6.1 Key Influencing Factors and Operational Recommendations
(1) Wastewater pH
pH is one of the critical factors affecting flocculation performance. Different flocculants have varying adaptability to wastewater pH. In food processing wastewater treatment, the applicable pH range for polyaluminum chloride (PAC) is typically between 6.0 and 8.5; for aluminum and iron salt coagulants, the optimum pH for turbidity removal in most natural waters is 6 to 8. When the wastewater pH falls outside the suitable range of the chosen chemical, it is advisable to adjust the pH via neutralization before the coagulation reaction unit.
In practice, it is recommended to measure the pH of the reaction zone at least once per shift and maintain it within the corresponding range according to the chemical type used.
(2) Flocculant Dosage
The dosage should be adjusted according to the level of wastewater contamination—insufficient dosage may lead to inadequate floc formation, while overdosing can increase chemical costs and negatively affect treatment efficiency. In actual engineering, the appropriate dosage should be determined primarily through on‑site jar tests. A standard jar test typically involves rapid mixing, slow mixing, and quiescent settling, with stepwise dosing to observe floc formation and supernatant turbidity, in order to identify a suitable dosage range.
It is advisable to maintain a certain adjustment margin during operation and to establish a regular verification schedule.
(3) Mixing Conditions
Proper mixing is essential for effective flocculation. According to the Technical Specification for Coagulation and Flocculation in Wastewater Treatment (HJ 2006-2010), the velocity gradient G value for coagulation is generally 600–1000 s⁻¹, and for flocculation it is 20–70 s⁻¹. The mixing time should be controlled between 30 and 120 seconds, and the flocculation time between 10 and 20 minutes.
The rapid mixing stage aims to disperse the chemical evenly, while the slow mixing stage promotes further collision and growth of micro‑flocs. Excessive slow‑mixing intensity may break up already formed flocs.
(4) Water Temperature and Salinity
Under low‑temperature conditions, the dissolution rate and chain extension of certain polymeric flocculants may decrease. If performance fluctuations are observed during winter operation, consider extending the reaction time or adjusting the chemical preparation concentration. In high‑salinity wastewaters (e.g., from pickled vegetable processing), electrolytes may interfere with flocculation; it is recommended to verify parameters through on‑site testing under such conditions.
6.2 Common Problems and Troubleshooting Approaches
Issue (1): Small, Fine Flocs with Slow Settling
Possible causes:
- Mismatch between flocculant type and wastewater characteristics
- Insufficient or excessive dosage (overdosing may cause colloidal restabilization)
- Improper mixing conditions (excessively high slow‑mixing speed may break flocs)
Troubleshooting suggestions:
- Check whether the reaction zone pH is within the suitable range for the chemical used
- Re‑evaluate the current dosage by conducting a jar test
- Observe floc morphology in the slow‑mixing zone; if flocs appear to be sheared, consider reducing the mixing intensity
Issue (2): Poor Oil and Grease Removal
Possible causes:
- Oil is present in emulsified form, which is difficult to break by simple flocculation
- The chosen flocculant has insufficient charge neutralization capacity for emulsified oil droplets
Troubleshooting suggestions:
- Evaluate whether an additional demulsification pretreatment step is needed before coagulation
- Try adjusting the pH to a neutral or slightly alkaline range (for animal fats) and observe any change in oil‑water separation efficiency
Issue (3): Fluctuating Treated Effluent Quality
Possible causes:
- Significant variations in influent quality due to different production batches
- Chemical dosing is not adjusted in time to follow influent changes
Troubleshooting suggestions:
- Install on‑line turbidity or suspended solids (SS) monitors at the equalization tank outlet to establish an early warning system
- Develop a feed‑forward control strategy that adjusts dosing proportionally based on monitoring data, minimizing lag effects
- Regularly check the stability of chemical preparation concentration; polymer flocculants should not be stored for prolonged periods after mixing, especially in summer when degradation is faster
Important note: The pH ranges, mixing parameters, and other reference values mentioned above are derived from relevant technical specifications and publicly available literature. Actual wastewater characteristics may vary, leading to different optimum conditions. Before applying these reference values to a specific project, it is strongly recommended to collect representative wastewater samples and conduct jar tests, using the test results as the primary basis for process design.