Immobilized oleaginous microalgae as effective two-phase purify unit for biogas and anaerobic digester effluent coupling with lipid production
Graphical abstract
Introduction
Currently, the environmental concerns and the increasing energy demand have stimulated scientists to find for environmentally friendly and renewable energy. Biogas has been considered as suitable and renewable source of energy because it is less harmful to the environments. Biogas produced from anaerobic digestion process is mainly composed of methane (CH4, 40–75%), carbon dioxide (CO2, 25–60%) with other minor components such as hydrogen sulfide (H2S, 0.005–2%), nitrogen (N2, 0–2%), oxygen (O2, 0–1%) and ammonia (NH3, <1%) (Posadas et al., 2015). The composition and concentration of biogas vary depending on its source. To be effective in energy conversion, the methane content in biogas should be upgraded higher than 90% before combustion (Zhao et al., 2015). Physical absorption, chemical conversion, membrane separation, pressure swing adsorption, and cryogenic separation have been applied for biogas upgrading process. However, these technologies are costly and effective only removing CO2 from biogas not converting (Meier et al., 2015). Photosynthetic CO2 fixation by microalgae is an alternative way to remove CO2 from biogas (Posadas et al., 2015). Microalgae have attracted increasing interest in removing CO2 from biogas because CO2 in biogas is converted into microalgal biomass through photosynthesis and high-value products such as lipids, pigments and protein can be generated (Meier et al., 2015). Previous studies have demonstrated the feasibility of utilizing biogas as carbon source for cultivation of microalgae (Posadas et al., 2015, Zhao et al., 2015, Meier et al., 2015, Srinuanpan et al., 2017, Srinuanpan et al., 2018). These strategies have successfully attained methane content more than 95% (v/v) with a low O2 content of 0.1–2.0%. The upgraded methane content meets the standard (>90%) for efficient combustion with an increased calorific value (Zhao et al., 2013). Interestingly, previous studies (Srinuanpan et al., 2017, Srinuanpan et al., 2018) found that two oleaginous microalgae, marine Chlorella sp. and Scenedesmus sp. had high potential to upgrade biogas and simultaneously accumulate lipids higher than 27% of their dry biomass. The fatty acid profiles of extracted microalgal lipids were mainly long-chain fatty acids (C16-C18 > 90%), indicating their potential use as biodiesel feedstocks. These results indicate that the oleaginous microalgae are not only able to remove CO2 from biogas but also generate the 3rd generation renewable biofuels.
The palm oil industry is one of the vital agro-industry in Thailand, which is the third largest palm oil producer in the world. In the process of crude palm oil (CPO) production from fresh fruit bunches, palm oil mill effluent (POME) was generated about 2.5 tons per ton of crude oil processed mainly from the process of sterilizing the fruits, extraction and clarification of oil (Pechsuth et al., 2001). POME is a thick brownish viscous liquid waste and characterized by high organic and inorganic contents that cause environmental concerns. POME normally is used as feedstock for biogas production through anaerobic digester. As the effluent from anaerobic digester still contains high levels of carbon, nitrogen and phosphorus, it needs to be treated in a secondary treatment system. On the other hand, they are suitable and cost-effective for microalgae cultivation. Several researches have indicated that the microalgae could be one of the candidates for treating effluent, especially secondary effluent due to their capability of converting the organic and inorganic compounds in the effluent into their biomass (Cheirsilp et al., 2017b, Xie et al., 2018). However, as the dark color of the effluent decreased the light penetration and resulted in low microalgal biomass production (Ding et al., 2016) the appropriate dilution of the effluent and suitable light illumination should be determined to achieve the high productivity of the microalgal biomass.
In recent years, with the aim of simplifying recovery of microalgal biomass for biofuel production, the use of immobilized microalgae has gained increasingly attentions. The immobilized microalgae can be recovered by a simple separation method (e.g. sieving) which requires low energy input. This technique also increases the retention time of microalgae cells within bioreactors and promotes the metabolism of the microalgae. Gel entrapment method is one of the most common ways to immobilize microalgae cells, in which natural polysaccharides such as agars, carrageenans and alginates are preferably used due to their low toxicity and high transparency (Lam and Lee, 2012). Alginate is commonly used for immobilization of microalgae and it also maintains the high viability of cells for extended periods of time (Ruiz-Marin et al., 2012) and recently the re-calcification of alginate beads has been proven to strengthen the beads (Lam and Lee, 2012, Castro-Ceseña and Sánchez-Saavedra, 2015). Moreover, it has been reported that the immobilized microalgae could also be cultivated in non-sterile secondary effluent (Covarrubias et al., 2012). Although there are several reports on immobilization using alginate beads for pollution control, none of them integrate biogas purifying ability and lipid producing ability of the microalgae.
This study aimed to evaluate the feasibility of using immobilized oleaginous microalgae for purification of biogas and phytoremediation of anaerobic digester effluent in one unit. The suitable bead volume and inoculum size were determined. The optimal effluent concentration and light intensity on pollutants removal, biogas upgrading, and lipid production, were investigated. Furthermore, the fuel properties of the microalgal lipids were also evaluated.
Section snippets
Microalgae strain and culture medium
The microalgae strain used in this study was oleaginous Scenedesmus sp. from Bioprocess Engineering Laboratory at Prince of Songkla University, Thailand. The modified Chu13 medium used as the basic medium for this study consisted of 0.8 g KNO3 as a nitrogen source, 0.04 g K2HPO4 as a phosphorus source, 0.1 g citric acid, 0.01 g Fe citrate, 0.1 g MgSO4·7H2O, 0.036 g NaHCO3, and 1 mL of trace metal solution per 1 L. The trace metal solution consisted of 2.85 g H3BO3, 1.8 g MnCl2·4H2O, 0.02 g ZnSO4
Effect of initial cell concentration
Among available, the oleaginous microalga Scenedesmus sp. has been selected as the most potential strain for removing CO2 from biogas and simultaneously producing lipids (Srinuanpan et al., 2017). In this study, to increase the performance of the microalgae and simplify the harvesting process, Scenedesmus sp. was immobilized in alginate gel beads and used for biogas purification. The results were compared with those using free cells. The immobilized microalgae cells at low initial cell
Conclusions
The immobilized oleaginous microalgae could be used as an effective two-phase purify unit for biogas upgrading and phytoremediation of anaerobic digester effluent. Through this strategy, the CO2 in biogas was effectively removed and the methane content was upgraded to be >95%. The immobilized microalgae also simultaneously removed nitrogen and phosphorus in the effluent. Moreover, the immobilized microalgae could be harvested by a simple sieving method without involving huge amounts of energy
Acknowledgments
This work was supported by Prince of Songkla University and the Southern Palm Oil Co., Ltd. The first author was financially supported by Thailand Research Fund (TRF) under the Research and Researchers for Industries (RRI) project (Grant No. PHD 58I0030) and Energy Conservation Promotion Fund. The second and fourth authors are supported by Thailand Research Fund under Grant No. RTA6080010.
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