Cristina González-Fernández

Enhancing methane production of Chlorella vulgaris via thermochemical pretreatments

To enhance the anaerobic digestion of Chlorella vulgaris, thermochemical pretreatments were conducted. All pretreatments markedly improved solubilisation of carbohydrates. Thermal treatments and thermal treatments combined with alkali resulted in 5-fold increase of soluble carbohydrates while thermal treatment with acid addition enhanced by 7-fold. On the other hand, proteins were only solubilized with thermo-alkaline conditions applied. Likewise, all the pretreatments tested improved methane production. Highest anaerobic digestion was accomplished by thermal treatment at 120°C for 40 min without any chemical addition. As a matter of fact, hydrolysis constant rate was doubled under this condition. According to the energetic analysis, energy input was higher than the extra energy gain at the solid concentration employed. Nevertheless, higher biomass organic load pretreatment may be an option to achieve positive energetic balances.

Linking microalgae and cyanobacteria culture conditions and key-enzymes for carbohydrate accumulation.

Microalgae are regarded as a potential biomass source for biofuel purposes. With regard to bioethanol production, microalgae seem to overcome traditional substrate drawbacks. Enzymatic activities are responsible for carbon allocation and hence for carbohydrate profiles. Enzyme activities may be manipulated by metabolic engineering; however, this goal may also be achieved by controlling environmental conditions of the culture system. We outline the key-enzymes as well as the main operational conditions applied to microalgae growth (inorganic nutrient supplementation, irradiance and temperature) that affect carbohydrate synthesis on microalgae and cyanobacteria. Normally, harsh conditions are needed for such a goal and thus, arrested microalgae growth may occur. Potential strategies to avoid arrested growth, while enhancing carbohydrate accumulation, were also pointed out in this review.

Nitrogen transformations under different conditions in open ponds by means of microalgae–bacteria consortium treating pig slurry

Four open ponds inoculated with microalgae-bacteria consortium treating different swine slurries (fresh and anaerobically digested) were evaluated in terms of nitrogen transformation under optimal and real conditions of temperature and illumination. Ammonium complete depletion was not achieved. Ponds operated under real conditions presented lower ammonium removal. Elimination capacities were around 26 mg N/Ld and were subsequently increased with increasing inlet ammonium loading rate. Different nitrogen transformation was observed depending on substrate source. When anaerobically digested slurry was fed to the ponds, nitrification followed by biomass uptake and denitrification were the main nitrogen transformation taking place depending on inlet ammonium loading rate and operational conditions. Ponds fed with fresh slurry exhibited denitrification as the main nitrogen removal mechanism for the pond operated under real conditions while under optimal conditions stripping, denitrification and biomass uptake contributed similarly. Therefore, this study confirmed that the so-claimed nitrogen recovery by microalgae biomass is frequently overestimated.

Performance comparison of two photobioreactors configurations (open and closed to the atmosphere) treating anaerobically degraded swine slurry

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.

Microalgae autoflocculation: an alternative to high-energy consuming harvesting methods

With increasing concerns regarding energy and environment, algae biofuel is generating considerable interest around the world. Nevertheless, the harvesting step required before downstream biomass processing is a major bottleneck. Commonly employed methods include addition of chemicals or use of mechanical equipment that increase dramatically the biofuel production cost. This review deals with naturally occurring processes that can help offset those costs by causing microalgae flocculation. Interaction theories are briefly reviewed. In addition, operational parameters such as pH, irradiance, nutrients, dissolved oxygen, and temperature effect on microalgae flocculation are evaluated. Finally, microalgae flocculation is also considered from an ecological point of view by taking advantage of their interaction with other microorganisms.

Anaerobic digestion of microalgal biomass: Challenges, opportunities and research needs

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.

Anaerobic co-digestion of livestock wastes with vegetable processing wastes: A statistical analysis

Anaerobic digestion of livestock wastes with carbon rich residues was studied. Swine manure and poultry litter were selected as livestock waste, and vegetable processing waste was selected as the rich carbon source. A Central Composite Design (CCD) and Response Surface Methodology (RSM) were employed in designing experiments and determine individual and interactive effects over methane production and removal of volatile solids. In the case of swine manure co-digestion, an increase in vegetable processing waste resulted in higher volatile solids removal. However, without a proper substrate/biomass ratio, buffer capacity of swine manure was not able to avoid inhibitory effects associated with TVFA accumulation. Regarding co-digestion with poultry litter, substrate concentration determined VS removal achieved, above 80 g VSL(-1), NH(3) inhibition was detected. Statistical analysis allowed us to set initial conditions and parameters to achieve best outputs for real-scale plant operation and/or co-digestion mixtures design.

Algaculture integration in conventional wastewater treatment plants: Anaerobic digestion comparison of primary and secondary sludge with microalgae biomass

This study evaluated the feasibility of using microalgae biomass as feedstock for anaerobic digestion together with other biomasses (primary and secondary sludge) normally generated in WWTP. Raw microalgae biomass anaerobic biodegradability (33%) was higher than that of secondary sludge (23%). Thermal pretreatment enhanced 62% and 16% methane yield for Chlorellavulgaris and secondary sludge, respectively. When both substrates were codigested, methane yields remained low. On the other hand, primary sludge supported the highest anaerobic biodegradability (97%) and when combined with thermally pretreated C. vulgaris, methane yields were higher (13-17%) than the ones expected theoretically. Despite the high protein content of those substrates and the high nitrogen mineralization, no ammonia inhibition was detected. Thereby, this study showed that algae biomass is a potential cosubstrate for biogas production together with municipal wastewater sludge.

Protease cell wall degradation of Chlorella vulgaris: Effect on methane production

In order to optimize the enzymatic dosage and microalgae biomass loads subjected to enzymatic hydrolysis prior anaerobic digestion of Chlorella vulgaris, organic matter solubilisation and methane production were investigated. Experimental data using protease dosage of 0.585 AU g DW(-1) showed that increasing biomass loads up to 65 g L(-1) did not affect markedly the hydrolysis efficiency (51%). Enzymatically pretreated biomasses subjected to anaerobic digestion enhanced methane production by 50-70%. The attempt of decreasing the enzymatic dosages revealed diminished hydrolysis efficiency concomitantly with a decreased methane production enhancement. In agreement with the good results observed for organic matter conversion into biogas, total nitrogen mineralization was attained for enzymatically pretreated biomass. Despite the high protein content of the biomass and the biocatalyst used in the present study no ammonia inhibition was detected.

Enzymatic pretreatment of Chlorella vulgaris for biogas production: influence of urban wastewater as a sole nutrient source on macromolecular profile and biocatalyst efficiency.

Two biocatalysts, namely carbohydrases and proteases, were assessed for organic matter solubilisation and methane yield enhancement of microalgae biomass. This study evidenced Chlorella vulgaris carbohydrate accumulation (40% on VSS basis) when grown in urban wastewater. Despite of the carbohydrate prevailing fraction, protease pretreatment showed higher organic matter hydrolysis efficiency (54%). Microscopic observation revealed that carbohydrases affected slightly the cell wall while protease was not selective to wall constituents. Raw and pretreated biomass was digested at 1.5 kg tCOD m(-3) day(-1) organic loading rate (OLR1) and 20 days hydraulic retention time (HRT). The highest methane yield (137 mL CH4 g COD in(-1)) was achieved in the reactor fed with protease pretreated C. vulgaris. Additionally, anaerobic digestion was conducted at OLR2 (3 kg tCOD m(-3) day(-1)) and HRT (15 days). When compared to raw biomass, methane yield increased 5- and 6.3-fold at OLR1 and OLR2, respectively. No inhibitors were detected during the anaerobic digestion.