Lab-Scale Treatment of Anaerobic Co-Digestion Liquor from Kitchen Waste, Human Feces, and Municipal Sludge Using Partial Nitritation-Anammox Process

Abstract

Effective nitrogen removal from anaerobic co-digestion is a major challenge to achieving dual-carbon goals. This study explored the acclimatization process of a lab-scale two-stage partial nitritation and anammox process of a stepwise increase in the percentage of raw anaerobic co-digestion liquor from kitchen waste, human feces, and municipal sludge in a venous industrial park in China, which has not been reported yet. Under limited dissolved oxygen (below 0.5 mg/L) and high ammonia levels (200–1500 mg/L), based on adjusting aeration rates, partial nitritation rapidly started up in 50 days. After acclimatization, partial nitritation still performed efficiently and stably, with the final total nitrogen loading rate (TNLR) of 1.24 ± 0.09 gN/L/d, nitrite accumulation rate of 99 ± 4%, and ratio of eff. nitrite/ammonia of 1.32 ± 0.13. In the anammox process, the final total nitrogen removal efficiency, total nitrogen removal rate, and TNLR reached 94 ± 5%, 1.27 ± 0.03 gN/L/d, and 1.36 ± 0.05 gN/L/d, respectively. Chemical oxygen demand (COD) was also reduced in both reactors, with COD removal rates of 0.7 gCOD/L/d in the partial nitritation and 0.4 gCOD/L/d in the anammox process. Overall, the PNA system demonstrated its feasibility in adapting to high ammonia, salinity, and iron levels, when treating anaerobic co-digestion liquor, particularly regarding resource recovery in venous industrial parks.

1. Introduction

The anaerobic co-digestion of kitchen waste, human feces, and municipal sludge offers significant advantages in pollution reduction and carbon removal, serving as a critical approach for the resource recycling and value-added treatment of urban organic solid waste in alignment with dual-carbon goals (carbon peaking and carbon neutrality) [1]. Emissions from the wastewater treatment industry account for approximately 1% of the total global carbon emissions. Achieving carbon neutrality in organic solid waste treatment requires energy self-sufficiency in wastewater treatment processes (WWTPs). Based on the consensus on the resource recovery from organic solid waste, the current mainstream approach emphasizes the separate removal of organic carbon and nitrogen [2]. In this case, the first stage involves capturing and recovering the energy contained in the influent organic matter as methane, meanwhile shifting the carbon footprint from “pollutant degradation in aerobic treatment or denitrification” to “energy recovery”. In the second stage, traditional nitrification and denitrification are abandoned due to the lack of organic carbon sources, allowing autotrophic nitrogen removal processes to take precedence, such as the anaerobic ammonium oxidation (anammox) process [3].

With stricter nitrogen emission standards being introduced, nitrogen removal technology has become a hot topic and a challenge in wastewater treatment. Additionally, the traditional nitrification-denitrification process has become unsatisfactory due to its high energy consumption. Anammox offers distinct benefits for treating high-strength ammonia wastewater with a low carbon-to-nitrogen (C/N) ratio. In comparison, autotrophic anammox can reduce oxygen demand by 60%, eliminate the need for external organic carbon, decrease excess sludge production by 90%, and lower N₂O greenhouse gas emissions [4]. In practical applications, anammox needs to be coupled with partial nitrification (PNA). Anammox-based processes are already applied at full scale to treat the supernatants of anaerobic sludge digester [5,6]. The application allowed for the reduction of the total electrical consumption to the WWTP by 40–50% [7]. It was found that the PNA reduced energy consumption by 0.416 KWh/m³, decreased CO₂ emissions by 75%, and saved cable expenses of 800,000 yuan per year [8].

Venous industry refers to the industry that recycles the waste generated during production and consumption, using solid waste as its main resource. According to the resource recovery theory within the context of dual-carbon goals, venous industrial parks have been widely constructed in many cities across China. Venous industry refers to the industry that recycles the waste generated during production and consumption, using solid waste as its main resource. According to the resource recovery theory within the context of dual-carbon goals, venous industrial parks have been widely constructed in many cities across China. Municipal sludge, kitchen waste, and feces are the three primary types of organic solid waste generated in daily urban life. According to statistics from the Ministry of Ecology and Environment (MEE), in 2020, China produced approximately 72.88 million tons of sludge, 91 million tons of kitchen waste, and 21.4 million tons of urban feces. Moreover, these numbers are expected to increase further with urbanization. These three types of solid waste are readily biodegradable. Anaerobic co-digestion can improve nutrient balance, dilute toxic components, enhance buffering capacity, and leverage the synergistic effects of various microorganisms. However, effective nitrogen removal from anaerobic co-digestion has become a major challenge in achieving dual-carbon goals.

The complex chemical composition also brings challenges for nitrogen removal in wastewater treatment. In a specific venous industrial park in Henan, China, the solid waste from kitchen waste, human feces, and municipal sludge undergoes anaerobic co-digestion. The effluent liquor is characterized by high ammonia, a low C/N ratio, high salinity, and the presence of high iron ions. Traditional nitrogen removal processes typically involve ammonia stripping as a pretreatment step, followed by a two-stage anaerobic/oxic (A/O) process. However, high chloride ions can corrode and clog equipment, inhibiting the activity of functional microbial communities in biological wastewater treatment [9,10]. Elevated salinity also hinders the ammonia stripping pretreatment, leading to a high nitrogen loading rate in the subsequent A/O process. Additionally, the toxicity of iron is largely based on its ability to catalyze the generation of radicals, which attack and damage cellular macromolecules, ultimately promoting cell death [11]. The high levels of salinity and iron ions in the effluent from anaerobic co-digestion also significantly impair the flocculation and sedimentation efficiency when using polyaluminum chloride and polyacrylamide as flocculating agents.

In this study, a lab-scale two-stage PNA process was used to treat anaerobic co-digestion liquor from kitchen waste, human feces, and municipal sludge in a venous industrial park. The feasibility, acclimatization process, and nitrogen removal performance were investigated. The autotrophic nitrogen removal technology in this study will serve as an important complement to anaerobic co-digestion technology in the context of dual-carbon goals in venous industries applications.

2. Materials and Methods

2.1. Experimental Setup

The partial nitritation process was conducted in a sequencing batch reactor (SBR) (Figure 1). The SBR was made of polymethyl methacrylate, with a total volume of 5 L (diameter of 14 cm and height of 35 cm). The temperature was maintained at 30 ± 1 °C by a hot water jacket. The aeration flow was controlled by air pressure using a high-precision electronic air pressure controller (Obkzn, China), with observed dissolved oxygen (DO) levels of 0.5–1.2 mg/L. The pH was automatically maintained at 7.5 by adding a solution of 1 M NaHCO₃ (MIK-PH6.0, Meacon). An 8-h SBR cycle consisted of 10 min of filling, 7 h of aerated reaction, 45 min of settling, and 5 min of drawing, all automatically controlled by a Programmable Logic Controller (PLC). The SBR drainage rate was set at 50%.

The anammox process was operated in an expanded granular sludge bed (EGSB) (Figure 1). The EGSB was also made of polymethyl methacrylate, with a total volume of 5 L and a working volume of 3 L. The temperature was maintained at 32 ± 1 °C through hot water circulation. The pH was automatically regulated at 7.8 by adding 0.1 M HCl (MIK-PH6.0, Meacon). An up-flow velocity of approximately 8 m/h was maintained to ensure sufficient shear stress for the granules. The SBR and EGSB were connected by a 30 L intermediate water tank, and both reactors were covered with a blackout cloth.

2.2. Inoculated Sludge

Activated sludge was taken from the aerobic tank of the anaerobic-anoxic-oxic (A²/O) process at the Shouchuang WWTP in Anyang, China, and served as the inoculum for the partial nitritation process. The mixed liquor suspended solids (MLSS) was 3570 mg/L, and the mixed liquor volatile suspended solid (MLVSS) was 2392 mg/L, resulting in a MLVSS/MLSS ratio of 0.67.

The anammox granules used in this study were sourced from another bigger EGSB in our lab. The EGSB maintained consistent operational stability for three months, achieving a total nitrogen removal rate (TNRR) of 2.2 gN/L/d and a total nitrogen removal efficiency (TNRE) of 86%. The MLVSS was recorded at 3.7 gVSS/L. In the anammox reaction, the theoretical stoichiometric ratio of ΔNO₂⁻-N/ΔNH₄⁺-N (removed nitrite to removed ammonia, referred to as ratio 1) is 1.32, and the ratio of ΔNO₃⁻-N/ΔNH₄⁺-N (produced nitrate to removed ammonia, referred to as ratio 2) is 0.26 [12]. In this EGSB, the observed ratio 1 and ratio 2 were 1.15 and 0.20, respectively.

2.3. Wastewater

The synthetic wastewater used in this study contained 0.15 g/L CaCl₂, 0.025 g/L KH₂PO₄, 0.2 g/L MgSO₄, 0.3 g/L NaHCO₃, and 1 mL trace elements solution. In synthetic wastewater, NH₃Cl was the only source of inorganic nitrogen for the partial nitritation process, while both NH₃Cl and NaNO₂ provided the inorganic nitrogen for the anammox process. The initial pH was adjusted to 7.5.

The raw wastewater came from anaerobic co-digestion liquor of kitchen waste, human feces, and municipal sludge from a specific venous industrial park in Henan, China. In this venous industrial park, approximately 200 tons of kitchen waste, 400 tons of municipal sludge, and 100 tons of feces were subjected to anaerobic co-digestion daily. The kitchen waste, human feces, and municipal sludge were organic solid wastes generated from the actual daily life and production processes in the city. The raw wastewater was directly taken from the effluent of the anaerobic co-digestion process associated with this project in October. This liquor had a chemical oxygen demand (COD) of 700–1100 mg/L, NH₄⁺-N of 700–1200 mg/L, and total phosphorus of 60–100 mg/L. In addition, it also contained 3000–8000 mg/L Cl⁻ and 20–50 mg/L iron ions, which came from kitchen waste and treatment agents in the anaerobic co-digestion process.

2.4. Reactor Operation and Experimental Design

2.4.1. The Fast Start-Up of Partial Nitritation Process in SBR

The activated sludge was aerated in a sealed condition with 300 mg/L NH₄⁺-N for 24 h. The sludge was then washed with tap water to remove floating matter and supernatant. This washed sludge was added into the SBR at a volume ratio of approximately 20%, with an observed MLSS of 2500 mg/L. The start-up process consisted of two stages, stage 1 of the activity elevation stage, and stage 2 of the stable operation stage. Synthetic wastewater was used throughout the entire start-up process. In stage 1, the activated sludge was acclimated by gradually increasing the influent ammonia nitrogen concentration from 200 mg/L to 1500 mg/L, based on the evaluation of nitrite accumulation rate (NAR) and ammonia removal efficiency (ARE). This adjustment facilitated the accumulation of ammonia oxidizing bacteria (AOB), enabling the achievement of the nitritation process [13]. In stage 2, by regulating the aeration rate, the nitritation process was controlled at the partial nitritation stage with influent ammonia of 1500 mg/L. The goal was to maintain the effluent ammonia to nitrite ratio at approximately 1:1.

2.4.2. The Acclimatization of Partial Nitritation Process to Raw Anaerobic Co-Digestion Liquor

After achieving stable operation of partial nitritation in the SBR, a mixture of synthetic wastewater and raw anaerobic co-digestion liquor with 800–900 mg/L NH₄⁺-N was used as the influent to acclimate the partial nitritation process. The percentage of raw anaerobic co-digestion liquor in the influent was stepwise increased to 10%, 20%, 40%, 60%, 80%, and finally 100%. Each time the effluent nitrite/ammonia ratio approached the anammox theoretical value of 1.32, the percentage of anaerobic co-digestion liquor was increased accordingly. This partial nitritation process in the SBR aimed to gradually adapt and stabilize to treat the raw anaerobic co-digestion liquor after the acclimatization process. The effect of aeration rate on the ammonia removal, eff. nitrite/eff. ammonia, and nitrite accumulation rate in the acclimatization process was investigated.

2.4.3. The Acclimatization of Anammox Process to Raw Anaerobic Co-Digestion Liquor

After achieving stable operation of the anammox process in the EGSB, the anammox process was acclimated using effluent from the partial nitritation feed with raw anaerobic co-digestion liquor. It was important to note that, in this study, for stable operation of the anammox process, a small amount of (NH₄)₂SO₄ or NaNO₂ would be added to maintain an appropriate NO₂⁻-N/NH₄⁺-N ratio in the anammox influent (1.1–1.2). Once the effluent concentrations of NH₄⁺-N and NO₂⁻-N were both below 10 mg/L, the percentage of raw anaerobic co-digestion liquor was stepwise increased (10%, 20%, 40%, 60%, 80%, and finally 100%).

2.5. Chemical and Data Analytical Methods

NH₄⁺-N and NO₂⁻-N were measured by N-(1-naphthalene)-ethylenediamine photometric method and ultraviolet spectrometry method, respectively. Measurements of total suspended solids (TSS) and volatile suspended solids (VSS) were performed following standard methods [14]. DO was measured according to the manufacturer’s instructions (Multi 3430, WTW, Munich, Germany). COD was measured using the fast digestion spectrophotometric method [15]. Significant statistical analysis of Pearson’s correlation analysis was conducted using SPSS software (IBM, v23), with the statistical significance level set at p < 0.05.

3. Results

3.1. Start-Up Process of Partial Nitritation in the Sequencing Batch Reactor

The partial nitritation was successfully started up within 30 days by a continuous aeration method with a low DO concentration of 0.3–0.5 mg/L (Figure 2). This start-up process was divided into three stages. Stage 1 was the complete nitration stage, aimed to selectively enrich and cultivate nitritation activity, thereby transforming the activated sludge into nitritation sludge. In stage 1 (0–15 days), the influent NH₄⁺-N was set at 200 mg/L, with a total nitrogen loading rate (TNLR) of 0.32 gN/L/d. During the first 1 to 5 days, the ARE was unstable, varying within the range of 57–65%. However, the NAR rapidly increased from 71% to 98%, suggesting that most nitrite was produced and retained in this system. During 6–15 days, ARE gradually increased to 83%, and NAR remained stable at 0.97 ± 1%, with low produced nitrate of 3.7 ± 1.3 mg/L, suggesting that the nitritation process had dominated this SBR. The successful suppression of nitrite oxidizing bacteria (NOB) was considered important in partial nitritation, to avoid further oxidation of nitrite [16]. In stage 2 of the activity elevation stage (16–33 days), there was a notable enhancement in nitritation performance, with a consistent improvement observed. The influent NH₄⁺-N was continuously increased from 200 mg/L to 1500 mg/L, resulting in a TNLR reaching 2.3 gN/L/d, allowing the system to adapt to high ammonia levels in potential raw wastewater. Meanwhile, the ratio of eff. nitrite/eff. ammonia was regulated towards the theoretical anammox influent ratio of nitrite/ammonia of 1.32 by adjusting the aeration rate. This adjustment was essential for optimizing the conditions for subsequent anammox processes [17]. In stage 3 of the stationary stage (34–50 days), a high influent NH₄⁺-N was maintained at 1500 mg/L, and the SBR demonstrated stable operation for 15 days. NAR reached 99.7%, indicating excellent partial nitritation performance at high ammonia.

3.2. The Acclimatization of Partial Nitritation to Raw Anaerobic Co-Digestion Liquor

In stage 4 of the acclimatization stage (51–121 days), the increasing percentage of raw anaerobic co-digestion liquor (10%, 20%, 40%, 60%, 80%, and 100%) was introduced into the SBR for 71 days in order to acclimate the partial nitritation system to raw wastewater (Figure 2). Each time the percentage of the raw anaerobic co-digestion liquor was raised, the ratio of nitrite/ammonia was quickly adjusted to stabilize around 1.32. As the percentage of anaerobic co-digestion liquor increased, the value of TNLR, NAR, and nitrite removal efficiency (NIRE) also showed an upward trend, with Pearson correlation coefficients of 0.712 (p = 0.000), 0.752 (p = 0.000), and 0.752 (p = 0.000), respectively. Conversely, the TNRE (−0.414, p = 0.013) and TNRR (−0.414, p = 0.044) were negatively related to this percentage. With 100% of influent raw anaerobic co-digestion liquor, the average TNLR, NAR, and eff. nitrite/eff. ammonia achieved 1.24 ± 0.09 gN/L/d, 99 ± 4%, and 1.32 ± 0.13, respectively, meaning that this effluent had already met the influent requirements of the anammox process. Meantime, the TNRR and TNRE were almost zero, indicating an almost absence of the denitrification process in the SBR. With the final average COD of 1100 mg/L, COD removal efficacy (CRE), COD loading rate (CLR), and COD removal rate (CRR) were 45 ± 3%, 1.65 gCOD/L/d, and 0.74 ± 0.05 gCOD/L/d, respectively. A similar study reported the feasibility of long-term stable nitrite accumulation in a partial nitritation treating diluted real sanitary landfill leachate with extremely high ammonia concentrations of 160 mM (dilution ratio of 50%) and a C/N of 0.13 [18]. The experiment demonstrated that despite the high ammonia levels, salinity, and iron ions present in the raw anaerobic co-digestion liquor, the partial nitritation SBR still performed efficiently and stably.

3.3. The Effect of Air Supply on Partial Nitritation Performance in the Sequencing Batch Reactor

In two-stage PNA, one of the most critical issues was the stable control and precise regulation of partial nitritation to produce a suitable effluent nitrite/effluent ammonia ratio, ideally close to 1.32, to meet the stoichiometric requirements of the anammox process [19]. Therefore, the effect of the control strategy based on aeration rate on partial nitritation performance was investigated (Figure 3). The values of ARE and the ratio of eff. nitrite/ eff. ammonia was positively and linearly correlated with aeration rate, with a correlation coefficient of 0.56. However, NAR showed almost no correlation with aeration rate, indicating that excellent NAR values close to 100% could be achieved at different aeration rates, without concerns about nitrate accumulation, particularly at a specific aeration rate of 3.2 m³/h/m² and an observed DO concentration of 0.5–1.0 mg/L. The ability to achieve high NAR values across a range of aeration rates without nitrate accumulation suggests that the system is robust and adaptable to varying operational conditions [20]. Thus, the stable operation of partial nitritation could be achieved by continuously adjusting the aeration rate. An aeration rate of 0.4–0.8 L/min was determined to be suitable for this partial nitritation process. In this case, the average ARE ranged from 50% to 60%, the ammonia removal loading rate fluctuated around 0.7 gN/L/d, and the ratio of nitrite/ammonia was relatively stable and close to the theoretical ratio of 1.32. These findings highlight the importance of precise aeration control in maintaining the desired nitrite/ammonia ratio, which is crucial for the efficiency of the subsequent anammox process [20].

3.4. The Acclimatization of Anammox Process to Raw Anaerobic Co-Digestion Liquor

The acclimatization test of the anammox process was conducted over 51 days and divided into two stages (Figure 4). Stage 1 of the baseline stage (1–10 days) served as the control stage fed with synthetic wastewater to establish a baseline performance over 10 days. In this EGSB system, the influent NH₄⁺-N was maintained at 16 mM, with an influent ratio of nitrite to ammonia of 1.32. The final observed TNRR and TNRE were 1.18 gN/L/d and 85%, respectively. The assessment of ratio 1 (1.29) and ratio 2 (0.29) indicated the predominance of a stable anammox process as the primary biological activity in this EGSB system.

In stage 2 of the acclimatization stage, the effluent from the partial nitritation process, fed with increasing percentages of raw anaerobic co-digestion liquor (10%, 20%, 40%, 60%, 80%, and 100%), was introduced into the anammox process as the influent to acclimate the anammox process to raw anaerobic co-digestion liquor. At the early part of stage 2, due to the influent ammonia concentration of 375 ± 27 mg/L, which was higher than in stage 1, the hydraulic retention time (HRT) was increased from 10 h (7 L/d) to 15 h (4.8 L/d) to maintain consistency in the TNLR between these two stages. The main general acclimatization strategy based on load regulation involved stepwise increasing the percentage of raw anaerobic co-digestion liquor once the concentrations of NO₂⁻-N and NH₄⁺-N in the effluent were both below 10 mg/L. During the acclimatization process, stoichiometric ratios were significantly affected by the increasing percentage of co-digestion liquor, resulting in periodic fluctuations. Compared to the control test, due to the supply of 160–570 mg/L COD from anaerobic co-digestion liquor, the stoichiometric ratio 1 continued its downward trend from 1.29 in stage 1 to 1.15 ± 0.03 after 40 days, while ratio 2 decreased from 0.29 in stage 1 to 0.14 ± 0.03 in entire stage 2. Research has shown that mixing different sources of organic matter can enhance nitrogen removal rates in anaerobic reactors [21]. ARE and NIRE values remained relatively stable throughout the acclimatization process, with values of 97.1 ± 0.9% and 99.5 ± 0.6%, respectively. Moreover, the stable ARE and NIRE values indicate that the system can maintain high performance even with fluctuating influent conditions, which is crucial for real-world applications. Consequently, the TNRE improved by about 9% due to the decline of effluent nitrate. The percentage of anaerobic co-digestion liquor was positively related to TNRE with a Pearson correlation coefficient of 0.872 (p = 0.00). The anammox process remained dominant in this EGSB system, and successfully adapted to 100% percent of partial nitritation effluent of raw anaerobic co-digestion liquor on day 46, achieving a stable operation for one week. The final TNRE, TNLR, and TNRR reached 94 ± 5%, 1.36 ± 0.05 gN/L/d, and 1.27 ± 0.03 gN/L/d, respectively. It was reported that long-term operations of various case studies show stable process performance of full-scale anammox reactors treating mono-digestion liquor from municipal and industrial effluents (such as fermentation industry and food industry), achieving ARE in excess of 90% at low and high TNLR up to 2.5 gN/L/d, which was relatively higher than the results observed in this study [22]. Additionally, 34% of influent COD was also removed via potential denitrification or anaerobic process in the EGSB, resulting in a final CRL of 0.9 gCOD/L/d and CRR of 0.4 gCOD/L/d. The values of CLR (0.966, p = 0.000) and CRR (0.769, p = 0.000) were also positively related to the percentage of anaerobic co-digestion liquor. These findings underscore the importance of a well-regulated acclimatization strategy to ensure the stability and efficiency of the anammox process. Furthermore, the ability of the anammox process to adapt to raw anaerobic co-digestion liquor demonstrates its robustness and flexibility, making it a viable option for treating complex wastewater streams.

4. Discussion

The anaerobic digestion treatment of the organic fraction of municipal solid wastes is gaining increasing interest, and its full-scale application is spreading worldwide. Anaerobic co-digestion offers an opportunity to overcome the drawbacks of mono-digestion by simultaneously digesting two or more feedstocks [23]. However, the treatment of anaerobic co-digestion supernatant is recognized as a major bottleneck in the widespread adoption of co-digestion technology. The PNA process can guarantee an effective solution for the treatment of the anaerobic co-digestion supernatant [5,24]. Most co-digestion studies have primarily focused on sources such as animal manure and sewage sludge [23]. However, the nitrogen removal from the anaerobic co-digestion liquor from kitchen waste, human feces, and municipal sludge by the PNA process has yet to be reported. Moreover, the treatability of similar types of wastewater must be demonstrated as feasible, given their complex chemical composition (high content of ammonium, metals, and solids, along with a low presence of readily biodegradable and recalcitrant organics).

Several studies have focused on the anammox treatment of anaerobic co-digestion liquor. A pilot test of the anammox process for treating the anaerobic digestion liquor of municipal sludge and food industry waste was carried out in Guillarei, Spain. It was found that the anaerobic co-digestion of various wastes did not have a negative impact on the removal efficiency of autotrophic denitrification, and the obtained TNRR was as high as 1.1 gN/L/d [5]. In another study, the short-term impact of 20 types of supernatants from anaerobic digestion of urban solid waste on anammox activity was tested, revealing that the anammox activity in undiluted raw wastewater could only achieve 73% to 89% [25]. The first anammox demonstration plant in South Korea used the anammox process combined with sulfur autotrophic denitrification process to treat the anaerobic co-digestion liquor of livestock and poultry wastewater, food wastewater, and sewage sludge, achieving a TNRR of 0.45 gN/L/d, with 80% of the total nitrogen (TN) removed by the anammox reactor [26]. In this study, the anammox process obtained a final TNRR of 1.27 ± 0.03 gN/L/d when treating raw anaerobic co-digestion liquor, which was relatively higher than the reported in similar studies.

The stable achievement of partial nitritation has been reported in numerous studies. The dominance of AOB is crucial for the long-term operation of partial nitritation. This can be accomplished by enriching AOB while washing out or suppressing NOB, to avoid NOB overgrowth and the consequent nitrate accumulation. The reported key factors included pH, DO, temperature, substrate concentration, and other inhibitors [27]. In this study, several parameters benefited the effective suppression of NOB, including a low DO of less than 0.5 mg/L, a high temperature of 30 ± 1 °C, a high pH value of 7.5, high influent ammonia of 200–1500 mg/L with a calculated free ammonia (FA) concentration of 6–45 mg/L, and a high effluent nitrite of 460 with a calculated free nitrite (FNA) concentration of 95 μg/L in stage 2. Although it is reported that NOB is inhibited at FA concentrations ranging from 0.1–4.0 mg/L, and FNA concentrations of 26–220 μg/L can completely inhibit NOB [27].

Salinity is well known as a significant inhibitor of anammox bacteria, particularly because high nitrogen-rich wastewater often has high salinity levels. As reported, ammonia oxidation and nitrite accumulation were not affected by 5.0–37.7 g/L of NaCl [28]. Additionally, it was reported that the anammox process, enriched from low-salinity water, could adapt to saline wastewater containing 29 g/L of NaCl after a long-term stepwise acclimatization [29]. Furthermore, a half maximal inhibitory concentration (IC₅₀) value of 6.1 mS/cm was determined for exposure to the liquid fraction of biowaste digestate from the anaerobic digestion of the organic fraction of municipal solid waste in a short-term test [25]. Another study showed that chlorides also reduced the removal rates of COD and TN in the biofilm process [30]. In addition, high total iron content will bring a high concentration of Fe²⁺ or Fe³⁺. It was proved that around 5 mg/L iron (Fe²⁺ or Fe³⁺) promoted the nitrogen removal process [31]. Higher concentrations inhibited nitrogen removal with a ratio of over 80% [32]. However, the critical iron concentration for anammox bacteria has not been clearly defined due to the different operation environments. Fe³⁺ had been found to promote nitrification and inhibit denitrification [33]. The critical Fe³⁺ concentration for a classical A²O process was reported to be 10 mg/L [34]. In this study, obvious iron attachment to the sludge was observed, and the color of partial nitritation sludge and anammox sludge turned reddish brown after the acclimatization process. In this study, partial nitritation and anammox process could also adapt to the salinity of 7 g/L on average and 20–50 mg/L total iron in the raw wastewater, at least for the duration of our experiment.

Therefore, by adapting to conditions of high ammonia, high salinity, and high iron under a C/N ratio of 1.28, the lab-scale partial nitritation and anammox system was able to achieve relatively stable and efficient operational performance. This demonstrates the feasibility of using PNA to treat anaerobic co-digestion liquor from kitchen waste, human feces, and municipal sludge in the venous industrial park.

5. Conclusions

In this study, the feasibility and performance of a lab-scale two-stage PNA process treating anaerobic co-digestion liquor from kitchen waste, human feces, and municipal sludge in a venous industrial park were investigated. By a stepwise acclimatization process, the PNA process successfully adapted to the raw wastewater characterized by high ammonia, high salinity, and high iron. Partial nitritation was achieved in the SBR by continuously adjusting the aeration rate, with TNLR, NAR, and eff. nitrite/eff. ammonia of 1.24 ± 0.09 gN/L/d, 99 ± 4%, and 1.32 ± 0.13, respectively. In anammox EGSB, the final TNRE, TNLR, and TNRR reached 94 ± 5%, 1.36 ± 0.05 gN/L/d, and 1.27 ± 0.03 gN/L/d, respectively. COD was removed in both reactors, with a total CRR of about 1.1 gCOD/L/d. Overall, these findings illustrated the feasibility of utilizing the PNA process for treating anaerobic co-digestion liquor from kitchen waste, human feces, and municipal sludge.