Yes. Your friction loss is dependent on your velocity and not the elevation.
The elevation will help determine your velocity by changing where the pump runs on the curve.
If you have significant downhill sections, you will have to analyze each section to determine the flows.
Probably the viscosity may become increasingly important for friction losses estimation/calculation if contaminations are higher& ambient temperatures are on lower side.
You should avoid high and low spots in the force main profile. Ideally the force main should slope upward from the discharge of the pump to the outlet of the forcemain. Otherwise, you will need air/vacuum valves at the high spots ( to avoid air/gas blocks ) and drains ( blowoff valves ) at the low points.
If this can't be avoided, you might consider a series of pumps, each pump lifting water to a high spot and then draining by gravity to a low spot where it can be lifted by the next pump.
You will also need to be concerned with the release of gases from the wastsewater; especially H2S.
For sanitary force mains temperature and viscosity variations are usually ignored.
But you do need to account for the friction (energy ) losses in all of the pipe, including the downward sloping sections.
