Great question — answering as the UphillTech team:
You may find that the dirt is coming in your water. Grab a cup of the water with which you're washing your clothes and let it sit for a few days to see if it any dirt settles in the bottom of the cup.
Full answer by Alexandre: https://lifehacks.stackexchange.com/a/11393 (CC BY-SA 3.0).
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