I am planning to use a modified biosand filter for my new AP system. I Have constructed a number of biosand filters and I can see some potential of it in AP system. Although I am new to AP and may have overlook some of the important matters to consider. In this link you can learn how a BSF works http://www.sswm.info/content/biosand-filter
and here is a qoute from the site
"There is a standing water height of 5 cm above the sand layer, which is maintained by adjusting the height of the outlet pipe. It is this design feature that allows the formation of a biofilm layer "
The modified BSF for AP system will have a sand depth of only 6 inches instead of the usual 20 to 24 inches and will allow some bacteria to pass through. It will have as large surface area as possible. Which can be under a media base grow bed. The biofilm will consist of bacterias that feed on the nutrients that is present in the feed water. I am also planning to add some high surface area media on top of the biofilm preferably matala to increase bacteria habitat. All of the waste from the fish tank will be trap in this sand filter and be converted to nutrients needed by the plant. The media base grow bed can also be use as biofilter but since it does not filter all of the solids it must be located on top of BSF and have its drain water go to the BSF.
please share your thoughts on this. thanks