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

Tolypothrix , a self-flocculating, fast growing, CO 2 and nitrogen-fixing cyanobacterium, can be cultivated in nutrient-poor ash dam waters of coal-fired power stations, converting CO 2 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 (CH 4) 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 °C for 10 h, (2) hydrothermal by autoclave at 121 °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 h at 10 min intervals with 20 s breaks and (5) freeze-thaw cycles at −80 °C for 24 h followed by thawing at room temperature. Thermal, hydrothermal and sonication pre-treatments supported high solubilization of organic compounds up to 24.40 g L−1. However, higher specific CH 4 production of 0.012 and 0.01 L CH 4 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 CH 4 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 CH 4 yields. Calculated theoretical CH 4 yields based on the elemental composition of the biomass were ~55% higher than actual yields. This highlights the complexity of interactions during AD which are not adequately represented by elemental composition. [Display omitted] • Pretreatment can improve biogas production especially for algae and cyanobacteria. • Sonication showed highest solubilization of organic compounds with a COD of 15.02 g L−1. • Thermal hydrolysis is optimal for Tolypothrix biomass giving high CH 4 yields (126.27 mL g−1 VS removed). • CH 4 yields were high despite low C/N 9.31, high VFA 9.57 g L−1 and ammonium-N 1.08 g L−1 contents. • AD system physico-chemical characteristics foiled investigated prediction accuracy. [ABSTRACT FROM AUTHOR]