How concentrated does the analyte solution need to be in order to perform a (chelatometric) titration I’m currently investigating a method to chemically pretreat cellulosic waste as bisorbents for enhanced uptake of lead in water. I intend to measure the initial and final concentration of water to record the change in concentration of aqueous lead via a chelatometric titration. 

In my experiment, I’ll be performing filtration in the batch mode at three different initial concentrations (5 mg/L, 20 mg/L, 50 mg/L). To visualize the magnitude of these concentrations, I’ve converted the units to the more conventional mol/L (M).

Initial concentrations of water samples I’ll be testing on in mol/L (M)

My question is, are these initial concentrations too low to be conducting a titration on. If so, what would be a good initial concentration of lead to be working with (considering after filtration, the concentration of lead will diminish significantly). If these initial (and very small post filtration) concentrations are measurable for a titration, what is a good  concentration for the EDTA titrant. Should the titrant concentration be significantly less than the analyte concentration?

All the best!
Sorry to tell you a bad news. Classical methods, like titrimetry or gravimetry do not work very well at low concentrations. Failure would be a strong word. The beauty of EDTA is that it forms 1:1 complex with most metals. Pb is not an exception. If your lead solution is 0.000001 M, your EDTA solution should be 0.000001 M. Such concentrations are never recommended in classical methods.

For ppm level, spectrophotometric methods can work quite well. Go to Google Scholar and search the methods in Google Scholar