Secondary Treatment
Biological Wastewater Treatment
The core of secondary treatment: using microbial communities to remove dissolved organic matter and nutrients from wastewater.
What is Biological Treatment?
Biological wastewater treatment is the engineered application of natural microbial processes to remove biodegradable organic matter and nutrients from wastewater. It forms the core of secondary treatment as defined by the EU Urban Wastewater Treatment Directive (91/271/EEC).
The directive requires secondary treatment for all agglomerations above 2,000 population equivalents (PE), achieving minimum removal rates of 70–90% for BOD₅ and 75% for COD. For sensitive areas, additional nitrogen and phosphorus removal is mandated.
Typical Design Parameters
BOD₅ Removal
EU UWWTD requirement
COD Removal
EU UWWTD minimum
Hydraulic Retention Time
Typical for activated sludge
Aeration Efficiency
Fine bubble diffusers
Process Categories
Aerobic Processes
Require dissolved oxygen. Microorganisms oxidize organic matter (BOD) and ammonia (nitrification). Main processes: activated sludge, MBBR, MBR, SBR. Typical DO requirement: 1.5–2.5 mg/L for BOD removal, >2.0 mg/L for nitrification.
Anoxic Processes
Absence of dissolved oxygen, but nitrate present as electron acceptor. Denitrifying bacteria convert NO₃⁻ to N₂ gas. Requires external or internal carbon source. Used for total nitrogen removal. Typical configurations: pre-anoxic, post-anoxic, or simultaneous.
Suspended Growth
Biomass is suspended in the mixed liquor (activated sludge, SBR, MBR). Characterized by MLSS concentration, SRT, and F/M ratio. Requires secondary clarification or membrane separation for solids–liquid separation.
Attached Growth
Biomass grows as biofilm on fixed or moving carrier media (MBBR, trickling filters, biofiltration). Not dependent on secondary clarification for biomass retention. Higher volumetric loading rates possible. Well-suited for retrofit applications.
Engineering Note
Secondary Treatment Requirements
The EU UWWTD defines secondary treatment requirements based on agglomeration size and receiving water sensitivity:
Discharge Limits per EU UWWTD
| Condition | Potential Issue | What to Investigate |
|---|---|---|
| BOD₅ | ≤ 25 mg/L | Minimum 70–90% removal required |
| COD | ≤ 125 mg/L | Minimum 75% removal required |
| TSS | ≤ 35 mg/L | For >10,000 PE; ≤60 mg/L for 2,000–10,000 PE |
| TN (sensitive areas) | ≤ 10–15 mg/L | Depends on PE class (≥10,000 or ≥100,000) |
| TP (sensitive areas) | ≤ 1–2 mg/L | Depends on PE class |
Secondary Treatment Design
Designing a secondary treatment system requires understanding the relationship between load, volume, and performance. Key design parameters include:
F/M Ratio
Extended aeration range
SRT for Nitrification
Temperature-dependent
MLSS
Conventional activated sludge
HRT
Typical for nutrient removal
Secondary Treatment Upgrade
When existing secondary treatment reaches its limits — whether through population growth, stricter regulations, or aging infrastructure — an upgrade assessment begins with understanding what is limiting performance.
Common upgrade pathways include adding carrier media (MBBR/IFAS) for biological capacity, membrane retrofits (MBR) for space-constrained sites, aeration upgrades for energy efficiency, and process reconfiguration for nitrogen removal.
Engineering Note
Frequently Asked Questions
Biological treatment cultivates microorganisms that consume biodegradable organic matter and nutrients as food. In suspended-growth systems (activated sludge) the biomass floats freely; in attached-growth systems (biofilm, MBBR) it grows on carriers. Oxygen is supplied by aeration so aerobic bacteria can oxidise organics into carbon dioxide, water and new biomass.
The Urban Wastewater Treatment Directive sets minimum secondary standards of BOD₅ 25 mg/L (or 70–90% reduction), COD 125 mg/L (75% reduction) and around 35 mg/L total suspended solids. Sensitive areas require additional nitrogen and phosphorus removal. These are the values in Directive (EU) 2024/3019.
Suspended-growth processes (conventional activated sludge, MBR) keep biomass in suspension and rely on clarifiers or membranes to separate it. Attached-growth processes (biofilm, MBBR) grow biomass on media that stay in the reactor, so they do not depend on sludge recirculation for biomass retention and can be robust to hydraulic peaks.
There is no single best process — it depends on load, temperature, footprint, target effluent quality and whether you are upgrading an existing plant. Activated sludge is proven and flexible, MBBR suits retrofits and capacity boosts, and MBR delivers high-quality effluent in a compact footprint. An engineering assessment matches the process to your constraints.
Engineering Assessment
Not sure what your plant needs?
Run a Secondary Treatment Engineering Assessment to identify process limitations and develop practical upgrade pathways.
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