Bioconversion of <i>Macrocystis pyrifera to methane</i>

Abstract

Abstract Methane production by anaerobic digestion of kelp is a relatively new approach to augmenting the national gas supply. Digestion of the giant marine kelp Macrocystis pyrifera , investigated in a series of bench‐scale experiments at the Institute of Gas Technology (IGT), is reported. Conducted with both untreated and chemically pretreated plants, the tests have evaluated the effects of major limits on bioconversion of kelp, including temperature, indigenous salt concentration, external nutrients, inocula source, loading, residence time, manner of mixing and frequency of feeding. A detailed chemical characterisation of the kelp samples is presented. Raw‐kelp digestion was not limited by nitrogen or phosphorus. Temperature ranges of 35–40°C and 50–55°C were found to be optimum for mesophilic and thermophilic digestion of raw kelp. The mesophilic methane yield in the semicontinuous mode was about 4.5 SCF/lb (280.94 cm 3 g −1 ) of volatile solids (VS) added compared with a thermophilic methane yield of 2.4 SCF/lb (149.83 cm 3 g −1 ) of VS added. The mesophilic organisms could be acclimatised to a salt concentration that has a conductivity of 43 000 μmho cm −1 without any deleterious effect. Thermophilic methanogens were more sensitive to high salt concentrations than the mesophilic organisms. Raw kelp had an estimated ultimate biodegradability of 80%. Mannitol and algin were the most biodegradable components.