Biological wastewater treatment is a core process in municipal and industrial wastewater management, using microorganisms to remove organic pollutants and nutrients efficiently. Its performance depends on proper process selection and precise control of key operating parameters to ensure stable and compliant operation.
What Is Biological Wastewater Treatment?
Biological wastewater treatment is a core process that uses microorganisms to degrade biodegradable organic pollutants in wastewater. Through microbial metabolism, organic matter is converted into carbon dioxide, water, biomass, and stable by-products, significantly reducing BOD, COD, nitrogen, and phosphorus concentrations.
This technology is widely applied in municipal wastewater treatment plants and in industrial sectors such as food & beverage, chemical processing, pharmaceuticals, textiles, and pulp & paper, due to its high efficiency, cost-effectiveness, and scalability.
Working Principle of Biological Wastewater Treatment
The biological treatment process can be summarized as a controlled microbial growth system:
- Organic pollutants act as a carbon and energy source
- Microorganisms degrade pollutants under aerobic, anoxic, or anaerobic conditions
- Microbial biomass is separated as sludge after treatment
- Treated effluent meets discharge or reuse standards
Proper control of operating conditions is essential to ensure stable performance and long-term compliance.
Main Types of Biological Wastewater Treatment
Aerobic Treatment
Aerobic systems operate with sufficient dissolved oxygen and are the most commonly used biological processes.
- Activated Sludge Process – Classic and widely adopted technology
- MBBR (Moving Bed Biofilm Reactor) – High resistance to load fluctuations
- SBR (Sequencing Batch Reactor) – Flexible operation and compact footprint
Typical applications:
Municipal sewage, food processing wastewater, and low-to-medium COD industrial effluents.
Anaerobic Treatment
Anaerobic systems operate without oxygen and convert organic matter into biogas (mainly methane).
- Upflow Anaerobic Sludge Blanket – High-rate anaerobic technology
- Anaerobic digesters
- Anaerobic membrane bioreactors (AnMBR)
Typical applications:
High-strength industrial wastewater and energy-recovery projects.
Anoxic Treatment
Anoxic processes are primarily used for denitrification, converting nitrate into nitrogen gas. They are commonly combined with aerobic zones to form biological nutrient removal (BNR) systems.
Pollutants Removed by Biological Treatment
- Biochemical Oxygen Demand (BOD)
- Biodegradable Chemical Oxygen Demand (COD)
- Ammonia nitrogen (NH₄⁺-N)
- Total nitrogen (TN)
- Phosphorus (with enhanced biological phosphorus removal or chemical support)
Key Operating Parameters for Biological Wastewater Treatment
Dissolved Oxygen (DO)
| Process | Typical Range |
| Aerobic activated sludge | 1.5 – 3.0 mg/L |
| MBBR aerobic zone | 2.0 – 4.0 mg/L |
| Nitrification zone | 2.0 – 3.0 mg/L |
| Anoxic zone | < 0.5 mg/L |
| Anaerobic zone | ≈ 0 mg/L |
Proper DO control balances treatment efficiency and energy consumption.
Sludge Retention Time (SRT)
| Treatment Objective | Typical SRT |
| BOD removal only | 3 – 8 days |
| Partial nitrification | 8 – 15 days |
| Complete nitrification | 15 – 25 days |
| Biological nitrogen removal | 12 – 20 days |
SRT directly influences microbial population structure and system stability.
Food-to-Microorganism Ratio (F/M)
| System Type | F/M (kg BOD/kg MLSS·day) |
| High-rate systems | 0.3 – 0.6 |
| Conventional activated sludge | 0.2 – 0.4 |
| Extended aeration | 0.05 – 0.15 |
| MBBR (based on carrier area) | 0.3 – 1.5 |
Balanced F/M ratios help prevent sludge bulking and excessive energy consumption.
Mixed Liquor Suspended Solids (MLSS)
| Process | Typical Range |
| Conventional activated sludge | 2,000 – 4,000 mg/L |
| Extended aeration | 3,000 – 6,000 mg/L |
| SBR | 3,000 – 5,000 mg/L |
| MBR | 6,000 – 10,000 mg/L |
pH Control
| Biological Process | Recommended pH |
| Aerobic degradation | 6.5 – 8.5 |
| Nitrification | 7.0 – 8.0 |
| Denitrification | 6.5 – 8.0 |
| Anaerobic treatment | 6.8 – 7.5 |
pH below 6.5 can significantly inhibit nitrifying bacteria.
Temperature
| Temperature Range | Biological Activity |
| < 10 °C | Strongly reduced |
| 10 – 20 °C | Normal operation |
| 20 – 35 °C | Optimal range |
| > 40 °C | Microbial inhibition |
Nutrient Ratio (C : N : P)
| System | Recommended Ratio |
| Aerobic treatment | 100 : 5 : 1 |
| Anaerobic treatment | 250 : 5 : 1 |
Industrial wastewater often requires nutrient supplementation to maintain biological stability.
Hydraulic Retention Time (HRT)
| Process | Typical HRT |
| Activated sludge | 4 – 8 h |
| SBR (per cycle) | 6 – 12 h |
| MBBR | 2 – 6 h |
| UASB | 6 – 24 h |
Summary
Biological wastewater treatment is a proven and essential technology for removing organic pollutants and nutrients from municipal and industrial wastewater through controlled microbial processes. By selecting appropriate aerobic, anoxic, and anaerobic treatment methods and maintaining key operating parameters such as DO, SRT, F/M ratio, pH, temperature, and HRT, stable performance and regulatory compliance can be consistently achieved. With integrated process design and real-time monitoring, GL Group helps clients implement reliable biological wastewater treatment systems that deliver long-term operational stability, optimized costs, and sustainable environmental outcomes. For biological wastewater treatment system design, parameter optimization, or plant upgrades, please contact GL Group.