The problem of pollution resulting improper management of municipal solid waste (MSW) in Nigeria needs to be eliminated by converting MSW to useful resources. In this study, we conducted lab-scale anaerobic digestion of OFMSW to optimize substrate concentration required to maximize biogas yield under wet ambient condition. After characterization, various concentrations of the substrate (OFMSW) ranging from 0% (wet process) to 45% (dry process) was subjected to One-Factor response design (using Design Expert version 9.0) as well as anaerobic digestion (using rumen juice as the source of microbial inoculum) inside one-stage 500 ml-capacity batch-type anaerobic digesters with useful volumes of 350 ml. Result showed that the highest and lowest volume of cumulative biogas production (596.4 ml and 107.6 ml) was recorded in the experimental set-up with 30% and 5% substrate respectively after 42days. However, the highest biogas yield (8.51 ml/gr. VS) was recorded in the experimental set- up with 5% substrate followed by the experimental set-up with 30% substrate (7.86 ml/gr. VS), while the lowest biogas yield (0.96 ml/gr. VS) was recorded in the experimental set-up with 45% substrate. Analysis of the response surface design showed that the optimum substrate concentration required to maximize biogas yield (~ 8.66 ml/gr. VS) in the wet process under ambient (lab) condition was approximately 5.52%. Confirmatory test for anaerobic digestion of the predicted optimum substrate concentration (5.52%) produced an average biogas yield of 7.03+1.453 ml/gr. VS. This result suggests that the true biogas yield under this wet process may lie between 5.58 ml/gr. VS and 8.48 ml/gr. VS. Finally, we isolated and identified bacteria species belonging to genera such as Bacillus, Bacteroides, Clostridium, Enterobacter, Escherichia, Lactobacillus, Micrococcus, Morganella, Propionibacterium, Pseudomonas, Providencia, Ruminococcus, Staphylococcus and Streptococcus inside the rumen juice, substrate and composite sample of the digestate respectively.