Tertiary treatment of the liquid fraction of pig manure with Phragmites australis

Since 2003, the pig industry in Flanders (Belgium) is obliged to process a portion of the nutrient overproduction. In general, pig manure processing occurs as follows: i) separation into liquid and solid fractions, ii) conversion of the solid fraction to an exportable product (e.g. composting) and iii) reduction of nutrient contents in the liquid fraction before discharge into surface water or spreading on arable land. The aim of this study was to evaluate the potential of constructed wetlands (CWs) planted with Phragmites australis to reduce nitrogen (N), phosphorus (P) and chemical oxygen demand (COD) in the liquid fraction to levels below discharge criteria. In addition, the removal efficiency of heavy metals (Cu, Zn) present at elevated levels in the liquid fraction was evaluated. A greenhouse experiment was conducted with subsurface flow (SSF) reed beds (2 × 0.125 × 0.11 m) filled with sand, loam, clayey sand or expanded clay (argex). The liquid manure load was set at 1 mm per day. Removal efficiencies varied between 64–75% for COD, 73–83% for N and 71–92% for P, depending on the matrix material used. However, effluent levels still remained significantly above the Flemish legal discharge criteria of 2 mgl^?1, 15 mgl^?1 and 125 mgl^?1 for P, N and COD respectively.