Performance Validation and Operational Optimization for Standard Elevation Projects

Abstract

Driven by stricter effluent discharge standards, many existing municipal wastewater treatment plants (WWTPs) face upgrading challenges including limited available land, non-stop operation requirements and near-future capacity expansion demands. This paper presents a full-scale upgrading project adopting a combined process of pure biofilm moving bed biofilm reactor (MBBR) and magnetic coagulation sedimentation (BFM process), supplemented by powdered activated carbon (PAC) adsorption for refractory COD control. Validated by one-year continuous operational data, the system delivers stable effluent with COD of 20.86±3.31 mg/L, ammonia nitrogen of 0.07±0.09 mg/L, total nitrogen (TN) of 8.09±1.16 mg/L and total phosphorus (TP) of 0.04±0.03 mg/L, meeting Class IV surface water quality requirements. Targeted operational strategies for seasonal water quality fluctuations, sudden COD spikes and energy optimization achieve over 20% reduction in total operational cost. This process route features compact footprint, stable effluent performance and non-stop construction adaptability, providing a highly feasible reference for similar WWTP upgrading projects.

1. Project Background and Technical Challenges

1.1 Upgrading Requirements and Site Constraints

Located in Shandong Province, China, the target WWTP was originally designed with a treatment capacity of 60,000 m³/d, adopting an AAO biochemical process followed by flocculation sedimentation, active sand filtration and UV disinfection, with effluent meeting Class A standard of GB 18918-2002. In actual operation, the plant had reached an average daily treatment volume of 55,900 m³/d, with 90% of operational time running at 62,500 m³/d under overload conditions, and the inflow is projected to rise to 100,000 m³/d by 2030.

To meet provincial regulatory requirements, the plant needed to upgrade its effluent to Class IV standard of GB 3838-2002 (with TN ≤ 12 mg/L). The upgrading project faced three core constraints:

• No production halt or capacity reduction allowed during construction

• Only 0.2 hectares of available land for new facilities

• Need to cover both current capacity and future 40,000 m³/d expansion

Before upgrading, the effluent TN and ammonia nitrogen had low compliance rates of 72.60% and 83.56% respectively, with unstable nitrification performance under low-temperature conditions. TP removal faced risks of floc flotation under hydraulic shock, and refractory COD fluctuations threatened stable compliance. The core objective of the upgrading was to strengthen deep removal of nitrogen, phosphorus and COD under strict land and operation constraints.

1.2 Technical Alternative Evaluation

For tertiary treatment of Class A effluent with low pollutant concentration, biofilm-based processes are the mainstream technical route. Two mature schemes were compared in terms of technical performance, land occupation and life-cycle cost.

The BFM process is an integrated innovative technology combining pure biofilm MBBR as the core biochemical unit and modified magnetic loaded sedimentation as the solid-liquid separation unit, with an intelligent cloud control platform for fine operation regulation. The B (biofilm) section enriches functional bacteria on suspended carriers to strengthen ammonia nitrogen, TN and COD removal; the M (magnetic) section replaces traditional secondary and tertiary sedimentation with a single high-efficiency separation unit to remove TP, SS and shed biofilm; the F (fine control) section integrates AI-based process regulation to adapt to rapid water quality changes.

Comprehensive evaluation confirmed that the BFM process could meet the 0.2-hectare land constraint, while the BAF+DNF scheme required 0.27 hectares even with additional high-efficiency sedimentation tanks. For refractory COD control, activated carbon adsorption was selected over ozone catalytic oxidation and Fenton processes due to its lower investment, smaller footprint and simpler operation, and the adsorption tank can be integrated with the BFM tank body to further save land.

2. Process Design and Technical Principle

2.1 Overall Process Flow

The original WWTP process remains unchanged, and the new upgrading system is built on the adjacent vacant land. The Class A effluent from the original plant is lifted to the BFM system, which consists of three functional zones in the B section: aerobic zone, anoxic zone and post-aerobic zone.

• The aerobic zone further removes residual ammonia nitrogen from the influent through nitrifying bacteria enriched on biofilm carriers

• The anoxic zone achieves deep denitrification with sodium acetate as supplementary carbon source

• The post-aerobic zone eliminates excess carbon source to avoid COD elevation in the effluent

An activated carbon adsorption tank is set after the B section to remove refractory COD. The effluent then enters the M section magnetic coagulation sedimentation unit for high-efficiency TP and SS removal. After contact disinfection, the final effluent is discharged to the receiving water body. Sludge from the M section is discharged to the sludge storage tank, dewatered to below 80% moisture content and transported off-site.

The B section is designed for the current 60,000 m³/d capacity, while the M section and disinfection tank are designed for the long-term 100,000 m³/d capacity, allowing the future 40,000 m³/d expansion to bypass the B section directly for subsequent treatment, fully accommodating phased construction requirements.

2.2 Core Process Design Parameters

B Section (Pure Biofilm MBBR)

• Structure: semi-underground reinforced concrete, 2 parallel trains

• Total hydraulic retention time (HRT): 2.9 h (aerobic zone 1.2 h, anoxic zone 1.2 h, post-aerobic zone 0.5 h)

• Suspended carriers: HDPE SPR-Ⅲ type, effective specific surface area ≥800 m²/m³

• Filling ratio: 45% in aerobic zone, 50% in anoxic zone

• Aeration system: 3 air suspension blowers (2 duty + 1 standby), design air-water ratio of 1.44

• Mixing system: dedicated mixers for carriers in anoxic zone, mixing power of 21 W/m³

Activated Carbon Adsorption Tank

• HRT: 0.5 h

• Equipped with 6 sets of hyperboloid mixers and 1 automatic powdered activated carbon dosing system

M Section (Magnetic Coagulation Sedimentation)

• Design capacity: 100,000 m³/d, 2 trains with 4 units

• HRT: 2.0 min (mixing), 2.0 min (magnetic loading), 4.9 min (flocculation)

• Inclined tube sedimentation tank: surface load of 14.2 m³/(m²·h), 80 mm diameter inclined tubes at 60° installation angle

3. Full-Scale Operational Performance

The system was commissioned and put into operation in December 2022. The following performance data is based on full-year continuous operation records from January to December 2023.

3.1 Nitrogen Removal Performance

Biofilm cultivation on suspended carriers was completed successfully within the commissioning period. Microscopic examination confirmed abundant rotifers, vorticella and epistylis in the biofilm, with a biofilm thickness of 200–400 μm. The attached growth form enables highly efficient enrichment of nitrifying bacteria, maintaining stable nitrification performance even under low substrate concentration conditions.

For ammonia nitrogen removal, the influent shows obvious seasonal characteristics: low concentration of 0.62±0.41 mg/L from June to October, and elevated concentration of 2.38±0.40 mg/L from November to the following May. The annual average influent ammonia nitrogen is 1.89±1.26 mg/L, while the effluent ammonia nitrogen stabilizes at 0.07±0.09 mg/L, fully meeting the Class IV standard throughout the year.

For TN removal, the annual average influent TN is 12.96±1.89 mg/L, with small seasonal fluctuation. The effluent TN stabilizes at 8.09±1.16 mg/L, with stable denitrification performance. The average denitrification rate of the anoxic zone biofilm reaches 0.3–0.4 g N/(m²·d), with an actual C/N ratio of 3–5 using 25% liquid sodium acetate as carbon source.

3.2 COD and Phosphorus Removal Performance

The annual average influent COD of the BFM system is 26.04±4.34 mg/L, with about 10% of operating days exceeding 30 mg/L. The powdered activated carbon dosing system is activated during high COD periods. The annual average effluent COD is 20.86±3.31 mg/L, with stable compliance. Even without activated carbon dosing, the process achieves 10–20% COD removal, superior to conventional coagulation sedimentation, due to efficient removal of suspended and colloidal COD by the magnetic separation unit.

For TP removal, the annual average influent TP is 0.23±0.06 mg/L, and the effluent TP stabilizes at 0.04±0.03 mg/L, with a maximum value of 0.19 mg/L. The addition of magnetic powder increases phosphorus removal efficiency by 15% compared with conventional PAC+PAM coagulation, as magnetic particles accelerate floc collision and form denser flocs with faster settling velocity.

4. Operational Optimization Strategies

4.1 Seasonal Water Quality Fluctuation Mitigation

The influent ammonia nitrogen of the BFM system rises significantly in winter due to reduced nitrification efficiency of the front-end AAO process, posing a challenge to stable nitrification of the biofilm system after long-term operation under low substrate conditions in summer.

Two optimization schemes were tested on site:

1. Directly introduce ~10% raw wastewater into the BFM system to increase substrate concentration and domesticate nitrifying bacteria. After 15 days of acclimation, the nitrification rate of the biofilm increased from 0.20 g/(m²·d) to 0.35 g/(m²·d)

2. Reduce dissolved oxygen in the front-end AAO process to control its nitrification rate and raise influent ammonia nitrogen for the BFM system

Field verification confirmed that direct raw water introduction has better operability and more stable effect. This strategy is adopted before winter every year to pre-improve the nitrification capacity of the biofilm system, ensuring stable effluent during low-temperature periods.

4.2 Emergency Response for Sudden COD Spikes

About 10% of annual operating days experience sudden COD elevation, mainly caused by refractory industrial wastewater entering the pipe network. The response mechanism includes three parts:

1. Establish an influent COD early warning system through the intelligent operation platform, which automatically triggers activated carbon dosing when COD exceeds the set threshold

2. Regularly conduct activated carbon type selection tests with actual wastewater samples. Field tests confirmed that activated carbon with an iodine value of 900 mg/g delivers the best COD removal effect for this project, with a dosage ratio of 6–7 g activated carbon per 1 g COD removed

3. Strengthen influent source tracing to reduce the frequency of abnormal COD inflow from the source

4.3 Energy and Chemical Consumption Reduction Measures

Given the large fluctuation of influent quality, the system has considerable energy-saving and consumption-reduction potential. Targeted optimization measures achieve over 20% reduction in total operational cost.

For energy consumption control:

• Implement linked control between blowers and online dissolved oxygen sensors; install vent pipes at blower outlets to reduce air supply during low ammonia nitrogen periods

• Adopt alternate operation of activated carbon tank mixers in groups during non-dosing periods, and arrange operation during off-peak electricity price periods

For chemical consumption control:

• Install online nitrate nitrogen monitors at the anoxic zone outlet, linked with carbon source dosing pumps to achieve precise carbon source dosing and avoid excessive dosage

• Adopt intermittent activated carbon dosing triggered by COD early warning, instead of continuous dosing

• Integrate chemical phosphorus removal from the original process into the M section, with PAC dosing linked to online TP data for precise control

• Magnetic powder dosing is automatically adjusted according to inflow volume through a quantitative feeder

The total direct operational cost of the system is 0.24 RMB/m³, including 0.09 RMB/m³ for electricity and 0.15 RMB/m³ for chemicals.

5. Engineering Application Analysis

5.1 Applicable Scenarios

This process route is particularly suitable for the following project types:

• Upgrading and reconstruction of existing municipal WWTPs with strict land constraints

• Standard elevation projects requiring non-stop construction without production reduction

• Deep treatment projects targeting Class IV or higher effluent quality from Class A influent

• Phased construction projects with near-term and long-term capacity differences

For industrial wastewater tertiary treatment, the process parameters and activated carbon selection need to be adjusted according to specific water quality characteristics.

5.2 Key Engineering Design Considerations

• Capacity phasing design: The biochemical section and solid-liquid separation section can be designed according to different construction stages, reducing initial investment while reserving expansion space

• Biofilm carrier selection: Carriers with high specific surface area should be selected to ensure sufficient enrichment of nitrifying bacteria under low substrate concentration conditions

• Mixing and aeration design: Specialized mixers for suspended carriers are required in the anoxic zone to ensure uniform fluidization of carriers without deposition

• Magnetic powder recovery system: A complete magnetic powder recovery and reuse system should be configured to reduce long-term chemical consumption

5.3 O&M Best Practices

• Establish seasonal biofilm acclimation mechanism to adapt to periodic changes of influent water quality

• Set multi-level early warning thresholds for key water quality indicators to realize precise linked dosing of chemicals

• Regularly test the performance of activated carbon and coagulants with actual water samples to optimize dosage strategy

• Maintain proper MLSS control in the front-end process to reduce SS load on the tertiary treatment system

5.4 Cost-Benefit Analysis

Compared with the traditional BAF+DNF process route, the BFM process reduces initial investment by about 50% and saves more than 60% of land occupation. The optimized operational cost is 20% lower than the conventional tertiary treatment scheme. For a 60,000 m³/d WWTP, the annual operational cost saving can reach more than 2.6 million RMB, with significant economic benefits over the full life cycle.

6. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that compact, modular and intelligent process solutions are the core direction for existing WWTP upgrading under increasingly strict land and effluent requirements. The pure biofilm MBBR + magnetic coagulation process fully aligns with our technical philosophy of efficient land use and data-driven operation.

This process fits perfectly with SYNERAQUA’s skid-mounted and modular equipment design capabilities. For upgrading projects with tight schedules and limited site space, we can deliver pre-fabricated modular BFM units to shorten on-site construction period and minimize impact on existing plant operation. Integrated with our SCADA automation system and smart water operation platform, the process can realize fully automatic fine regulation of aeration, carbon source dosing and chemical feeding, further improving operational stability and reducing energy and material consumption. We continue to optimize the biofilm carrier formulation and magnetic separation efficiency to provide more cost-effective deep treatment solutions for municipal and industrial wastewater upgrading projects.

7. Conclusion

The pure biofilm MBBR combined with magnetic coagulation sedimentation process provides a reliable technical route for municipal WWTP upgrading under land-constrained and non-stop operation conditions.

1. The process achieves stable effluent quality meeting Class IV surface water standard, with excellent removal performance for ammonia nitrogen, TN, TP and refractory COD, demonstrating strong adaptability to water quality fluctuations.

2. With a footprint of only 200 m² per 10,000 m³/d treatment capacity, the process solves the core pain point of insufficient land for upgrading projects, and supports phased construction to adapt to near-term and long-term capacity changes.

3. Targeted operational optimization strategies including raw water acclimation for seasonal fluctuations, COD early warning linked dosing and comprehensive energy saving measures reduce total operational cost by more than 20%.

4. The upgrading route of adding BFM process after Class A effluent is technically feasible, economically reasonable and operationally stable, and can be widely referenced for similar municipal wastewater treatment plant standard elevation projects.

FAQ

Q1: What is the core advantage of pure biofilm MBBR over conventional activated sludge processes for tertiary treatment?
Pure biofilm MBBR enriches functional bacteria exclusively on suspended carriers, achieving 10 times higher nitrifying bacteria concentration than conventional activated sludge. It maintains stable nitrification performance even under low substrate concentration and low temperature conditions, and requires no sludge return control with simpler operation.

Q2: How much land can the BFM process save compared with traditional tertiary treatment processes?
Compared with the conventional BAF + denitrification deep bed filter process, the BFM process reduces land occupation by about 60%, with only 200 m² of footprint required per 10,000 m³/d treatment capacity, making it ideal for upgrading projects with limited available land.

Q3: Can this process be implemented without halting the operation of the existing WWTP?
Yes. The process is built as an independent additional tertiary treatment system, with no need to modify or stop the original treatment process. The original plant maintains normal operation throughout the construction and commissioning period, fully meeting non-stop upgrading requirements.

Q4: How does magnetic coagulation improve phosphorus removal efficiency?
Magnetic powder added in the coagulation process acts as a crystal nucleus for floc formation, accelerating particle collision and forming denser flocs with higher settling velocity. It increases phosphorus removal efficiency by about 15% compared with conventional coagulation, and achieves more stable low-concentration TP effluent.