Abstract
Anaerobic co-digestion of pig slurry with four agricultural substrates (tomato, pepper, persimmon and
peach) was investigated. Each agricultural substrate was tested in co-digestion with pig slurry at four
inclusion levels: 0%, 15%, 30% and 50%. Inclusion levels consisted in the replacement of the volatile solids
(VS) from the pig slurry with the VS from the agricultural substrate. The effect of substrate type and inclusion
level on the biochemical methane potential (BMP) was evaluated in a batch assay performed at 35 C
for 100 days. Agricultural substrates chemical composition was also analyzed and related with BMP.
Additionally, Bacteria and Archaea domains together with the four main methanogenic archaeal orders
were quantified using quantitative real-time TaqMan polymerase chain reaction (qPCR) at the end of
the experiment to determine the influence of agricultural substrate on sludges microbial composition.
Results showed that vegetable substrates (pepper and tomato) had higher lipid and protein content
and lower carbohydrates than fruit substrates (persimmon and peach). Among substrates, vegetable substrates
showed higher BMP than fruit substrates. Higher BMP values were obtained with increasing addition
of agricultural substrate. The replacement of 50% of VS from pig slurry by tomato and pepper
increased BMP in 41% and 44%, respectively compared with pig slurry only. Lower increments in BMP
were achieved with lower inclusion levels. Results from qPCR showed that total bacteria and total
archaea gene concentrations were similar in all combinations tested. Methanomicrobiales gene concentrations
dominated over the rest of individual archaeal orders.
