Biomass pre-treatments of the N2-fixing cyanobacterium Tolypothrix for co-production of methane

Tolypothrbc, a self-flocculating, fast growing, CO2 and nitrogen-fixing cyanobacterium, can be cultivated in nutrient-poor ash dam waters of coal-fired power stations, converting CO2 emissions into organic biomass. Therefore, the biomass of Tolypothrix sp. is a promising source for bio-fertiliser production, providing micro- and macronutrients. Energy requirements for production could potentially be offset via anaerobic digestion (AD) of the produced biomass, which may further improve the efficiency of the resulting biofertilizer. The aim of this study was to evaluate the effectiveness of pre-treatment conditions and subsequent methane (CH4) production of Tolypothrix under out-door cultivation conditions. Pre-treatments on biogas and methane production for Tolypothrix sp. biomass investigated were: (1) thermal at 95 degrees C for 10 h, (2) hydrothermal by autoclave at 121 degrees C at 1013.25 hPa for 20 min, using a standard moisture-heat procedure, (3) microwave at an output power of 900 W and an exposure time of 3 min, (4) sonication at an output power of 10 W for 3.5 hat 10 min intervals with 20 s breaks and (5) freeze-thaw cycles at -80 degrees C for 24 h followed by thawing at mom temperature. Thermal, hydrothermal and sonication pre-treatments supported high solubilization of organic compounds up to 24.40 g L-1. However, higher specific CH4 production of 0.012 and 0.01 L CH4 g(-1) volatile solids(added). was achieved for thermal and sonic pre-treatments, respectively. High N- and low C-content of the Tolypothrix biomass affected CH4 recovery, while pre-treatment accelerated production of volatile acids (15.90 g L-1) and ammonia-N-accumulation (1.41 g L-1), leading to poor CH4 yields. Calculated theoretical CH4 yields based on the elemental composition of the biomass were similar to 55% higher than actual yields. This highlights the complexity of interactions during AD which are not adequately represented by elemental composition.

Automated summary

This study evaluated the effectiveness of pre-treatment conditions and subsequent methane production of Tolypothrix under outdoor cultivation conditions. Results showed that thermal, hydrothermal, and sonication pre-treatments supported high solubilization of organic compounds. However, high N-content and low C-content of the biomass affected methane recovery, and pre-treatment accelerated the production of volatile acids and ammonia-N accumulation, leading to poor methane yields. Calculated theoretical methane yields based on the elemental composition of the biomass were higher than actual yields, indicating the complexity of interactions during anaerobic digestion.