The secondary clarifier is an essential component in wastewater treatment, especially in biological wastewater treatment plants. It is used to separate solids, in particular activated sludge, which is produced during biological treatment. It also plays a key role in returning biological sludge to the cycle and removing excess sludge. A well-designed and operated secondary clarifier ensures high effluent quality and contributes to the stability of the entire plant.
This article provides a comprehensive explanation of how secondary clarifiers work, the technical principles behind them, the challenges that can arise and the technologies available to maximize efficiency and operational safety.
Table of contents
Function and tasks of the secondary clarifier
The secondary clarifier has several central functions in wastewater treatment:
Separation of solids:
- Removal of suspended solids (SS) and biological sludge flocs from wastewater.
Recirculation of the activated sludge:
- Return of the settled sludge to the biological treatment stage to stabilize the microbiology and optimize the degradation performance.
Excess sludge removal:
- Discharge of excess sludge that is no longer required for the biological process.
Clear water supply:
- Ensuring purified discharge water that meets the legal requirements.
Technical basics of the secondary clarifier
The main task of the secondary clarifier, the separation of sludge and water, is based on physical processes, in particular sedimentation. This process depends on several parameters:
Sedimentation principles
Particle properties:
- The sedimentation rate depends on the size, density and shape of the particles.
- Heavier and larger particles settle faster than smaller or less dense particles.
Hydraulic conditions:
- The flow velocity of the wastewater must be sufficiently low to allow the particles to settle.
Basin geometry:
- The shape and size of the tank influence the flow conditions and the retention time.
Sludge Volume Index (SVI)
The sludge volume index (SVI ) is an indicator for the settleability of activated sludge. It describes how much space 1 g of sludge takes up in a settling tank. An optimum SVI is typically between 80 and 150 ml/g.
Types of secondary clarifiers
Secondary clarifiers can be designed in various forms, depending on space requirements, wastewater composition and area of application.
1. circular clarifier
Structure:
- Central inlet structure for even distribution of wastewater.
- Funnel-shaped base that facilitates the settling of sludge.
- Scraper that conveys the settled sludge into the central sludge hopper.
Functionality:
- The wastewater flows radially from the center of the tank to the outside.
- Clear phase formation between sludge and water.
Advantages:
- Uniform hydraulic load.
- High operational reliability with fluctuating loads.
Application:
- Municipal wastewater treatment plants and large industrial plants.
Photo: Our secondary clarifier with circular scraper as part of our ALMA BHU BIO biological treatment plant
2. rectangular clarifier
Structure:
- Linear geometry with lateral or central sludge scrapers.
Functionality:
- Water flows from one end to the other.
- Solids settle on the floor and are collected by scrapers.
Application:
- Space-saving alternative to circular clarifiers, especially for industrial plants.
3. lamella clarifier
Structure:
- Consists of inclined lamellas that increase the effective sedimentation surface.
Functionality:
- Water flows through the lamella packs, which accelerates sedimentation.
Application:
- Compact secondary clarification systems for limited space or highly contaminated wastewater.
4. flotation systems (for light particles)
Structure:
- Use of dissolved air flotation or air injection for particle separation.
Functionality:
- Particles are carried to the surface by gas bubbles and skimmed off.
Application:
- For wastewater with light particles or particles that are difficult to sediment.
Photo: Our ALMA NeoDAF flotation plant
Challenges and optimizations in secondary clarification
Secondary clarification is susceptible to various operational problems, but these can be minimized through targeted measures.
1. sludge lift
Causes:
- Gas formation through denitrification.
- Excessive return sludge concentration.
Solution:
- Optimization of the oxygen supply in the biological stage.
- Reduction of the sludge retention time in the tank.
2. overload due to hydraulic peaks
Causes:
- Rainwater inflow or production peaks.
Solution:
- Use of buffer tanks for hydraulic relief.
3. sludge deposits and blockages
Causes:
- Insufficient flow guidance or defective sludge scrapers.
Solution:
- Regular inspection and maintenance of mechanical components.
Photo: Our secondary clarifiers designed as circular clarifiers with biomass recirculation into our anaerobic biogas plant ALMA BHU GMR
Conclusion
The secondary clarifier is an indispensable part of industrial wastewater treatment that not only helps to separate solids, but also optimizes the efficiency of biological treatment. With modern technologies such as lamella clarifiers and compact clarifiers, even highly contaminated wastewater can be treated efficiently. The right design, regular maintenance and targeted process optimization are essential to ensure high operational reliability and consistent water quality. In an increasingly resource-conscious industry, the secondary clarifier is a key technology for sustainable and efficient wastewater treatment.
For further information on our products, please feel free to contact us at any time!
