Secondary clarification is a central process step in wastewater treatment, which plays a key role in biological wastewater treatment plants in particular. It is used to separate biomass, suspended solids and particles that were not removed during biological treatment. In addition to its importance for the efficiency of the wastewater treatment plant, secondary clarification has a decisive influence on compliance with legal discharge limits and water quality. It also enables biomass to be fed back into the biological process and thus contributes to the stability and efficiency of biological treatment.
This article explains the technical basics, how it works and the challenges and solutions for secondary clarification, especially in industrial plants.
Table of contents
Objectives of secondary clarification
Secondary clarification pursues several central objectives, which can vary depending on the application:
Separation of solids:
- Removal of suspended solids (SS) and biomass from wastewater.
- Ensuring clear process quality.
Biomass recycling:
- Return of activated sludge to the biological treatment stage (recirculation).
- Stabilization and optimization of biological processes.
Discharge into the environment:
- Reduction of the solids load in the effluent to comply with legal discharge limits.
Sludge discharge:
- Removal of excess sludge from the circuit for further treatment.
Technical principles of secondary clarification
Secondary clarification is based on physical principles, in particular sedimentation, in which solids settle due to gravity. Efficient secondary clarification requires precise coordination between hydraulic load, sludge quality and plant geometry.
Key factors:
Sedimentation rate:
- Depending on the particle size, density and flow conditions in the clarifier.
Hydraulic dwell time:
- Sufficient time is required for the solids to settle.
Sludge Volume Index (SVI):
- Describes the settleability of the sludge. Values between 80 and 150 ml/g are considered optimal.
Compaction of the sludge:
- Effective separation of water and solids in the settled sludge.
Types of secondary clarification systems
Different secondary clarification systems are used depending on the type of plant and wastewater composition:
1. circular clarifier
Structure:
- Central inlet structure for even distribution of wastewater.
- Funnel-shaped geometry that enables sedimentation of the solids.
Functionality:
- Water flows radially from the inside to the outside.
- Solids settle to the bottom and are transported into the sludge hopper by scrapers.
Application:
- Frequently found in municipal sewage 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 in secondary clarification
Post-clarification is a complex process that is influenced by various factors. Common problems and possible solutions include:
1. sludge buoyancy (bulking):
Cause:
- Excessive gas formation due to biodegradation (e.g. nitrogen release during denitrification).
- Filamentous microorganisms that impair sludge settleability.
Solution:
- Optimization of aeration and return sludge routing.
- Use of biocides or sludge separation technologies.
2. overloading of the secondary clarifier:
Cause:
- High hydraulic load, e.g. due to rainwater or production peaks.
Solution:
- Use of buffer and expansion tanks.
- Reduction of the inflow rate through targeted process control.
3. insufficient separation of solids:
Cause:
- Unfavorable flow conditions or insufficient sludge compaction.
Solution:
- Optimization of the pool hydraulics through flow simulations.
- Use of flocculants to improve the settling properties.
Secondary clarification as part of integrated systems
Secondary clarification is often integrated into a comprehensive wastewater treatment system. It plays a key role in the coupling of biological and physical processes.
Recirculation of the activated sludge:
- The settled sludge is fed back into the aeration tank to stabilize the biological activity.
Removal of excess sludge:
- Excess sludge is discharged for sludge dewatering and recycling.
Polishing steps:
- After secondary clarification, a polishing step (e.g. filtration, activated carbon) may be necessary to remove fine particles or trace substances.
Photo: Our secondary clarifiers designed as circular clarifiers with biomass recirculation into our anaerobic biogas plant ALMA BHU GMR
Conclusion
Secondary clarification is an indispensable step in industrial wastewater treatment that ensures the quality of the treated water and optimizes the efficiency of the biological processes. By using modern technologies such as lamella clarifiers, circular clarifiers and flotation plants, even complex wastewater can be treated effectively. Careful process control and continuous optimization are crucial to reducing operating costs and ensuring compliance with legal requirements. Secondary clarification is therefore not only a key technical, but also an economic and ecological process in industrial water management.
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