A circular economy approach for utilization of tannery fleshing hydrolysate and kitchen wastes into organic fertilizer through enzymatic decomposition

This study presents a sustainable method for converting tannery fleshing waste (FH) into organic fertilizer using enzymatic decomposition with crude protease. After extracting fat from the enzymatic hydrolysis, the enzyme-rich residue was mixed with dried kitchen waste (KW) and allowed to decompose for 45 days, producing nutrient-rich fertilizer. FT-IR spectroscopy confirmed the presence of important functional groups, including hydroxyl, aliphatic hydrocarbons, esters, and amide-I linkages. The organic fertilizers had higher nutrient content, with nitrogen (1.08-1.67%), phosphorus (0.78–0.98 %), potassium (0.1–0.76 %), and magnesium (239–259.5 ppm) which is higher than commercial fertilizers. FESEM-EDX analysis revealed a dense, porous structure with a high surface concentration of calcium, which enhances nutrient release in the soil. Dissolution tests showed that nutrients from the organic fertilizer were released gradually over 36 hours, whereas commercial NPK fertilizers released nutrients within 150 minutes in simulated soil-water conditions. Field trials with a Randomized Complete Block Design (RCBD) demonstrated improved growth in Malabar spinach, particularly with fertilizer sample S-5, which had an optimal flesh to kitchen waste (FH to KW) ratio of 1:10. Although S-5 had a lower nitrogen content (1.08 %), it contained higher levels of phosphorus (0.98 %), potassium (0.765 %), and magnesium (259.5 ppm), contributing to enhanced plant growth. The organic fertilizer resulted in a shoot length of 38.8 ± 2.0 cm, root length of 16.33 cm, 31 ± 3 leaves, and 95.12 % dry matter of the plant. Heavy metal analysis of the plant confirmed that levels of chromium (Cr), iron (Fe), nickel (Ni), cadmium (Cd), copper (Cu), and lead (Pb) were within WHO safety limits. Phytotoxicity tests of the fertilizer also showed no negative impact on Malabar spinach seed germination. [Display omitted] •Sustainable method converts tannery waste into organic fertilizer via crude protease enzyme.•Fertilizers showed higher nutrient levels than commercial alternatives, boosting plant growth.•FT-IR confirmed key functional groups in organic fertilizers, enhancing nutrient availability.•Controlled nutrient release from organic fertilizers outperformed commercial NPK in tests.•Safe heavy metal levels and no phytotoxicity observed, ensuring environmental safety for plants.