In a water treatment, the optimum time of flocculation is usually given as 30 minutes. In case the time of flocculation is increased beyond this value, the flocs will 

The correct answer is option 3) Break up and defeat the purpose of flocculation  

Explanation: 

• Flocculation involves gentle mixing to promote the aggregation of particles into larger flocs.
• If the flocculation time exceeds the optimal duration (30 minutes), prolonged mechanical agitation can shear or break apart the formed flocs.
• This fragmentation results in smaller particles that are less likely to settle effectively in the subsequent sedimentation tank, undermining the goal of flocculation.  

Option 1 is incorrect because flocculation tanks are designed with slow mixing to keep flocs suspended until they reach sedimentation. Settling prematurely would require halted mixing, not extended time.  

Option 2 refers to dissolved air flotation, not sedimentation, and is unrelated to prolonged flocculation.  
Option 4 (sticking to paddles) is less critical compared to shear-induced breakup, which directly impacts process efficiency.  

Thus, over-flocculation leads to floc breakup, reducing sedimentation efficacy.