The energetic exploitation of the organic fraction of municipal solid waste treated in an existing Mechanical Biological Treatment plant was found to be successful by the Solid State Anaerobic Digestion. The amount of inoculum used per tonne of waste for starting the anaerobic process was shown to have a relevant effect on both biogas and biomethane production. For a waste-to-inoculum ratio ranging from 1:1 to 1:3 (w/w), the energy production increased from about 100 kWh/tonne to about 380 kWh/tonne of waste organic fraction. Consequently, the investment costs also rise, going from about 180 epsilon to more than 370 epsilon/tonne of treated municipal solid waste organic fraction. The economic optimization analysis showed that the waste-to-inoculum ratio that minimized the treatment cost, ranged from 1:1.5 to 1:2.5, being strongly influenced by the plant size (i.e. electrical efficiency) and by the Green Certificate value. Considering the cost of producing electrical energy, the optimum ratio was 1:2, leading to a cost of 0.26 epsilon/kWh, during the investment period, and of 0.14 epsilon/kWh after the investment period. (c) 2012 Elsevier Ltd. All rights reserved.