Biochar, zeolite, and ferric chloride effectively separate phosphorus and nitrogen (plus potassium) in swine manure: A coagulation-flocculation-sedimentation approach

Abstract

We studied the effectiveness of different coagulants (0.15 M FeCl₃, 0.40 M HCl, and 2.5% H₃PO₄) in separating key plant macronutrients (N as NH₄⁺, P, and K) from swine manure into solid and liquid fractions for their sustainable use in agriculture. This study assesses how these coagulants impact nutrient recovery and separated fraction's characteristics, aiming to improve the solid-liquid separation process using biochar and zeolite. The FeCl₃ treatment, biochar, and zeolite showed optimum nutrient recovery among the tested coagulants for solid (93% of total P) and liquid (90% and 50% of total NH₄⁺ and K, respectively) phase separation of swine manure. Most macronutrients present in swine manure Ca (99%), Mg (76%), and P (71%) exist in the solid fraction, while NH₄⁺ (100%) and K (72%) are predominantly present in the liquid fraction and their distribution changed under the influence of coagulating agents. Gibb's free energy (ΔG⁰) values of P (−19.7 kJ mol⁻¹) and K (−5.31 kJ mol⁻¹) were used to assess the applicability of flocculation followed by sedimentation process as opposed to the reverse one. Characterization of manure solids using E₄/E₆ ratio, FTIR, pH metric curve, and desorption with Mehlich-III solution (∼12% recycled of 93% separated P) suggest that P strongly complexed in the separated solid fraction, possibly through cation bridging and ligand-exchange reactions. Adsorption-desorption results indicate that the cation exchange was responsible for NH₄⁺ (99%) and K (73%) recycling using zeolite. Inclusively, the study presents a proper combination of coagulant and physical processes to recover and recycle nutrients from swine manure effectively.