Use of Steel Slag to Remove Soluble Phosphorus from Closed Marine Systems
Serge Parent1, Jean Bouvrette1, Rachel Léger1, Yves Comeau2
1Biodôme de Montréal, Montréal, CANADA H1V 1B3
2École Polytechnique de Montréal, Montréal, CANADA H3C 3A7
The accumulation of soluble phosphorus is a major problem in closed marine systems as it favors excessive algae growth, requiring constant efforts to maintain water quality. Experiments recently showed that steel slag produced in electric arc furnaces (EAF) retained up to 6 g P/kg slag in bench scale tests and efficiently removed phosphorus from the effluent of a freshwater fish farm. Our goal was to determine whether EAF slag could also be used to remove soluble phosphorus from a large marine aquarium. The study was conducted in a 3,000 m³ marine system at the Montreal Biodome. The system contained 20 mg P/L of orthophosphate (60 kg P). A 10 m³ pilot unit was built and filled with 9,500 kg of slag gravel 2-5 mm in size. Water was gravity fed into the unit and flowed through it in upflow mode at rates between 3 and 9 L/min. The unit removed 7 kg P (0.75 g P/kg slag) during its first two months of operation. Maximum daily removal was 165 g P/d at a flow rate of 9 L/min. Slag cementation and channelling were observed 30 days after startup. Operation of the unit had little impact on aquarium alkalinity and no adverse effect on fishes and invertebrates. Given a net annual input of 4 kg P, complete removal of soluble phosphorus from the system would take two years using four slag batches. The use of steel slag offers a promising solution to reduce soluble phosphorus accumulation in closed marine systems.