The improper management of swine wastewater is a major environmental concern for farmers due to the high phosphorus content of animal manures which deteriorates water quality. To minimize negative environmental effects resulting from improper animal waste disposal and to promote sustainability, the principles of circular economy encourage the recovery and reuse of valuable waste resources, such as struvite, a slow-release phosphorus mineral. However, due to the high levels of antibiotics residues in animal waste streams, the application of struvite as fertilizer could reintroduce antibiotics into the environment. In this study, we investigated the fate of five classes of antibiotics on struvite crystals from synthetic swine wastewater. Struvite precipitation was simulated in bench-scale experiments, by adding antibiotics at a concentration of 10 mg/L to a solution containing Mg²⁺, PO₄^3-, and NH₄⁺ in a 1.2:1:1 ratio. The solution was shaken for 60 min and allowed to stand for 90 min. The concentration of phosphate and antibiotic in the supernatant was measured to determine phosphate recovery and antibiotic fate. We observed that for comparable phosphate recovery, the sorption of chlortetracycline on struvite increased from 20.3% to 78.3% as pH increased from 8.5 to 10.5. An increase in the phosphate to chlortetracycline molar ratio from 250:1 to 1000:1 resulted in a 44% increase in chlortetracycline sorption on struvite. Likewise, increasing the contact time from 2 h to 6 h also resulted in a 44% increase in the amount of chlortetracycline sorbed on the struvite crystals. The mechanism of antibiotics sorption on struvite was mainly through electrostatic interactions and ternary complexation between multivalent cations and tetracycline. The outcomes of this study provided a deeper understanding of the fate of antibiotics during phosphate recovery processes.

Nathaniel Bolujoko

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