Evaluation of the co-digestion of fruit and vegetable residues with cow and pig manure

Now a day the garbage production, like fruit and vegetable residues is improving, we tested the co-digestion of both kind of residues with cow and pig manure to produce methane and an organic fertilizer. Residues were mixture with 20% cow or pig manure at 7% solids, one series was adjusted to pH 8; digesters were maintained in greenhouse during two months. Weekly were analyze pH, electrical conductivity, biogas and methane; every three weeks were analyzed the germination index using lettuce seeds.

Using vegetable residues and pig manure was produced more methane than co-digestion with cow manure, it improve in these digesters adjusted to pH 8 (43% methane was produced). The germination index was higher too. This could be explained because the pH during all the digestion process was between 6.5 and 8, such a way the methanogenic bacteria was not inhibited and the decomposition process was complete.  

Whereas using  fruit residues the pH diminish drastically lower then 6, even in those adjusted the initial pH to 8; this condition affect the methanogenic bacteria such a way the methane production was less than 10%, the decomposition process was incomplete and the germination index was lower than 80%, indicating that the digestate was phytotoxic.

Co-digestion of vegetable residues with pig manure, could be used to produce methane as an alternative energy source and to produce an organic fertilizer. The most important condition is the pH, it has to be hold up than 6.5.