Influences of dissolved organic matters on tetracyclines transport in the process of struvite recovery from swine wastewater.

Due to the extensive existence of tetracyclines (TCs), struvite (MgNH4PO4·6H2O) recovery from swine wastewater will pose TCs-pharmacological threats to the agricultural planting and environment. This study investigated the influences of dissolved organic matters (DOM), as an important medium in the wastewater, on TCs transport during struvite recovery from swine wastewater. Compared to TCs concentrations of 1.49-2.16 μg/g in the solids obtained from synthetic wastewater, the existence of DOM significantly enhanced TCs contents in the products with the values of 360-742 μg/g. DOM was fractionated into four size fractions with different molecular weight cut-off, i.e. FDOM1 (30 kDa-0.45 μm), FDOM2 (5-30 kDa), FDOM3 (1-5 kDa) and FDOM4 (<1 kDa). Results revealed that the destabilization and aggregation of FDOM1 and FDOM2 contributed major roles to TCs transport from the aqueous phase to the solid products. Meanwhile, the hydrolysis of certain parts of FDOM1 and FDOM2 led to the aqueous TCs re-partition among various DOM constituents, which presented a false appearance that FDOM4 with smaller molecular weight posed significant influences on TCs transport. Increasing pH values from 8.5 to 10.5 resulted with a stepwise increase of precipitated DOM, thereby enhancing TCs concentrations from 94.5 to 292.4 μg/g to 627.2-825.0 μg/g in the recovered solids. The outcomes provide a better understanding on the capability of DOM on TCs transport and abatement in the phosphate recovery process.