Ability of the aquatic fern Azolla to remove COD and polyphenols from olive mill wastewater

We investigated the biofiltration ability of the aquatic fern Azolla to remove polyphenols and chemical oxygen demand (COD) from olive mill wastewater (OMWw) collected from the traditional (TS) and continuous (CS) extraction systems. Azolla biomass was packed into five sequential Imhoff cones and five sequential columns. In both experiments, the filtrates collected from the 5th biofilter showed a decrease in polyphenol contents: from 7650 mg l-1 to 3610 mg l-1 in TS OMWw and from 3852 mg l-1 to 1351 mg l-1 in CS OMWw. The COD contents decreased from 110200 mg L-1 to 52400 mg L-1 in TS OMWw and from 41600 mg L-1 to 2300 mg L-1 in CS OMWw. A 5:1 OMWw to Azolla-fresh-weight ratio was optimal for both polyphenol and COD removal. The biofiltration ability of alfalfa was compared with that of Azolla, but the treatment with alfalfa did not result in the reduction of COD or polyphenols.