Dirty filter chlorine demand?

[I.Robert]

Quick and maybe a paranoid question... Can a dirty sand filter cause a chlorine demand?
I'm (hopefully) almost done SLAMming and I've been running my filter on recirculate just in case. Now I'm scared to turn the valve to filter, since I had clear water and high demand before - can the problem hide in the filter?

 

[woodyp]

Algae cannot grow inside a dark filter-----no light. Backwash it and filter.

 

[I.Robert]

Thank you!