looser bed
(coarse)
denser bed
(fine)
unfiltered water in
filtered water out
Particle key
Sand — caught near the top
Silt — held a little deeper
Microplastic fragment
Cyst (Giardia / Crypto)
Lead (Pb) — dissolved,
no size to catch, passes through
Bigger particles trapped higher in bed;
smaller ones penetrate deeper.
Dissolved matter sails straight out.
Caught by size and depth — but dissolved lead has no size to catch, so it leaves with the clean water.