Key Differences Between Primary and Secondary Sedimentation Tanks
Location in the Treatment Process
Primary Sedimentation Tank
Located after the grit chamber and before the aeration tank, the primary sedimentation tank removes settleable solids early in the treatment process. It can achieve over 90% removal of settleable solids, reducing the load on downstream biological treatment.
Secondary Sedimentation Tank
Positioned after biological treatment (such as the aeration tank), the secondary sedimentation tank separates activated sludge from treated water and allows sludge recycling. It is a core component of biological treatment systems.
Treatment Objectives
Primary Sedimentation Tank
Removes suspended solids (SS)
Reduces part of the organic load (mainly suspended BOD₅)
Improves conditions for biological treatment
Secondary Sedimentation Tank
Separates activated sludge from mixed liquor
Handles sludge with:
High concentration
Flocculent structure
Low density
Slow settling velocity
Typical Design Parameters
Primary Sedimentation Tank
Surface loading: 1.5–3.0 m³/m²·h
Settling velocity: 1.5–3.0 m/h
Retention time: 1.0–2.0 h
Weir loading: ≤ 2.9 L/s·m
Sludge moisture: 95%–97%
Secondary Sedimentation Tank (Activated Sludge)
Surface loading: 1.0–1.5 m³/m²·h
Settling velocity: 1.0–1.5 m/h
Retention time: 1.5–2.5 h
Weir loading: ≤ 1.7 L/s·m
Sludge moisture: 99.2%–99.6%
Secondary Sedimentation Tank (Biofilm Process)
Surface loading: 1.0–1.5 m³/m²·h
Retention time: 1.5–4.0 h
Sludge moisture: 96%–98%
Weir loading: ≤ 1.7 L/s·m
Operation and Maintenance Best Practices
Primary Sedimentation Tank
Optimize sludge scraping cycles to avoid sludge flotation or disturbance
Prefer intermittent sludge discharge; monitor carefully if continuous discharge is used
Ensure uniform effluent distribution and clean weirs regularly
Maintain proper scum removal system alignment
Inspect equipment for noise, corrosion, and mechanical issues
Flush sludge pipelines monthly to prevent clogging
Conduct annual cleaning and full inspection
Monitor key parameters: SS, COD, BOD₅, pH, temperature
Secondary Sedimentation Tank
Ensure even influent distribution across tanks
Regularly remove scum and flush scum hoppers
Maintain level effluent weirs to prevent short-circuiting
Monitor sludge blanket height and effluent clarity
Check scraper operation, vibration, and motor performance
Perform annual tank inspection, especially for submerged components
Take precautions when emptying tanks in high groundwater conditions
Verify automatic control systems (e.g., sludge level-based discharge)
Common Problems and Solutions
Primary Sedimentation Tank
Fine Suspended Solids in Effluent
Causes
Hydraulic shock load or overload
Sludge disintegration due to over-aeration
Poorly settling particles
Solutions
Improve flow equalization
Optimize inlet/outlet distribution
Adjust aeration parameters
Add flocculants if necessary
Sludge Floating
Causes
Septic influent
Denitrification from nitrate-rich sludge
Impact from sludge treatment return flows
Solutions
Adjust hydraulic parameters
Strengthen scum removal
Control sludge age and nitrification
Sludge Washout
Causes
Uneven weir loading
Scraper malfunction
Wind effects (for open tanks)
Solutions
Adjust weir levels
Repair equipment
Secondary Sedimentation Tank
Low Dissolved Oxygen in Effluent
Causes
Excess sludge retention
Equipment malfunction
High temperature
Solutions
Increase sludge return rate
Repair scrapers
Improve temperature control
High BOD₅ and COD in Effluent
Causes
Shock loading
Poor aeration control
Solutions
Strengthen monitoring
Optimize aeration tank operation
Sludge Floating
Causes
Anaerobic conditions (gas production such as H₂S)
Denitrification producing nitrogen gas
Solutions
Increase sludge discharge
Improve aeration at tank outlet
Control sludge retention time (SRT)
Primary and secondary sedimentation tanks serve different but complementary roles:
Primary tanks focus on removing settleable solids and reducing load
Secondary tanks ensure efficient sludge separation and recycling
Proper design, operation, and troubleshooting of both systems are essential for maintaining stable wastewater treatment performance.
