A comparison of aluminum dosing methods for reducing sediment phosphorus release in lakes

Aluminum (Al) treatment is one of the most commonly used approaches to reduce internal phosphorus (P) loading in lakes. However, the adequate amount of Al that should be added to permanently inactivate mobile (releasable) sediment P can be determined using many different methods. These methods differ substantially in their specified design sediment depth, targeted P pool(s), and expected binding ratio. In this study, Al doses for inactivating sediment P in Beung Gii Lake of Thailand were determined using the most commonly used methods reported in literature and then compared. Experimental procedures included sediment P fractionation, Al assay experiments, and a geochemical model. Mobile P was detected in the lake's sediment at 2.52, 5.42, and 7.65 g/m² in the upper 4, 10, and 15 cm, respectively, with additional P contained in labile organic form. Comparing the resulting Al doses for the lake, it was found they varied by nearly an order of magnitude (45-306 g Al/m²). This result highlights the importance of choosing a dosing method, because such a range of Al doses would likely result in highly variable levels of effectiveness and longevity, including both under- and overdosing. Based on the results of this study and a review of literature, a conservative, fixed ratio between Al and mobile plus labile organic sediment P (11:1) is recommended. All potentially releasable P (both mobile organic and inorganic forms) within the active sediment layer should be used to determine the total Al dose. Finally, the calculated Al dose in most cases will need to be split into sub-doses, based on lake morphology and total Al dose, to ensure maximum binding efficiency. Al dosing strategy should seek to minimize the risk for overdosing, maximize binding efficiency, and ensure all potentially releasable P forms are targeted during treatment.