ABSTRACT
Digestate is the residual by-product from anaerobic digestion. It is nutrient-rich and suitable for use as a fertiliser/soil amendment. However, the quality, safety and utility of digestate are dependent upon chemical composition and microbial community. This study was aimed at determining time and temperature required for the effective treatment of digestate used as a safe biofertilizer. Samples of cattle rumen digestate slurry was obtained from National Centre for Energy and Environment, University of Benin, Benin City, Edo State, Nigeria. The samples were treated with different temperatures of 28±2, 55, 60, 70 and 80oC at different exposure duration of 0, 20, 30, 40 and 60 mins. Microbiological, physicochemical and heavy metal analysis was carried out on the samples using standard methods. Bacterial isolates from the samples were screened for rhizobacterial potential. The results showed that digestate treated at 70 oC for 40 min had bacterial counts of 8.05 ± 0.71x102 cfu/ml, which fell below permissible limits of 10 3 cfu/ml. Salmonella and Escherichia counts were not observed at this temperature and exposure duration. The identified bacterial isolates were Pseudomonas sp., E. coli, Proteus sp., Enterobacter sp., Citrobacter sp., Klebsiella sp., Bacillus sp., Enterococcus sp., Streptococcus sp., Yersinia sp., Salmonella sp. and Shigella sp. The fungal isolates were Fusarium sp., Aspergillus fumigatus, Bipolaris sp., Penicillium sp., Aspergillus sp., Trichoderma sp. and Rhizopus sp. Temperature and time did not significantly influence the physicochemical parameters (pH, EC, TAN, TKN, NIT, TP and TK) of the cattle rumen digestate slurry (P>0.05) while the heavy metal (Zn, Cr, Cu, Ni, Pb and Cd) were within the acceptable limit of Test Method of Examination for Composting Compost. Screening for rhizobacterial potential revealed Pseudomonas sp., Proteus sp., Bacillus sp. and Streptococcus sp. to be positive for phosphate solubilization, nitrogen fixation, ammonia production and induction of indole acetic acid. It was concluded that treating digestate at 70 oC for 40 min confirmed the product to be safe and can be applied as biofertilizers.
