Collection Tank an Integral Part of Treatment but Ignored
Role of Collection / Equalization Tank Operating Conditions in Enhancing STP’s Performance
Why the Collection / Equalization Tank Matters in a Conventional STP?
In a conventional sewage treatment plant (STP) based on the Activated Sludge Process (ASP), the collection tank / equalization tank is often viewed only as a hydraulic buffer. However, the biological condition maintained in this tank has a direct influence on the efficiency, stability, and nutrient removal capability of the downstream aerobic treatment system.
When operated under controlled anaerobic conditions, the collection / equalization tank can significantly improve the overall performance of the activated sludge process by preserving the influent’s biological treatment potential.
Role of the Collection / Equalization Tank in an ASP-Based STP
For the activated sludge process to function effectively, the influent wastewater must retain:
Adequate biodegradable organic carbon
A favourable COD : N balance
A favourable COD : N balance
The collection / equalization tank acts as the first biological conditioning zone. Its operation determines whether the wastewater entering the aeration tank is biologically enriched or biologically depleted.
Benefits of Anaerobic Conditions in the Collection / Equalization Tank
When the collection / equalization tank is maintained without aeration and with only limited mixing:
Dissolved oxygen is quickly depleted
Organic matter is not aerobically oxidised
Carbon remains preserved for downstream biological processes
Partial fermentation may begin without full degradation
This ensures that the influent reaching the aeration tank still contains readily utilizable carbon, which is essential for:
Stable heterotrophic biomass growth
Effective denitrification in anoxic zones
Adverse Effects of Aeration in Collection / Equalization Tanks
In many STPs, the collection / equalization tank is aerated to prevent odour or avoid septicity. While well intentioned, this practice can adversely affect the activated sludge process.
Consequences of Aerating the Collection / Equalization Tank
Readily biodegradable COD is consumed prematurely
Organic carbon is oxidised to CO₂
Influent entering the aeration tank becomes carbon-limited
Biological nutrient removal efficiency is reduced
Aerating the collection / equalization tank effectively diverts carbon away from the aeration tank, where it is most needed.
Impact on Activated Sludge Process Performance
When carbon-deficient wastewater enters the aeration tank:
Denitrification efficiency in anoxic zones decreases
Nitrogen removal becomes inconsistent
Aeration energy demand increases
External carbon dosing may become necessary
Overall process resilience reduces
This results in higher operating costs and greater operator intervention.`
Anaerobic vs Aerated Collection / Equalization Tank
Comparison for ASP-Based STPs
Anaerobic Conditioning vs Septic Treatment
It is important to distinguish controlled anaerobic conditions in a collection / equalization tank from septic treatment, as the two differ significantly in both intent and outcome.
Controlled Anaerobic Conditioning (Preferred for ASP)
This represents short-term anaerobic retention, intended only for biological conditioning.
Key characteristics:
Short hydraulic retention time (hours)
No sludge digestion or stabilisation
No methane generation
Limited to acidogenic activity
Wastewater remains biologically active
Benefit to ASP:
Preserves organic carbon
Enhances denitrification potential
Improves biological stability in aeration tanks
Septic Conditions (Undesirable for STP)
Septic tanks are designed for long-term anaerobic digestion, not conditioning.
Key characteristics:
Long retention time (days)
Sludge accumulation and digestion
Methane and Sulphide Generation
Loss of biodegradable carbon
Production of reduced, inhibitory compounds
Impact on STP:
Reduced oxygen transfer efficiency
Inhibition of nitrifying organisms
Odour and Corrosion Problems
Poor Sludge Settleability
Longer recovery times after shock loads
Septic effluent is biologically exhausted, not biologically prepared, for aerobic treatment.
Design Philosophy for Conventional STPs
For STPs employing the activated sludge process, the collection/equalisation tank should be designed and operated as a carbon management and conditioning zone, not as a treatment reactor.
Avoid aeration unless absolutely necessary
Limit mixing to hydraulic requirements
Control retention time
Prevent sludge accumulation
This approach ensures that wastewater entering the aeration tank retains maximum biological value, leading to improved treatment efficiency, regulatory compliance, and long-term operational reliability.