In water treatment alum is being used as one of the coagulant to reduce the turbidity/TSS of raw water. For a same turbidity of raw water the alum consumption to maintain the treated water turbidity as 0.95 NTU (for ex.) is varing. What could be the reason. or what are all the factors that increases the alum consumption.

I can think of two things, from my experience. Alum demand is sometimes affected by pH and/or alkalinity. If the pH and/or alkalinity are higher, then alum demand may go up. A second factor is dissolved organics, which sometimes shows up as color. That is likely the biggest factor in what you are seeing. This is typically a seasonal issue with surface waters, depending on what might find its way into the water, such as decaying leaves in the fall and winter.