), making nitritation easier to achieve. However, with further studies on nitritation, researchers have found that the activity of NOB gradually recovers, and nitrate accumulates after 100–200 days of nitritation with a single NOB inhibitor. In other words, under these conditions, nitritation cannot operate stably for a long time, and nitrite cannot be supplied steadily, preventing the popularization and application of high-efficiency N-removal technology [1,2,3,4].
For both non-mainstream wastewater (toilet wastewater, landfill leachate, and sludge liquid) and mainstream wastewater (municipal wastewater), it is difficult to stably suppress NOB activity using a single factor. The mechanisms involved are the same (i.e., a short sludge retention time, low dissolved oxygen, high free ammonia, and free nitrous acid) due to the strong adaptability and resilience of NOB to stressful environments by shifting their community structure [3,5,6,7]. NOB are groups of bacteria that have the function of catalyzing the conversion of nitrite to nitrate. Different species of NOB have different tolerances to suppression factors. The prolonged use of a single factor to suppress NOB activity results in the increased suppression in several NOB, while other NOB that are slightly more resistant to the suppression factor gradually become the dominant NOB. For example, Nitrospira has a lower dissolved oxygen (DO) affinity than Nitrobacter and Ca. Nitrotoga and is, therefore, able to maintain activity at a low level of DO (0.4–0.7 mg/L), making it a dominant NOB genus [8]. In turn, the dominant genus is converted from Nitrospira to Ca. Nitrotoga to cope with the sludge retention time (SRT) of only 1 day, as the latter has a faster growth rate [9]. Therefore, even in the case of a low level of DO or short SRT, the different dominance patterns of NOB species and the ecological characteristics of dominant NOB that are able to adaptively transform make it difficult to guarantee the stability of a single factor in suppressing NOB activity. This has been recognized by researchers in the field.
The current research on methods through which to achieve the long-term stability of nitritation focuses on the multifactorial synergistic suppression of NOB activity. Factors that suppress NOB can be broadly categorized into three types, namely stressful factors, inhibitors, and substrate gradients [10,11]. Specifically, stressful factors include low DO levels, short SRTs, high temperatures, light, etc. Inhibitors include free ammonia (FA), free nitrous acid (FNA), and NH2OH, etc. The substrate gradient refers to the construction of microenvironments that inhibit NOB activity, such as using biofilm or granular sludge to form a DO concentration gradient, leading to the low DO suppression of NOB in the inner layer of a microbial system [12]. The multifactorial synergistic suppression of NOB is meant to establish conditions in which at least two types of suppression factors coexist, such as the stress factor of salinity with an N2H4 inhibitor [13], the stress factor of a short SRT with an NH2OH inhibitor [3,14], alternating inhibition using two different inhibitors, FNA and FA [15], or using biofilm with a substrate gradient or granular sludge with inhibitors [16,17,18]. The above suppression combinations have been proven in laboratory tests to be viable approaches for maintaining nitritation stability. However, most of the studies on the use of multifactorial stacking to inhibit NOB and obtain stable nitritation have focused on mainstream wastewater, while few studies have focused on non-mainstream wastewater that carries stress factors or can endogenize inhibitors. Moreover, most of the studies related to non-mainstream wastewater have only focused on the spontaneous achievement of nitritation within a short operation time or have described nitritation collapse [19].
Previous studies have demonstrated that toilet wastewater (TW) aerobic biofilm can spontaneously achieve nitritation using endogenous FNA [20]; at the same time, instability of the nitritation process also emerged [21]. Therefore, how to overcome NOB adaptation and maintain stable nitritation in the treatment of TW using multifactorial synergistic suppression is investigated in this study. Since biofilm (the condition for a substrate gradient) is the mainstream treatment process, this study focuses on the effects of a synergistic suppression strategy using an inhibitor FNA and limited available DO through a series of field experiments with gradient changes in the DO concentration of the nitritation unit. The concentration from low to high corresponds to the change in the DO from a suitable value for stable nitritation to an unsuitable one. This study explored how the above-mentioned synergistic suppression can achieve the stable nitritation of TW under different DO concentrations. In this study, we developed an endogenous and unregulated synergistic suppression strategy for stabilizing nitritation in non-mainstream wastewater to support the application of efficient N-removal technology.

2. Materials and Methods

2.1. Experimental Scenarios and System Design

In this study, five household three-chamber septic tanks in a rural area of the northern Anhui province were selected for this trial. The working volume of each tank was about 1.5 m3, and they received wastewater from conventional flush toilets (~6 L per flush) in houses with 3–4 persons. An aeration unit made of PVC pipe with a working volume of about 120 L was connected to the outlet of each septic tank. The aeration unit was filled with spherical filler (polypropylene, 25 mm in diameter, porosity of ~84%, filling rate of ~55%) [21]. The bottom of the aeration unit was equipped with a microporous aerator, and the aeration amount could be adjusted. Images of the experimental scenario and a schematic diagram of the experimental system are shown in Figure 1.

2.2. Experimental Design

Four of the septic tanks were selected as the test group to investigate how the synergistic suppression strategy of FNA (inhibitor) and a low-oxygen micro-environment (substrate gradient) supports the stable nitritation of TW under different DO concentrations and to determine the working mechanism and engineering characteristics. The remaining tank was designated as the validation group (V1) to verify the applicability and accuracy of the working mechanism. The four tanks in the test group were designated as S1, S2, S3, and S4, respectively. The DO concentrations in the aeration unit of the test groups were controlled by adjusting the aerator gear, with average values of 0.65, 1.89, 3.83, and 6.17 mg/L, respectively (Figure 2a). There was no specific regulation for the DO in the aeration unit of the validation group.
Regular monitoring of the five systems was conducted, averaging 1–3 times a week. The sampled units were the third chamber of the septic tanks and the aeration units. The monitored indicators included the pH, total ammonia nitrogen (TAN), nitrite, nitrate, total nitrogen (TN), and COD of the two functional units, and the DO in the aeration unit. The characteristics of the wastewater in the third chamber of the septic tanks and the aeration units are shown in Table 1.
Since the aeration units of the five septic tanks were not installed at the same time, the experimental systems began non-simultaneously; therefore, the sampling was not synchronized. However, all important operation phases of each system were consistently documented.

2.3. Analytical Methods

The nitrogen compounds (TAN, NO2−-N, NO3−-N, and TN) were measured using Nasser’s reagent spectrophotometry, diazo-coupled spectrophotometry, phenol disulfonic acid ultraviolet spectrophotometry, and alkaline potassium persulfate ultraviolet spectrophotometry. The chemical oxygen demand (COD) was measured using a reagent bottle method (20–1500 mg/L of COD reagent, Hach, Ames, IA, USA). The pH, DO, and temperature were monitored using a pH/DO meter (HQ30D53000000-PHC10103/LDO10103, Hach, Ames, IA, USA).

2.4. Data Analysis and Mechanism Elaboration

Based on the data from the long-term operation of the four test septic tanks, the variation rules of the parameters that may affect nitritation stability were comprehensively analyzed. Combined with correlation analysis among the parameters, the mechanism of stability–collapse–recovery of TW biofilm nitritation was illustrated. Subsequently, the operation results of the four test septic tanks were further analyzed according to the elaborated mechanism, and the applicability and accuracy of the mechanism were verified based on the long-term operation data of the validation group.

3. Results and Discussion

3.1. Toilet Wastewater Nitritation Under Varying DO Concentrations

The average DO concentrations of the aeration units were regulated to 0.65, 1.89, 3.83, and 6.17 mg/L for the four test septic tanks. The NO3−-N concentrations and NO2−-N/NO2−-N+NO3−-N (NAR, nitrite accumulation rate) were used to determine the stability of TW nitritation. The nitritation stability monitoring results, including the NO3−-N, NAR, FNA, and COD/TN, are given in Figure 3.
Figure 3a shows the nitritation stability when the DO concentration was controlled to less than 1 mg/L. For the first 150 days, the NO3−-N concentration stabilized at 1 mg/L. At days 150–170, TW temporarily did not enter S1, and the aeration unit was shut down, resulting in a reduction in the FNA in the aeration unit to nearly zero. Thus, the dual-factor suppression of NOB coupled with low DO disappeared. After the resumption of feedstock on day 174, NOB activity was temporarily increased, resulting in an increase in the NO3−-N concentration up to 8.5 mg/L and a decrease in the NAR down to 78.0%. With the increase in the FNA concentration above 0.02 mg/L, the NOB was suppressed again. The NO3−-N concentration decreased to 3.3 mg/L, and the NAR reached 95.4%. Thus, the stability of nitritation was restored.
Figure 3b shows the results at the DO concentration of 1.89 mg/L. No records were available from days 1 to 75 due to no sampling. At days 76–200, the mean concentration of NO3−-N was 1.9 mg/L. After day 200, the NO3−-N concentration gradually, but not significantly, increased to a maximum of 4.7 mg/L, and the NAR was 95.6%. Subsequently, the NO3−-N concentration decreased again to 2.7 mg/L. Although the DO concentration controlled by the aeration unit of S2 was higher than that of S1, high FNA and limited available DO dual-factor suppression basically occurred, and the nitritation process was stable overall, with a minimum NAR of 95.6%, which was similar to the ratio of S1.
As shown in Figure 3c, the average DO concentration was controlled to 3.83 mg/L. During the first 200 days, the mean NO3−-N concentration was 4.1 mg/L, and the NAR was 97.5%. After day 200, the NO3−-N concentration continued to increase to a maximum of 15.3 mg/L, and the NAR decreased to the minimum of 90.4%. Based on these results, the higher DO concentration in the aeration unit of S3 made it difficult for the FNA-limited available DO dual-factor suppression condition to occur and, thus, the nitritation process was less stable.
Figure 3d shows the results during the period of the DO concentration being controlled to 6.17 mg/L. Before day 100, the NO3−-N concentration was low, with a mean value of 2.1 mg/L and a NAR of 96.7%. After day 100, the NO3−-N concentration increased rapidly, reaching a maximum of 62.4 mg/L, and the NAR decreased to a minimum of 57.3%, which indicates nitritation collapse. Subsequently, the nitritation then gradually recovered, and the NO3−-N concentration eventually stabilized at 8.9 mg/L, with a NAR of 88.7%.
The process of nitritation stability from collapse to recovery was analyzed, taking system S4 as an example. At such a high DO concentration, FNA is the only factor that suppresses NOB activity. Therefore, the nitritation under this condition was unlikely to remain stable for a long period. On day 100, the nitritation started to destabilize. The triggering condition for the sustained increase in the NO3−-N concentration could have been the decrease in the COD/TN from 0.82 to 0.69 and then to less than 0.50, which made it difficult for a low-oxygen microenvironment to form under the biofilm [18,22]. In contrast, the triggering condition for the restoration of the nitritation stability after day 175 may have been the gradual increase in the COD/TN from below 0.50 to 0.84, which in turn re-established the condition of low available DO.

3.2. High FNA–Low Available DO Dual-Factor Suppression to Maintain the Stability of Toilet Wastewater Biofilm Nitritation

Based on the above results and discussion, the stability maintenance mechanism of TW biofilm nitritation can be explained by the high FNA–low available DO dual-factor suppression model. The model is shown below (Figure 4a). High FNA and a low concentration of NOB-available DO are the two factors that suppress NOB activity where the available DO is the amount of DO available to the NOB located on the biofilm. When these two factors occur simultaneously, they can effectively maintain the stability of the nitritation reaction. Specifically, high concentrations of FNA can be achieved spontaneously based on the characteristics of non-mainstream wastewater, while low concentrations of DO can be achieved through biofilm processes and the regulation of wastewater COD/N. It was reported that heterotrophic aerobic bacteria coexist with autotrophic aerobic bacteria, such as nitrifying bacteria like AOB and NOB, in the outer layer of biofilm [23]. Since autotrophic bacteria multiply more slowly than heterotrophic bacteria, when the concentration of organic matter in wastewater is high, it is overpopulated by heterotrophic bacteria. Autotrophic bacteria predominantly exist in the inner layer of biofilm. Microorganisms in the inner layer only have access to DO that penetrates the biofilm. The amount of DO penetrating the biofilm to reach a position available to the autotrophic bacteria is influenced by the wastewater DO concentration and the biofilm’s DO penetrability. The latter is mainly related to the thickness and structural characteristics of the biofilm, which can be characterized by the COD/N [24,25,26]. Therefore, the factors that determine the DO available to NOB in biofilm include two aspects—the wastewater DO concentration and the COD/N.
Under conditions where the inhibitor FNA can be steadily obtained through TW nitritation, the DO available for NOB needs to be controlled to achieve dual-factor suppression with high FNA that maintains the long-term stability of nitritation. Based on the results of this paper, without strict control of the DO concentration in the nitritation (aeration) unit, the COD/TN in the unit needs to be maintained at no less than 0.50. The data from the long-term operation of the experimental system also showed a significant (p ≤ 0.05) negative correlation between the COD/TN and the NO3−-N/TIN (Figure 4b); the latter could indicate the stability of nitritation. These results are in line with the principle that a higher COD/TN indicates more organic matter in the wastewater that consumes DO, which means a lower DO concentration in the wastewater. This is conducive to the stable maintenance of nitritation, resulting in a low value of NO3−-N/TIN. Therefore, there was a significant negative correlation between the COD/TN and the NO3−-N/TIN. Since the COD/TN in the nitritation unit was generally hard to determine and depended directly on the COD/TN of the influent to the nitritation unit, a COD/TN correlation analysis between the influent and the treated wastewater in the nitritation unit was carried out (Figure 4c). When the influent COD/TN was greater than 1.57, there was a 100% probability that the COD/TN in the nitritation unit was greater than 0.50, which enabled the high FNA–low available DO dual-factor suppression. Thus, TW nitritation stability was achieved, as was self-recovery from unstable operation.

3.3. Comprehensive Analysis of Toilet Wastewater Nitritation Using the Dual-Factor Suppression Mechanism

Based on the mechanism established in Section 3.2, the stable operation conditions of the four test septic tanks were discussed comprehensively to summarize the engineering features of TW biofilm nitritation (Figure 5). First, the single-factor suppression of NOB using FNA during the first 100 to 200 days of the operation period was able to achieve nitritation stability. Based on the experimental data (Figure 3), an FNA concentration greater than 0.02 mg/L could effectively suppress NOB, even if the average DO concentration reached 6.17 mg/L. As the operation period continued, i.e., when the single-factor suppression of NOB activity using FNA was maintained for more than 100–200 days, stable nitritation could not be achieved. At this time, several strains in the NOB adapted to the FNA suppression and their activity gradually recovered, requiring another suppression factor in conjunction with the FNA to maintain the TW nitritation stability.
For the S1 and S2 systems, controlling the wastewater DO to not exceed 2 mg/L was another factor in suppressing NOB activity. The amount of DO that penetrated the biofilm and reached the NOB-available position was low (Figure 4a), and the NOB activity was steadily suppressed by the high FNA–low available DO, which in turn controlled the NO3−-N/TIN to less than 3%. For the unit in which the wastewater DO exceeded 2 mg/L, the NOB activity gradually recovered after 100–200 days of operation, which directly led to nitritation collapse. In parallel, the COD/TN in the nitritation unit did not exceed 0.5. The NOB activity recovered because only one suppression factor—high FNA—was available during this period, and the low available DO concentration suppression condition was not established, resulting in a maximum fluctuation of the NO3−-N/TIN up to 25.9%. In contrast, when the COD/TN exceeded 0.5 and the low available DO suppression condition was re-established, the NOB was gradually suppressed and the NO3−-N/TIN decreased to less than 5% (Figure 3d).
The NO3−-N/TIN concentration of 8.2% when the nitritation fluctuated and then returned to stability in the nitritation (aeration) unit of S3 was higher than that of S4, probably since the latter had a COD/TN of 0.84, exceeding the threshold of 0.5. In addition, both S1 and S4 went through a shutdown phase, during which the system was not fed and the nitritation unit stopped working. In this state, the concentrations of both the FNA and DO in the nitritation unit were almost zero (Figure 3a,d), with only one suppression factor—low DO. Thus, when the system resumed feeding and aeration, a transient nitrate increase occurred in the nitritation unit, with a cycle of about 10 days. When FNA was generated and accumulated, the high FNA–low available DO dual-factor suppression condition was re-established, and the stability of nitritation was restored.

3.4. Validation of the Dual-Factor Suppression Mechanism with High FNA–Low Available DO

To verify the applicability and accuracy of the elaborated mechanism and engineering characteristics, we synchronously designed a demonstration system to validate the long-term operation data; the results are shown in Figure 6. It was found that from the beginning up to day 60, the NO3−-N/TIN did not exceed 1.0%. The mean concentration of FNA was 0.07 mg/L, and the COD/TN in the nitritation unit was significantly greater than 0.50. After day 60, the NO3−-N/TIN began to rise to 16.6%, and the COD/TN was less than 0.50. When the demonstration system had operated for 88 days, and the stability of nitritation was restored, the NO3−-N/TIN was about 1.5%, the COD/TN increased to more than 0.5, and the average concentration of FNA was 0.60 mg/L. The above parameters are in complete agreement with the engineering characteristics (Figure 5). Specifically, the COD/TN gradually decreased from 0.75 to 0.50 by day 60, predicting the instability of the nitritation.
In addition, no feed (a stopping phase) to the system was demonstrated during the trial period. When the feed was resumed, a sudden increase in NOB activity occurred, and the nitritation immediately returned to a stable state. This suggests that the system possesses a negative feedback mechanism for stability maintenance and self-regulation under destabilizing conditions when high FNA–low available DO dual-factor suppression occurs. The average DO concentration in the nitritation unit of the validation system was 6.5 mg/L, which demonstrates type a four engineering characteristic (Figure 5). Additionally, when the influent COD/TN was greater than 1.57, the COD/TN in the nitritation unit was significantly greater than 0.50.

4. Conclusions

In this study, based on previous research that toilet wastewater (TW) as a non-mainstream wastewater can endogenously generate free nitrous acid (FNA) and spontaneously carry out nitritation, a high FNA–low concentration of dissolved oxygen (DO) dual-factor suppression approach was newly developed by adopting a biofilm process. Under varying DO concentrations of wastewater in the nitritation unit, the following conclusions were obtained regarding the effects and mechanism of the dual-factor suppression approach to maintaining TW nitritation stability:
(1) The high FNA–low available DO dual-factor suppression of NOB enables the long-term stable operation of TW biofilm nitritation and possesses a negative feedback regulation capability to continuously maintain the activity of the biofilm when nitritation instability occurs.
(2) For TW, FNA is an existing normalized inhibitor of nitrite-oxidizing bacteria (NOB), and therefore, a low concentration of available DO is a key factor for the dual-factor suppression of NOB activity in the biofilm process. By adjusting the DO of the nitritation unit to be no more than 2 mg/L and/or the COD/TN in the unit to be no less than 0.50, a low concentration of DO available for NOB can be established and stable nitritation can be achieved.
(3) When the influent COD/TN of a TW nitritation unit is greater than 1.57, the probability that the COD/TN in the unit will be greater than 0.50 is nearly 100%.

Author Contributions

A.L.: Conceptualization, Methodology, Software, Writing-original draft, Writing—review & editing; M.L.: Writing—review & editing, Validation; X.J.: Writing—review & editing; H.W.: Data curation, Writing—review & editing; H.F.: Funding acquisition, Project administration, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

We are grateful to the Young Scientists Fund of National Nature Science Foundation of China (NO: 42407511), Joint Study on Ecological Environment Protection and Restoration of the Yangtze River (Phase II, 2022-LHYJ-02-0404).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Images of experimental scenario; (b) schematic diagram of experimental system.
Figure 1. (a) Images of experimental scenario; (b) schematic diagram of experimental system.
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Figure 2. (a) The DO concentrations in the aeration units of the test group; (b) the wastewater temperature during the test period.
Figure 2. (a) The DO concentrations in the aeration units of the test group; (b) the wastewater temperature during the test period.
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Figure 3. Changes in the relevant indicators of the nitritation (aeration) units in the test group: (a) S1, (b) S2, (c) S3, and (d) S4.
Figure 3. Changes in the relevant indicators of the nitritation (aeration) units in the test group: (a) S1, (b) S2, (c) S3, and (d) S4.
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Figure 4. (a) The high FNA–low available DO dual-factor suppression mechanism (COD/TN > 0.5); (b) correlation analysis among the main indicators of nitritation; (c) COD/TN relationship between the influent and the treated wastewater in the aeration unit.
Figure 4. (a) The high FNA–low available DO dual-factor suppression mechanism (COD/TN > 0.5); (b) correlation analysis among the main indicators of nitritation; (c) COD/TN relationship between the influent and the treated wastewater in the aeration unit.
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Figure 5. Engineering characteristics of stability conditions for TW biofilm.
Figure 5. Engineering characteristics of stability conditions for TW biofilm.
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Figure 6. (a) The DO concentration and (b) main indicators in the nitritation unit of the validation group.
Figure 6. (a) The DO concentration and (b) main indicators in the nitritation unit of the validation group.
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Table 1. The characteristics of the wastewater in the third chamber of the septic tanks and the aeration units.
Table 1. The characteristics of the wastewater in the third chamber of the septic tanks and the aeration units.
ItemS1S2S3S4V1
InfEffInfEffInfEffInfEffInfEff
pH9.067.17 (0.30)8.58 (0.10)6.90 (0.32)8.60 (0.08)7.03 (0.54)8.30 (0.35)7.05 (0.80)-7.21 (0.67)
COD *682119 (43)471 (167)114 (50)439 (174)108 (45)430 (228)152 (61)401 (111)133 (49)
TAN *35653 (20)290 (49)118 (24)294 (65)87 (40)228 (61)117 (61)243 (55)118 (50)
TN *385121 (37)314 (59)252 (48)313 (67)251 (47)258 (51)216 (47)255 (56)215 (56)
NO2−-N *-65 (21)-126 (31)-153 (31)-76 (30)-80 (41)
TAN: total ammonia nitrogen; TN: total nitrogen. * Unit, mg/L, standard deviation in parentheses. S1–S4 represent the four test groups in sequence; V1 represents the validation group; Inf and Eff represent the wastewater in the third chamber of the septic tanks and the aeration units, respectively.
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Li, A.; Lv, M.; Jing, X.; Wang, H.; Feng, H. The Suppression of Nitrite-Oxidizing Bacteria Using Free Nitrous Acid and Limited Available Dissolved Oxygen to Maintain the Stability of Toilet Wastewater Biofilm Nitritation. Sustainability 2024, 16, 10165. https://doi.org/10.3390/su162310165

AMA Style

Li A, Lv M, Jing X, Wang H, Feng H. The Suppression of Nitrite-Oxidizing Bacteria Using Free Nitrous Acid and Limited Available Dissolved Oxygen to Maintain the Stability of Toilet Wastewater Biofilm Nitritation. Sustainability. 2024; 16(23):10165. https://doi.org/10.3390/su162310165

Chicago/Turabian Style

Li, Ao, Minghuan Lv, Xue Jing, Hongliang Wang, and Huijuan Feng. 2024. "The Suppression of Nitrite-Oxidizing Bacteria Using Free Nitrous Acid and Limited Available Dissolved Oxygen to Maintain the Stability of Toilet Wastewater Biofilm Nitritation" Sustainability 16, no. 23: 10165. https://doi.org/10.3390/su162310165

APA Style

Li, A., Lv, M., Jing, X., Wang, H., & Feng, H. (2024). The Suppression of Nitrite-Oxidizing Bacteria Using Free Nitrous Acid and Limited Available Dissolved Oxygen to Maintain the Stability of Toilet Wastewater Biofilm Nitritation. Sustainability, 16(23), 10165. https://doi.org/10.3390/su162310165

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