Nutrient recovery from freshwater aquaculture effluent by employing seawater driven zeolite-assisted forward osmosis process

Abstract

This study focused on recovering nutrients from inland freshwater aquaculture effluent which contains dilute nutrients such as ammonium (NH4+) and phosphate (PO43-) ions. Forward osmosis (FO) process with the assistance of Mg-zeolite Y was employed to recover PO43- through chemical precipitation while NH4+ recovery was achieved by regenerating the zeolite. It showed that zeolite-assisted FO process requires 56.56 ± 2.88 h to dewater 87.5% of the feed solution while standalone FO process took only 51.08 ± 2.99 h. The difference in duration was due to the supplement of Mg2+ that reduces osmotic pressure gradient. The ammonium sorption process of Mg-zeolite Y introduced Mg2+ into the feed solution and facilitated the precipitation of PO43-. Phosphate recovery from the Ca-rich solution from standalone FO process resulted in amorphous calcium phosphate (ACP) with co-precipitation of calcite and dolomite. The zeolite-assisted FO process at pH 10.5 yielded optimal results with 98.46% phosphate recovery, forming mainly amorphous magnesium phosphate (AMP). Subsequently, a two-step precipitation process was proposed to recover Mg2+ as brucite. The concentrated feed solutions from the cycles underwent the proposed two-step precipitation study, resulting in 0.507 g/L of AMP and 5.4077 g/L of brucite. An average of 88.7% NH4+ recovery with high reuse efficiency of Mg-zeolite Y using magnesium sulfate as regenerating solution could be achieved. Reusability studies showed consistent or slightly higher water flux during the dewatering process. The composition of spent regenerant was investigated, and its reuse as liquid fertilizer was feasible after dilution.