| Cultivation of microalgae as renewable energy source with plentiful nutrients in treated effluents from wastewater treatment plants (WWTPs) has been intensively investigated. Generally, CO2 in treated effluent was a limiting factor for microalgae growth, and CO2 addition as carbon source is effective to enhance the growth. Meanwhile, utilisation of commercial CO2 (CC) raises the cost of the cultivation. Anaerobic digesters emits substantial amount of CO2 in biogas, and it is a way to utilize the CO2 as carbon source to reduce the cultivation cost. In this study, indigenous microalgae were cultivated with treated effluent. Moreover, CO2 in biogas (CB) was purified by newly developed two-stage membrane separation system which consisted of commercial follow fiber membrane modules made from polyimide (NM-B01A, Ube Industries, Japan), and effects of CB utilisation on the cultivation were evaluated by indoor experiments, as shown in Fig.1. Firstly, optimum CO2 injection condition was decided with CC (CO2 content: 99.95%). CC was intermittently injected to three reactors to keep pH of the culture at 7, 8 and 9, respectively. CC was not injected with the other reactor (median pH was 10.8). It was revealed that CO2 injection not only increased biomass production indicated by suspended solids (SS; n=5) but also enhanced energy content indicated by higher heating values (HHV; n=1) to which high carbon and hydrogen content contributed, represented as 163±18 mg/L and 21.0 kJ/g at pH of 8, and 55±10 mg/L and 11.6 kJ/g at 10.8. The suitability of the CO2 addition to keep pH at 8 was suggested.Actual biogas (AB) from a digestion tank passed through the dry desulfurization process in a WWTP was treated by the separation system. CO2 and CH4 contents (n=3) in AB were 39.6±0.6% and 59.9±1.3%, respectively, and H2S (n=2; 2.5±0.8 ppm) was quite low due to the dry desulfurization process. CO2 and CH4 contents (n=3) in CB were 98.3±1.1% and 0.902±0.467%, and high purity of CO2 was obtained. H2S (n=2; <0.2 ppm) was also low.Two comparative cultivation experiments in which CB or CC was intermittently injected to keep pH of the culture at 8 were conducted. High CO2 assimilation ratio by indigenous microalgae (91.8%) was obtained. SS (n=7) in the reactor with CB and CC distributed 166±23 and 176±16 mg/L, respectively, and there was no significant difference between them (p>0.05). The no significant negative effects by CB injection attributed to the low contents of CH4 and H2S, which are known as inhibitors for microalgae growth, due to the membrane separation and pass through the desulfurization process. There were also no significant differences (p>0.05) in not only HHV (n = 3; CB: 21.1±0.46 kJ/g-dry; CC: 21.2±0.90 kJ/g-dry) but also fatty acid methyl ester (FAME) content (n = 3; CB: 40.1±14.5 mg-FAME/g-dry; CC: 48.3±7.1 mg-FAME/g-dry). Moreover, the FAME composition (principal compound: palmitic acid methyl ester) was fairly similar between them. Based on the results, this study revealed that CB utilisation has no significant negative effects on biomass production or their energy content, and its effectiveness for indigenous microalgae cultivation was demonstrated. |
