What is the mechanism behind pin floc formation during the coagulation-flocculation process and how can it be rectified?

Pin floc formation during coagulation-flocculation is characterized by the formation of very small, weak, and poorly settling flocs, often described as being about the size of a pinhead. This is usually due to either under-coagulation or over-coagulation, leading to unstable particles that don't aggregate effectively. The mechanism behind pin floc formation typically involves one or more of the following issues: 1) Insufficient Coagulant Dosage: When the coagulant dosage is too low, the charge neutralization of the colloidal particles is incomplete. This means the particles still retain some of their negative charge, causing them to repel each other and hindering the formation of larger flocs. 2) Over-Coagulation and Restabilization: Conversely, an excessive coagulant dosage can also cause pin floc. Adding too much coagulant can reverse the charge of the particles, making them positively charged and causing them to repel each other again. This is known as restabilization. 3) Improper pH: The pH of the water affects the speciation and effectiveness of the coagulant. If the pH is outside the optimal range for the coagulant being used, it may not be able to effectively neutralize the charge of the particles. 4) Insufficient Mixing: Inadequate mixing during the rapid mix stage can prevent the coagulant from being properly dispersed throughout the water, leading to uneven charge neutralization and pin floc formation. 5) High Turbidity or Organic Matter: Very high levels of turbidity or natural organic matter (NOM) can increase the coagulant demand, requiring a higher dosage to achieve effective coagulation. If the dosage is not increased sufficiently, pin floc may result. To rectify pin floc formation, the following steps can be taken: 1) Jar Testing: Perform jar tests to optimize the coagulant dosage and pH. Jar tests simulate the coagulation-flocculation process in the laboratory, allowing operators to determine the optimal conditions for floc formation. 2) Adjust Coagulant Dosage: Based on the jar test results, adjust the coagulant dosage to the optimal level. If the pin floc is caused by under-coagulation, increase the dosage. If it is caused by over-coagulation, decrease the dosage. 3) Adjust pH: Adjust the pH to the optimal range for the coagulant being used. This may involve adding acid or base to the water. 4) Improve Mixing: Ensure adequate mixing during the rapid mix stage to properly disperse the coagulant. This may involve increasing the mixing intensity or improving the design of the rapid mix basin. 5) Add Polymer: Adding a polymer can help to bridge the small flocs together and form larger, more settleable flocs. 6) Monitor Raw Water Quality: Regularly monitor the raw water quality, including turbidity, organic matter, and alkalinity, to anticipate changes in coagulant demand and adjust the treatment accordingly. For example, if jar tests reveal pin floc, the operator should first check the pH and then slowly adjust the coagulant dosage up or down until larger, faster settling flocs are observed.