Case Study of MBBR+ACCA Process for Upgrading and Reconstruction of an Urban Wastewater Treatment Plant
Against the backdrop of China's booming economy, the pace of industrialization and urbanization has accelerated significantly. This process is inevitably accompanied by a year-on-year increase in the discharge of industrial wastewater and domestic sewage, exacerbating water pollution issues and impacting the sustainable ecological civilization construction of China. With the comprehensive implementation of the Water Pollution Prevention and Control Action Plan, stricter discharge requirements have been imposed on urban wastewater treatment plants across the country. Local standards in some cities have reached quasi-Class IV water quality, and for effluents discharged into sensitive water bodies, certain individual indicators are gradually approaching the Class III standard for surface water. However, the residual pollutants in urban wastewater after biological treatment are primarily non-biodegradable organic compounds with poor biodegradability. Relying solely on traditional biological enhancement technologies has become insufficient to meet the increasingly stringent emission standards.
Activated coke possesses a highly developed mesoporous system capable of adsorbing macromolecular pollutants in water. With high mechanical strength, stability, good adsorption performance, and relatively economical cost, it has been widely applied in the treatment of industrial wastewater that is difficult to biodegrade. In recent years, filtration technology using activated coke as a medium has also found certain applications in the advanced treatment of municipal wastewater plants, achieving good results in the ultimate removal of pollutants. Combining an engineering example from an upgrading project at a wastewater treatment plant in Henan Province, the author adopted the MBBR+ACCA (Activated Coke Circulating Adsorption) process to upgrade the treatment of urban wastewater. The effluent COD, NH₃-N, and TP indicators met the GB 3838-2002 Class III water standard, providing a reference for upgrading projects at other wastewater treatment plants.
1. Basic Situation of the Wastewater Treatment Plant
The total design capacity of this wastewater treatment plant is 50,000 m³/d, comprising a Phase I design capacity of 18,000 m³/d and a Phase II design capacity of 32,000 m³/d. It primarily treats urban domestic sewage and a small amount of industrial wastewater. An upgrade was completed in 2012, with the effluent meeting the Grade 1A standard of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants GB 18918-2002. The main process is multi-stage AO + denitrification filter + high-density sedimentation tank. The process flow is shown in Figure 1.
Currently, the wastewater treatment plant is operating near full capacity. Based on current operational data, under good plant maintenance, the effluent quality can be stably maintained at the GB 18918-2002 Grade 1A standard. The effluent concentrations for COD, BOD₅, NH₃-N, TN, and TP range from 21.77-42.34 mg/L, 1.82-4.15 mg/L, 0.13-1.67 mg/L, 8.86-15.74 mg/L, and 0.19-0.42 mg/L, respectively.
Prior to the upgrade, the plant faced the following issues: 1) Aging and damaged screens in the pretreatment section allowed some floating debris into the biological tanks, easily clogging pumps and affecting subsequent treatment; 2) Unstable TN removal during low winter temperatures and significant fluctuations in water quality and quantity; 3) Insufficient tank volume in the Phase I biological tanks and unreasonable anoxic zone partitioning, leading to poor TN removal efficiency and high chemical dosage for subsequent carbon source addition; 4) The original aeration system used outdated traditional centrifugal blowers with high energy consumption; 5) Severe clogging of filter media in the denitrification filters, incomplete backwashing, and difficulty in stable operation; 6) Frequent failures of mixing and stirring equipment in the high-density sedimentation tanks; 7) Frequent failures of the two existing belt filter presses for sludge dewatering, high moisture content of dewatered sludge, large sludge volume, and high sludge disposal costs; 8) Lack of odor control facilities for the pretreatment and sludge treatment systems; 9) Obsolete central control system with limited data storage capacity and loss of most remote operation functions.
2. Design Water Quality
Considering years of operational water quality data from the plant, with a 90% confidence level and including a certain margin, the design influent quality was determined. Based on the receiving water body's environmental quality requirements, the upgraded effluent COD, BOD₅, NH₃-N, and TP must meet the GB 3838-2002 Class III water standard, while TN and SS will adhere to the original standard. The design influent and effluent qualities are shown in Table 1.
3. Upgrading Concept and Process Flow
3.1 Upgrading Concept
According to the design effluent quality, this upgrade sets higher requirements for COD, BOD₅, NH₃-N, and TP. Considering the plant's current process, water quality characteristics, and existing problems, the focus is on enhanced removal of COD, NH₃-N, and TP while ensuring stable TN removal. Furthermore, limited available space within the existing plant necessitates fully exploiting the potential of existing structures through equipment renewal, process intensification, and renovation, aiming for effective removal of COD, NH₃-N, TN, and TP. Therefore, utilizing the original multi-stage AO tanks and adding suspended carriers to form a hybrid biofilm-activated sludge MBBR process can effectively improve treatment stability and shock load resistance. The long sludge age of biofilm on carriers is suitable for nitrifier growth and maintaining high nitrifier concentrations, significantly enhancing system nitrification capacity. The dense biofilm inside the carriers has a long sludge age, hosting substantial populations of nitrifying and denitrifying bacteria, enabling simultaneous nitrification-denitrification (SND) and thus strengthening TN removal. Hence, the MBBR process is well-suited for this plant's upgrade.
Based on similar upgrade project experience, to ensure stable compliance for COD and TP, additional safeguard treatment facilities are still required on top of the existing process coupled with MBBR. Activated coke, as a porous material, exhibits more significant adsorption performance compared to activated carbon, effectively removing COD, SS, TP, color, etc. Moreover, biologically activated coke can utilize attached microorganisms to degrade organic matter, enabling regeneration of adsorption sites while adsorbing pollutants. This dynamic equilibrium mechanism allows for sustained and stable system operation. The Activated Coke Circulating Adsorption (ACCA) process uses activated coke as the medium, integrating filtration and adsorption. It employs compressed air to lift and clean the filter media. Through reverse-flow zoning and uniform flow design, it ensures full contact between activated coke and wastewater, achieving ultimate water quality improvement and guaranteeing stable effluent compliance.
For the plant's aging and faulty equipment, they will be replaced with technologically advanced, energy-efficient equipment to reduce operational costs. Specifically, the pretreatment screens will be replaced with internally fed fine screens to intercept hair and fibers, preventing clogging of MBBR carrier retention screens.
3.2 Process Flow
The upgraded process flow is shown in Figure 2. To meet head requirements, a new lift pump station was added. A newly constructed V-type filter serves as the pretreatment unit for the subsequent activated coke adsorption, ensuring ACCA system stability. Raw water passes through screens and grit chambers to remove floatables, hair, and particulates before entering the hybrid MBBR biological tanks for enhanced nitrogen removal. The mixed liquor then enters secondary clarifiers for solids separation. The supernatant is lifted via the new pump station into denitrification filters and high-density sedimentation tanks. The effluent is then lifted by the new pump station into the V-type filter and two-stage activated coke adsorption tanks for advanced treatment, further removing COD, TP, SS, color, etc. The final effluent is disinfected before discharge.
4. Design Parameters of Major Treatment Units
4.1 Biological Tanks
The existing Phase I biological tanks are divided into two groups with relatively small tank volume but sound structure. Therefore, for this upgrade, while meeting head requirements, the tank walls were raised by 0.5 m. After renovation, the total effective volume is 10,800 m³, with a total HRT of 14.4 h and an anoxic zone HRT of 6.4 h, increasing anoxic retention time to improve TN removal. The existing Phase II biological tanks have an effective volume of 19,600 m³, a total HRT of 14.7 h, and an anoxic zone HRT of 6.8 h. This project involved replacing the aeration systems and some aging submersible mixers in both Phase I and II biological tanks, and adding suspended carriers and retention screens. The carriers are made of polyurethane or other high-performance composite materials, with a cubic specification of 24 mm, a specific surface area of 4,000 m²/m³, and a filling ratio of 20%. The biological treatment system's AOR is 853.92 kg O₂/h, with an air supply rate of 310.36 Nm³/min.
4.2 Lift Pump Station and Wastewater Tank
A new lift pump station was constructed to pump effluent from the high-density sedimentation tanks to the V-type filter for further treatment. A wastewater tank stores backwash wastewater from the filters. Small pumps are used to evenly pump the backwash wastewater into the Phase II biological tanks to avoid shock loading. Three secondary lift pumps were installed (2 duty + 1 standby, Q=1,300 m³/h, H=12 m, N=75 kW), with variable frequency drive (VFD) control. The backwash wastewater tank is equipped with 2 transfer pumps (1 duty + 1 standby, Q=140 m³/h, H=7 m, N=5.5 kW) and one submersible mixer (N=2.2 kW) to prevent sedimentation.
4.3 V-Type Filter
A new V-type filter was constructed with structural dimensions of 36.9 m (L) × 29.7 m (W) × 8.0 m (H). It uses homogeneous quartz sand filter media. The filter is divided into 6 cells arranged in two rows. Each cell's outlet pipe has an electric regulating valve to control constant water level operation. The backwashing process can be regulated via PLC. The design filtration rate is 7.0 m/h, the forced filtration rate is 8.4 m/h, and the single-cell filtration area is 49.4 m². Backwash water intensity is 11 m³/(m²·h), backwash air intensity is 55 m³/(m²·h), and surface sweep intensity is 7 m³/(m²·h). Backwash duration is 10 minutes. The backwash cycle is 24 hours (adjustable), washing one cell at a time. Quartz sand media size is 1-1.6 mm with k₈₀ < 1.3. Cast-in-place monolithic filter plates are used.
4.4 Activated Coke Adsorption Tanks
A new activated coke adsorption tank was constructed with structural dimensions of 49.5 m (L) × 30.15 m (W) × 11.0 m (H). It employs a two-stage filtration configuration with a total of 36 cells, 18 cells per stage. The maximum filtration rate is 6.02 m³/(m²·h), with an average of 4.63 m³/(m²·h). First-stage single-cell dimensions are L×W×H = 5.0 m × 5.0 m × 11.0 m, with an empty bed contact time (EBCT) of 1.4 h. Second-stage single-cell dimensions are L×W×H = 5.0 m × 5.0 m × 9.5 m, with an EBCT of 1.08 h. The system uses 2,000 tons of activated coke with particle size 2-8 mm,equipped with mobile coke washers, water distributors, inlet/outlet weirs, etc.
4.5 Activated Coke Building
A new activated coke building was constructed for storing activated coke and supplying it to the adsorption tanks. Structural dimensions are 33.5 m (L) × 13.0 m (W) × 6.5 m (H). Main ancillary equipment includes: 1 activated coke dewatering vibrating screen, 3 coke feeding pumps (2 duty + 1 standby, Q=40 m³/h, H=25 m, N=7.5 kW), 2 filtrate discharge pumps (1 duty + 1 standby, Q=120 m³/h, H=20 m, N=18.5 kW), 2 air compressors (1 duty + 1 standby, Q=7.1 m³/min, N=37 kW), and an air receiver tank (V=2 m³, P=0.8 MPa).
4.6 Plate-and-Frame Dewatering Room
A new plate-and-frame dewatering room was built next to the existing sludge dewatering room. Due to space limitations, one set of plate-and-frame filter press (filter area 300 m²) was configured, serving as a backup to the belt filter press. Ancillary facilities include one conditioning tank (effective volume 80 m³). The sludge quantity is 6,150 kg DS/d, with thickened feed sludge moisture content of 97% and dewatered cake moisture content of 60%. Main ancillary equipment includes: 2 feed pumps (1 duty + 1 standby, Q=60 m³/h, H=120 m, N=7.5 kW), 2 press water pumps (1 duty + 1 standby, Q=12 m³/h, H=187 m, N=11 kW), 1 washing pump (Q=20 m³/h, H=70 m, N=7.5 kW), 2 dosing pumps (1 duty + 1 standby, Q=4 m³/h, H=60 m, N=3 kW), 1 air compressor (Q=3.45 m³/min, N=22 kW), 1 set of air receiver tank (V=5 m³, P=1.0 MPa), and 1 set of PAM preparation unit (Q=2 m³/h, N=1.5 kW).
4.7 Odor Control System
A new biofiltration odor control system was added with a design air flow rate of 12,000 m³/h. Glass reinforced plastic (GRP) pipes are used to collect and treat odors from the pretreatment and sludge treatment systems. Stainless steel frames and PC endurance boards are used to seal pretreatment equipment.
4.8 Other Facility Updates
- Replaced with 2 internally fed fine screens with 5 mm aperture,with screw conveyors and wash water tank, V=10 m³ and 2 wash water pumps (1 duty + 1 standby, Q=25 m³/h, H=70 m, N=11 kW).
- Replaced with 4 more efficient air suspension blowers, VFD controlled (3 duty + 1 standby, Q=130 m³/min, P=63 kPa, N=150 kW).
- Replaced the filter media in the existing denitrification filters with 1,800 m³ of ceramic media (particle size 3-5 mm).
- Replaced 2 mixing agitators in the high-density sedimentation tanks (speed 60-80 rpm, N=5.5 kW), 4 flocculation agitators (speed 10-20 rpm, N=2.2 kW), and the tube settlers (260 m²).
- Replaced the belt filter press with a 2 m wide beltand matching air compressor, 1 set.
- Utilizing the original central control room, updated equipment, instruments, and established centralized control,established a plant-wide data communication system to achieve data communication between the central control room and substations, as well as automation of production process control.
5. Operational Performance and Technical-Economic Indicators
5.1 Operational Performance
After the completion of this upgrade project, all treatment units have been operating stably. The influent and effluent water quality monitoring data for 2023 are shown in Table 2.
As shown, the average effluent concentrations for COD, NH₃-N, TN, TP, and SS were 11.2, 0.18, 8.47, 0.15, and 2.63 mg/L, with average removal rates of 95.16%, 99.45%, 77.31%, 94.75%, and 97.38%, respectively. The effluent COD, NH₃-N, and TP consistently met the GB 3838-2002 Class III water standard.
The upgraded project has been in operation for nearly two years. Results indicate that the MBBR+ACCA process is stable, efficient, and produces high-quality effluent, demonstrating strong resistance to shock loads and low-temperature conditions. Even with a minimum winter water temperature of 9.4 °C and significant water quality fluctuations, the effluent quality remained stable and met discharge standards. Before and after the upgrade, the carbon source dosage did not increase, yet TN removal was significantly enhanced. This is because, on one hand, nitrifying microorganisms attached to the MBBR carriers grow and accumulate in a stable aerobic environment, leading to more complete nitrification. On the other hand, nitrate was further removed in the upgraded MBBR tanks and anoxic tanks. The final ACCA system acts as a safeguard, further adsorbing and removing recalcitrant COD, TP, SS, etc., making the effluent quality more stable. Moreover, after project implementation, the plant can produce high-quality reclaimed water, laying the foundation for future water reuse.
5.2 Technical-Economic Indicators
The total investment for this project was 86,937,600 RMB, comprising construction and installation costs of 74,438,500 RMB, other expenses of 7,593,500 RMB, contingency costs of 4,101,600 RMB, and initial working capital of 804,000 RMB. After stable system operation, the additional electricity cost for the entire plant is 0.11 RMB/m³, the activated coke cost is 0.39 RMB/m³, resulting in a total increase in operating costs of approximately 0.50 RMB/m³.
6. Conclusion
- This project implemented equipment renewal, process intensification, and renovation at the existing wastewater treatment plant, and added advanced treatment, improving the removal efficiency for COD, NH₃-N, TN, and TP.
- After the upgrade, using the main "MBBR+ACCA" process, the effluent COD, NH₃-N, and TP stably improved from Grade 1A to the surface water Class III standard, and TN removal was significantly enhanced.
- Practice shows that this process operates stably and efficiently, is resistant to load shocks, produces high-quality effluent, and adds an operating cost of approximately 0.50 RMB/m³. It can serve as a reference for upgrading projects and water reuse initiatives at other wastewater treatment plants.