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

Abstract

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 Sargassum 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 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 17.5 mg L-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.

Sargassum Inundations in Turks and Caicos: Methane Potential and Proximate, Ultimate, Lipid, Amino Acid, Metal and Metalloid Analyses

“…The recalcitrance of some of the organic polymers within the pelagic Sargassum may be a major reason for the low yields. Alginate and other hydrocolloids are difficult to breakdown without pre-treatment prior to AD [130][131][132]. Buffiere et al [133] found fibrous content of terrestrial wastes to be inversely proportional to BI and MP.…”

“…Phenolics have been implicated as the inhibitor of AD in some seaweed studies [25,130,[148][149][150][151][152]. The phenolic may impede the hydrolysis of complex molecules in the early stages of AD, but this process will depend not only on the phenolic but also the substrate [130,153,154]. Although the information is still somewhat limited, especially on Sargassum, phenolics appear to be a significant factor in the low methane yield from Sargassum and in particular, the mixed mats where the phenolic level was highest.…”

A Brief Review of Anaerobic Digestion of Algae for Bioenergy

“…Phenolic extracts from S. muticum have displayed antimicrobial activity against various aerobic bacteria [190]. High seasonal phenolic levels in Ascophyllum nodosum have been linked to decreased methane yields [191], and when polyphenols were 'fixed' in Ascophyllum nodosum by formaldehyde biogas production was improved [178,192]. Milledge et al [192] found that although phenolic compounds did not appear to inhibit the breakdown of the simple and readily digested compounds, phenols could inhibit the breakdown of more complex molecules in the preliminary hydrolysis stage of AD.…”

“…High seasonal phenolic levels in Ascophyllum nodosum have been linked to decreased methane yields [191], and when polyphenols were 'fixed' in Ascophyllum nodosum by formaldehyde biogas production was improved [178,192]. Milledge et al [192] found that although phenolic compounds did not appear to inhibit the breakdown of the simple and readily digested compounds, phenols could inhibit the breakdown of more complex molecules in the preliminary hydrolysis stage of AD. Similarly, polyphenols showed a negative impact on the degradation of seaweed using microbes from the rumen of sheep ewes [145].…”

A Review of Seaweed Pre-Treatment Methods for Enhanced Biofuel Production by Anaerobic Digestion or Fermentation

“…These differences are also likely to influence the effects of pre-treatments and biofuel yields. Alginic acid and its sodium salt were found to be recalcitrant to AD with average methane yields equivalent to only 23%-28% of their theoretical methane potential, considerably below that of cellulose [69].…”