Sewer gradients

I’ve been told that simplified sewerage projects haven’t worked well in Sub-Saharan Africa. Is this because they’ve been run by NGOs with little or no previous experience of sewerage? Possibly. Here’s a test for you: due to ground conditions you have to have a sewer gradient larger than 1 in 200. Would you go for 1 in 300 or 1 in 100? [Simple if you’re an engineer, but maybe not if you’re not?]