Results and discussion Physico chemical characteristics of

3.3. Effect of OLR on the production of intermediate metabolites during co-digestion
VFAs and NH3 are key intermediate metabolites that are produced during AD. VFAs including acetate, propionate, butyrate, iso-butyrate, valerate, and iso-valerate, are both products of hydrolysis and acidogenesis as well as substrates of methanogenesis. Stable concentration of VFAs indicates a correct balance between hydrolysis/acidogenesis and methanogenesis. For stable AD, the concentration of VFAs stabilizes at a lower value, since the VFAs produced from hydrolysis and acidogenesis can be consumed by KY02111 in time. VFAs inhibit methane production when excessive accumulation occurs.
Ammonia originates from the degradation of proteins, peptides, and amino acid. Ammonia is an important nitrogen source for the growth of biogas-producing microorganisms and is a key pH-stabilizing agent for the neutralization of VFAs. However, high concentrations of ammonia would inhibit methanogens. Analysis of the ammonia concentration is shown in Fig. 5(b). During the early days of digestion, the consumption of ammonia by the growing microorganisms exceeded its formation by the degradation of protein, which resulted in a net decrease in the ammonia concentration. In the late fermentation stages, the microbial growth reached a stable phase, whereas the proteins continued to degrade. Therefore, the ammonia concentration increased gradually. The maximum NH4+-N concentration was 1200 mg/L, which was much lower than the inhibition level of 3860 mg/L (Benabdallah El Hadj et al., 2009).