Effect on substrate temperature
The mean substrate temperature during digestion was 28.5 ± 2.5 oC. This temperature was within the mesophilic range of 25-35 oC considered optimal for the support of biological-reaction rates (Tchobanoglous et al., 2003). It was observed that MR 0:1 and 1:1, which had lower average temperatures (Table 3), had longer digestion times
and significant (p 0.05) correlation between substrate temperature and biogas production (Table 4). The variation of substrate temperature with time, as shown in Fig. 1, indicated that temperatures of MRs 3:7 and 7:3 increased gradually, while MRs 1:0, 0:1 and 1:1 had fluctuations of ±0.50, ±1.03 oC and ±1.50 oC, respectively during the digestion period. The non-significant (p > 0.05) correlations between ambient and substrate temperatures (Table 4) showed that there was no heat exchange through the digesters wall. Also, there was no significant (p
> 0.05) cor relation between substrate temperature and pH in any of the treatments (Table 4).