With the information from the first stage was elaborated an experiment with three treatments and three repetitions, were used three bioreactors of 2000 liters each them. The treatments were integrated by three values of duration to acid pretreatment (3, 7 and 10 days) and three values of pH to operation of the bioprocess (4,5 — 5,0; 5,1 — 5,5 and 5,6 — 6,0). The materials used were wastes of different fruits and vegetal from Central Wholesaler of Antioquia. The wastes were

triturated and mixed with water in a relation of 1:2,5. In each test were taken samples of wastes and sludge to determinate the organic load, in addition was recorded daily the pH and the gas production. When the pretreatment of acidification ended, agricultural lime was added like at the first stage. The methodology employed to obtain the quantity of gas generated was the same of the first stage, was used a gas flow meter Metrex G2,5 with accurate of 0,040 m3/h and maximum pressure of 40 kPa. Samples of gas were collected in Tedlar bags and then were analyzed in a chromatographic gas (Perkin Elmer) to determinate its composition.

The organics load of wastes was obtained at the beginning and end of bioprocess; this included the total suspended solids (TSS), total solids (TS), volatile fatty acids (VFAs), chemical oxygen demand (COD) and biochemical oxygen demand (BOD). The analytic method employed was the Standard Method by water and residual water like the first stage. Was calculated the production of gas (liters/day), hydrogen percentage (% de H2) and yield of biohydrogen (liters of H2/day).