Vermicomposting of biosolids with cow manure and oat straw

Biosolids, mainly from textile industries and the rest from households, were vermicomposted with Eisenia fetida, cow manure and oat straw for 2 months at three different moisture contents (60%, 70% and 80% dry weight base) in triplicate to reduce pathogens and toxic organic compounds, and to find the best medium for growth of E. fetida. The vermicompost with the best stability and maturity and a weight loss of 18% was obtained with 1,800 g biosolid, no straw and 800 g manure at 70% water content. This vermicompost had the following properties: pH 7.9; organic C content of 163 g kg^–1; an electrolytic conductivity of 11 mS cm^–1; a humic-to-fulvic acid ratio of 0.5 (HA/FA); total N content of 9 g kg^–1; water soluble C (C_w) less than 0.5%; cation exchange capacity of 41 cmol_c kg^–1; a respiration rate of 188 mg CO₂-C kg^–1 compost-C day^–1; a NO₃^–/CO₂ ratio greater than 8; and a NH₄⁺/NO₃^– ratio lower than 0.16. The vermicompost gave a germination index for cress (Lepidium sativum) of 80% after 2 months while the earthworm production increased 1.2-fold and volatile solids decreased five times. In addition, the vermicompost contained less than 3 CFU g^–1Salmonella spp., no fecal coliforms and Shigella spp. and no eggs of helminths. Concentration of sodium was 152 mg kg^–1 dry compost, while concentrations of chromium, copper, zinc and lead were below the limits established by the USEPA.