Vermistabilization of paper mill sludge by an epigeic earthworm Perionyx excavatus: Mitigation strategies for sustainable environmental management

The present study demonstrates the vermistabilization of paper mill wastewater sludge (PMS) spiked with cow dung (CD) employing indigenous epigeic earthworm Perionyx excavatus Perrier. A total of six treatments were prepared along with a positive control (PC) and negative control (NC). Twenty earthworms were released into each treatment including PC and NC without earthworms. The different proportions viz., T1 (CD – 100%), T2 (PMS:CD – 1:3), T3 (PMS:CD – 1:2), T4 (PMS:CD – 1:1), T5 (PMS:CD – 3:1), T6 (PMS:CD – 2:1), PC (PMS – 100%) and NC (PMS – 100%) and changes in chemical parameters and microbial properties were recorded during the course of 60 days. Vermistabilization caused a significant decrease in the level of heavy metals: Cd (2.9–27.8%), Cu (0.22–42.3%), Pb (1.3–56.3%) and Cr (0.8–46.2%). The bioconcentration factor (BCFs) was also calculated and great amount of heavy metals accumulated in their body (mg kg−1) that ranged from 0.31 ± 0.003–0.45 ± 0.007 for Cd, 0.12 ± 0.005–0.24 ± 0.003 for Cu, 0.15 ± 0.005–0.31 ± 0.006 for Pb and 0.29 ± 0.007–0.56 ± 0.001 mg kg−1 for Cr, accumulation of heavy metals are in the order: Cr > Cd > Pb > Cu. The physicochemical parameters of earthworm treated substrate such as electrical conductivity, total nitrogen, total phosphorus and total potassium were significantly increased; whereas, pH, total organic carbon, C:N ratio (carbon: nitrogen) and C:P ratio (carbon: phosphorus) were reduced after 60 days of vermistabilization. The vermistabilized materials also had a higher population of bacteria (98.90 ± 0.30 CFU × 106 g−1), fungi (43.75 ± 0.55 CFU × 103 g−1) and actinomycetes (67.65 ± 0.45 CFU × 105 g−1) than initial mixtures. Moreover, several histopathological changes were observed in earthworm tissues viz., disintegration of cells, irregular surface of epidermis, cellular debris, irregular cellular compartmentation, and oval-shaped nucleus. Higher level of histopathological abnormalities was recorded in PC (PMS-100%) while none were detected in lower concentrations of PMS. Furthermore, the study concludes that the paper mill sludge in a mixture of cow dung (1:1 ratio) can be a useful proposition for utilizing this hazardous waste through the adoption of vermitechnology.