Hobbyman,
Not "gravity flowing through the filter," but we are on the same page as far as float controlled flow tied to the well pump. I found an example of the concept with cisterns that collect rainfall but need t be augment with "mains" water if the level gets too low. Even found one website that sells the hardware ready to install. This one apparently uses weigths instead of floats. (Haven't actually investigated it yet.)
Tank/pump wtr lev control My thoughts were if the epdm pit is sufficently large enough to allow enough retention time, after dropping through a packed column via gravity for aeration, then a pump is used to pump the ferric iron laden water from the pit through a DIY Sand & Gravel filter like the one below. It's good for up to 2000 gph ( gpm). Still in the air on what to do there but if the tank(s) are lw enough the fresh water can enter via gravity and the rest of the flow may be pumped from a sump tank.
S & G filter The above filter is used in koi ponds to produce gin clear water after some kind of primary filter to remove most of the feces and uneaten feed. Should work great for water that is otherwise clean except for the ferric iron.
The gravity operated packed column I already have set up, and consists of five gallon buckets in a vertical connected chain full of media. I used it to aerate the well water before it went to the trout pond. It has the added benefit of blowng off hydrogen sulfide gas. And the media promotes catalizing the ferric iron from its ferrous bicarbinous state with a coating of hydrogen oxide.
Keep in mind this is all still fluid but my focus at the moment is iron removal.
