A comparison of two methods to reduce internal phosphorus cycling in lakes: aluminium versus phoslock

Abstract: Sediment nutrient inactivation through P binding agents is one of the most common management strategies used to prevent potential P release to lake water. Al-based compounds (aluminium sulphate, Al2(SO4)3·18(H2O)) and a modified lanthanum-based clay material called Phoslock are among the products that can be used to chemically bind P permanently in sediment. In the present thesis, aluminium sulphate (Alum) and Phoslock were compared in terms of dose application and P binding efficiency in a controlled setting in order to develop dosing models that can be further used in lake restoration and management. Furthermore, a literature review of past and current research was performed to be able to compare both products in terms of potential non-target side effects and treatment longevity. Results showed that Al was able to bind 93-95% of mobile P at the highest dose (Al:Mobile P 150:1). Moreover, cost-effective Al dose ratios were 75:1 for samples with mobile P ranging from 1.55-0.64 mg/g and 25:1 for samples with mobile P ranging from 0.33-0.12 mg/g. By comparison, Phoslock was only able to bind a maximum of 25-35% of mobile P in sediments at the Phoslock:Mobile P dose ratio of 150:1. A comparison between theoretical and real binding ratios also showed that Al was more effective than Phoslock at removing mobile P from sediments. P partitioning after Al treatment showed an increase in the Al-P fraction, whereas Phoslock treatment increased P content in the Ca-P fraction. Treatment costs were lower for Al than for Phoslock and were 83 €/kg of P and 1227 €/kg of P inactivated, respectively. Obtained results, together with potential non-target side effects and treatment longevity gathered from literature, showed that, in general terms, Al is a better product, but that Phoslock may be better in shallower systems with frequent sediment resuspension.