Natural covers usually consist of blown chopped straw or peat moss. The natural covers tend to sink after a while. There is some ongoing research into spraying straw with mineral oil to help keep it afloat longer. Manmade covers of geotextiles are available commercially. Filling liquid manure storages from the bottom minimizes agitation and preserves the crust to help reduce odours.
High fibre feeds also promote crusting.
Many existing storages can easily be altered to be filled from the bottom . The air ventilated from the barn is screened for dust and pressurized in a plenum under the biofilter. The air is then forced through a box usually containing a filter medium of peat moss or compost, achieving odour reductions of 60 - 80 percent. As the air flow tends to dry out the filter medium, it needs to be continuously sprayed with water. The filter medium also has to be changed every 2 - 5 years.