Analysis of the Effect of MBBR Process Retrofit in a Southern Wastewater Treatment Plant
The "2022 China Urban Construction Status Bulletin" released by the Ministry of Housing and Urban-Rural Development of the People's Republic of China in October 2023 shows that by the end of 2022, the treatment capacity of wastewater treatment plants in China had reached 216 million m³/d, a year-on-year increase of 4.04%. The total volume of wastewater treated has been on a growth trend for 10 consecutive years since 2013. The rapid development of cities is accompanied by an increase in wastewater discharge, and the contradiction between the land required for the expansion and renovation of wastewater treatment plants and urban development land is becoming increasingly prominent.
For expanding the capacity of existing wastewater treatment plants, the conventional activated sludge process generally adopts the method of plant expansion. As the expansion volume increases, land acquisition costs gradually rise, and the construction period is extended. Deepening the tapping of treatment capacity within the existing wastewater treatment plant is currently an effective measure to further enhance urban wastewater treatment capacity and alleviate the contradiction between urban development and land use. The Moving Bed Biofilm Reactor (MBBR) originated in Norway in the late 1980s. It enhances the enrichment of functional bacteria and thereby improves the system's treatment capacity by adding suspended carriers to the biological tank to form biofilms. Due to its characteristic of being able to be "embedded" into the original biological system, it is widely used in the upgrading and renovation of wastewater treatment plants, achieving in-situ capacity enhancement without adding new land. Furthermore, compared to other land-saving retrofit processes such as Membrane Bioreactor (MBR) and High Concentration Composite Powder Carrier Biological Fluidized Bed (HPB), the MBBR process does not require periodic replacement or replenishment of carriers, making it more economically advantageous.
This article takes the capacity expansion retrofit using the MBBR process at a wastewater treatment plant in southern China as an example. It analyzes the operational performance of the plant before and after the retrofit, the nitrification performance of the MBBR zone, and the microbial community structure, clarifying the practical role of the MBBR process in in-situ capacity expansion. The aim is to provide references and suggestions for the design and operation of similar wastewater treatment plants.
1 Project Overview
A wastewater treatment plant in southern China has a total designed treatment capacity of 7.5×10⁴ m³/d, with Phase I capacity at 5×10⁴ m³/d and Phase II at 2.5×10⁴ m³/d. Both phases initially used the Modified Bardenpho process. The main treatment targets are domestic wastewater from the collection area and partial industrial wastewater from an industrial park. The effluent quality must comply with the Grade A standard specified in the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918-2002). With the rapid development of urban construction and the economy, wastewater discharge has been increasing, and the project has been operating at or beyond full capacity. In 2021, as required by government authorities, the project needed to expand its capacity by an additional 2.5×10⁴ m³/d based on the original scale, reaching a total treatment capacity of 1×10⁵ m³/d. The effluent standard remained Grade A of GB 18918-2002. The designed influent and effluent quality are shown in Table 1.
The area surrounding this project is agricultural land, and there was insufficient reserved land for expansion within the original plant site. Additionally, during the initial construction of Phase II, the pretreatment units were already built according to a capacity of 5×10⁴ m³/d. Therefore, the focus of this retrofit project was to fully tap the treatment potential of the existing biological tanks and minimize land occupation for modifying the biological tanks. The MBBR process is widely used in in-situ capacity expansion and renovation of wastewater treatment plants due to its "embedded" characteristic. For example, a wastewater treatment plant in northern China used the MBBR process for capacity increase, maximizing the use of existing tank volumes and process flow, achieving a 20% in-situ capacity expansion with effluent stably meeting Grade A standards. Another plant in Guangdong used the MBBR process for in-situ enhancement of biological treatment performance, achieving a good effect of 50% in-situ capacity expansion with effluent stably better than the discharge standard. Therefore, considering the actual needs of the wastewater treatment plant and comprehensively evaluating factors such as land use and operation, the MBBR process was ultimately selected as the treatment process for this capacity expansion retrofit.
2 Process Design
2.1 Process Flow
The core of this capacity expansion retrofit was to enhance the treatment capacity of the biological tanks in-situ through MBBR, ensuring stable compliance with effluent standards despite a 100% increase in flow. Since the original pretreatment and advanced treatment units were already constructed for a capacity of 5×10⁴ m³/d, this retrofit focused on reusing existing facilities. The core modification was the biological tanks, along with the construction of a new secondary sedimentation tank set to meet the treatment demand after the flow increase. The process flow after retrofit is shown in Figure 1. Influent undergoes pretreatment through coarse/fine screens and a grit chamber, then enters the Modified Bardenpho-MBBR tank for removal of carbon, nitrogen, phosphorus, and other pollutants. The effluent from the biological tanks passes through sedimentation tanks and a high-efficiency clarifier to ensure stable compliance with SS and TP standards. After disinfection, the final effluent is discharged into the receiving river for ecological water replenishment.
2.2 Biological Tank Retrofit
The biological tank retrofit plan is shown in Figure 2. While doubling the treatment flow, the volumes of the original anaerobic and anoxic zones remained unchanged. 20% of the volume from the original aerobic zone was partitioned to create an additional anoxic zone, expanding the overall anoxic zone volume to meet denitrification demand. Suspended carriers were added to the remaining volume of the aerobic zone to form the aerobic MBBR zone. Supporting inlet/outlet screening systems and MBBR-specific mixers were installed. The original chain aeration system was replaced with a bottom perforated aeration system to ensure good fluidization of the suspended carriers and prevent their loss with the water flow. After retrofit, the total Hydraulic Retention Time (HRT) of the biological tanks is 8.82 h, with anaerobic zone HRT at 1.13 h, anoxic zone HRT at 3.05 h, and aerobic zone HRT at 4.64 h. The total system internal recycle ratio is 150%, and the Sludge Age is 16 days.
Regarding equipment, 4 sets of submersible mixers were added to the anoxic zone (Power P = 4 kW, Impeller Diameter D = 620 mm). SPR-III type suspended carriers were added to the aerobic MBBR zone, with a diameter of (25.0 ± 0.5) mm, height of (10.0 ± 1.0) mm, effective specific surface area >800 m²/m³, and density of 0.94 ~ 0.97 g/cm³. The density approaches that of water after biofilm attachment, complying with the industry standard "High-density Polyethylene Suspended Carrier Fillers for Water Treatment" (CJ/T 461-2014). The fill ratio is 45%. Two sets of suspended carrier-specific submersible mixers were added (P = 5.5 kW). Twenty-two sets of liftable aeration systems, 4 sets of fixed aeration systems, and 45 sets of fine bubble aerators were added. Two internal recycle pumps were replaced (Flow Q = 1600 m³/h, Head H = 0.60 m, P = 7.5 kW).
2.3 Construction of New Secondary Sedimentation Tank
Due to the increased flow, the existing secondary sedimentation tanks could not meet the effluent requirements. A new secondary sedimentation tank was needed to support the increased treatment capacity. The new tank is consistent with the original ones, using a rectangular horizontal flow type. The effective tank volume is 4900 m³, with HRT = 7 h. One pump-type sludge scraper was added (Operating Speed V = 0.8 m/min). Six submersible axial flow pumps (external recycle pumps) were added (Q = 180 m³/h, H = 4 m, P = 5.5 kW). Two waste sludge pumps were added (Q = 105 m³/h, H = 11 m, P = 7.5 kW).
3 Analysis of MBBR Retrofit Effect
The operational performance before and after the Phase II retrofit, the simultaneous operational performance of Phase I and Phase II, the water quality changes along the process in Phase II, and the nitrification capacity of the biofilm and suspended sludge phases in Phase II were analyzed to assess the enhancement effect of the MBBR retrofit on the system's treatment capacity.
3.1 Operational Performance Comparison
Before the retrofit, Phase II was already operating above its designed flow, with an actual average flow of (3.02 ± 0.46) ×10⁴ m³/d. After retrofit, the flow further increased to (5.31 ± 0.76) ×10⁴ m³/d, an actual increase of approximately 76%. The maximum operational flow reached 7.61×10⁴ m³/d, 1.52 times the design value. Influent and effluent quality before and after retrofit are shown in Table 2 and Figure 3. Regarding influent loading, after retrofit, the ammonia nitrogen (NH₃-N), total nitrogen (TN), COD, and TP loadings increased to 1.61, 1.66, 1.60, and 1.53 times the pre-retrofit levels, respectively. In terms of actual influent/effluent quality, influent NH₃-N and TN before/after retrofit were (22.15±3.73)/(20.17±4.74) mg/L and (26.28±4.07)/(23.19±3.66) mg/L, respectively. Effluent NH₃-N and TN before/after retrofit were (0.16±0.14)/(0.14±0.08) mg/L and (8.62±1.79)/(7.01±1.76) mg/L, with average removal rates of 99.28%/99.31% and 67.20%/69.77%, respectively. Despite the substantial increase in flow and influent loading after retrofit, effluent quality was still better than before retrofit. The increased anoxic zone volume ensured good TN removal, with effluent TN further reduced after retrofit. The aerobic zone achieved a significant enhancement in nitrification capacity through the suspended carrier biofilm. Even with a 20% reduction in aerobic zone volume compared to pre-retrofit and significant increases in flow and influent loading, highly efficient NH₃-N removal was maintained. Influent COD and TP before/after retrofit were (106.82±34.37)/(100.52±25.93) mg/L and (2.16±0.54)/(1.96±0.49) mg/L, respectively. Effluent COD and TP before/after retrofit were (10.76±2.04)/(11.15±3.65) mg/L and (0.14±0.07)/(0.17±0.05) mg/L, with average removal rates of 89.93%/93.52% and 88.91%/91.33%, respectively. After retrofit, effluent quality remained stably better than the design discharge standard.
Operational data from November to January of the following year (post-retrofit) were further selected to compare the performance of Phase I and Phase II under low-temperature conditions (minimum temperature 12°C). Influent and effluent pollutant concentrations for both phases are shown in Figure 4. Under winter low-temperature conditions, effluents from both processes were stably better than the design discharge standard. Particularly for NH₃-N removal, which is susceptible to low temperatures, with an influent NH₃-N concentration of (18.98±4.57) mg/L, Phase I effluent NH₃-N was (0.27±0.17) mg/L and Phase II was (0.29±0.15) mg/L, both demonstrating good resistance to low temperatures. Notably, after the MBBR retrofit in Phase II, the aerobic zone HRT was only 66.07% of that in Phase I, achieving a significant improvement in nitrification performance.
3.2 Performance Analysis of MBBR Zone
To further determine the actual effect of each functional zone, water samples from the end of each functional zone in Phase I and Phase II were taken for parallel measurement. Results are shown in Figure 5. Influent NH₃-N concentrations were 18.85 mg/L and 18.65 mg/L, and effluent NH₃-N concentrations were 0.35 mg/L and 0.21 mg/L, with NH₃-N removal rates of 98.14% and 98.87%, respectively. From the nitrogen profile changes, NH₃-N removal in Phase II mainly occurred in the aerobic MBBR zone. The NH₃-N concentration at the MBBR zone effluent was 0.31 mg/L, contributing 99.46% to the overall NH₃-N removal, already better than the design discharge standard. The subsequent aerobic activated sludge zone served a safeguarding role. Furthermore, wastewater treatment plants using MBBR in the aerobic zone commonly exhibit Simultaneous Nitrification and Denitrification (SND). However, in this project, no Total Inorganic Nitrogen (TIN) removal was observed in the aerobic MBBR zone, which may be related to the relatively low influent substrate concentration in this project.
To further investigate the effect of adding suspended carriers on the system's nitrification performance, supernatant from the anoxic zone effluent of Phase I was taken. Nitrification performance tests were conducted on Phase I pure sludge, Phase II pure sludge, Phase II pure biofilm, and Phase II combined biofilm-sludge system. Under conditions consistent with the actual project (carrier fill ratio, sludge concentration, water temperature), with DO controlled at 6 mg/L to determine the optimal nitrification performance. Results are shown in Table 3. The nitrification rates for Phase I pure sludge, Phase II pure sludge, Phase II pure biofilm, and Phase II combined biofilm-sludge system were 0.104, 0.107, 0.158, and 0.267 kg/(m³·d), respectively. The addition of suspended carriers enhanced the system's nitrification performance. The nitrification rate of the Phase II combined biofilm-sludge system reached 2.57 times that of the Phase I pure activated sludge system. Moreover, the pure biofilm load was already higher than the activated sludge load, significantly improving the system's shock load resistance. In the Phase II combined system, the biofilm contributed 59.92% to the nitrification, holding a dominant position.
3.3 Rationality Analysis of the Retrofit
To analyze the rationality of using the combined biofilm-sludge MBBR process for this retrofit, calculations were performed regarding the effect of carrier addition, the system's shock load resistance, and the correlation between flow increase and carrier addition. If Phase II of this project had not been retrofitted and used the traditional activated sludge process, based on the designed influent/effluent NH₃-N and the optimal volumetric nitrification rate of Phase I activated sludge (DO=6 mg/L), the calculated effluent NH₃-N concentration would be 5.55 mg/L, failing to meet the effluent standard. If calculated based on the optimal nitrification rate obtained from the Phase II combined system test, at the designed influent flow, Phase II could tolerate a maximum influent NH₃-N concentration of up to 55 mg/L, which is 2.20 times the design value, significantly enhancing the system's shock load resistance. Therefore, using MBBR for this retrofit is rational and effectively ensures stable compliance with effluent standards. If Phase I were also retrofitted with the MBBR process, based on the designed influent/effluent pollutant concentrations, the treatment flow could be increased by more than 1-fold, providing the possibility for wastewater treatment plants to match rapid urban development and achieve smooth upgrades.
4 Biofilm Attachment Status and Microbial Analysis
The biofilm attachment on the suspended carriers in this project is shown in Figure 6. Biofilm uniformly coated the inner surface of the carriers, being dense without flocculent material in the carrier pores. The average thickness was (345.78 ± 74.82) μm. The average biofilm biomass was (18.87 ± 0.93) g/m², the Volatile Suspended Solids (VSS)/SS ratio was stable at 0.68 ± 0.02, and the average VSS was (12.77 ± 0.61) g/m².
To further explore the enhancement effect of the MBBR retrofit on system treatment capacity from a microscopic perspective, samples of Phase I activated sludge, Phase II activated sludge, and biofilm were taken for 16S amplicon high-throughput sequencing. The relative abundance of microorganisms at the genus level within the system is shown in Figure 7.
The dominant nitrifying genera on the suspended carrier biofilm were Nitrospira and Nitrosomonas, with relative abundances of 7.98% and 1.01%, respectively. In contrast, the dominant nitrifying genus in both Phase I and Phase II activated sludge was Nitrospira, with relative abundances of 1.05% and 1.27%, respectively. Nitrospira is the most common nitrifying genus in wastewater treatment plants. Many of its species have been proven to possess complete ammonia oxidation (comammox) capability, meaning a single microorganism can complete the process from ammonia to nitrate. The MBBR process, in the form of biofilm, achieved efficient enrichment of Nitrospira, with a relative abundance 7.58 times that in activated sludge, providing a microscopic foundation for the enhancement of system nitrification performance. It can also be observed that the relative abundance of nitrifying bacteria in the activated sludge from the same system as the biofilm (Phase II) was slightly higher than in the Phase I pure activated sludge system. This may be because shedding biofilm from the suspended carriers inoculated the activated sludge during dynamic renewal, increasing the relative abundance of nitrifying bacteria in the sludge.
The dominant denitrifying genera in both systems were mainly enriched in the activated sludge and were relatively similar in composition, including Terrimonas, Flavobacterium, Dechloromonas, Hyphomicrobium, etc. The relative abundances of denitrifying genera in Phase I and Phase II were 8.76% and 7.52%, respectively. From a functional perspective, in addition to denitrification, some species within Terrimonas can degrade anthracene-like substances; Flavobacterium can degrade biodegradable plastics (e.g., PHBV); Hyphomicrobium can utilize various toxic and hard-to-degrade organic compounds for denitrification, such as dichloromethane, dimethyl sulfide, methanol, etc. The influent of this project contains some industrial wastewater, leading to the specialization of functional microbial communities under long-term acclimation. Although this project did not exhibit significant macroscopic SND effects, some denitrifying functional groups were still found on the suspended carrier biofilm, including Hyphomicrobium, Dechloromonas, Terrimonas, and OLB13, with a total proportion of 2.78%. This indicates that after the biofilm reaches a certain thickness, the anoxic/anaerobic microenvironments formed inside can provide conditions for the enrichment of denitrifying bacteria, also offering the possibility for SND occurrence in the aerobic MBBR zone. Furthermore, Proteiniclasticum was detected in both Phase I and Phase II sludge, with relative abundances of 1.09% and 1.18%, respectively. This genus has good capability for decomposing and transforming proteinaceous substances. Its enrichment may be related to the presence of numerous dairy product enterprises within the collection area of this project.
Notably, the relative abundance of Candidatus Microthrix in Phase I activated sludge reached 3.72%. It is a common filamentous bacterium in activated sludge, often associated with sludge bulking. However, its relative abundance in Phase II sludge and biofilm was only 0.57% and 1.03%, respectively. After retrofitting with the MBBR process, the fluidization of suspended carriers has a shearing effect on filamentous bacteria, reducing the likelihood of filamentous bulking in the activated sludge.
5 Economic Analysis
The electricity consumption per cubic meter before and after this retrofit was 0.227 kWh/m³ and 0.242 kWh/m³, respectively. At an electricity price of 0.66 RMB/(kWh), the operational electricity costs were 0.150 RMB/m³ and 0.160 RMB/m³. The increase in electricity consumption was mainly due to new anoxic zone mixing and additional electrical equipment from the new secondary sedimentation tank. The phosphorus removal chemicals used in this project are Polyferric Chloride (PFC) and Polyacrylamide (PAM). Dosage remained consistent before and after retrofit: PFC dosage 2.21 t/d, cost 0.014 RMB/m³; PAM dosage 17.081 kg/d, cost 0.0028 RMB/m³. This project fully utilizes the carbon source in the raw influent for denitrification. No external organic carbon source was added before or after retrofit. The direct electricity and chemical costs per cubic meter before and after retrofit were 0.167 RMB/m³ and 0.177 RMB/m³, respectively.
6 Conclusions and Outlook
(1) Phase II of a southern wastewater treatment plant used the MBBR process for capacity expansion retrofit, addressing issues such as land shortage. After retrofit, the treatment flow increased from (3.02±0.46) ×10⁴ m³/d to (5.31±0.76) ×10⁴ m³/d, achieving 76% in-situ capacity expansion. The maximum operational flow reached 1.52 times the design value, with effluent stably better than the design discharge standard.
(2) By embedding the MBBR process in the biological stage, highly efficient and stable NH₃-N removal was achieved under winter low-temperature conditions, even though the aerobic HRT was only 66.07% of that in the activated sludge process. The MBBR zone contributed 99.46% to NH₃-N removal. If Phase II had not been retrofitted, under the same flow and water quality, the effluent NH₃-N would reach 5.55 mg/L. Therefore, using MBBR for this retrofit was necessary and rational.
(3) The suspended carrier biofilm enhanced the enrichment effect of the core nitrifying genus Nitrospira. Its relative abundance in the biofilm was 7.58 times that in the activated sludge, providing a microscopic foundation for the improvement of system nitrification performance. Additionally, the enrichment of denitrifying genera in the biofilm offers the possibility for SND occurrence.
This project used the combined biofilm-sludge process to achieve in-situ capacity increase. However, actual operation is still limited by the retention and recovery of activated sludge, preventing further enhancement of treatment capacity. Currently, pure biofilm processes have been applied in actual projects, completely abandoning activated sludge and utilizing the high-load characteristics of biofilm for efficient pollutant removal, unrestricted by activated sludge limitations. This provides a new solution for the new construction, renovation, or expansion of wastewater treatment plants.