Comparing the performances in various stages, the specific
biogas production was the largest in Stage 2 and least
in Stage 1, while the methane content was the highest in
Stage 1. These results indicate that the addition of garbage
could raise the biogas production and lower the methane
content; the performance of the methane fermentation
could be improved by the co-digestion of the swine manure
and the garbage. The methane content in the swine waste
was higher than that in the presence of garbage probably
due to the higher concentration of ammonia in the swine
waste, as shown in Eqs. (1)–(3). As Fig. 2 depicts, the varying
tendency of ammonia concentration was in agreement
with that of the methane content. In contrast, the removal
of VS and COD was higher with the addition of garbage
than without, because the garbage contained more degradable
organic substances than the swine waste did. The H2S
in the biogas was 2000–3000 ppm throughout all the stages,
similar to the general methane fermentation of the swine
waste (Livestock Industry’s Environmental Improvement
Organization, 2001). Therefore, 99% of the H2S was
removed before it was supplied to the electric generator
in this study.