Study on the treatment of black and odorous water bodies by aeration combined with sulfur-iron autotrophic denitrification MBBR

In recent years, with considerable attention from various levels of the Chinese government, point-source pollution from rural domestic sewage has been effectively mitigated. However, the rapid expansion of modern agriculture in China has resulted in a significant problem of agricultural surface pollution. A considerable amount of nutrients and organic matter are being discharged excessively into rivers, lakes, and ponds, surpassing their natural purification capacity. Consequently, the presence of foul-smelling and contaminated water has emerged, posing threats to both the ecological environment and human health (Cao et al. 2020). In January 2022, the Chinese government launched the Action Programme for the Battle of Agricultural and Rural Pollution Control (2021–2025) in response to the issue of black and odorous water bodies. This program explicitly aims to eliminate large areas of black and odorous water bodies by 2025 and, as outlined in the Fourteenth Five-Year Plan, achieve the basic elimination of inferior class V surface water by the same year. Consequently, there is a critical demand for technology capable of efficiently managing black and odorous water bodies.

Aeration diminishes the concentration of sulfur-containing odor compounds, including hydrogen sulfide (HS), methyl mercaptan (MT), and dimethyl sulfide (DMS), within the water body (Luo et al. 2021). Furthermore, aeration has the potential to influence nitrogen cycling (Holmroos et al. 2016) and nutrient removal (Ilyas & Masih 2018) in the water body. Hence, aeration technology has been extensively employed for the remediation of black and odorous water bodies both domestically and internationally, including notable cases like the Thames River in UK, the Ruhr River in Germany, and the Baima Branch River in Fuzhou City, China (Peng et al. 2017). Several researchers have investigated this topic. For instance, Liu et al. (2020) have highlighted that aeration alone can achieve removal rates ranging from 3.2 to 45.6% for ⁠, 19.5 to 84.8% for COD, and 56.4 to 88.2% for BOD₅ in black and odorous water bodies. It is evident that aeration is effective in managing COD and BOD₅ levels in water bodies; however, the removal efficiency for ammonia nitrogen remains suboptimal. Wang et al. (2008) utilized solar aeration technology in malodorous rivers, achieving maximum removal rates of 16.9% for TN and 33.5% for TP in the water body. The study indicates that standalone aeration technology struggles to address total nitrogen pollution encompassing ammonia nitrogen, nitrate nitrogen, and other total phosphorus (TP) pollution in black and odorous water bodies. Hence, a more efficient technical approach is required to tackle this issue.

The Moving Bed Biofilm Reactor (MBBR) is a well-established technology for Simultaneous Nitrification Denitrification (SND) in biological nitrogen removal processes (Wang et al. 2006), leveraging biofilm for efficient nitrogen removal (Bhattacharya & Mazumder 2021). Currently, MBBR technology is predominantly employed in wastewater treatment and is less commonly utilized for addressing black and odorous water bodies. Peng et al. (2013) implemented intermittent aeration MBBR treatment for wastewater, achieving impressive average removal rates of 92.22% for ammonia nitrogen, and 76.73% for TN. Pan et al. (2022) achieved the highest removal efficiencies for TN and TP in wastewater using MBBR at a C/N (nitrogen to carbon ratio) of 5, reaching 78.94 and 59.64%, respectively. Previous studies have effectively removed TN from wastewater. However, it is crucial to note that water bodies emitting foul odors typically exhibit low carbon-to-nitrogen (C/N) ratios. Some researchers have employed MBBRs to treat low C/N ratio wastewater. For instance, Zhang et al. (2024) utilized a novel Pure Biofilm Moving Bed Biofilm Reactor (PH-MBBR) based on heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria, achieving a TN removal rate of 10.02 mg/(L h) at a C/N ratio of only 4. Luan et al. (2022) proposed an intermittently aerated MBBR for the removal of nitrogen and carbon from low C/N rural wastewater. It provides an applicable solution for low C/N and low-cost rural wastewater treatment. Additionally, Wei et al. (2023) studied the impact of total organic carbon (TOC) on the formation of malodorous water bodies. The experimental findings revealed that malodorous water formation occurs when the TOC level exceeds 150 mg/L, corresponding to a CODcr of approximately 290 mg/L and a C/N ratio of 3:1. Xu et al. (2022) analyzed that a C/N ratio greater than 4 indicates sufficient carbon sources for denitrifying bacteria, enabling complete denitrification reactions. Additionally, Zeng et al. (2024) highlighted that a decrease in the C/N ratio led to significant nitrate nitrogen accumulation, resulting in a reduced SND efficiency of 26.1–65.3% and severe denitrification inhibition, ultimately yielding an average total nitrogen removal rate of 25.1–67.3%. Hence, achieving denitrification in black water bodies with low carbon-to-nitrogen ratios becomes challenging without the addition of carbon sources. This situation often results in elevated levels of total nitrogen, primarily in the form of NO₃-N, posing a significant pollution characteristic. Consequently, meeting the surface water Grade Ⅳ standard (GB3838-2002) becomes problematic. The removal of TP from wastewater often involves the use of MBBR (Rudi et al. 2019; Zhong et al. 2023). Luan et al. (2024) introduced an innovative approach known as the Anaerobic/Intermittent Aerated Moving Bed Biofilm Reactor (AnIA-MBBR). In their study, the efficiency of TP removal using different intermittent aeration modes (short and long aeration) was found to be 74 and 59%, respectively, when treating wastewater with a ratio of approximately 3. Limited research has been conducted on the application of MBBR for phosphorus removal in low C/N ratio black and odorous bodies. This highlights the pressing need for effective nitrogen and phosphorus removal strategies in low C/N ratio black and odorous water bodies.

In summary, aeration technology alone struggles to remove nitrogen and phosphorus effectively from black and odorous water bodies. Although integrating the MBBR method can enhance denitrification efficiency, the low C/N ratio of such water bodies, as in the case of the black and odorous water used in this study (BOD₅/TN = 1.5), hinders effective denitrification and phosphorus removal by biofilms. To address these challenges, this study suggests combining MBBR with sulfur-iron autotrophic denitrification to develop a novel ‘aeration + sulfur-iron autotrophic denitrification MBBR’ treatment process. A comparative experimental study between this new approach and the existing ‘aeration + MBBR’ treatment was conducted, focusing on enhancing the treatment of black and odorous water bodies. The goal is to offer a more effective solution for nitrogen and phosphorus removal.

The experimental setup of this study is detailed in Table 1, comprising three distinct groups: a new treatment process group (aeration + sulfur-iron autotrophic denitrification MBBR), an existing treatment process group (aeration + MBBR), and a blank control group. Each group included two replicate samples, resulting in a total of six microcosms simulating black and odorous water bodies. These microcosms were created using experimental buckets filled with black and odorous water bodies collected from the field, a black-smelling river ditch in a rural area of Jimei District, Xiamen City, China, longitude: 118.086 latitude: 24.617, the river ditch is always sewage inflow, and the water body of the river ditch is not flowed and shows black color while emitting a bad smell. The experimental barrel, with dimensions of 32 cm in diameter and 51 cm in height, has a capacity of 31 liters. Each barrel was filled with 30 liters of black and odorous water bodies. For control purposes, two parallel experiments, labeled as #1 and #2, were established using only black and odorous water bodies without any treatment. The existing treatment group, comprising experiments #3 and #4, introduced an aeration device and MBBR suspended ball fillers into the barrels containing black and odorous water bodies. Meanwhile, the new treatment process group, involving experiments #5 and #6, utilized an aeration device along with sulfur-iron autotrophic denitrification MBBR suspended ball fillers in the barrels to treat the water.

This study primarily focused on key water quality indicators for black and odorous water bodies, including olfaction, turbidity, DO, pH, COD, BOD₅, NH₃-N, NO₃-N, TN, and TP (Table 4). To evaluate the efficacy of different treatment processes for black and odorous water bodies in rural areas, we compared the average values of these indicators between the blank control group and the experimental groups. Sampling was conducted daily at 16:00, with the sampling point located 10 cm below the water surface at the center of the bucket. The study was divided into two detection stages: before the water quality met the Surface Water Quality Standard V, samples were collected every 24 hours; once the water quality reached this standard, the interval between samplings was extended to 48 hours until it met the Surface Water Quality Standard IV, marking the end of the experiment.

Black and odorous water bodies used in each experimental bucket came from a rural ditch near the school and were characterized by its foul odor and blackened appearance.

The initial water quality parameters of this water body are listed in Table 5, in which the mean value of dissolved oxygen content was 1.82 mg/L, and the mean value of NH₃-N content was 8.136 mg/L. At the same time, the approximate value of BOD₅/TN of this black smelly water was 1.5, and the biochemical properties were extremely poor. According to the ‘Rural Black And Odorous Water Bodies Control Work Guidelines,’ this water body is a mildly malodorous black odor water body (Yu et al. 2020; https://www.gov.cn).

We employed the Flavor Rating Analysis (FRA) for evaluating the odor intensity of malodorous water bodies, engaging multiple evaluators to rate and describe the odor of the water samples. Table 6 demonstrates that the black and odorous water bodies initially exhibit a high level of odor and significant malodorous characteristics. The experimental group, which underwent the new treatment process (aeration + sulfur-iron autotrophic denitrification MBBR), and the existing treatment process (aeration + MBBR) were compared to the blank control group. The results showed that both treatment processes were able to reduce the odor of the black and odorous water within days 1–3, and both demonstrated a better ability to reduce the olfactory level. Following aeration, the new treatment process and the experimental group of the existing treatment process exhibited sufficient dissolved oxygen, enabling the rapid oxidation of H₂S, FeS, and other malodorous compounds generated during the anaerobic degradation of organic matter, this effective oxidation contributed to a notable improvement in alleviating the malodor of the water body (Luo et al. 2021). The olfactory level of the blank control group was also reduced, but the time used was longer, so it can be seen that the additional aeration device can effectively reduce the olfactory level of black and odorous water bodies. As shown in Figure 5, the turbidity of the water body was reduced rapidly in the 1st–5th day, which was due to the natural sedimentation of the water body and the decomposition of organic pollutants by aerobic microorganisms on the one hand, and the adsorption effect of MBBR filler in the experimental group on the other hand.

In Figure 6(a), the experimental groups comparing the new treatment process (aeration + sulfur-iron autotrophic denitrification MBBR) with the existing treatment process (aeration + MBBR) showed a significant increase in DO concentration within 1 day, rising from 1.86 to 8 ± 0.5 mg/L while maintaining consistent saturation levels. This enhancement resulted in a more than fourfold increase in water DO concentration. In contrast, the DO concentration in the blank control group remained persistently low throughout the experiment. Throughout the experimental process, the water temperature in the experimental groups for both the new and existing treatment processes fluctuated within the range of 22–25 °C, while the DO concentration varied between 7 and 9 mg/L, showing a negative correlation with temperature changes. This pattern aligns with the findings of Rao & Huang (2017) that the decomposition of organic matter in the water column will consume oxygen in the water, the higher the temperature, the greater the rate of oxygen depletion, the dissolved oxygen saturation value decreases.

As indicated in Figure 6(b), both the new and existing treatment processes resulted in a significant increase in pH within 1 day, particularly when compared to the blank control group. Aeration played a key role in enhancing DO levels through complete oxidation of acidic reducing substances like HS and increasing gas over-saturation pressure, leading to carbon dioxide extrusion and ultimately elevating the pH (Muller et al. 2022). In contrast, when comparing the experimental groups of the new and existing treatment processes, it was observed that the pH of the experimental group in the new treatment process was slightly lower than that of the experimental group in the existing treatment process at the end of the experiment. After the 10th day of the experiment, this difference occurred despite both groups having similar DO levels, and it was attributed to the acidity generated by sulfur autotrophic denitrification. Notably, the pH values of both the blank control group and each experimental group ranged between 6 and 9, all within compliance with the surface IV water standard outlined in the Environmental Quality Standard for Surface Water (GB3838-2002).

During the 18-day experiment, the COD of the black and odorous water bodies in both the experimental groups of the new treatment process (aeration + sulfur-iron autotrophic denitrification MBBR) and the existing treatment process (aeration + MBBR) decreased to 12.30 and 14.295 mg/L from the initial concentration of 52.67 mg/L, respectively, as illustrated in Figure 7(a). The fluctuation of COD in each experimental group within 5 days of the experiment may be due to the influence of outdoor dust factors, at this time the microorganisms decomposing organic matter have not completed a certain amount of accumulation, resulting in insufficient decomposition capacity. As the experiment proceeded, the new treatment process and the existing treatment process experimental group DO rapidly increased to 8 ± 0.5 mg/L (Figure 6(a)) and maintained a stable aerobic system to play a role, additionally, the COD gradually reduced after 5 days of the experiment. At the conclusion of the experiment, the concentrations of COD in both experimental groups were found to be lower than 30 mg/L, reaching surface Ⅳ water standard (GB3838-2002).

Figure 7(b) indicates that the experimental groups utilizing innovative treatment processes and traditional treatment processes achieved average COD removal rates of 78.6 and 72.86%, respectively, mirroring the treatment outcomes reported in Safwat (2018). This indicates an increase in the average removal rate of 5.74% for the experimental group undergoing the new treatment process compared to the group using the existing treatment process. Moreover, the new treatment process exhibited significantly higher removal rates of 46.15% in comparison to the blank control group, demonstrating its efficacy in COD removal.

At the onset of the experiment, the BOD₅ value in the black and odorous water bodies was recorded at 14 mg/L. On the 5th day of the experiment, samples were collected from each group of water bodies for testing. The BOD₅ values in different experimental groups are presented in Table 7.

The average BOD₅ values in the experimental groups undergoing the new treatment process and the existing treatment process were 4.05 and 4.25 mg/L, respectively. These values indicate a reduction in BOD₅ to 6 mg/L or less, aligning with the Grade IV water quality standards (GB3838-2002). The removal efficiency of BOD₅ was significantly higher in the treatment groups compared to the blank control group, showing a positive correlation with the COD trends in the water bodies, consistent with findings from the study by Xu et al. (2022) that the trends of both COD and BOD₅ show a certain positive spatial correlation. The average removal rates of BOD₅ in the experimental groups following the new treatment process and the existing treatment process were 71.07 and 69.64%, respectively. The average removal rate of the experimental group of the new treatment process was increased by 65.36% compared with the blank control group, which has a better removal effect.

At the onset of the experiment, the initial NH₃-N concentration in the black and odorous water bodies was 8.136 mg/L. As illustrated in Figure 8(a), In the 18-day experiment, the new treatment process experimental group (aeration + iron autotrophic denitrification MBBR), the existing treatment process experimental group (aeration + MBBR), and the blank control group can reduce the NH₃-N concentration in the black and odorous water bodies to 0.132, 0.1, and 0.734 mg/L, respectively, which means that it reaches the standard of the surface Ⅳ class water of 1.5 mg/L below. Notably, both the new treatment process and the experimental group of the existing treatment process rapidly achieved levels below 1.5 mg/L within the first 5 days of the experiment. This rapid reduction can be attributed to the high dissolved oxygen levels of 8 ± 0.5 mg/L, which facilitated the swift nitrification reaction, leading to the oxidation of NH₃-N to NO₃-N and ⁠, thus promoting NH₃-N removal. The experimental group of the new treatment process exhibited a comparatively superior treatment effect compared to the experimental group of the existing treatment process. This can be attributed to the enhanced adsorption effect on elemental nitrogen provided by the filling materials of zeolite and coconut shell activated carbon (Bhattacharya & Mazumder 2021; Zhang et al. 2022; Li et al. 2024). In contrast, NH₃-N concentrations in the blank control group were reduced to less than 1.5 mg/L after 16 days. This longer period of control was achieved due to the positive correlation between the biodegradation coefficients of NH₃-N and its integrated degradation coefficient. In this case, biodegradation played a major role and relied on the biological self-purification capacity of the river (Krasnoperova 1994).

According to Figure 8(b), while all experimental groups consistently achieved a removal rate of over 90% throughout the experiment, both the new treatment process and the existing treatment process experimental group demonstrated an exceptionally rapid removal of NH₃-N compared to the blank control group. By the 18th day of treatment, the average removal rates of NH₃-N in the experimental group of the new treatment process and the existing treatment process reached 98.77 and 98.39%, respectively, indicating a significant removal effect. The experimental group of the new treatment process was 7.78% higher than the 90.99% removal rate of the blank control group.

By the end of the experiment, the NO₃-N concentrations in the new treatment process, existing treatment process, and blank control group were 0.701, 8.571, and 6.485 mg/L, respectively. The removal rates of NO₃-N were −169.23, −3,198.08, and −2,394.23% (Figure 9(b)). The average removal rate of the experimental group of the new treatment process was 3,029.05% higher than that of the experimental group of the existing treatment process, and the average removal rate of the new treatment process was 2,225% higher than that of the blank control group, showcasing the effectiveness of the new treatment process in denitrification.

The changes in TN concentration and the removal rate of TN in the water in the control group and the experimental group with two different treatment processes are shown in Figure 10.

The initial TN concentration in the black and odorous water bodies at the commencement of the experiment was notably high at 9.34 mg/L. As depicted in Figure 10(a), the new treatment process (aeration + sulfur-iron autotrophic denitrification MBBR) exhibited fluctuations when compared to the existing treatment process (aeration + MBBR) and the blank control group within the initial 4 days of the experiment. This fluctuation may be attributed to the use of zeolite and coconut shell activated carbon as filling materials in the new treatment process, both of which possess adsorption capabilities for elemental nitrogen (Bhattacharya & Mazumder 2021; Zhang et al. 2022; Li et al. 2024). Consequently, there was an initial reduction in TN within the experimental group of the new treatment process within 1 day, while the subsequent rebound is likely due to the presence of nitrogen in the organic matter (Sharma et al. 2022). The black and odorous water bodies contain a substantial amount of organic matter, and Figure 7(a) illustrates the rapid decrease in COD at the outset of the experiment, leading to a subsequent increase in TN. Following the 4th day of the experiment, there was a decline in TN levels within the experimental group undergoing the new treatment process, coinciding with the activation of sulfur-iron autotrophic denitrification. During this period, the denitrification rate initially lagged behind the nitrification rate, as illustrated in Figure 9(a) depicting the dynamics of NO₃-N alteration. Subsequent to the 8th day of the experiment, the denitrification rate surpassed the nitrification rate. As a result of the accelerated reduction in NO₃-N, the TN levels experienced further decreases, culminating in an average TN concentration of 1.34 mg/L in the experimental group following 18 days of treatment. These results align with the standards set for surface Grade IV water quality (GB3838-2002). Contrary to expectations, the TN content in the experimental group undergoing the existing treatment process did not decrease; instead, it showed a slight increase compared to the baseline control group. Moreover, the TN content remained higher overall in the experimental group. This observation, coupled with the dynamic conversion of NH₃-N to NO₃-N, indicates that high levels of dissolved oxygen can hinder the denitrification processes while facilitating the breakdown of organic matter, ultimately exacerbating eutrophication within the water body (Yang et al. 2008). Consequently, the TN levels in the experimental group undergoing treatment consistently exceeded those in the control group.

At the end of the experiment, the mean TN concentrations in the experimental group of the new treatment process, the experimental group of the existing treatment process and the blank control group were 8.195, 9.555, and 1.34 mg/L (Figure 10(a)), respectively. Figure 10(b) illustrates the TN removal rates for the experimental group using the new treatment process, the experimental group employing the existing treatment process, and the blank control group, which were 85.65, −2.30, and 12.26%, respectively. The new treatment process significantly enhanced the TN removal rate by 87.95% compared to the existing treatment process and by 73.39% compared to the control group, demonstrating a substantial improvement in nitrogen removal efficiency.

At the start of the experiment, the initial TP concentration in the black and odorous water bodies was 0.555 mg/L. Figure 11(a) illustrates that the new treatment process (aeration + sulfur-iron autotrophic denitrification MBBR), effectively decreased the TP levels throughout the experiment. In this process, sulfur-iron autotrophic denitrifying bacteria utilize elemental sulfur and sponge iron as electron donors to facilitate autotrophic denitrification. Concurrently, sponge iron adsorbs through both physical and chemical adsorption on its surface, under reducing conditions, interacts with Fe³⁺ and Fe²⁺ to form stable precipitates such as FePO₄ and Fe₄(PO₄)₃(OH)₃ (Yuan et al. 2023), thereby achieving simultaneous denitrification and phosphorus removal. After 18 days of treatment, the TP concentration in the experimental group undergoing the new treatment process decreased significantly from the initial 0.555 mg/L to below 0.15 mg/L, surpassing the regulatory threshold of 0.3 mg/L as specified in the Grade IV surface water standards (GB3838-2002). In contrast, both the blank control group and the experimental group following the existing treatment method (aeration + MBBR) experienced an increase in TP concentration from the initial 0.555 mg/L to over 0.55 mg/L. This concentration exhibited fluctuations throughout the experimental period, initially declining possibly due to elevated DO levels promoting phosphorus uptake by phosphorus-collecting bacteria, the subsequent rise in TP levels could also be attributed to increased DO levels, which facilitate the absorption of phosphorus by phosphorus-collecting bacteria (Zheng et al. 2023). In the initial 3–6 days of the experiment, the experimental group of the existing treatment process exhibited slightly lower TP levels compared to the blank control group, attributed to higher DO levels. However, as ammonia nitrogen converted to nitrate in both groups, nitrate accumulation (Figure 10(a)) hindered the metabolism and growth of phosphorus-aggregating organisms (PAOs) within the biological phosphorus removal particles (Torresi et al. 2019). Concurrently, endogenous phosphorus within the organic matter deposited in the anaerobic sediment was reciprocally transformed and released into the water column. The phenomenon of endogenous phosphorus deposition in anaerobic sediments is the result of a combination of factors, such as changes in redox conditions, decomposition of organic matter, biogeochemical processes, and mineralization rates. Together, these processes promote the release, transformation, and accumulation of phosphorus, making anaerobic sediments important phosphorus reservoirs (Jin et al. 2020). This ultimately leads to a gradual increase in TP.

In Figure 11(b), the TP removal rates after 18 days of treatment were 78.02% for the experimental group undergoing the new treatment process, −4.05% for the experimental group following the existing treatment method, and −0.45% for the blank control group. The TP removal rate in the new treatment process's experimental group significantly reached 78.02%. Compared to the existing treatment process experimental group, this represented an 82.07% increase in TP removal, and a 78.47% increase compared to the blank control group. These results highlight the superior performance of the new treatment process in TP removal efficiency.

This thesis introduces a novel treatment process for addressing black and odorous water bodies termed ‘aeration + sulfur-iron autotrophic denitrification MBBR’. It conducts a comparative experimental study on the water purification efficacy of this new treatment process, the existing treatment process (aeration + MBBR), and the blank control group. After 18 days of continuous monitoring, the following primary conclusions are drawn:

• (1) The novel treatment process exhibits notable superiority over the existing treatment process in nitrogen and phosphorus removal. The TN and TP removal rates in the experimental group of the new treatment process achieved 85.65 and 78.02%, respectively, surpassing those of the existing treatment process by 87.95 and 82.07%. Furthermore, they were 73.39 and 78.47% higher than the removal rates observed in the blank control group.

• (2) The new treatment process demonstrates effectiveness in removing organic pollutants, with a removal rate that is not only comparable to the existing treatment process but slightly improved. Specifically, the COD removal rate in the experimental group of the new treatment process reached 78.60%, surpassing that of the existing treatment process by 5.74% and exceeding that of the blank control group by 40.41%.

• (3) In comparing the experimental group of the new treatment process with the existing treatment process, there are no significant differences in terms of DO, turbidity, and pH levels. Both processes exhibit the ability to increase DO by over fourfold, reduce turbidity by a factor of seven, and maintain pH within the range of 6–9.

• (4) The experimental group of the new treatment process underwent an 18-day treatment period, resulting in average values of 1.295 NTU for turbidity, 8.41 mg/L for DO, 12.305 mg/L for COD, 4.05 mg/L for BOD₅, 0.1 mg/L for NH₃-N, 1.34 mg/L for TN, and 0.122 mg/L for TP in the water sample. These values all met the criteria for surface Grade Ⅳ water quality. Conversely, the TN and TP levels in the experimental group of the existing treatment process did not meet the specified standards.

Yannan Xue: Conceptualization, Writing-original draft, Data curation, Visualization, Investigation, Validation, Methodology, Formal analysis. Minghong Zhang: Visualization, Validation, Investigation. Mulan Zhu: Conceptualization, Writing-review & editing, Supervision, Resources, Funding acquisition, Investigation, Validation, Methodology, Formal analysis. Zhengwei Chen: Visualization, Validation, Investigation.

This work was Supported by Fujian Science and Technology Program (No. 2023I0042).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict