The conversion of renewable resources into value-added products such as bioenergy is one of the growing concerns of bioeconomy strategy. Within this concept, assessing the proper combination of local wastes has major importance. This study set out to assess the feasibility of using spent tea waste as a single and co-substrate on anaerobic digestion and to explore the influence of the amount of microorganisms on the digester performance. For this purpose, biomethane potentials tests were conducted for seven different mixing ratios of spent tea waste and cow manure on a mass basis. The reactors operated under mesophilic conditions for 20 days with two inoculum/substrate ratios. The results revealed that using spent tea waste as a co-substrate did not reveal a significant effect on biomethane production in the reactors. Contrarily, the amount of inoculum had a remarkable effect on biomethane production, resulted in an increase in methane production between 28 and 32%. While the biomethane yields were in the range of 129-138 mL(N)CH(4)g(VS)(-1)for the co-digesters operated with inoculum/substrate ratio of 1, the range was 165-181 mL(N)CH(4)g(VS)(-1)for the co-digesters operated with inoculum/substrate ratio of 2. These findings represent the potential usage of the spent tea waste as a co-substrate within the sustainable waste management approach and are relevant to plant operators.