Forward osmosis treatment of thermal evaporator brine stream

Abstract

Forward osmosis technology was evaluated for treating evaporator brine stream from a petrochemical industry at bench scale using ammonium bicarbonate as a draw solution. Calcium carbonate scaling that forms from the interaction between the calcium ions in the feed solution and carbonate ions from the draw solution (reverse salt diffusion) leads to the reduction in water flux and water recoveries achievable (feed TDS concentration of ~60 000 mg‧L−1, calcium ~545 mg‧L−1 and draw solution (ammonium bicarbonate) concentration of ~240 000 mg‧L−1). Fouling can be prevented by softening the feedwater before it is treated in forward osmosis. Without calcium and magnesium, permeate fluxes and water recoveries of up to 6 L‧m−2‧h−1 and 60%, respectively, could be achieved. It was also observed in this study that the concentration of the calcium ions in the feed does have an impact on the formation of the calcium carbonate scale, implying that some hardness can be tolerated in the feed to the forward osmosis process. It can, therefore, be concluded that without some hardness removal, ammonium bicarbonate draw solution is not suitable for treating concentrated brine streams (e.g., evaporator brine) that contain a high concentration of calcium ions. FO technology using ammonium bicarbonate as a draw solution can be considered an alternative technology to treat concentrated brine streams from inland industries, provided some pre-treatment to remove scaling precursors such as calcium is incorporated in the flow scheme.