Mining phosphorus from anaerobically treated dairy manure by forward osmosis membrane

We investigated the effect of draw solution type on the forward osmosis (FO) performance for enriching nutrients from anaerobically treated dairy manure (ATDM) followed by chemical precipitation for phosphorus recovery as struvite crystal. The FO membrane significantly rejected COD (>97%) and phosphate (>98%) whereas there was only 70%-73% and 73%-76% rejection of ammonia-nitrogen and total nitrogen, respectively. The draw solution type had little impact on the retention of the aforementioned wastewater constituents by the FO membrane. At 60% water recovery, a water flux decline of 82%-96% was observed due to increasing contaminant concentration in the concentrated feed solution. Water flux decline was less for MgCl2 than EDTA-2Na and NaCl as draw solutes. On the contrary, the reverse solute flux was higher for NaCl than MgCl2, which in turn was higher than EDTA-2Na. Pre-concentration of ATDM by FO facilitated struvite precipitation. MgCl2 as the draw solute provided a higher purity of struvite than EDTA-2Na and NaCl. This is because when MgCl2 was used as the draw solute, the reverse salt flux of magnesium to the concentrated ATDM provided favourable conditions for struvite crystal formation. This study demonstrates the technical feasibility of phosphorus recovery from ATDM using the FO process.