Are you looking to buy one?
Do you need one for the outlet as the saniflow has an air admittance valve already
it does not have one need one to fit 21.5 waste outlet pipe to stop syphoning when flushed
1.5" is usually the smallest durgo valve you can get so you may have to upsize on the highest point and cut a tee in before the drop to drain
macerator is only three foot above ground
Product Code: 42968 - Screwfix