What is the role of 'velocity gradient' (G value) in the flocculation process, and how does it affect floc size and strength?

The velocity gradient (G value) in the flocculation process is a measure of the mixing intensity, representing the rate of change of velocity within the water. It plays a critical role in promoting particle collisions and the formation of larger, more settleable flocs. Flocculation is the process of gently mixing destabilized particles (resulting from coagulation) to encourage them to collide and aggregate into larger flocs. The G value is calculated using the formula G = (P/μV)^0.5, where P is the power input, μ is the dynamic viscosity of the water, and V is the volume of the flocculation basin. A higher G value indicates a higher mixing intensity, while a lower G value indicates a lower mixing intensity. The G value affects both the size and the strength of the flocs. Initially, a moderate to high G value is beneficial to promote frequent collisions between the destabilized particles, leading to the formation of microflocs. However, if the G value is too high, the shear forces generated by the intense mixing can break apart the flocs, resulting in smaller, weaker flocs that are difficult to settle. This is often referred to as floc shearing. On the other hand, if the G value is too low, there may not be enough collisions between the particles, resulting in slow floc growth and the formation of small, weak flocs. Therefore, the optimal G value is a balance between promoting particle collisions and preventing floc breakage. Tapered flocculation, where the G value is gradually reduced as the floc size increases, is often used to optimize floc formation. For example, a typical tapered flocculation system might start with a G value of 50 s^-1 for microfloc formation and then gradually decrease to 10 s^-1 to promote gentle floc growth without breakage.