Valorization of pulp and paper mill sludge through vermiculture: A nutritional assessment of Eisenia fetida as sustainable feed alternative

Abstract

Pulp and paper mill sludge (PPMS), a significant byproduct of the paper industry, poses considerable environmental and disposal challenges. However, its organic content offers potential for reuse as a nutrient-rich substrate in vermiculture. This study aimed to evaluate the suitability of PPMS for the cultivation of Eisenia fetida, for enhancing the nutritional quality of earthworm biomass for potential application in animal feed and as a sustainable bioresource. E. fetida were cultured in PPMS amended with various substrates; vegetable peels (TSV), soil (TSS), fresh cow manure (TSCF), and composted cow manure (TSCC) in a 2:1 ratio while PPMS alone (TS) served as the control. After eight weeks of vermiculture, harvested E. fetida were analyzed for proximate composition; moisture and dry matter content, total soluble protein, lipids, total phenolics and antioxidants, total soluble sugar, vitamins, and minerals contents. Moisture (86.7–89.6%) and dry matter content (10.4–13.3%) differed significantly (p < 0.05). In addition, significant differences (p < 0.05) were found in soluble protein (17.8–21.7%), lipid (7.8–13.3%), total phenolics (14.3–23.5 mg g⁻¹), antioxidant capacity (5.2–27.6 μmol TE g⁻¹), and total soluble sugar (0.8–1.3%) contents. A strong positive correlation (R² = 0.92) was observed between total phenolics and antioxidant activity, with the highest values in TS, followed by TSV, and TSS. Although the TS yielded significantly (p < 0.05) lower nutritional indices for total soluble protein and sugar, PPMS (TS) exhibited significantly reduced (p < 0.05) concentrations of potentially toxic elements (Co, Pb, Cd) and enhanced vitamins compared to amended treatments. These findings suggest that E. fetida cultured in PPMS, especially without amendments, can accumulate essential nutrients while exhibiting a lower risk of toxic element bioaccumulation. E. fetida tested from PPMS amended with cow manure, vegetable peels and soil may serve as an alternative protein source for poultry and aquaculture feed, as well as a viable approach for industrial PPMS valorization.