What Is Biological Wastewater Treatment?
Biological wastewater treatment refers to the targeted use of microorganisms to remove biodegradable substances from wastewater. The microorganisms use organic compounds as a source of nutrients and energy, converting them into biomass, carbon dioxide, water or – in anaerobic processes – biogas.
Important assessment parameters include chemical oxygen demand (COD) and biochemical oxygen demand (BOD5). They describe the organic load of wastewater and provide an indication of its biodegradability.
How Does Biological Wastewater Treatment Work?
A biological treatment plant generally consists of several coordinated process steps:
- Mechanical pretreatment to remove coarse materials and solids.
- Homogenization and buffer storage to equalize fluctuations in flow rates and pollutant loads.
- Biological reaction stage using microorganisms.
- Separation of biomass.
- Further treatment or discharge of the treated water.
The aim is to reduce the organic load and, where necessary, remove nitrogen and phosphorus compounds. The biological stage often forms the core of an industrial wastewater treatment plant.
Aerobic Processes
In aerobic processes, biological degradation takes place with the supply of oxygen. The necessary aeration is therefore one of the most important operating systems of a treatment plant.
Typical aerobic processes include:
- Activated sludge process
- Sequencing Batch Reactor (SBR)
- Moving Bed Biofilm Reactor (MBBR)
- Membrane Bioreactor (MBR)
In the activated sludge process, microorganisms are present as suspended biomass. Following biological degradation, the biomass is separated from the treated water in a secondary clarifier.
MBBR systems use freely moving plastic carriers on whose surfaces a biofilm grows. This allows high biomass concentrations to be achieved. Membrane bioreactors combine biological treatment with membrane filtration. The membrane replaces conventional secondary clarification and produces largely solids-free permeate.
In addition to carbon removal, aerobic processes also enable nitrification and denitrification for nitrogen elimination.
Anaerobic Processes
Anaerobic processes operate without dissolved oxygen. Organic compounds are converted into methane-rich biogas through various microbial metabolic stages.
Typical reactor concepts include:
- UASB reactors (Upflow Anaerobic Sludge Bed)
- EGSB reactors (Expanded Granular Sludge Bed)
- Anaerobic fixed-bed reactors
- High-rate reactors with biomass carriers
Anaerobic treatment is particularly suitable for highly polluted industrial wastewater with high COD loads. Advantages include comparatively low sludge production and the possibility of generating energy from biogas. Since the effluent quality is often insufficient, the anaerobic stage is frequently combined with downstream aerobic treatment.
Important Operating Parameters
The performance of biological processes depends on numerous influencing factors:
- COD and BOD5 load
- Temperature
- pH value
- Oxygen supply
- Nutrient ratio of carbon, nitrogen and phosphorus
- Sludge age
- Conductivity and salt content
- Toxic or biologically inhibitory substances
Nitrogen and phosphorus deficiencies in particular can limit biological degradation. Likewise, high salt loads, solvents, disinfectants or other inhibitory substances can impair the activity of microorganisms.
Importance for Water Recycling
Biological wastewater treatment often forms the basis for industrial water recycling concepts. It reduces organic loads and thereby creates the conditions for subsequent treatment stages.
However, additional processes are often required before the water can be reused, for example:
- Filtration
- Activated carbon adsorption
- Oxidation processes
- Ultrafiltration
- Reverse osmosis
- Disinfection
The required combination of processes depends on the quality requirements of the intended use. Cooling water, process water and cleaning water have different water quality requirements.
Industrial Challenges
Fluctuating Wastewater Loads
Problem: Changes in production, cleaning processes or batch operations lead to significant fluctuations in pollutant loads.
Impact: The biological stage may be overloaded or underloaded.
Solution: Mixing and equalization tanks, together with adapted process control, stabilize operation.
Inhibitory and Toxic Substances
Problem: Certain substances can damage microorganisms or reduce their metabolic activity.
Impact: Reduced treatment efficiency, disrupted nitrification or biomass loss.
Solution: Sidestream treatment, pretreatment of critical wastewater streams and continuous monitoring.
Energy and Sludge Management
Problem: Aerobic processes require energy for aeration and generate excess sludge.
Impact: Higher operating and disposal costs.
Solution: Optimized aeration, energy efficiency measures and, where appropriate, the use of anaerobic pretreatment stages for biogas production.
Conclusion
Biological wastewater treatment is a key technology in industrial wastewater treatment. Aerobic processes enable high treatment efficiency and the elimination of nitrogen compounds. Anaerobic processes are particularly suitable for highly polluted wastewater streams and enable the use of organic loads for biogas production.
The selection of the appropriate process depends on wastewater composition, treatment objectives and operating conditions. For water recycling and demanding effluent quality requirements, biological treatment is often combined with physicochemical or membrane-based processes.
