The aquatic fern Azolla property as biofilter for olive mill wastewater purification

The use of Azolla biomass as biofertiliser or as feed supplements for aquatic and terrestrial animals have been known for a long time; its use as biofilter is a more recently practise. The aim of this study was to test the capability of Azolla (vegetable matrix) as a biofilter to reduce polyphenols and organic matter contents in the olive mill wastewater (OMWw). Experiments were carried out under laboratory conditions using either fresh or dry Azolla biomass. In order to build the biofilters, fresh or dry Azolla biomass was packed into Imhoff's cones; before packing dry biomass was rewetted. We produced the fern Azolla outdoors in a synthetic Hogland's medium, 2.5 times concentrated, under the climatic conditions of Florence (experimental area of the Ecosystem Study Institute, Department of Florence). Azolla production technology is easy and cheap and during summer months we achieved the average yield of 10 g (dw) m-2 d-1. OMWw treatments were carried out using five packed biofilters in series. At the output of each biofilter, before transferring OMWw in the following cone, a sample of OMWw was taken and analysed to determine both polyphenols and organic matter contents; this last was expressed as chemical oxygen demand (COD). On coming out of the 5th packed biofilter, polyphenols were reduced from 7,650 mg l-1 to 3,610 mg l-1 and COD from 110,200 mg l-1 to 52,400 mg l-1. The removal capability of each Azolla-packed biofilter, from the 1st to the 5th was constant and an average COD removal of 11.600 mg l-1 was achieved. The OMWw to Azolla-fresh-weight ratio of 5:1 was optimal for both polyphenols and COD removal. We did not find any significance difference on efficiency between the uses of dry or fresh Azolla biomass. Due to the presence on the N2-fixing Anabaena azollae in Azolla's leaflets mucilaginous cavities, the fern could grow in a culture medium without nitrogen content, reducing the production cost.