High Suspended Solids in Secondary Clarifier Effluent: What Are the Causes
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- Kate
- Issue Time
- Jul 22,2025
Summary
Inadequate design parameters (e.g., insufficient HRT, excessive surface overflow rate) in secondary clarifiers can lead to sludge flotation, resulting in effluent SS violations. Over-optimization for cost reduction is a common cause, requiring balanced design and operational considerations.
By: Kate Nana
Post Date: March 30th, 2024
Post Tags: Sedimentation tank, Secondary Wastewater Treatment, sewagetreatment，Tube Settler
High Suspended Solids in Secondary Clarifier Effluent: What Are the Causes?
The appropriateness of the design parameters for secondary clarifiers is a critical factor in determining whether the suspended solids (SS) in the effluent will exceed standards. Many municipal wastewater treatment plants, in an effort to reduce construction costs during the design phase, significantly shorten the hydraulic retention time (HRT) and maximize the hydraulic surface loading. This often leads to sludge flotation during operation, resulting in excessive suspended solids in the effluent. So, what causes high suspended solids in secondary clarifier effluent? Today, let’s explore this issue together!
1. Hydraulic Retention Time (HRT) in Secondary Clarifiers
The HRT of wastewater in the secondary clarifier is a key operational parameter. Only with sufficient retention time can effective flocculation be achieved, ensuring high sedimentation efficiency. Therefore, it is recommended to set the HRT of secondary clarifiers at approximately “3–4 hours”.
2. Hydraulic Surface Loading of Secondary Clarifiers
For any sedimentation tank, when the inflow is constant, the size of particles it can remove is also fixed. Among these removable particles, the settling velocity of the smallest particle is exactly equal to the hydraulic surface loading of the clarifier. Thus, the lower the hydraulic surface loading, the more particles can be removed, improving sedimentation efficiency and reducing effluent suspended solids. Designing secondary clarifiers with a lower hydraulic surface loading facilitates effective sedimentation of sludge and other suspended solids. Generally, it is recommended to control the hydraulic surface loading of secondary clarifiers within “0.6–1.2 m³/(m²·h)”.
3. Incomplete Sedimentation
The secondary clarifier may fail to effectively settle suspended solids, leading to high SS levels in the effluent. This could be due to excessive flow velocity, insufficient sedimentation time, or infrequent sludge removal from the tank.
To address this, ensure that the secondary clarifier has “adequate sedimentation time” and “proper inflow velocity control”, allowing sufficient time for suspended solids to settle. Additionally,“regularly clean the settled sludge” to maintain the effective volume of the sedimentation zone.
4. Influent SS/BOD₅ Ratio
The proportion of MLVSS (Mixed Liquor Volatile Suspended Solids) in the activated sludge of the biological system is closely related to the influent “SS/BOD₅ ratio”. When the SS/BOD₅ ratio is high, the MLVSS proportion in the activated sludge decreases, and vice versa. Operational experience shows that:
- When “SS/BOD₅ < 1”, the MLVSS proportion can remain above “50%”.
- When “SS/BOD₅ > 5”, the MLVSS proportion may drop to “20–30%”.
A low MLVSS ratio requires the system to maintain a “higher sludge age” to ensure nitrification efficiency, accelerating sludge aging and leading to excessive SS in the effluent.
5. Sludge Recirculation
If sludge is recirculated back into the secondary clarifier, it may carry suspended solids, increasing the SS concentration in the effluent. This could be due to design or operational issues in the sludge return system.
Therefore, “inspect the design and operation of the sludge return system” to ensure it effectively returns sludge to the beginning of the treatment process without introducing additional suspended solids. Repair or replace any faulty equipment or piping to prevent sludge recirculation issues.