Thermophilic and mesophilic methane production from anaerobic degradation of the cyanobacterium Spirulina maxima

Abstract Methane production from fermentation of the cyanobacterium, Spirulina maxima , as the sole substrate was investigated in 200 mL working-volume anaerobic digesters maintained at 35 and 55°C. Digesters were fed once-per-day with a feed concentration ( S t 0 ) of 22.5 g volatile solids (VS) per liter, at retention times (θ) of 8, 12, and 16 days. Digester contents were mixed for one min before and after feeding. After 3 volume turnovers, effluent samples were obtained on four consecutive days. Methane production rate (L CH 4 /L-day) and methane yield ( B ) (L/g of COD fed) at 35°C were 0.47 and 0.09, 0.41 and 0.15, and 0.31 and 0.15 at the respective θ of 8, 12, and 16 days; at 55°C they were 0.20 and 0.05, 0.31 and 0.11, and 0.19 and 0.09, respectively. The ultimate methane yield ( B 0 ) after 105 days of batch fermentation was 0.22 L CH 4 per g COD fed (0.33 L CH 4 /g VS fed). COD degradation at these retention times and temperatures was between 23 and 40%, ammonia nitrogen between 1.12 and 1.86 g/L, and alkalinity between 7.0 and 7.8 g CaCO 3 /L. The concentration of total volatile acids at 35°C was 4.07, 2.58 and 3.13 at θ = 8, 12, and 16 days, respectively; at 55°C they were 6.78, 4.18, and 4.07, respectively. These results indicate that the biomass of S. maxima can be used as a sole nutrient for methane production at mesophilic and thermophilic temperatures. However, the methane production rates at these retention times are higher at the mesophilic temperature. These rates are significantly lower than those obtainable with cattle or swine wastes because greater maximum loading rates can be obtained with these wastes before digester failure occurs.