Effect on substrate pH
The range of substrates pH during digestion (6.25-6.98) was within 6-8 considered suitable for bacteria involved in anaerobic digestion. The pH variation was characterized by increase and decrease in pH values (Fig. 2) of the treatments. According to Macias-Corral et al. (2008), decrease in pH could be attributed to hydrolysis of the easily degradable fraction of the manures and conversion to volatile fatty acids (VFA), while the increase could be attributed to subsequent transfer and consumption of VFA by methanogenesis. The pH fluctuations were highest and least in MR 1:0 (±0.54) and MR 3:7 (±0.33), respectively. The pH of MR 0:1 continued to increase throughout the experiment after a temporary decrease during the first week. MRs 3:7, 7:3 and 1:0 had the same pattern of variation during the experiment.