Yesterday I backwashed my sand filter until the water coming coming out was almost clear. I then took off the multiport valve and rinsed the sand with running water from the hose for 15 min. The water that was overflowing from the filter was very dirty and brownish at first. After rinsing I did another back wash where it was though I had not backwashed 30 min earlier. The question is why does so much dirt build up in a sand filter even though it is being backwshed when needed, and how often do sand filters need this deep cleaning?