The cumulative biogas production from the pretreated rice straw after 60 days of incubation is presented in Table 5

The cumulative biogas production from the pretreated rice straw after 60 days of incubation is presented in Table 5. Retention time, pressure and moisture level were considered as pretreatment variable parameters, and the effects of the pretreatments were evaluated using the accumulated methane production as the response variable. The initial production rates were determined as the average of the methane production per day during the first ten days of incubations, and are shown in Table 5.
The amount of produced methane by inoculum was deducted from the total of produced methane from each sample. The highest initial production rate was after pretreatment of rice straw in the cycle 10 bar-10 min with an increase of 59% from 92 to 146 N ml/g VS and followed by 144 N ml/g VS, compared with that of the unpretreated (Fig 1c). The unpretreated rice straw resulted in almost the same initial production rate in methane production as compared to some the pretreated rice straw in the blank moisture with low pressure and time (Fig. 1a). The highest initial production rate was obtained in pretreated rice straw in the cycle 15 bar-10 min with moisture 35%, with an increase of 166% from 92 to 245 N ml/g VS followed by 244 N ml/g VS (Fig. 2c). The maximum initial production rate was after the pretreatment of rice straw in the cycle 10 bar-15 min with moisture 70% with an increase of 141% from 92 to 222 N ml/g VS followed by 221 N ml/g VS (Fig. 3b).
The highest methane production was obtained for pretreated rice straw in the cycles 10 bar-10 min and 15 bar-5 min with blank moisture 418 and 370 N ml/g VS compared with that of the unpretreated rice straw 196 N ml/g VS (Table 5) and this means increased by 114% of the methane yield. The pretreated rice straw in the cycles 15 bar for 15-10 min with moisture 35% was obtained the highest methane production 399 and 387 N ml/g VS, respectively. Methane yield increased by 196 to 399 N ml/g VS, and this means increased by 104% compared to the unpretreated rice straw. The highest methane production was reached for the pretreated rice straw after pretreatment in the cycles 15 bar for 10-5 min with moisture 70% to 496 and 450 N ml/g VS, respectively (Fig. 3c). The methane yield of rice straw increased by 196 to 496 N ml/g VS, and this means increased by 154% compared to the unpretreated sample.
According to the improvement of methane production yield, Structural modification of rice straw was performed by steam explosion pretreatment, and methane production in primary days of the anaerobic digestion process was increased for the pretreated rice straw. According to the initial production rate of pretreated rice straw in the cycles 10 bar-10 min, 15 bar-15 min with moisture 35% and 15 bar-10 min with moisture 70%, were about 35%, 57% and 37% of methane production yield occurred in 10 days of incubation, respectively.
The lowest initial production rate was observed, in the cycles 5 bar-15 min with blank moisture, leading to a methane level of 91 N ml/g VS than to the unpretreated rice straw 92 ml/g VS (Fig. 1b). The lowest methane production was in the cycle 10 bar-15 min with blank moisture, leading to a methane production of 178 N ml/g VS that the less was than to the unpretreated rice straw 196 ml/g VS (Fig. 1d). The lowest initial production rate and methane production was observed, for pretreated rice straw in the cycle 5 bar-1 min with moisture 35%, leading to a methane production of 166 and 267 N ml/g VS compared with that of the unpretreated 92 and 196 ml/g VS, respectively (Fig. 2a and 2b). Pretreated rice straw in the cycle 15 bar-10 min with moisture 70% resulted in the lowest initial production rate of 180 N ml/g VS (Fig. 3c). The lowest methane production was in cycle 5 bar-5 min with moisture 70% leading to a methane production of 272 N ml/g VS (Fig. 3a).
Parameters effects, confidence intervals of steam explosion pretreatment were calculated on the anaerobic digestion experiments. The response surface after steam explosion pretreatment was with two levels of each parameter. The parameters were used as covariates to be able to measure the size of the effects of the pretreatments. The parameters were considered significant when the probability (p-value) was less than 0.05. The ANOVA analysis of the response surface design model (Table 6) showed pressure and moisture as significant factors in increasing methane yield, while the time was not statistically significant (fig. 4).
According to the results of fits and diagnostics for unusual observations, two pretreatments in the cycles 10 bar-10 min and 15 bar-10 min with the moisture level of the blank were the best and worst pretreatment conditions considering standard residual 2.50 and -2.91 and fit 413.6 and 447.5 respectively.