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

Abstract

AbstractOverland 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 PO4‐P was determined by adsorption isotherms, and the data were compared with the actual decreases in P loads obtained in the field. The PO4‐P retention curves obeyed rather well the conventional adsorption models, which suggests that sorption can be an important mechanism removing PO4‐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 PO4‐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.