Operational Performance and Nitrification Characteristics of Hybrid Sludge-Biofilm MBBR Process in Municipal Wastewater Treatment Plant

Operational Performance Evaluation of a Hybrid Sludge-Biofilm MBBR Process in a WasteWater Treatment Plant

The Moving Bed Biofilm Reactor (MBBR) originated in Northern Europe in the 1990s. Due to its advantages of high denitrification efficiency, strong shock load resistance, and ease of retrofitting into existing tanks, it has been widely applied in new construction, expansion, and upgrading of wastewater treatment plants in China. The domestic application volume has exceeded 26 million m³/d, making it one of the mainstream wastewater treatment processes. In 2021, MBBR was included in the "Outdoor Drainage Design Standard" (GB 50014-2021) as a "widely used and reliable" process. The core of the MBBR process is to form a biofilm on suspended carriers, enabling targeted cultivation and efficient enrichment of functional bacteria. In a hybrid sludge-biofilm system, it can compensate for the insufficient treatment capacity of activated sludge, thereby improving the overall performance of the biological system. Wu Di et al. found that the relative abundance of nitrifying bacteria in the biofilm on suspended carriers after retrofitting was more than 10 times that in the activated sludge system. By adding suspended carriers, the nitrification load of the system increased by 124.69% compared to before the retrofit, significantly enhancing the system's nitrification capacity. The MBBR process is characterized by the fluidization of suspended carriers within a specific zone under the restriction of screens. Unlike the activated sludge system, the microbial composition of the biofilm is closely related to the environment in which the suspended carriers are placed, directly affecting the treatment performance of the biofilm. Zhou Zhengxing et al. used a six-stage MBBR to treat river water. Affected by substrate availability, the relative abundance of biofilm decreased stepwise from 2.86% to 0.76%, and the nitrification load correspondingly decreased from 31.7 g/(m³·d) to 1.3 g/(m³·d). Han Wenjie et al. found that the relative abundance of nitrifying bacteria in the main aerobic zone of a suspended carrier system differed by more than two times among different stages, resulting in different shock load resistance capacities. It can be seen that although the MBBR process can enhance the treatment capacity of a system, the location of carrier addition and the staging of the MBBR zone can lead to significant differences in final performance. Existing research has mostly focused on the design and overall performance of the MBBR process, with few studies analyzing influencing factors such as the location and staging of the MBBR zone, making it difficult to judge the rationality of the design.

This paper takes a hybrid sludge-biofilm MBBR system in the Yangtze River Delta region that has been in operation for seven years as an example. The long-term operational performance of the system is analyzed, and the effects of different carrier addition zones and different staging on the nitrification performance of the biofilm are investigated, aiming to provide references and recommendations for the design and operational control of MBBR processes in wastewater treatment plants.

1 Project Overview

A wastewater treatment plant in the Yangtze River Delta region was constructed in three phases. Phase I and Phase II were commissioned in 2007 and 2011, with design capacities of 100,000 m³/d and 50,000 m³/d, respectively. In 2017, the plant underwent upgrading and expansion. The existing Phase I and Phase II were retrofitted in-situ to meet the Class A discharge standard of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant" (GB 18918-2002). A new Phase III was built with a capacity of 50,000 m³/d, bringing the total treatment capacity of the plant to 200,000 m³/d. The influent water quality for Phases I, II, and III is the same, but each phase operates independently without mutual interference. The design influent and effluent water quality are shown in Table 1. The new Phase III adopted an anaerobic-anoxic-aerobic (AAO) process with a hydraulic retention time (HRT) of 17.5 hours in the biological section. However, the existing Phase I and II had biological HRTs of 12.0 hours and 16.5 hours, respectively, which were insufficient and could not meet the Class A standard through operational optimization alone. Additionally, there was no reserved land for expansion on the original site, so treatment capacity had to be improved based on the existing structures. Therefore, the mature MBBR process was selected for the upgrading. Different upgrading schemes were developed based on the actual operating conditions of Phase I and II. Neither Phase I nor Phase II changed the tank structure or the HRT of the functional zones. Suspended carriers were added in the aerobic zones. In Phase I, carriers were added in the middle of the aerobic zone, using a two-stage addition with three parallel trains. In Phase II, carriers were added in the front part of the aerobic zone, using a single-stage addition. The layouts of the biological sections of Phases I, II, and III after retrofitting are shown in Figure 1, and the design parameters for each phase are shown in Table 2. All three phases use high-efficiency sedimentation tanks for advanced treatment, and the effluent is discharged into the receiving water body after disinfection.

2 Materials and Methods

To investigate the actual operational performance of the biological systems after MBBR retrofitting, the performance of the activated sludge and biofilm was evaluated using along-the-flow-path sampling and in-situ laboratory-scale tests.

2.1 Along-the-Flow-Path Data Measurement

Water quality samples were taken from the effluent of each functional zone along the flow path of Phases I, II, and III, including the effluent of the anaerobic tank, anoxic tank, activated sludge zones of the aerobic tank, and MBBR zones. Influent samples were taken from the effluent of the primary sedimentation tank. Sampling was performed every 2 hours for a total of 3 times. After rapid sedimentation, the supernatant was collected and promptly filtered for pretreatment. The three samples were mixed in equal amounts, and ammonia nitrogen, nitrate nitrogen (NO₃⁻-N), and soluble COD were determined.

2.2 Nitrification Performance Measurement

Using the supernatant of the anoxic zone effluent after sedimentation, the nitrification performance of different biological phases in Phases I, II, and III was measured. In the same batch of tests, three systems were set up: a pure-biofilm system (only suspended carriers), a pure-activated sludge system (only activated sludge), and a hybrid sludge-biofilm system (both carriers and sludge). The suspended carriers were taken from the respective addition zones in Phase I and II. The activated sludge was taken from the end of the aerobic zone of each phase. The laboratory-scale system had an effective volume of 25 L, and the sludge concentration and carrier filling ratio were kept consistent with actual operation. During the tests, the temperature was 18–21°C, and the dissolved oxygen concentration was 6.0–7.5 mg/L. Samples were taken at regular intervals to measure ammonia nitrogen and NO₃⁻-N, and the volumetric loads of the activated sludge system, the suspended carrier system, and the hybrid system were analyzed. The nitrification contribution rate of the suspended carriers in the hybrid system was calculated according to equation (1).

Where: Ø — nitrification contribution rate of suspended carriers;
A_hybrid — volumetric load of the MBBR hybrid system, kg NH₄⁺-N/(m³·d);
A_S — volumetric load of the activated sludge system, kg NH₄⁺-N/(m³·d).

2.3 Biofilm Thickness and Biomass Measurement

Biofilm thickness was measured using an AOSVI optical instrument. Biomass was determined by the gravimetric method.

2.4 Analysis of Microbial Community Structure

High-throughput sequencing of 16S rDNA amplicons was used to determine the microbial communities in the suspended carriers and activated sludge. Activated sludge samples were taken from the biological tanks of Phases I, II, and III. Biofilm samples were scraped from the suspended carriers in the first and second stages of the MBBR zone in Phase I, and from the suspended carriers in the MBBR zone of Phase II. The specific method for high-throughput sequencing was consistent with the report by Han Wenjie et al.

3 Project Operational Performance

The actual influent and effluent conditions from January 1, 2022, to January 1, 2023, are shown in Table 3. The influent consists of municipal wastewater and industrial wastewater from the collection area. The actual influent concentrations were slightly lower than the design values. The effluent quality of Phases I, II, and III was consistently better than the design discharge standards. In terms of carbon removal, Phase III achieved the best COD removal, with an average effluent COD concentration of only (18.97±0.53) mg/L. However, the effluent BOD₅ concentrations of Phases I, II, and III were basically the same, indicating that although the HRTs of the biological tanks differed significantly, the efficiency of utilizing easily degradable organic matter was similar. However, because Phase III had the longest HRT, it performed better in removing refractory organic compounds. In terms of nitrogen removal, the effluent ammonia nitrogen concentrations of Phases I, II, and III were essentially the same, consistently better than the Class A standard. Based on the nitrification performance, the calculated nitrification volumetric loads in the aerobic zones of Phases I, II, and III were 0.111, 0.082, and 0.076 kg NH₄⁺-N/(m³·d), respectively. The actual nitrification capacities of Phase I and Phase II were 1.3 and 1.1 times that of Phase III, respectively. The addition of suspended carriers greatly improved nitrification performance. The less pronounced improvement in Phase II was mainly due to its original HRT already being 16.5 hours, and this project was an in-situ retrofit that did not change the actual HRT. On the basis of good nitrification, all three phases achieved good denitrification, with TN removal rates exceeding 70%. The system denitrification loads were 0.118, 0.082, and 0.075 kg TN/(m³·d), respectively. Even Phase I, which had the shortest anoxic retention time, still met the denitrification requirements, indicating that the raw water had sufficient carbon sources for denitrification, which also validated the original intention of not modifying the anoxic zones during the Phase I and II retrofits.

4 Discussion and Analysis

4.1 Comparison of Nitrification Enhancement by Suspended Carrier Addition

To further investigate the impact of suspended carrier addition on the biological system, laboratory-scale tests were conducted to measure the nitrification performance of the hybrid systems in Phase I and II and the activated sludge systems in Phases I, II, and III. The suspended carriers for Phase I were taken from zone 1-1. The results are shown in Figure 2.

As shown in Figure 2, the average volumetric loads of the hybrid systems in Phase I and II were 0.392 and 0.189 kg NH₄⁺-N/(m³·d), respectively, which were significantly higher than the 0.137 kg NH₄⁺-N/(m³·d) of the Phase III activated sludge system, representing increases of 186.13% and 37.96%, respectively. The MBBR process achieves targeted cultivation by adding suspended carriers into the biological tank, especially enhancing the enrichment of autotrophic nitrifying bacteria. The enrichment capacity is far superior to that of the activated sludge system, thereby achieving in-situ improvement of nitrification performance. Furthermore, studies have shown that extending HRT within a reasonable range is more favorable for the enrichment of nitrifying bacteria. In this project, the HRT of Phase I was significantly lower than that of Phase II and III. However, from the nitrification test results, the average sludge loads of Phase I and II were basically the same, at 0.035 and 0.036 kg NH₄⁺-N/(kg MLSS·d), respectively, which were higher than the 0.024 kg NH₄⁺-N/(kg MLSS·d) of Phase III. In the MBBR process, there is dynamic renewal of the biofilm. Detached biofilm enters the activated sludge and can "re-inoculate" the activated sludge, thereby enhancing its performance. Therefore, even with an HRT reduced by 46.5%, the nitrification performance of the activated sludge in the hybrid system of Phase I was still higher than that of the pure activated sludge system. This is similar to the study by Sun Xiao et al., where the nitrification capacity of activated sludge in the MBBR zone of a hybrid system was still significantly higher than that of the activated sludge system when the HRT was only 70% of that of the activated sludge system, with a 63% improvement in nitrification performance, confirming that detached biofilm can act as an inoculum to significantly promote the nitrification performance of activated sludge in hybrid systems.

4.2 Comparison of Nitrification Performance of Different Stages in the MBBR Zone

In Phase I, the aerobic MBBR zone consisted of three parallel trains, each with two-stage carrier addition. To investigate the nitrification performance of the biofilm on the carriers in different stages, the first-stage and second-stage carriers from each train were taken for laboratory-scale nitrification tests. The nitrification performance, thickness, and biomass of the biofilm on the carriers in each stage are shown in Figure 3 and Table 4. Under the same test conditions, the average volumetric loads of the first-stage biofilm were 0.21, 0.22, and 0.25 kg NH₄⁺-N/(m³·d), respectively, which were slightly higher than those of the second-stage biofilm (0.17, 0.19, and 0.20 kg NH₄⁺-N/(m³·d)), approximately 1.2 times higher. Along-the-flow-path measurements showed that the ammonia nitrogen concentration entering the first stage of the aerobic MBBR zone was (24.11±3.11) mg/L, and after treatment, the ammonia nitrogen concentration entering the second stage was only (9.39±2.12) mg/L, with an ammonia nitrogen removal ratio of approximately 60%:40%. The first stage of the aerobic MBBR zone bore a higher influent ammonia nitrogen load, and the biofilm was always in a relatively high-ammonia environment. In contrast, the second stage had a lower influent ammonia nitrogen concentration, and the biofilm was in a relatively low-ammonia environment, resulting in lower nitrification performance. The biofilm thicknesses of the first and second stages of MBBR Train 1 were (477±135) μm and (307±115) μm, respectively; for Train 2, they were (551±94) μm and (438±94) μm; for Train 3, they were (459±208) μm and (429±74) μm. The biofilm thickness of the first stage was significantly higher than that of the second stage in all trains. In terms of biomass, the first and second stage biomasses of Train 1 were 20.79 and 17.75 g/m²; of Train 2, 21.49 and 18.55 g/m²; of Train 3, 24.03 and 17.92 g/m². The results for nitrification performance, biomass, and biofilm thickness were consistent, all showing higher values in the first stage than in the second stage. This indicates that within different stages of the same MBBR train, the first stage has a higher substrate availability, which promotes biofilm growth, while the second stage has a lower influent load, resulting in lower biomass. Based on the operational performance of staged MBBR zones, aeration control can be staged. When the influent load is low, the aeration rate in the first stage can be appropriately reduced to allow some substrate to pass to the second stage, ensuring biofilm growth in the second stage while reducing energy consumption. When the influent load increases or low winter temperatures reduce nitrification performance, the aeration rate in the first stage can be increased, using a "first-stage load reduction + second-stage compliance guarantee" control strategy to achieve stable compliance. Through functional staging and aeration rate control, both compliance and energy savings can be achieved.

4.3 Effect of Carrier Addition Zone on Nitrification Performance

In this project, Phase I and Phase II both underwent in-situ retrofitting using the MBBR process, but the locations of the MBBR zones were different. Therefore, to determine the effect of carrier addition location on biofilm nitrification performance, the effluent from the functional zones of Phase I and II was measured to assess the actual performance of the MBBR zones. The results are shown in Figure 4.

In Phase I, carriers were added in the middle of the aerobic zone. Ammonia nitrogen removal occurred almost entirely in the aerobic MBBR zone (DO about 2 mg/L). The volume of the MBBR zone accounted for 60% of the total aerobic zone volume, but it contributed 94% of the ammonia nitrogen removal, playing the primary nitrification role. The post-aerobic activated sludge zone (DO about 4 mg/L) only served as a safety buffer. In Phase II, carriers were added in the front part of the aerobic zone. The MBBR zone (DO about 2 mg/L) showed no significant ammonia nitrogen removal, with only 5.78% removal. The nitrification process mainly occurred in the aerobic activated sludge zone (DO about 2 mg/L), which contributed 94.03% of the ammonia nitrogen removal. The reason for this phenomenon may be related to the high influent organic matter concentration in the MBBR zone. Along-the-flow-path monitoring showed that the COD concentration in the anoxic zone effluent was still 56.77 mg/L. After entering the aerobic zone, heterotrophic bacteria compete more strongly for dissolved oxygen than autotrophic bacteria, so carbon removal occurs first. As shown in Figure 4, the pre-aerobic activated sludge zone in Phase I and the MBBR zone in Phase II both removed about 10 mg/L of COD. Therefore, for the MBBR zone in Phase I, the effect of organic matter had been largely eliminated, and it did not affect biofilm nitrification. In contrast, the MBBR zone in Phase II directly received the anoxic zone effluent and was significantly affected by organic matter, lacking a favorable nitrification environment, resulting in weak biofilm nitrification performance. The nitrification laboratory-scale tests showed that the nitrification load of the Phase I MBBR biofilm was 0.207 kg NH₄⁺-N/(m³·d), while that of the Phase II MBBR biofilm was only 0.067 kg NH₄⁺-N/(m³·d), a decrease of about 67.63% compared to Phase I, which corresponded to the macroscopic nitrification performance of the systems. Therefore, for MBBR enhancement of activated sludge biological performance, if carrier addition is to be applied in a selected zone, it is recommended to add carriers in the middle of the aerobic zone to avoid inhibition of microbial nitrification by influent organic matter. For low C/N wastewater or situations where organic matter in the anoxic zone is fully utilized, the aerobic MBBR zone can also be directly applied in the front part.

4.4 Effect of Carrier Addition on Nitrifying Functional Microbial Communities

To further confirm the effect of carrier addition on nitrifying functional microbial communities, high-throughput sequencing of 16S rDNA amplicons was performed on the activated sludge of Phases I, II, and III and the biofilm on the carriers in each zone. The relative abundances of microorganisms at the genus level in different phases and biological phases are shown in Figure 5.

As shown in Figure 5, the dominant nitrifying genus in all systems was Nitrospira, a common nitrifying bacterium in wastewater treatment plants that can oxidize nitrite to nitrate, and some strains also have the ability to directly oxidize ammonia to nitrate. The high-throughput results showed that the relative abundance of Nitrospira on the Phase I 1-1 biofilm was the highest (6.66%), reaching 6.34 times that of the activated sludge in the same system (1.05%), achieving efficient enrichment of long-sludge-age nitrifying bacteria. The relative abundance of Nitrospira on the Phase II biofilm was 1.86%, significantly lower than that on the Phase I biofilm, indicating that the enrichment of nitrifying bacteria was limited by organic matter, corresponding to the macroscopic nitrification performance of the Phase I and II MBBR zones. It can further be seen that the relative abundance of Nitrospira in the Phase I activated sludge was 1.05%, higher than that in Phase II (0.89%) and Phase III (0.85%), validating the "re-inoculation" effect of detached biofilm on the activated sludge system. The lower relative abundance of Nitrospira on the Phase II biofilm also indirectly affected the relative abundance of Nitrospira in the Phase II activated sludge, failing to achieve a good "re-inoculation" process. Meanwhile, the relative abundances of Nitrospira in the Phase I MBBR zone 1-1 and 1-2 biofilms were 6.66% and 5.27%, respectively. With more abundant substrate, the enrichment of nitrifying bacteria was higher in the Phase I MBBR zone 1-1 biofilm, corresponding to the nitrification laboratory-scale results. The relative abundance of nitrifying bacteria on the Phase I MBBR zone 1-2 biofilm was also much higher than that in the activated sludge, ensuring stable effluent compliance during influent shock events. In addition, norank_f_JG30-KF-CM45 had a relatively high abundance in different phases and biological phases, with relative abundances in Phase I biofilm (1-1, 1-2), Phase II biofilm, Phase I sludge, Phase II sludge, and Phase III sludge being 10.36%, 11.25%, 7.17%, 2.86%, 2.73%, and 3.37%, respectively. Studies have shown that norank_f_JG30-KF-CM45 has nitrification or denitrification capability, but no clear conclusion has been reached yet. The high enrichment efficiency in this project may be related to specific substances in the influent water quality. The dominant denitrifying bacterium in all systems was Terrimonas, with relative abundances in Phase I biofilm (1-1, 1-2), Phase II biofilm, Phase I sludge, Phase II sludge, and Phase III sludge of 0.19%, 0.13%, 0.73%, 2.08%, 2.48%, and 1.56%, respectively. Additionally, Dechloromonas is also a typical complete denitrifying bacterium, but it was mainly enriched in the Phase II biofilm, with a relative abundance of 2.81%. Studies have shown that the abundance of this genus is closely related to the degradation of organic matter in wastewater, and its content is significantly positively correlated with COD removal rate, which is consistent with the fact that the Phase II MBBR zone received COD that was not fully utilized in the anoxic zone.

5 Conclusions

(1) A wastewater treatment plant in the Yangtze River Delta region adopted the MBBR process for upgrading. Without adding new land, it achieved in-situ enhanced denitrification by adding biofilm in the existing tanks. The effluent quality was consistently better than the design discharge standards, and the system has been operating stably for more than seven years. Laboratory-scale tests showed that the volumetric load of the hybrid sludge-biofilm system was 186.13% higher than that of the activated sludge system in the same plant, confirming the rationality of the retrofit.

(2) The MBBR retrofit improved the enrichment capacity of functional microbial communities. The relative abundance of the dominant nitrifying genus Nitrospira in the biofilm was 6.34 times that in the activated sludge of the same system, ensuring stable treatment performance and shock load resistance. The detached biofilm "re-inoculated" the activated sludge in the same system, increasing the relative abundance of nitrifying bacteria in the sludge.

(3) The MBBR zone can be arranged in stages. The first stage reduces the pollutant load, and the second stage ensures stable effluent compliance. In actual operation, the aeration rate can be adjusted according to the influent pollutant load. Under low influent load, reducing the aeration in the first stage saves energy and ensures biofilm growth in the second stage. Under high influent load, increasing aeration ensures compliance. This meets both compliance and energy-saving requirements. At the same time, the design of the aerobic MBBR zone should fully consider the effect of COD in the anoxic zone effluent. When the COD in the anoxic zone is not fully utilized, the MBBR zone can be appropriately moved downstream to avoid COD affecting the nitrification performance of the biofilm.