The influence of amino acids structure on their anaerobic digestion and the strategy to enhance biotransformation of refractory ones

Abstract Protein is an important component of organic waste, and it is first hydrolyzed to amino acids (AAs) during anaerobic digestion (AD). Until now, however, the effect of AAs structure on their AD has never been documented. In this paper the influence of AAs structure on their two-phase AD and the strategy to enhance biotransformation of refractory ones were investigated. Firstly, the AD performance of nineteen water-soluble AAs was compared. The methane yield of polar AAs was higher than that of non-polar ones (162.3–308.1 verse 10.7–140.5 mL/gCODadd) except glycine (284.1 mL/gCODadd). Further studies showed that the order of methane production with polar AAs, according to their carbon number, was 4-C   Alkyl AAs > Ph AAs. The mechanism investigation revealed the relationship between the structure of AAs and the amount of VFAs excluding propionate was consistent with the influence of AAs structure on methane yield. The microbes in AD system of refractory AAs had higher cell surface hydrophobicity (CSH) and lower cell membrane permeability (CMP), which led to lower enzyme activities and amino acid degradation as well as VFAs generation, and thus less methane production. Finally, the strategy to increase the biotransformation of refractory AAs was studied, and the methane yield was increased up to 139.8% by the application of rhamnolipid to change CSH and CMP in the digestion system. These findings elucidated the relationship between AAs structure and AD performance and provided new ideas to improve the biotransformation of refractory AAs.