I use a tub which is 1/3rd of a plastic 44gallon ad blue drum, I usually seive in my pan using the hose and poor off silty water onto lawn , then dip into tub and swirl to silt water up again and poor onto lawn, continue this until most the silt is gone then pan into tub letting rocks and sand wash out into tub.
This way the water In the tub stays clean long enough to get a back and arm ache and until im sick of panning. Easily get through 20 litres of dirt , plus I usually use a 1/2inch sieve on a 20 liter buckets in the river when getting pay dirt which washes alot of silt away and saves bringing larger stones home , in most cases half's the material brought home.
Thanks aussiefarmer, some good ideas there. I've done something similar hosing out my bucket of material for a while before scooping into the pan but I like your idea of dumping the dirty water out on the go.