Phosphate Removal by Peat from Peat Mining Drainage Water during Overland Flow Wetland Treatment

Overland flow technique, which involves conducting wastewater across a natural mire, has during the recent years been widely used in Finland to decrease the loading of P and other elements from peat mining areas to the watercourses. In this study the applicability of laboratory tests in estimating the P retention by overland flow areas was assessed at a site in northern Finland. The ability of peat to retain PO 4 ‐P was determined by adsorption isotherms, and the data were compared with the actual decreases in P loads obtained in the field. The PO 4 ‐P retention curves obeyed rather well the conventional adsorption models, which suggests that sorption can be an important mechanism removing PO 4 ‐P from the peat mining drainage water during overflow treatment. The Freundlich equation gave a better fit to the P sorption than did the one‐surface Langmuir model. The constants describing the sorption characteristics of the peat showed spatial and temporal variation. They indicated, however, that the theoretical effective P sorption time for the overland flow area can be 20 to 25 yr. The ability of peat to retain P increased with increasing concentrations of oxalate extractable Fe and Al. Sorption reactions were found to be important in the annual decreases in PO 4 ‐P from drainage waters in an overland flow area, but their efficiency in removing P from the peat mining drainage water appears to depend on hydrological conditions. In the first experimental year, the sorption calculated from the isotherm data amounted to 60% of the retention obtained in the field. Owing to the high flow conditions in the second year, the decrease in the loading of P in the field was decisively lower than estimated.