The inhibition of anaerobic digestion by model phenolic compounds representative of those from Sargassum muticum

Practical yields of biogas from the anaerobic digestion of macroalgae and, Sargassum muticum in particular, are substantially below the theoretical maximum. There is considerable conjecture about the reasons for the relatively low practical methane yields from seaweed, and polyphenols are suggested as one of the elements in the low yield of methane from brown seaweeds. However, there appears to be little information on the effect of specific phenolics on defined substrates. This paper examines the effect of some simple phenolic compounds, representative of those reported in S. muticum on methane production from a range of model substrates. Three simple phenolics were selected, gallic acid, epicatechin and phloroglucinol; at four addition levels, 0, 0.5, 3.5 and 7.5% w/w of substrate; for four substrates, a readily digested simple organic substance, glycerol, and three polymers found in seaweed, cellulose, alginic acid and the sodium salt of alginic acid. Alginic acid and its sodium salt were found to be recalcitrant with average methane yields of equivalent to only 23-28% of their theoretical methane potential. Methane yield was further reduced by the presence of high concentrations (7% of substrate equivalent to 17.5mgL(-1)) of phloroglucinol and epicatechin. None of the phenolic compounds studied appeared to inhibit the breakdown of the simple and readily digested compound, glycerol. Low methane yield in seaweed may be due to the recalcitrance of complex hydrocolloids and phenolic inhibition of the breakdown of more complex molecules in the initial hydrolysis stage of anaerobic digestion, but further research is required.