Optimization of Stocking Density of Eisenia fetida in Bioconversion Process of Pulp and Paper Mill Sludge (PPMS) and Its Population Dynamics

The pulp and paper mill industry in Corner Brook, Newfoundland and Labrador, Canada produces 150 Mg/day of pulp and paper mill sludge (PPMS) with a high moisture content (70–80%). Instead of the current practice of burning, PPMS may be repurposed as vermicompost. However, the optimum population density of Eisenia fetida to maximize the bioconversion process and its influence on population dynamics are largely unknown. The current study aimed at determining how stocking densities affect the vermicomposting time, vermicompost quality, and population dynamics of E. fetida. By using three stocking densities—1.13 g (TL), 2.04 g (TM), and 3.01 g (TH)—the E. fetida per kg of PPMS and the quality of the vermicompost were determined by chemical analysis followed by a germination test and bioassay on Raphanus sativus seedlings. The total vermicompost produced in TL (66.3%) and TM (68.8%) made the bioconversion process quicker in 60 days. The addition of more E. fetida (TH) delayed the process of vermicomposting by up to 90 days (p < 0.003). Overall, 0.87 (TM) E. fetida/L of PPMS was found to be the optimum population density for obtaining the best quantity and quality of vermicompost. The highest earthworm biomass was harvested in TL, followed by TM and TH, as 3.2, 1.3, and 0.9-fold, respectively, compared to the introduced biomass in 60 days (p < 0.008). The mean growth rate (2.6 mg/worm/day), biomass gain (3.49 mg/g), and reproduction rate (3.9 cocoon/worm) were also significantly higher (p < 0.023) in TL compared to TM and TH in 60 days. Therefore, the present study shows the importance of using an optimum stocking density to maximize the bioconversion process in PPMS, especially in cooler regions.