Forward Osmosis with High-performing TFC Membranes for Concentration of Digester Centrate prior to Phosphorus Recovery

Forward osmosis (FO), using thin-film composite (TFC) membranes, was used to concentrate digester centrate for improved phosphorus recovery. Digester centrate, obtained after the dewatering of digested sludge, contains phosphorus that can be precipitated as struvite (MgNH ₄PO ₄·6H ₂O) and used as a fertilizer. By concentrating phosphorus in the digester centrate, it is possible to increase the efficiency of struvite precipitation by decreasing reaction time, reduce tank volume, and minimize magnesium addition. A lab-scale setup FO unit with TFC membranes was used to concentrate phosphorus. Both synthetic and real digester centrate were used as feed solution, and both synthetic and real seawater were used as draw solution. The TFC membranes demonstrated a high water flux, around 17 LMH for pure water and 5 LMH for real digester centrate, emphasizing its viable use even when using digester centrate – a solution high in solids. It was possible to concentrate digester centrate to a volume concentration ratio (VCR) of 7 using seawater with a salinity of 22PSU; this correspond to an expected reduction in magnesium demand of >30%. A phosphorus rejection >99% was achieved without pH adjustment. Rejection of ammonium and ammonia was lower, and decreased with increasing pH. Aeration caused 57% decrease in water permeability most likely due to membrane scaling at higher pH, while cleaning with tap water could restore water permeability. Thus, FO using TFC membranes is a potential method for concentration phosphorus in digester centrate, but pH should be kept below 8 to avoid ammonia transport to the draw solution and to avoid scaling.