Figure 2: Concentration of COD, of PO43- and of nitrogen pollution in the influent and in the effluent sampled at 75 cm filter depth. Legend: C: Center of the filter basin (cm), P: Periphery of the filter basin (cm), COD: Chemical oxygen demand (mg O2/l ), PO43-: Phosphorus (PO43-) (mg/l ), NTK: Kjeldahl nitrogen (NTK) (mg/l ), NH4+: Ammonium (N–NH4) (mg/l ).