Study on Influencing Factors and Chemical Kinetics in the High-Concentration Simultaneous Nitrification and Denitrification (SND) Process

Abstract

High concentrations of activated sludge are an excellent biological treatment; in particular, simultaneous nitrification and denitrification play a huge role in nitrogen removal. However, the influencing factors of SND have not been fully elucidated. The effects of sludge concentration and dissolved oxygen (DO) concentration on the performance of SND in a high-concentration activated sludge reactor assisted by chemical agents were investigated, and the SND reaction effect was de-termined by analyzing the along-stream changes of elemental nitrogen in the reactor. The results showed that the SND phenomenon in the reactor was most obvious when the system activated sludge concentration (MLSS) was maintained at 7–9 g/L and DO concentration at 1–2 mg/L. When MLSS decreases within the range of 5–9 g/L, the nitrification reaction improves, but the SND phe-nomenon decreases or even disappears; the SND phenomenon diminishes with increasing DO con-centration. Thus, high sludge concentrations and low dissolved oxygen concentrations are im-portant influences associated with SND and promote unconventional nitrogen removal pathways. In addition, the average value of MLVSS/MLSS for the high-concentration activated sludge process was 0.586, which indicates that the system has a higher activated sludge volume and better sludge activity, which is very effective in enhancing SND. In addition, this study also further investigated the influencing factors of SND in the high-concentration method by exploring the kinetic modeling of the SND reaction in the high-concentration method.

1. Introduction

Biological and chemical treatments are the two most prevalent methods used in water treatment. Among these, biological treatment is generally considered to be more cost-effective and environmentally sustainable compared to chemical treatment [1]. Among these methods, chemically-assisted high-concentration activated sludge is an effective bi-ological treatment technique. The high-concentration activated sludge method has gar-nered significant attention from researchers due to its large microbial population, strong resistance to shock loads, and excellent nitrogen removal capabilities at low temperatures [2]. There are valuable lessons to be gained when constructing or retrofitting a new wastewater treatment plant.

The mixed liquor suspended solids (MLSS) in current domestic and international ac-tivated sludge methods typically average approximately 3 g/L, whereas high-concentrat-ion activated sludge systems usually have an MLSS above 4 g/L [3]. It was observed that aerobic granular sludge, characterized by its unique multilayer structure in high-concen-tration activated sludge reactors, possesses a dense configuration and high biomass conc-entration. This results in a significant treatment capacity, rapid settling, and the presence of diverse microbial communities [4]. High sludge concentrations restrict oxygen transfer and create a gradient in dissolved oxygen distribution within the sludge floc. This results in the formation of multiple zones that enable the coexistence of nitrifying and denitrify-ing microorganisms [5]. SND, which involves the simultaneous nitrifying and denitrifying of microbial reactions occurring within the same reactor, is a promising alternative to bi-ological nutrient removal (BNR) [6]. Various SND mechanisms have been proposed [7]. SND offers several advantages, including high removal efficiency, a compact footprint, and low nitrogen removal costs within a single unit [8]. SND is a biological process in which nitrification and denitrification occur simultaneously within a single reactor, with-out distinctly defined aerobic and anoxic zones. This approach has reportedly resulted in approximately 25% savings in aeration and a reduction of approximately 30% in carbon requirements [9]. SND-related reactions can occur spontaneously and are widespread in the environment [10]. SND also includes several nitrogen removal pathways, such as con-ventional nitrification/denitrification, heterotrophic nitrification, and aerobic denitrifica-tion (HNAD), all catalyzed by specific microorganisms. Additionally, it encompasses complete ammonia oxidation from the genus Nitrospiraea (comammox) [11]. Chai et al. [12] identified two HNAD microorganisms, such as Pseudomonas and Fusobacterium, in the SND reactor. SND, characterized by simple operation and low energy consumption, demonstrates high denitrification efficiency and promising application prospects. It has played a significant role in various wastewater treatment facilities and denitrification re-actors [11]. Therefore, the implementation of SND in high-concentration activated sludge systems is crucial for effective nitrogen removal from wastewater.

First, the higher concentration of activated sludge maintains the system under low load for an extended period, which promotes the colonization of autotrophic nitrifying bacteria and denitrifying bacteria that utilize PHA as their electron donor. This enhances the simultaneous removal of nitrogen and phosphorus from the system [13]. Therefore, sludge concentration is a critical factor to consider in this study of SND. Given the varying redox conditions necessary for nitrification and denitrification associated with SND in high-concentration activated sludge processes, the concentration of dissolved oxygen (DO) emerges as a key factor influencing SND [14]. According to the microenvironment theory, the gradient of dissolved oxygen concentration within the sludge or biofilm establishes varying redox conditions that enable the coexistence of nitrifying, denitrifying, and par-thenogenic anaerobic bacteria. This creates a favorable environment for SND [6]. The gra-dient of dissolved oxygen concentration is also influenced by sludge particle size. Theref-ore, applying different levels of dissolved oxygen to stimulate microorganisms at varying sludge concentrations is essential for determining the balance between nitrification and denitrification. This understanding is crucial for the SND process in high-concentration activated sludge systems. Studies have demonstrated that SND can significantly enhance nitrogen removal. For instance, Zhou et al. combined the high-concentration activated sludge method with a dissolved oxygen control technique, achieving a mixed liquor suspended solids (MLSS) concentration exceeding 6 g/L and a dissolved oxygen (DO) level below 1 mg/L at the aerobic front end. This approach strengthened the SND process in the system and enabled effective nitrogen removal from municipal wastewater, even under conditions of limited carbon sources and short hydraulic retention times [15]. Therefore, it is feasible to respond to the occurrence of the SND phenomenon by controlling the concentrations of activated sludge and dissolved oxygen in the reactor. This approach allows for the study of changes in nitrogen content in both the influent and effluent water.

Studies have shown that higher sludge concentrations contribute to an increase in the number of functional microorganisms [16], and they also affect the filamentous growth of biocake and bulk sludge [17], thereby promoting SND processes in anaerobic environments to enhance nitrogen removal efficiency. At the same time, lower sludge concentrations lead to reduced denitrification in aerobic tanks, resulting in a decreased amount of nitrate-nitrogen being converted and utilized through denitrification. The combination of enhanced nitrification and weakened denitrification causes a significant increase in ni-trate-nitrogen levels at reduced sludge concentrations. This indicates that within a certain range, as sludge concentration decreases, the nitrification reaction improves, but the SND phenomenon diminishes or may even disappear. Therefore, determining the optimal con-centration of activated sludge in the aeration tank is crucial for promoting SND produc-tion in high-concentration activated sludge systems.

The optimal SND performance was reported at a dissolved oxygen level of 0.7 ± 0.1 mg/L, achieving a total nitrogen (TN) removal efficiency of 72.28% and an SND efficiency of 73.69% [18]. Increased dissolved oxygen levels significantly inhibited denitrification, particularly at 4.5 mg/L, which resulted in reduced SND efficiency [19]. Sriwiriyarat et al. [20] showed that SND requires a dissolved oxygen concentration of 6 mg/L to sustain ef-fective nitrification, thereby generating more nitrous oxide for denitrification in high-con-centration activated sludge systems. As a result, there is no consensus on the impact of dissolved oxygen concentration on SND in these methods. It is essential to identify the optimal dissolved oxygen concentration to enhance SND performance in high-concentration activated sludge processes.

Currently, there are numerous studies related to SND; however, research on the high--concentration activated sludge method remains limited, with very few studies focusing specifically on SND reactions. Therefore, a detailed analysis of the factors influencing the SND reaction from the perspective of the high-concentration activated sludge method is essential. This research holds significant practical importance for sewage treatment plants that utilize this method for denitrification.

To address these issues, this study examined the effects of sludge concentration and dissolved oxygen on the performance of SND in high-concentration activated sludge pro-cesses. Therefore, the objectives of this study were: (1) to examine the effects of dissolved oxygen (DO) concentration and sludge concentration on denitrification performance and SND; (2) to determine the optimal concentrations of activated sludge and dissolved oxyg-en (DO) necessary for the effective operation of a high-concentration activated sludge reactor; and (3) to analyze the sludge sedimentation performance and the SND mechanism. Additionally, we derived the kinetic model for simultaneous nitrification and denitrifica-tion in a high-concentration activated sludge system, as well as the reaction kinetic equat-ion for this experiment, based on the classical kinetic model of the activated sludge method.

2. Materials and Methods

2.1. Test Water Quality

This test was conducted at a sewage treatment plant located in Chengdu City, which processes effluent from the regulating pool. The sewage plant manages discharges from an area that utilizes a combined rainwater and sewage system. The concentration levels of COD and BOD5 are low, indicating a limited organic carbon source. The water quality parameters are presented in

Table 1. Test for water quality.

Serial Number	Sports Event	Realm	Average Value
1	CODcr (mg/L)	45.2–373	138
2	BOD5 (mg/L)	29.5–169	67.3
3	NH3-N (mg/L)	9.54–49.8	23.7
4	TN (mg/L)	12.9–87.8	36.1
2	TP (mg/L)	1.24–7.39	3.52

.

2.2. Test Equipment

In this experiment, a chemically-assisted high-concentration activated sludge method was employed, utilizing new anoxic and mixed reaction tanks in comparison to conventional water treatment methods. The system diagram is shown in Figure 1.The di-mensions of the anoxic tank are 2.3 m × 2.3 m, with a height of 2.8 m, resulting in an effec--tive volume of 13.2 m³. Additionally, there is a return piping system connecting the an-oxic and aerobic tanks. The mixing reaction tank is divided into three compartments: one for mixing and two for flocculation. The mud-water mixture from the aeration tank is pumped into the mixing tank while simultaneously introducing CPAM and PAC via a dosing pump. This ensures thorough mixing of the two types of chemicals with the mud--water mixture. The mixture then flows through the two flocculation tanks in succession and ultimately returns to the aeration tank due to gravity. The dimensions of the mixing tank are 0.5 m × 0.5 m with a height of 1.4 m, while the flocculation tanks each measure 0.7 m × 0.7 m with a height of 1.4 m.

2.3. Testing Instruments

The main instruments and equipment, along with their models, are shown in

Table 2. Main equipment.

Equipment Name	Equipment Model
PH meter	Hash HQ-40d Portable Monitor
dissolved oxygen concentration meter	Hash HQ-40d Portable Monitor
pump	SCN35.65.037.4
stirrer	CMJB-0.25A
dosing pump	Solenoid pump
electromagnetic flow meter	HXLD-DN40 electromagnetic flow meter
electromagnetic flow meter	HXLD-DN15 electromagnetic flow meter
electrically heated drying oven	101-1AB
muffle furnace	SX-4-10

.

2.4. Test Reagents

The chemical reagents used during the test are outlined in

Table 3. Drugs and reagents.

Reagent Name	Fineness
PAC	AR
CPAM	AR
potassium dichromate	AR
concentrated sulfuric acid	GR
ferrous sulfate	AR
caustic soda	AR
anhydrous ethanol	AR
chlorine residual powder kit	DPD
total chlorine powder package	DPD

. Among these, PAC and CPAM serve as flocculants essential for forming high-concentration activated sludge. After daily manual dosing, these chemicals are added to the solution barrel and subseq-uently delivered to the mixing reaction tank via a dosing pump. The CPAM dosage was maintained at 5 mg/L, while the PAC dosage was set at 60 mg/L.

2.5. Analytical Items and Test Methods

The water quality indicators and testing methods are shown in

Table 4. Water quality testing and analysis methods.

Water Quality Indicators	Detection Methods
COD	Potassium dichromate
NH4+-N	Nasher’s reagent photometric method
NO3-N	ultraviolet spectrophotometry
NO2-N	N-(1-Naphthyl)-ethylenediamine photometric method
TN	Alkaline potassium persulfate-ultraviolet spectrophotometry
TP	Molybdenum antimony spectrophotometry
PH, DO, temperature	Hash HQ-40d Portable Monitor

.

The sludge testing items and measurement methods are shown in

Table 5. Sludge testing items and corresponding measurement methods.

Sludge Testing Program	Measurement Methods
MLVSS	cauterization
MLSS	gravimetric method
SV%	sedimentation

.

3. Results and Discussion

3.1. Effect of Sludge Concentration on SND

To investigate the impact of sludge concentration on the system’s SND effect, this study controlled the sludge concentration through the discharge pipe under the aeration tank, maintaining ranges of 7–9 g/L and 5–7 g/L. The system operated stably under opti-mal nitrogen removal conditions, allowing for the simultaneous monitoring of nitrogen fluctuations in the influent and effluent of the aerobic tank. The results, shown in Figure 2, indicate that when the sludge concentration in the aerobic tank decreased from 7–9 g/L to 5–7 g/L, the total nitrogen reduction in the aerobic tank dropped from 1.27 mg/L to 0.23 mg/L, while the reduction in ammonia nitrogen concentration increased from 4.05 mg/L to 4.78 mg/L. This suggests that a significant decrease in nitrogen loss occurs in the system when the sludge concentration is reduced. Additionally, the C/N ratio is a crucial factor influencing the SND efficiency of activated sludge [21]. A lower sludge concentration also results in a reduced C/N ratio. This reduction facilitates the enrichment of slow-growing nitrifying bacteria, which contributes to a lower overall growth rate. Consequently, this leads to the formation of smaller-sized aerobic particles (0.41–0.48 mm) that exhibit high stability [22]. Biological denitrification operates by reducing the nitrate content in water and converting it into nitrogen gas [23]. Simultaneously, the oxygen diffusion efficiency in the aerobic tank improves, increasing the space available for effective nitrification. This results in enhanced nitrification and improved ammonia-nitrogen removal. In the experi-ment, the nitrate nitrogen concentration increased from 2.64 mg/L to 3.21 mg/L. Zhou’s study indicated that as the C/N ratio decreased, the dominant denitrifying bacteria shifted from incomplete denitrification to complete denitrification [24]. This suggests that when the activated sludge concentration was reduced from 7–9 g/L to 5–7 g/L, nitrification was boosted within the system, allowing for a greater conversion of ammonia nitrogen to ni-trate nitrogen. At high sludge concentrations, the conversion of nitrate to nitrite and the subsequent conversion of nitrite to ammonia during the reduction of assimilated nitrate are inhibited [18], which in turn enhances the denitrification effect. Conversely, the de-crease in sludge concentration weakens denitrification in the aerobic tank, reducing the amount of nitrate nitrogen converted and utilized through this process. The combination of enhanced nitrification and diminished denitrification leads to a significant increase in nitrate nitrogen at lower sludge concentrations. Additionally, a higher C/N ratio results in shallower oxygen penetration, which in turn enhances SND performance [25]. This occ-urs because high sludge concentrations restrict oxygen transfer and create a gradient in dissolved oxygen distribution within the sludge floc. This results in the formation of mul-tiple zones that allow nitrifying and denitrifying agents to coexist. The internal anoxic conditions within the floc promote the reverse nitrification process, thereby enhancing SND [26]. In summary, the concentration of activated sludge in the aeration tank is a cru-cial factor influencing the generation of SND in high-concentration activated sludge systems.

3.2. Effect of DO Concentration on SND

Oxygen permeation is one of the key factors influencing denitrification within parti-cles or biofilms [27]. To investigate the effect of DO concentration on SND in high-concen-tration activated sludge systems, this study controlled the DO levels in the aeration tank at three intervals: 1–2 mg/L, 2–3 mg/L, and 3–4 mg/L. The changes in inlet and outlet water quality within the aerobic tank were analyzed, with results presented in Figure 3. The concentration of ammonia nitrogen rose from 4.16 mg/L at the lowest DO level to 4.82 mg/L and 5.11 mg/L as the DO concentration increased to the higher intervals. This sug-gests that a higher DO concentration in the aerobic tank progressively enhances nitrificat-ion and improves ammonia nitrogen removal. Additionally, the presence of a DO conc-entration gradient may facilitate the coexistence of autotrophic nitrifying bacteria and het-erotrophic denitrifying bacteria [28]. Therefore, the ammonia-nitrogen removal rate imp-roved. At the same time, the concentration of nitrate nitrogen increased with rising DO levels, changing from 2.74 mg/L to 3.47 mg/L and then to 3.59 mg/L. Nitrate nitrogen can be selectively removed by microorganisms through denitrification and the reduction of nitrates to ammonium (DNRA) [29]. The conventional biological nitrogen removal process includes two steps: aerobic nitrification and anoxic denitrification. In the second step, ni-trate nitrogen is reduced to nitrogen by denitrifying bacteria [30]. This indicates that changes in DO concentration influence the nitrification rate in the aerobic tanks and subs-equently affect the concentrations of ammonia and nitrate nitrogen. When the DO conc-entration increased from 1–2 mg/L to 2–3 mg/L, the total nitrogen reduction decreased from 1.27 mg/L to 0.61 mg/L. With a further increase in DO concentration to 3–4 mg/L, the total nitrogen reduction dropped to 0.21 mg/L. This suggests that nitrogen loss due to denitrification decreases as the DO concentration rises in the aerobic tank, leading to a gradual decline in the total nitrogen removal rate. This is because complete nitrification is primarily accomplished at dissolved oxygen concentrations exceeding 5 mg/L, whereas short-path nitrification typically takes place at DO levels ranging from 0.3 to 1.5 mg/L [31]. In the denitrification process, dissolved oxygen levels below 0.5 mg/L typically do not inhibit the process, while concentrations above 1 mg/L can partially suppress denitrifica-tion [32]. Huang et al. found that dissolved oxygen levels of 1 mg/L and 2 mg/L resulted in higher simultaneous nitrification and denitrification (SND) and improved nitrogen re-moval efficiency through both complete nitrification and denitrification pathways [33]. Thus, the denitrification process can achieve denitrification by controlling the concentrat-ion of DO [34]. An increase in dissolved oxygen concentration can enhance the rate of nitrification in the aerobic cell, thereby influencing the accumulation of nitrate nitrogen. However, when the DO concentration is too high, it can cause the flux of nitrite and nitrate to be released from the particles into the surrounding liquid, resulting in less effective TN removal [35]. This would further limit denitrification, thereby reducing the effectiveness of simultaneous nitrification and denitrification in aerobic tanks. However, excessively low dissolved oxygen concentrations are also unacceptable. Research shows that both ex-cessively high and insufficient dissolved oxygen concentrations can reduce TN removal efficiency [36]. This study demonstrated that the SND phenomenon in the reactor was most pronounced when the dissolved oxygen concentration in the aerobic tank was maint-ained at 1–2 mg/L.

3.3. Analysis of Sludge Concentration and Sludge Settling Performance

In wastewater treatment, mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) are commonly used to assess the sludge content and quality in aeration tanks. A higher ratio of MLVSS to MLSS indicates improved activated sludge activity. Figure 4 illustrates the variation of MLSS and MLVSS in the high-concen-tration activated sludge process during system operation. The average MLSS value for this process was approximately 7.6 g/L, while the average MLVSS value was about 4.6 g/L. The resulting MLVSS/MLSS ratio was 0.586, suggesting a substantial amount of activated sludge and good sludge activity within the system.

During system operation, the changes in SV30 and SVI are illustrated in Figure 5 As shown in Figure 5, the SV30 of the high-concentration household sludge system ranges from 57.3% to 77.2%, while the SVI values fluctuate between 88% and 99%. The sludge settling ratio is higher than that observed in conventional activated sludge methods, at-tributed to the elevated concentration of activated sludge in the aeration tank. Addition-ally, particle size plays a crucial role in determining the formation of hypoxic zones [37]. Larger particles maintain a hypoxic zone for a longer duration. Particle size is significant because it influences the volume and duration of the anoxic zone within the particle. A larger diameter also restricts the rapid diffusion of oxygen, preventing it from penetrating too deeply and too quickly [38]. Therefore, a high sludge settling ratio can enhance the aggregation of sludge particles, facilitating SND. Additionally, the effluent suspended solids (SS) remained consistently low throughout the experimental period, with no issues affecting the sedimentation tank’s settling performance. This indicates that the addition of the flocculants CPAM and PAC significantly improved the sludge settling performance of the high-concentration activated sludge system, helping to maintain high activity levels and ensuring stable system operation.

3.4. SND Mechanism Analysis

The main metabolic pathway of SND nitrogen removal is NH₄⁺→NH₂OH→NO₂⁻→NO₃⁻→NO₂⁻→NO→N₂O→N₂ [39]. By analyzing the changes in nitrogen in each stage of the system, we can understand the SND reaction mechanism. As shown in Figure 6, the concentration of ammonia nitrogen in the influent water is approx-imately 23.50 mg/L. After entering the anoxic pool, this concentration decreases to 5.25 mg/L, primarily due to the dilution from mixed liquid reflux, while nitrogen concentra-tions remain relatively stable in the anoxic ponds. During the aerobic pond phase, the ammonia nitrogen concentration further decreases due to nitrification, ultimately reach-ing 0.89 mg/L in the effluent of the aerobic pond. The nitrate nitrogen concentration in the influent water starts at 1.2 mg/L and rises to 2.94 mg/L upon entering the anoxic tank, influenced by the mixed liquid reflux. Subsequently, denitrification in the anoxic pond significantly reduces the nitrate concentration. In the aerobic pond, nitrification causes the nitrate nitrogen concentration to increase, with a final concentration of 3.97 mg/L in the aerobic pond effluent. It is noteworthy that there was no significant accumulation of nitrite nitrogen in any of the reactors. The variations in nitrite concentrations throughout the process closely mirrored those of nitrate, albeit at much lower levels.

In the anoxic pond, TN and NO₃⁻-N concentrations decreased by 3.12 mg/L and 1.71 mg/L, respectively, while the ammonia nitrogen concentration remained relatively un-changed. This indicates that denitrification was the primary process occurring in the an-oxic pond. In the aerobic pond, the effluent ammonia nitrogen concentration decreased by 4.26 mg/L compared to the influent, while the nitrate concentration increased by 2.74 mg/L. Notably, the total nitrogen concentration in the aerobic pond effluent was substan-tially lower than that of the influent, measuring approximately 1.27 mg/L. This suggests that nitrogen loss occurred during the aerobic pond stage and further supports the occur-rence of simultaneous nitrification and denitrification in that phase. Overall, the denitrifi-cation in the anoxic pond contributed to a reduction in TN of 3.12 mg/L, while the simul-taneous nitrification and denitrification in the aerobic pond further decreased TN by 1.27 mg/L. This highlights the significant effectiveness of synchronous nitrification and deni-trification processes.

The mechanism of simultaneous nitrification and denitrification in the high-concen-tration activated sludge process can be analyzed from the following two perspectives:

(1) Due to the high concentration of activated sludge in the aerobic pond, oxygen transfer becomes uneven when dissolved oxygen levels are low, leading to the formation of local anoxic environments. This creates a microenvironment that is conducive to bio-logical denitrification [40], resulting in the SND phenomenon.

(2) As shown in Figure 7, following the addition of flocculant, some microbial pellets appear in the high-concentration activated sludge system. These pellets are large and com-pact in structure. This particular structure creates greater resistance to oxygen transfer within the pellets, leading to a dissolved oxygen gradient. Consequently, an anoxic region develops inside the pellets, providing a microenvironment that is favorable for denitrify-ing bacteria.

3.5. SND Reaction Kinetics of High-Concentration Activated Sludge Method

A seven-hour batch experiment was conducted for this study. The influent to the aer-ation basin was turned off, and water samples were collected continuously every hour to analyze changes in NH₄⁺-N and NO₃⁻-N concentrations. These observations were evalu-ated using the SND kinetic model equation.

$${\left({\displaystyle \frac{{\mathrm{d}}_{\mathrm{N}{\mathrm{H}}_4^{+}-\mathrm{N}}}{\mathrm{d}\mathrm{t}}}\right)}_{\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}=-{\left({\displaystyle \frac{{\mathrm{d}}_{\mathrm{N}{\mathrm{O}}_3^{-}-\mathrm{N}}}{\mathrm{d}\mathrm{t}}}\right)}_{\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}=-\mathrm{R}$$

(1)

The kinetic constant R represents the nitrification reaction rate during SND and indi-cates a linear relationship between the NH₄⁺-N concentration in the reaction system over time. Therefore, in this study, linear regression analysis was conducted to evaluate the NH₄⁺-N versus time curve, with the results presented in Figure 8. Based on the regression analysis, the value of R was found to be 0.9326 (R² = 0.9567).

During the SND reaction system, nitrite nitrogen is produced only minimally, and the ammonification process can be considered negligible. Consequently, ammonia nitro-gen is fully converted to nitrate nitrogen through nitrification, which is subsequently de-graded through denitrification. Therefore, the sum of the degradation of ammonia nitro-gen per unit time (∆t) (∆N) and the increase of nitrate nitrogen in the reaction system ($∆{\mathrm{N}\mathrm{O}}_{\mathrm{M}}$) ($∆\mathrm{N}+∆{\mathrm{N}\mathrm{O}}_{\mathrm{M}}$) is the actual degradation of nitrate nitrogen per unit time ∆t(${\mathrm{d}}_{\mathrm{N}{\mathrm{O}}_3^{-}-\mathrm{N}}$). Therefore, the denitrification formula:

$${\left({\displaystyle \frac{{\mathrm{d}}_{\mathrm{N}{\mathrm{O}}_3^{-}-\mathrm{N}}}{\mathrm{d}\mathrm{t}}}\right)}_{\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}=-\mathrm{Z}{\displaystyle \frac{{\mathrm{S}}_{\mathrm{N}}}{{\mathrm{K}}_{\mathrm{N}}+{\mathrm{S}}_{\mathrm{N}}}}$$

(2)

Perform the following transformation:

$${\displaystyle \frac{1}{\frac{(-∆\mathrm{N}-∆{\mathrm{N}\mathrm{O}}_{\mathrm{M}})}{∆\mathrm{t}}}}={\displaystyle \frac{{\mathrm{K}}_{\mathrm{N}}}{\mathrm{Z}}}\left(-∆\mathrm{N}\right)+\mathrm{Z}$$

(3)

Linear regression analysis was conducted using the absolute value of ∆N as the hor-izontal coordinate, and the absolute value of 1/(((∆N + ∆NO_M))/∆t), which represents the inverse of the sum of ammonia nitrogen degradation and the increase in nitrate nitrogen, as the vertical coordinate. The results are presented in Figure 9.

This results in a Z value of 0.3175 and a ${\mathrm{K}}_{\mathrm{N}}$ of 0.0182 (R² = 0.8577). These kinetic constants were then substituted into the SND kinetic model.

$${\left({\displaystyle \frac{\mathrm{d}\mathrm{E}}{\mathrm{d}\mathrm{t}}}\right)}_{\mathrm{S}\mathrm{N}\mathrm{D}}=\mathrm{R}-\mathrm{Z}{\displaystyle \frac{{\mathrm{S}}_{\mathrm{N}}}{{\mathrm{K}}_{\mathrm{N}}+{\mathrm{S}}_{\mathrm{N}}}}$$

(4)

The kinetic equation for the SND reaction in the aerobic tank of the high-concentration activated sludge system can be expressed as follows:

$${\left({\displaystyle \frac{\mathrm{d}\mathrm{E}}{\mathrm{d}\mathrm{t}}}\right)}_{\mathrm{S}\mathrm{N}\mathrm{D}}=0.9326-0.3175{\displaystyle \frac{{\mathrm{S}}_{\mathrm{N}}}{0.0182+{\mathrm{S}}_{\mathrm{N}}}}$$

(5)

The saturation constant KN for the single-stage activated sludge denitrification pro-cess ranges from 0.1 to 0.2 mg NO₃⁻-N/L. In contrast, the present kinetic model indicates that the saturation constant for nitrate is lower than that observed in the single-stage acti-vated sludge denitrification process. The lower KN value can be attributed to the fact that the data used for model fitting were derived from actual wastewater reaction systems. In these systems, the SND rate is influenced by a combination of factors, including sludge concentration, nitrate concentration, and oxygen diffusion. In this context, nitrate reduct-ion is regarded as the primary nitrogen metabolism process [9]. On one hand, this reac-tion system employs the high-concentration activated sludge method, which features elev-ated sludge concentrations. In the microenvironment of the SND process, this con-tributes significantly to the overall dynamics; the concentration of NO₃⁻-N in the anoxic zone is lower than that produced through actual nitrification due to mass transfer limita-tions. This results in a reduced concentration of NO₃⁻-N available for the denitrification process, which ultimately restricts the rate of denitrification in this system. On the other hand, the SND process is further constrained by the competition between oxygen and NO₃⁻-N for electrons during the reaction. Through the derivation of the SND kinetic model, it can be demonstrated that in this reaction system, the reaction rate of the SND denitrification process utilizing the high-concentration activated sludge method is solely limited by the denitrification rate.

4. Conclusions

This study demonstrated that the high-concentration activated sludge reactor exhibi-ted optimal denitrification performance at a sludge concentration of 7–9 mg/L and a dis-solved oxygen concentration of 1–2 mg/L, during which the SND efficiency reached its peak. A decrease in sludge concentration results in an increase in the dissolved oxygen concentration within the reactor. During this period, total nitrogen removal declines significantly, while the removal of ammonia and nitrate nitrogen increases. This indicates enhanced nitrification and a clear inhibition of denitrification, ultimately leading to a red-uction in SND efficiency. In addition, during the stable operation of the system, the aver-age value of MLVSS/MLSS was approximately 0.586. This reflects good sludge activity and a high number of active microorganisms in the system, indicating that sludge concent-ration and dissolved oxygen levels significantly impact biomass and microbial activity. The kinetic modeling of the SND reaction revealed that the reaction rate of the SND deni-trification process, achieved using the high-concentration activated sludge method, is con-strained by the denitrification rate. This study offers a new perspective for optimizing efficient SND through the control of sludge concentration and dissolved oxygen levels. However, current research on the microbial mechanisms involved in pollutant removal by high-concentration activated sludge is still insufficiently clear. Future studies should focus on the microbial population structure and the composition of denitrifying microor-ganisms within the high-concentration activated sludge method. This will help to eluci-date the microbial community structure and the denitrification reaction mechanisms as-sociated with this approach.