The GEEB Team of the University of Reunion directed this research towards the optimization of the anaerobic digestion of solid and liquid waste by injection of leachate. The objectives are to assess the performances of this type of process (to accelerate the process of biological breakdown) and to require a better control of the physicochemical mechanisms involved. During anaerobic digestion, the reactional medium desiccates and induces a deceleration of the process. The injection of leachate methanogene makes it possible to humidify the medium and to maintain the medium reactional under ideal physicochemical and biochemical conditions and to bring an additional organic load. In this paper, the optimization of the process of Co-digestion is presented through an experimental and a theoretical study. The experimentation is carried out on 10L laboratory bioreactors and is based on obtaining leachate methanogene by using liquid waste (STEP mud, liquid pigmanure,residuary liquor) and solids. Several parameters are studied: the total volume of the mixture (5 or 10 L), the volume and the state of each waste in the mixture (always a solid waste and a liquid waste). A sample of leachate is taken every 8 days and a liquid addition of waste of the same quantity is carried out in the bioreactor. The experimentation is stopped when it is no longer possible to distinguish waste and leachate. Then, the leachate is characterized from a physicochem ical point of view by spectrometric techniques: Ntotal, AGV, NO2-, NH4+, TOC, PO43-, K+,COD, BOD, DM, VM but also pH. Lastly, they will be reinjected in a Co-digestion. The experimental data obtained will be used as inputs parameters of the numerical mathematical model (biphasic: Acidogenesis and Methanogenesis). Simulations obtained make it possible to follow the behavior of the medium through the organic load (COD, TOC) and the biogas produced with or without addition of leachate