Evaluation of different strategies to maximize biogas production from algae

Abstract

Seaweeds (macroalgae) are a promising substrate for biogas production due to the high percentage of carbohydrates and high growth rate. Therefore, the biogas produced from the anaerobic digestion of seaweeds is a sustainable and renewable alternative source of bioenergy. Seaweeds are available in coastal areas and may also be produced in aquacultures. This work presents results of the biochemical methane potential (BMP) of the wild seaweed, Gracilaria vermiculophylla, as well as the effect of physical and thermochemical pre-treatments on their biodegradability. The codigestion with glycerol and sewage sludge was also studied. The BMP of G. vermiculophylla after a physical pre-treatment (washing and maceration) reached 481±9 L CH4 kg-1 VS, corresponding to a methane yield of 79±2%. Regarding the thermochemical pre-treatments, it was found that the increase of temperature (from 20 to 90ºC), NaOH concentration (from 0.1 to 0.5 g NaOH per g TS) and pressure (from 1 to 6 bar) caused an increase in the seaweeds solubilisation up to 44%. However, the subsequent methane production was not increased as expected, although a faster methane production rate was observed. The co-digestion of G. vermiculophylla with glycerol or sewage sludge has proved to be quite effective for increasing the methane production. Addition of 2% glycerol (w:w) increased the methane production by 18% (599±16 L CH4 kg-1 VS) and methane yield by 22%, achieving almost complete substrate methanation. Moreover, the codigestion of seaweed and secondary sludge (15:85%, TS/TS) caused an increase of 25% in the BMP (605 ± 4 L CH4 kg-1 VS), relatively to the individual digestion of algae. The addition of glycerol in this assay did not cause significant improvements.