Composting of tannery effluent with cow manure and wheat straw

Wastewater from the leather industry in León (Guanajuato, México) is discharged into the Turbio river without treatment. Tannery wastewater contains utilizable nutrients, but also toxic organic compounds which might affect soil processes and plant growth, and pathogens, which might pose a threat to the local farming community. Tannery effluent was composted with cow manure and wheat straw for 90 days to reduce pathogens and toxic organic compounds and monitored. The compost was characterized by an electrolytic conductivity (EC) of 28.1 mscm^-1, cation exchange capacity of 17.7 meq 100 g^-1, an absorbance at 645 nm of 0.0175, a respiration rate of 0.062 mg CO₂-C kg^-1 compost-C day^-1, pH 8.5 and C:N ratio 7:1 with a germination index for cress (Lepidium sativum) of 48% after 90 days. Less than 10 faecal coliforms and no Salmonella sp., Shigella sp. or eggs of helminthes were detected in the compost while total coliforms decreased by log₁₀ of 2. Total concentrations of lead (Pb) were 8.9 mgkg^-1 dry compost, chromium (Cr) 77 mgkg^-1 dry compost, cadmium (Cd) 0.4 mgkg^-1 dry compost, copper (Cu) 10.3 mgkg ^-1 dry compost and sodium (Na) 14,377 mgkg^-1 dry compost. The compost characteristics indicated that it was mature, but the germination index for cress of less than 50% suggested possible remaining phytotoxic compounds. However, the large salt concentrations (especially Na), might have inhibited cress development and thus reduced the germination index. The large salt concentration might thus limit the use of this kind of compost.