appropriate interval for methanogenic activity
(12)
. Lowest pH
values were observed in set 3 due to operational problems and
high VFA concentrations produced by higher OLR. High pH values
and total alkalinity due to high TAN concentration provided
enough buffering capacity. Therefore, no inhibitory effect of acid-
i
fi
cation was observed in RA and RM during the whole operation
period.
In set 4 and set 5, the TS content of the chicken manure was
increased to 5.5% and 8.25%, respectively. High TS contents
resulted in mixing and heating problems in RA with the magnetic
hot plate stirrer. Therefore, mixing of RA was maintained by an
automatic shaker to overcome this problem. However, the in-
stabilities in the acid reactor did not affect the operation of the
methanogenic reactor, but total biogas production decreased.
When the system recovered, the biogas production improved
again. Although, VFA concentration was higher than the normal
range for methanogens, signi
fi
cant biogas productions were
achieved without any inhibition and with pH ranging from 7 to 8.
This shows that the mesophilic acidogenic-thermophilic meth-
anogenic anaerobic digestion of chicken manure can proceed
successfully in two stage systems.
The study concludes that it is possible to produce high biogas
yields from chicken manure at TS loadings higher than 5%, by using
a two stage mesophilic acidogenic
e
thermophilic methanogenic
anaerobic digestion system without using any other carbon rich
wastes and without using any chemical additives to control pH. The
stage separation was bene
fi
cial for higher biogas conversion rates
(426
e
554 mL/g VS
feed
) at lower HRT (12 days) values and at high TS
(3.0
e
8.25%) values. Higher biogas yields were achieved than the
previous studies using higher HRTs (20
e
30 days) and lower OLRs.
Methane content of the two stage system together were 74% in
average that is slightly higher than the previous studies. Acclima-
tization of reactors to the substrate and TAN concentration as high
as 3500 mg NH
4
e
N/L resulted in more stable biogas production.