Management of hydrogen sulfide in anaerobic digestion of enzyme pretreated marine macro-algae

Enzymatic pretreatment of algae by means of cellulose degrading enzyme was evaluated through lab-scale and pilot-scale experiments. The degradation efficiency of the enzyme depended on the initial physical quality of the algae. Lab-scale batch anaerobic digestion experiments showed comparatively low methane potential for the pretreated algae at both mesophilic and thermophilic temperatures. However, the raw algae (cut into small pieces) were found to be hardly hydrolysable. The methane potential of raw algae in thermophilic and mesophilic digestion was about 17 NmL/g VS and –36 NmL/g VS respectively. Presence of inhibitory agent(s) was obvious at both temperatures. Very fast growth of sulfate-reducing bacteria was noticed in the continuous digestion, so that in less than 20 days, hydrogen sulfide concentrations over 10000 ppm were observed in both meso- and thermophilic reactors. Inhibition of methanogenesis in the thermophilic reactor occurred at unionized dissolved sulfide concentration of about 22 mg/L (10000 ppm in the biogas) while it was mainly non-SRB acetogens that were inhibited in the mesophilic reactor at unionized sulfide concentrations as high as 50 mg/L (17000 ppm in the biogas). This shows that probably thermophilic digestion is more prone to be inhibited at high sulfide concentrations. Micro-aeration was found to be more efficient in the thermophilic reactor while its effect on the mesophilic process was negligible. (Less)