Bioconversion of sewage sludge and industrially produced woodchips

Platinum mines produce large amounts (750 m(3) per month) of woodchips as by-product during ore extraction, posing environmental hazards but also a possible source of organic material to be applied in revegetating tailing dams. The aims of this study were to assess the effectiveness of different bioconversion strategies viz. composting, vermicomposting and a commercial microorganism inoculant (EM). Woodchips (WC) and sewage sludge (SS) with a mixing ratio of 3: 1 were composted and vermicomposted for 112 days and the generated products evaluated in terms of their chemical and physical characteristics. Despite low temperatures (33degreesC) in all the treatments, no Escherichia coli or Salmonella was detected in any of the end-products. No significant (P > 0.05) difference in the mean percentage change of C in the different bioconversion strategies was observed and it was only in the SS+WC and SS+WC+EM groups that the soil available P (P-Bray 1) increase was statistically significant (P < 0.05). Treatments containing SS showed significant (P < 0.05) decreases in NH4+, whereas NO2- and NO3- increased significantly (P < 0.05). The NH4+:NO3- ratios in the treatments containing SS were lower than 0.16, ranging from 0.011-0.0016, which is an indication of the maturity of the compost. Total solids and ash contents showed an increase, while the volatile solids and the lignin decreased, but it was only in the vermicomposted treatments that these changes were statistically significant (P < 0.05). The percentage neutral detergent fibre (%NDF) and %cellulose decreased significantly (P < 0.05) in all the treatments containing SS and particle size analysis indicated higher reductions in the vermicomposted treatments. It is concluded that vermicomposting of industrially produced woodchips and sewage sludge is more effective than composting and that the addition of a microbial inoculant did not have a significant effect on the decomposition process.