Step 4: Basin
Number one...it is a lid for the Grey Water Bucket. It keeps you from staring at bucket of dirty, soapy water from the last guy, while you wash your hands. It also keeps that same dirty, used water from splashing back up on your clean hands while your are washing.
The Basin also allows hands to be washed inside the basin's high side walls - keeping the falling spout water from splashing off your hands onto your pants.
Because the Basin sits almost entirely above the grey water bucket it does not compromise the storage capacity of the gray water bucket. This means that the gray water bucket does not need to be emptied until the fresh water bucket is exhausted and ready to be refilled.
Hand washing could be done directly over an open Gray Water Bucket without the Basin but, as it fills; imagine trying to wash-up with water splashing into a nearly full bucket of used water.
A similar commercial product (Wishy Washy) selling for $90 (buckets not included, shipping extra) is set up with an open gray water bucket.
The addition of the basin shown here and the provisions for stacking the two buckets to bring hand washing to counter height, improves on the comercial product and go a long way to make the system more practical than splashing water from a waist high spout into a open bucket of gray water on the ground.
By the way, the material to make the system shown here did not cost anywhere near $90..nor even Half that! And both systems use similar Marine Outboard fuel primer squeeze bulb and hose assemblies that cost around $15-20. (or buy cheaper hardware hose (Chemical resistant hose is not necessary with water) and the squeeze bulb with check valve is around $12 by itself). Another $15 for plumbing, tubing & fittings; throw in some scrap wood and it will land far less than $90!