Vermicomposting of Sewage Sludge: Earthworm Population and Agronomic Advantages

Abstract

Optimal substrates for vermicompost should ensure survival and reproduction of earthworms, as well as vermicompost quality. This concerns especially waste material substrates. Therefore, the objectives of this study were to estimate the adaptation of Eisenia foetida to mixtures of sewage sludge and composted cow dung, and also to evaluate the quality of the vermicompost as fertilizer. Four treatments contained sewage sludge and composted cow dung in ratios of 70:30 (A), 80:20 (B), 90:10 (C) and 95:5 (D), with 60-70% moisture. The samples were incubated at 17 ± 2°C with 50 adult earthworms each. Following 50 days of composting, cocoons, juveniles and adult earthworms were counted and several physicochemical parameters were determined. E. foetida could not survive treatment D. Treatments A and B had the highest survival and reproduction. However, treatment C showed the highest amounts of available N and P (2189.80 and 2310.63 mg kg−1, respectively) and a C:N ratio of 12.46, making it a better fertilizer. The values of bulk density (0.64 g cm−3), organic matter (54.82%), organic C (31.80%), available K (1801.92 mg kg−1) and total N (2.55%) for treatment C were within the ranges for vermicompost recommended in the Mexican guidelines. Significant differences were found in all physical and chemical parameters (p<0.05), except for P. Fractionation of P in treatment C and sewage sludge showed that the largest proportion of P is slightly soluble in water and so, available to plants. Overall, this study suggests that sewage sludge and cow dung vermicomposted with E. foetida can improve fertility and nutrient availability, as well as earthworm population growth, compared with the sewage sludge alone.