3.2. CH4 production
The cumulative CH4 yield of blue algae inoculated with swine
manure or granular sludge, as a function of time under different
ISRs, are shown in Fig. 1. The contribution of background CH4
production by inoculum of swine manure or granular sludge was
deducted from the entire cumulative CH4 yield of codigestion.
The cumulative CH4 yield after 22 days was highest for codigestion
of blue algae with swine manure at ISR of 2.0, with the value of
212.7 mL g1 VS. As can be seen from Fig. 1, the cumulative
CH4 yield of blue algae during codigestion increased from 48.2 mL g1 VS to 212.7 mL g1 VS when the ISR increased from
0.5 to 2.0, representing a 341.3% increase in CH4 conversion efficiency.
However, when ISR reached 3.0, the volume of CH4 produced
decreased to 190.3 mL g1 VS, showing the oversaturation
of the inoculum. The same conclusion was previously achieved
by other researchers using different substrates [35]. In order to
evaluate the methanogens efficiency thoroughly, the experimental
data obtained from the cumulative CH4 production were fitted to
the modified Gompertz equation with R2 > 0.97. The regression
results exhibited in Table 2 showed that Pmax, Rmax and k were all
dependent on the ISR. The Pmax value predicted from the modified
Gompertz equation seemed to be slightly higher than those of the
experimental cumulative CH4 yield. The Pmax and Rmax reached the
highest level of 219.99 mL g1 VS and 19.44 mL g1 VS d1 at ISR
2.0, demonstrating the optimized inoculum substrate ratio for
the codigestion of blue algae with swine manure. Interestingly,
the k value showed the similar trend with Pmax, with the highest
value of 1.61 d at ISR 2.0. This might be because the microorganisms
in the inoculum needed a period to adapt a new environment
when they are transferred to the new condition [36].
Comparing to the codigestion of blue algae with swine manure,
digestion of blue algae inoculated with granular sludge showed less
effective in CH4 production. The cumulative CH4 yield of blue algae
increased from 32.8 mL g1 VS to 73.5 mL g1 VS when the ISR
increased from 0.5 to 3.0 during digestion. Pmax had the also similar
tendency as that obtained in the experiments. With the increase of
ISR from 0.5 to 3.0, Rmax, increased from 2.32 mL g1 VS d1 to
7.40 mL g1 VS d1. The k ranged from 0.04 d to 0.11 d with the
shortest lag phase time at ISR 0.5. The lower CH4 production rate
might be due to the inferior hydrolysis efficiency derived from the
barriers of algae cell, which will be discussed as follows.