Prior to water quality analysis using monitoring instruments, biological matter is typically subjected to sedimentation treatment in a specialized tank known as the secondary sedimentation tank (SST), whose core function is sludge-water separation. Any operational anomalies in this process directly compromise water quality accuracy. This paper investigates the key causes of SST abnormalities during water quality monitoring, as outlined below:

Presence of Fine Suspended Particles in Effluent  

Four primary mechanisms underlie this issue:  

1. Formation of needle-like flocs: Induced by suboptimal operational parameters or raw water quality fluctuations.
2. Excessive aeration in the aeration tank: Disrupts floc structure and stability.
3. Hydraulic overload: Exceeds the SST’s designed sedimentation capacity.
4. Short-circuiting: Reduces hydraulic retention time (HRT), causing flocs to be discharged before complete sedimentation.

Mitigation strategies:  

1. Dosing chemical flocculants to enhance floc aggregation.
2. Optimizing aeration tank operational parameters (e.g., DO levels, sludge age) to improve sludge settleability.
3. Reducing hydraulic load to match the SST’s design capacity.
4. Adjusting the effluent weir level to eliminate flow short-circuiting.

Floating Sludge Lumps  

This phenomenon arises from sludge clumping and accumulation, which triggers sludge deflocculation and subsequent floatation to the surface.

Mitigation strategies:  

1. Implementing frequent sludge discharge to prevent excessive accumulation (avoiding over-frequent operation).
2. Inspecting and replacing damaged sludge scrapers promptly to ensure effective sludge removal.

Abnormal Dissolved Oxygen (DO) Levels in Effluent  

Both low and high DO levels in SST effluent stem from the following factors: 

1.Low DO Due to Prolonged Sludge Retention  

Prolonged residence of activated sludge in the SST leads to continuous oxygen consumption by aerobic microorganisms, reducing effluent DO.

Solution: Increase return sludge flow to shorten sludge retention time (SRT).

2.Low DO Due to Malfunctioning Sludge Suction Equipment  

Faulty sludge suction machines hinder timely sludge return, prolonging SRT for part of the sludge and exacerbating oxygen depletion.

Solution: Conduct immediate maintenance to restore normal equipment operation.

3.Low DO Due to Elevated Water Temperature  

Higher temperatures reduce oxygen solubility in water.

Solution: Extend sewage retention time in treatment facilities, utilize the regulating tank volume, and promote water evaporation to lower temperature.

4.DO Fluctuations Linked to Aeration Tank Operation  

Low influent organic load or excessive aeration in the aeration tank disrupts effluent DO balance.

Solution: Adjust influent flow via the regulating tank to increase organic load, or reduce blower output to lower aeration intensity.

5.High DO Due to Mixed Liquor Toxicity  

Toxic substances in the aeration tank inhibit microbial oxygen utilization, leading to elevated residual DO.

Solution: Identify and remove the source of toxic contaminants.