• Energy Research
• 6. Clean water close

Integration of anaerobic digestion (AD) with microalgae processes has become a key topic to support economic and environmental development of this resource. Compared with other substrates, microalgae can be produced close to the plant without the need for arable lands and be fully integrated within a biorefinery. As a limiting step, anaerobic hydrolysis appears to be one of the most challenging steps to reach a positive economic balance and to completely exploit the potential of microalgae for biogas and fertilizers production. This review covers recent investigations dealing with microalgae AD and highlights research opportunities and needs to support the development of this resource. Novel approaches to increase hydrolysis rate, the importance of the reactor design and the noteworthiness of the microbial anaerobic community are addressed. Finally, the integration of AD with microalgae processes and the potential of the carboxylate platform for chemicals and biofuels production are reviewed.

The purpose of the study was comparison of two configurations of photobioreactors an open-type photobioreactor open to atmosphere and a tubular type photobioreactor closed to the atmosphere. Organic matter was fairly removed under both configurations at 50-60% and biomass carbon content on dry weight basis accounted for 45%. Both configurations were able to completely exhaust ammonium, however different mechanism removals were responsible for the different influent loads applied. In terms of nitrogen recovery by biomass assimilation, the open configuration ranged 38-47% whereas the closed type presented 31%. It is worth to mention that nitrification-denitrification was taking place under both photobioreactor configurations. Approximately 80% phosphate removal was achieved regardless the configuration and biomass P content was slightly higher in the closed-type reactor. For nutrient recycling, biomass harvesting is described as the key issue of this technology. Nevertheless, the closed configuration highlighted the great potential of the biofilm formation by retaining 96% of the total biomass produced.

Short-chain fatty acids (SCFAs) are considered building blocks for bioproducts in the so-called carboxylate platform. These compounds can be sustainably produced via anaerobic fermentation (AF) of organic substrates, such as microalgae. However, SCFAs bioconversion efficiency is hampered by the hard cell wall of some microalgae. In this study, one thermal and two enzymatic pretreatments (carbohydrases and proteases) were employed to enhance Chlorella vulgaris biomass solubilization prior to AF. Pretreated and non-pretreated microalgae were assessed in continuous stirred tank reactors (CSTRs) for SCFAs production. Aiming to understand microorganisms' roles in AF depending on the employed substrate, not only bioconversion yields into SCFAs were evaluated but microbial communities were thoroughly characterized. Proteins were responsible for the inherent limitation of raw biomass conversion into SCFAs. Indeed, the proteolytic pretreatment resulted in the highest bioconversion (33.4% SCFAs-COD/CODin), displaying a 4-fold enhancement compared with raw biomass. Population dynamics revealed a microbial biodiversity loss along the AF regardless of the applied pretreatment, evidencing that the imposed operational conditions specialized the microbial community. In fact, a reduced abundance in Euryarchaeota phylum explained the low methanogenic activity, implying SCFAs accumulation. The bacterial community developed in the reactors fed with pretreated microalgae exhibited high acidogenic activities, being dominated by Firmicutes and Bacteroidetes. Firmicutes was by far the dominant phylum when using protease (65% relative abundance) while Bacteroidetes was prevailing in the reactor fed with carbohydrase-pretreated microalgae biomass (40% relative abundance). This fact indicated that the applied pretreatment and macromolecule solubilization have a strong effect on microbial distribution and therefore in SCFAs bioconversion yields.

Anaerobic degradation of enzymatically pretreated Chlorella vulgaris was aimed in an upflow anaerobic sludge blanket reactor (UASB) to evaluate the organic loading rate (OLR) effect on biomass valorization. Low OLRs resulted in high methane yields (171 mL CH4/g CODin) at low hydraulic retention time (HRT of 6 days). Firmicutes (35-43%), Bacteroidetes (17-18%) and Euryarchaeota (11%) dominated at low OLRs, promoting methanogenic activity. On the contrary, the highest OLRs resulted in low methane yield (86 mL CH4/gCODin) with a concomitant short-chain fatty acids (SCFAs) accumulation of 37% SCFAs-COD/CODin. The highest OLR decreased UASB reactor biodiversity, hampering Euryarchaeota population development (2.5%) and boosting Firmicutes (55%) and Proteobacteria (14%). These results demonstrated the suitability of UASB reactor configuration to reach high bioprocess efficiency for both, biogas and SCFAs production, with lower energetic and area requirements than those normally needed in continuous stirred tank reactors.

Abstract The aim of the present study was to compare cyanobacteria strains ( Aphanizomenon ovalisporum , Anabaena planctonica , Borzia trilocularis and Synechocystis sp.) and microalgae ( Chlorella vulgaris ) in terms of growth rate, biochemical profile and methane production. Cyanobacteria growth rate ranged 0.5–0.6 day −1 for A . planctonica , A . ovalisporum and Synecochystis sp. and 0.4 day −1 for B . tricularis . Opposite, C . vulgaris maximum growth rate was double (1.2 day −1 ) than that of cyanobacteria. Regarding the methane yield, microalgae C . vulgaris averaged 120 mL CH 4 g COD in −1 due to the presence of a strong cell wall. On the other hand, anaerobic digestion of cyanobacteria supported higher methane yields. B . trilocularis and A . planctonica presented 1.42-fold higher methane yield than microalgae while this value was raised to approximately 1.85-fold for A . ovalisporum and Synechochystis sp. In the biogas production context, this study showed that the low growth rates of cyanobacteria can be overcome by their increased anaerobic digestibility when compared to their microalgae counterpartners, such is the case of C. vulgaris .

Unspecific microorganisms consortia are normally used in anaerobic biodegradation of solid wastes. However, these consortia can be tuned to optimally obtain determined bioproducts. In this study, high value-added products and biogas were obtained via an innovative two-stage anaerobic bioprocess from microalgae biomass. The anaerobic fermentation (AF) entailed the production of short-chain fatty acids (SCFAs) and subsequently, only the solid spent of AF effluent was valorized for methane production via conventional anaerobic digestion (AD). Applied conditions in AF (25 °C, HRT 8 days) favored Firmicutes predominance (64%) enabling a conversion efficiency of 32.1% g SCFAs-COD/g CODin. Opposite, a wider microbial biodiversity was determined in the AD reactor (35 °C, HRT 20 days), being mainly composed by Firmicutes (28.6%), Euryarchaeota (17.7%) and Proteobacteria (15.3%). AD of the AF-solid spent reached 168.9 mL CH4 /g CODin. Strikingly, operational conditions imposed mediated a microbial specialization that maximized product output.

The techno-environmental performance of a medium-scale wastewater treatment system using high-rate algal ponds was evaluated through mass and energy balances and life cycle assessment. The system involves wastewater primary treatment, microalgae-based secondary treatment, thermal hydrolysis with steam explosion of microalgae, anaerobic co-digestion of pre-treated microalgal biomass and primary sludge, and biogas cogeneration. Furthermore, two scenarios based on alternative biogas uses were considered: (i) biogas for heat and electricity, and (ii) biogas for heat, electricity, and biomethane. Pumping wastewater to the primary settler arose as the main source of electricity consumption. When compared to conventional activated sludge plants, a large decrease in the energy consumption was observed for the secondary treatment. Moreover, a favourable life-cycle performance was generally found for the microalgae-based systems when displacing conventional energy products. Finally, the preference for a specific scenario on biogas use was found to be highly conditioned by the techno-environmental aspects prioritised by decision-makers.

In an attempt to propose an integrated system to treat swine manure, the codigestion of this residue and microalgal biomass was evaluated. Response surface methodology showed that chemical oxygen demand/volatile solids (COD/VS) and COD algae supplement presented a significant effect on methane yield. Second-order polynomial model fitted quite well with the experimental results. Nevertheless, methane yield values achieved were not expected. Highest methane yield was exhibited by swine manure as a sole substrate, while algal biomass digestion reported the lowest. A closer examination to methane production rates showed that methane production was taking place in a higher extent on samples with higher proportion of algae. In agreement with that, nitrogen organic mineralization was low for those trials; hence the recalcitrant nature of the algal cell wall was evidenced. Even though methane production, hence breakage of the cells, was steadily occurring, the need of an algal biomass pretreatment seems to be imperative for the feasibility of this integrated system.