Overviw
Characteristics of MBBR Technology
The Moving Bed Biofilm Reactor (MBBR) demonstrates unique advantages in petrochemical wastewater treatment. This technology utilizes specially designed suspended carriers that allow microorganisms to grow both as biofilm (attached to carriers) and suspended activated sludge, combining the benefits of long sludge age and high shock-load resistance from biofilm processes with the mass transfer efficiency of activated sludge systems. In practical petrochemical applications, MBBR systems show remarkable techno-economic performance: compact modular design saves 30-40% footprint; high specific surface area carriers (500-800 m²/m³) support richer microbial communities; and flexible expansion capability accommodates flow and load variations.
Challenges in Petrochemical Applications
However, increasing feedstock diversification in petrochemical operations has led to more complex wastewater characteristics: wider COD fluctuations (200-800 mg/L), higher refractory organics content, and intermittent toxic shocks (e.g., phenols, sulfides). Operational data from a major refinery showed conventional MBBR's COD removal efficiency drops below 50% (from normal 65%) when influent COD exceeds 600 mg/L, with ammonia removal showing ±30% fluctuations. Stricter emission standards (GB31571-2015), especially for total nitrogen (≤40 mg/L), have created urgent needs for MBBR upgrades.
While PAC-enhanced activated sludge systems are well-documented (showing 15-20% COD removal improvement), systematic industrial-scale studies on MBBR-PAC hybrid systems remain scarce. This case study validates the synergistic mechanisms at a 10-million-ton refinery, providing replicable technical solutions.
Process Modification and Performance Enhancement
Pre-modification MBBR Performance
The 80 m³/h MBBR system (20h HRT) treating high-salinity (TDS>5000 mg/L), low-B/C (<0.3) petrochemical wastewater showed:
- Average COD removal: 51.9%
- Effluent COD frequently exceeded 150 mg/L (peak 215 mg/L)
- Thin, sparse biofilm with few protozoa indicated poor shock resistance
PAC-Enhanced MBBR Modifications
Key upgrades included:
- New 4h-HRT anoxic tank (DO<0.5 mg/L) with 50% sludge/100% supernatant recycle
- PAC dosing (75 mg/L) at MBBR inlet
- Operational optimization: MBBR HRT adjusted to 16h, DO maintained at 2-4 mg/L
- Enhanced clarification with PAC-assisted flocculation (retaining 3h settling time)
Post-modification Performance
After 14-month operation:
✓ COD removal increased to 65.8% (p<0.01), effluent COD<150 mg/L (avg. 97.2 mg/L) even at 662 mg/L shock loads
✓ Biofilm thickness increased 30-50%; protozoa (e.g., Vorticella) abundance rose 5-8×
✓ Phenol tolerance improved 40%
✓ Incremental operating cost: only ¥0.15/m³ (total ¥1.43/m³)
Mechanisms:
- PAC's "buffer-regeneration": >1000 m²/g PAC rapidly adsorbs toxins (120-150 mg/g capacity) with biological regeneration
- Microbial community optimization: 25% increased Nitrospira abundance
- Enhanced mass transfer: 35-40% improved biofilm-sludge interface transfer
Techno-Economic Analysis
Operating Costs
Main cost increments:
Power: New recycle pumps + PAC system (¥0.63/m³)
- Chemicals: PAC dosing (¥0.067/m³)
- Maintenance/monitoring (¥0.08/m³)
- Total: ¥1.43/m³ (excl. depreciation/labor)
Environmental Benefits
✓ Stable effluent COD<150 mg/L for advanced oxidation
✓ TN removal reached 54.8%
✓ Reliable compliance with emission standards
✓ Reduced environmental risks
Conclusions
① PAC dosing (75 mg/L) significantly enhanced MBBR performance under:
- Anoxic: 4h HRT
- MBBR: 16h HRT
- Load: COD≤600, NH₃-N≤40, TN≤45 mg/L
while maintaining ¥1.43/m³ treatment cost.
② Key enhancement mechanisms:
- Toxin adsorption/buffering
- Biofilm structure optimization
- Adsorption-biodegradation synergy
③ Key Operational Notes:
✓ Add 0.5-1 mg/L PAM for sludge settling
✓ Maintain >3h clarifier HRT
✓ Monitor effluent PAC (<5 mg/L)
✓ Prevent PAC accumulation in aeration systems
This cost-effective solution demonstrates strong potential for petrochemical wastewater upgrades.