I've started building a sand filter for our rain water collection system. It's to be a non-backflushable slow sand filter. The vessel is a 4000 litre (1000 gal) plastic cistern (I made a mistake on the tank size when I first posted - fixed now). I'll be using drain rock, pea gravel, and 20/50 filter sand as media and would like to hear what options I have for a barrier between the pea gravel and filter sand to stop migration of the sand into the gravel. Is landscape cloth food grade? Would a cotton sheet stand the test of time?