The impact of competing anions on arsenate removal in dissolved air flotation

• Arsenic in water treatment sludge can limit land disposal. • Control of anions during flotation treatment can inhibit arsenic removal. • The low arsenic sludge generated is more readily disposable to land. • Arsenic can then be removed in a separate process and disposed of separately. Arsenic is removed in drinking water treatment along with suspended and dissolved solids using coagulation, flocculation and clarification technology. However, the result is a sludge with elevated arsenic concentrations which cannot readily be recycled to land. Flotation technology has found widespread use in industrial and domestic water treatment. Arsenic removal efficiency during flotation can be influenced by the ionic composition of the water, presenting a potential way to generate a sludge essentially free of arsenic which can then be removed in a separate treatment stage. To assess the impact of anions on arsenate removal from drinking water using flotation, a batch coagulation, flocculation and dissolved air flotation (C/F/DAF) process with poly aluminium chloride (PAC) as a coagulant was examined. Batch processes were conducted at three different ion concentration levels (1, 5, and 10 mM), within a pH range of 5 to 9, and under a saturated pressure of 4 bar. It was observed that the presence of anions inhibited arsenate removal to differing degrees, with phosphate ions exhibiting the most significant inhibitory effect at each pH level while still maintaining high suspended and dissolved solids removal. The results show that at 10 mM concentration of phosphate anions at pH 6, high removal of dissolved and suspended solids can be achieved (79 % reduction in turbidity, 76 % reduction in UV254 nm) using flotation with minimal removal of arsenate. This presents an opportunity to remove the arsenic separately leaving the bulk of the sludge generated more amenable to land recycling.