Washing Posidonia oceanica with treated urban wastewater for biofilter and agricultural use

Posidonia oceanica is an important higher plant in shallow waters of the Mediterranean coast. It plays a crucial role not only in marine ecosystems but also in protecting shores from surf impact thanks to the deposition of important volumes of leaves and wave damping. However, these deposits interact with touristic activities and they are removed from the beaches producing important management problems. Many different uses have been proposed to take advantage of this material, but the high salt and sometimes, heavy metal content hinder its applicability. The aim of this study is to evaluate the potential use of treated wastewater to wash the posidonia remains to transform the remains into valuable resources, which, in this way, could also act as a biofilter in supplementary water treatments. We proposed a set of 6 alternatives combining factors such as flooding/natural leachate, natural/forced airing bed or frequency in water application: once-daily percolated washing with natural convection; multiple-daily percolated wash with natural convection, once-daily percolated wash with forced convection, multiple-daily percolated wash with forced convection, once-daily flood wash with emptying weekly and once-daily flood wash with overflow. Our results show that less than 8 days are required to remove the initial salt content for all 6 treatments, although once-daily flood wash with emptying weekly treatment was the one that caused less Pb and Zn retention on posidonia residues and more P accumulation, which makes it more interesting for agricultural use. Regarding the ability of Posidonia for water treatment, suspended solids do not show a clear reduction for any treatment, and even small increases were produced. In addition, anaerobic processes increased COD and BOD5. These results suggest that more research is needed prior to use posidonia residues as a biofilter.