Effect of the Population Size of the Earthworm Eudriluseugeniae and the Composting Time on Heavy Metal Content during the Vermicomposting of Cashew Residues
Quality organic fertilizers contain low heavy metals content. This study aimed to determine the population size of the earthworm Eudriluseugeniae necessary to obtain low levels of heavy metals in vermicompost produced from cashew residues and cow dung. Thus, in 300 g of each waste and their mixtures with cow dung, four population sizes of the African nightcrawler E. eugenia (5, 10, 15 and 20) were tested. Heavy metal contents of Zn, Cu, Pb, Cd, Cr and Ni were measured at 30, 60 and 90 days of the vermicomposting. Results showed that heavy metals such as Zn and Cu were more concentrated in the cashew residues. Ni contents were higher than the recommended level in organic fertilizer after mixing them with cow dung. Heavy metal contents were influenced by the composting time and the earthworm’s population numbers. The batches of 15 and 20 individuals of E. eugeniae in 90 days showed better reduction in heavy metals content. These results indicate that when using cashew residues and cow dung for vermicomposting, 15 earthworms can be recommended to mineralize 300 g of waste in 90 days.
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