You’ll hear the term pre-wash a lot. You may think, why? Why? Why wash twice? Essentially you’re not washing twice. Let me explain.
When you put your nappies in a pre-wash they’ll have urine in them and although you’ll wash any poopy residue off them there may be remnants remaining.
If you were to just wash them they will be washing in dirty water, in poo and pee water. So you need to pop them in a short pre-wash of 30-60 minutes in warm to hot water. They are now ready to go into the main wash or put in the dry pail until you’re ready for the main wash.
Now they’ll be washed in CLEAN water.
In the pics below shows water from a pre-wash and a main wash
More info to be found https://www.clothnappyhelp.com.au/washing-101.html