Greenalgae as a substrate for biogas production - cultivation and biogas potentials

Algae is regarded as a good potential substrate for biogas production, due to high cells productivity, low cellulose and zero lignin content. Two parts were included in this study: first, cultivations of micro-algae (Chlorella sorokiniana and Tetraselmis suecica) at two different nitrate concentrations, also the effect of addition of CO2 on algae grow was investigated in this first part. Second, batch fermentations of the cultivated micro-algae as well as a powder Chlorella (obtained from Raw Food Shop) and a dry mix filamentous algae (collected in the pounds in the park at the back of the Tema-building and then dried) were performed. In this part also effects of thermo-lime pretreatment (room temperature, 80oC, 105oC and 120oC) on the algae biogas potentials was investigated. Both strains of micro-algae cultured at low nitrate gave more CH4 yield: 319 (±26) mL and 258 (±12) mL CH4 per added gVS was obtained during the degradation of Chlorella sorokiniana grown at 0.4mM-N and 2mM-N level, respectively. For Tetraselmis suecica 337 (±37) mL and 236 (±20) mL CH4 per added gVS was obtained at 2.4mM-N and 12mM-N level, respectively. Powder Chlorella gave the highest biogas production (719 ±53 mL/added gVS) and CH4 yields (392 ±14 mL/added gVS), followed by the dry filamentous algae (661 ±20 mL biogas and 295 ±9 mL CH4 per added gVS) and Tetraselmis suecica (12 mM-N; 584 ±7 mL biogas and 295 ±9 mL CH4 per added gVS). A negative effect of lime treatment at room temperature on CH4 yield of algal biomass was obtained. Lime treatment at 120oC showed the fastest degradation rate for Tetraselmis suecica and powder Chlorella during the initial 5 days of incubation. Chlorella sorokiniana and Tetraselmis suecica cultures flushed with biogas containing 70% and also CO2 enriched air (5% CO2) did not increase cells growth (measured as OD600) if compared to references grown under air. On the contrary, a clearly inhibition effect on the algal cells growth was observed in some cultures