Anaerobic digestibility of marine microalgae Phaeodactylum tricornutum in a lab-scale anaerobic membrane bioreactor

Growth of microalgae adapted to high-light temperature conditions in different types of wastewater

To obtain microalgae strains adapted to wastewater in the Mediterranean region, microalgae present in the Nile River were cultivated at both high-light temperature (HLT) and low-light temperature (LLT) conditions. It was found that the species which became dominant under HLT was chlorophyta of the genus Scenedesmus. In contrast, under LLT, bacillariophyta became dominant. The microalgae strain (Scenedesmus arcuatus) was subsequently isolated and cultivated in different types of primary treated wastewater under HLT and LLT conditions. The different types of primary treated wastewater were black water (BW), grey water (GW), and sewage water (SW). Growth rates reached during the exponential phase at HLT using GW, BW, and SW were between 0.38 and 0.4 (day-1). At LLT, 1.5-2.7 folds of lower growth rates were determined due to limitation by CO2. Removal of COD and inorganic N and P from sewage wastewater reached up to 88, 96, and 100%, respectively. Results thus indicate that microalgae adapted to the climate conditions can be efficiently used for COD removal and nutrient recovery from wastewater in the Mediterranean.

Solids retention time dependent, tunable diatom hierarchical micro/nanostructures and their effect on nutrient removal

The hierarchical three-dimensional (3D) micro/nanostructures of diatoms make them a promising biomaterial for fabricating nanomaterials, producing bioactive pharmaceuticals or nutraceuticals, and removing micropollutants. For diatom production in a continuous flow system, little is known how bioreactor operating parameters, especially solids retention time (SRT), affect the 3D structures of diatoms. This study demonstrated that tunable diatom micro/nanostructures could be produced by varying the SRT of membrane bioreactors (MBRs). A diatom strain (Stephanodiscus hantzschii) was cultivated in two identical MBRs with a fixed hydraulic retention time (HRT) of 24 h and staged SRTs from 5, to 10, and to 20 d. As SRTs increased from 5 to 20 d, important characteristics of diatom micro/nanostructures showed linear decreases: the diameters of foramina on the areola layer decreased from 170 ± 10 to 130 ± 12 nm, the numbers of nanopores per cribrum layer decreased from 20 ± 3 to 12 ± 2, and the specific surface areas of the diatoms decreased from 36.01 ± 1.27 to 12.67 ± 2.45 m²·g^-1. However, the average diatom heights increased from 2.9 ± 0.3 to 3.9 ± 0.4 µm, while diatom cell diameter (5 µm) and nanopore size (20 nm) remained unchanged. The silicon content of diatoms also linearly increased with SRT. The decrease in diatom porosity and increase in silicon content were probably due to the reduced diatom growth rates (likely resulting in less pores) at increasing SRTs, which also facilitated silica deposition as the overall diatom population stayed longer in the MBRs. As the SRTs increased from 5 to 10, and to 20 d, the nitrate (NO₃^-) removal efficiency decreased from 75% to 70%, and to 60%, respectively, whereas phosphorus (P) removal efficiency increased from 74% to 80%, and to 90%, respectively. The opposite trends in efficiencies were because NO₃^--N was removed by cellular uptake and biomass waste whereas P was mainly removed through diatom-assisted chemical precipitation.

Co-digestion of harvested microalgae and primary sludge in a mesophilic anaerobic membrane bioreactor (AnMBR): Methane potential and microbial diversity

Anaerobic co-digestion of primary sludge and raw microalgae (Scenedesmus and Chlorella) was performed in a lab-scale semi-continuous anaerobic membrane bioreactor to assess the biological performance and identify the microbial community involved in the co-digestion process. The reactor was operated at 35 °C for 440 days, working at a solids retention time of 100 days. The system achieved 73% biodegradability and showed high stability in terms of pH and volatile fatty acids. An enriched microbial community was observed. Of the several phyla, Chloroflexi and Proteobacteria were the most abundant. Cellulose-degraders phyla (Bacteroidetes, Chloroflexi and Thermotogae) were detected. Syntrophic microorganisms played an important role in intermediate degradation, enhancing methane production, mainly carried out by Methanosaeta. A nutrient-rich effluent (400 mg NH₄-N·L^-1 and 29 mg PO₄-P·L^-1) and digestate (860 mg N·L^-1 and 151 mg P·L^-1) were obtained. The bio-nutrients released from anaerobic co-digestion could be reused for microalgae cultivation or agricultural applications.

Unveiling microbial structures during raw microalgae digestion and co-digestion with primary sludge to produce biogas using semi-continuous AnMBR systems

Methane production from microalgae can be enhanced through anaerobic co-digestion with carbon-rich substrates and thus mitigate the inhibition risk associated with its low C:N ratio. Acclimated microbial communities for microalgae disruption can be used as a source of natural enzymes in bioenergy production. However, co-substrates with a certain microbial diversity such as primary sludge might shift the microbial structure. Substrates were generated in a Water Resource Recovery Facility (WRRF) and combined as follows: Scenedesmus or Chlorella digestion and microalgae co-digestion with primary sludge. The study was performed using two lab-scale Anaerobic Membrane Bioreactors (AnMBR). During three years, different feedstocks scenarios for methane production were evaluated with a special focus on the microbial diversity of the AnMBR. 57% of the population was shared between the different feedstock scenarios, revealing the importance of Anaerolineaceae members besides Smithella and Methanosaeta genera. The addition of primary sludge enhanced the microbial diversity of the system during both Chlorella and Scenedesmus co-digestion and promoted different microbial structures. Aceticlastic methanogen Methanosaeta was dominant in all the feedstock scenarios. A more remarkable role of syntrophic fatty acid degraders (Smithella, Syntrophobacteraceae) was observed during co-digestion when only microalgae were digested. However, no significant changes were observed in the microbial composition during anaerobic microalgae digestion when feeding only Chlorella or Scenedesmus. This is the first work revealing the composition of complex communities for semi-continuous bioenergy production from WRRF streams. The stability and maintenance of a microbial core over-time in semi-continuous AnMBRs is here shown supporting their future application in full-scale systems for raw microalgae digestion or co-digestion.

Acclimatised rumen culture for raw microalgae conversion into biogas: Linking microbial community structure and operational parameters in anaerobic membrane bioreactors (AnMBR)

Ruminal fluid was inoculated in an Anaerobic Membrane Reactor (AnMBR) to produce biogas from raw Scenedesmus. This work explores the microbial ecology of the system during stable operation at different solids retention times (SRT). The 16S rRNA amplicon analysis revealed that the acclimatised community was mainly composed of Anaerolineaceae, Spirochaetaceae, Lentimicrobiaceae and Cloacimonetes fermentative and hydrolytic members. During the highest biodegradability achieved in the AnMBR (62%) the dominant microorganisms were Fervidobacterium and Methanosaeta. Different microbial community clusters were observed at different SRT conditions. Interestingly, syntrophic bacteria Gelria and Smithella were enhanced after increasing 2-fold the organic loading rate, suggesting their importance in continuous systems producing biogas from raw microalgae.

A review of membrane fouling and its control in algal-related membrane processes

Membrane technologies have received much attention in microalgae biorefinery for nutrients removal from wastewater, carbon dioxide abatement from the air as well as the production of value-added products and biofuel in recent years. This paper provides a state-of-the-art review on membrane fouling issues and its control in membrane photobioreactors (MPBRs) and other algal-related membrane processes (harvesting, dewatering, and biofuel production). The mechanisms of membrane fouling and factors affecting membrane fouling in algal-related membrane processes are systematically reviewed. Also, strategies to control membrane fouling in algal-related membrane processes are summarized and discussed. Finally, the gaps, challenges, and opportunities in membrane fouling control in algal-related membrane technologies are identified and discussed.

Performance and methane fermentation characteristics of a pilot scale anaerobic membrane bioreactor (AnMBR) for treating pharmaceutical wastewater containing m-cresol (MC) and iso-propyl alcohol (IPA)

In this study, a pilot scale anaerobic membrane bioreactor (AnMBR) was operated for 80 days to treat pharmaceutical wastewater containing m-cresol (MC) and iso-propyl alcohol (IPA). The aim of the study is to investigate the performance and methane fermentation characteristics of AnMBR at different hydraulic retention time (HRT) (48, 36, 24, 18 and 12 h). The average total removal efficiencies of MC and IPA were 95%, 96% during the 80 days, which demonstrated that the AnMBR system performed well in the MC and IPA removal. The major volatile fatty acid (VFA) was found to be acetic acid, propionic acid, butyric acid, besides, the VFA accumulated apparently when HRT decreased to 12 h. The decrease of HRT led to an increase of relative abundance of methanosarcina from 13 to 33% and a decrease in biogas yield from 0.19 to 0.05 L/gCOD_removal. The biogas production was found to increase dramatically at HRT of 36 h. The trend of methane content kept stable at this stage with the average value of 78.5% which higher than other HRTs. The investigation of methanogen community showed that methanosarcinaceae was always dominant acetoclastic methanogens and methanomicrobiales was the dominant hydrogen utilizers throughout the operational period. When the HRT dropped to 12 h, the growth of the methanosarcinaceae and methanomicrobiales was observed, the amount of the methanosarcinaceae and methanomicrobiales sharply increased. After the overall research, HRT of 36 h was chosen as the most suitable operating condition due to the comprehensively preferable performance and more economical.

Effect of long residence time and high temperature over anaerobic biodegradation of Scenedesmus microalgae grown in wastewater

Anaerobic digestion of indigenous Scenedesmus spp. microalgae was studied in continuous lab-scale anaerobic reactors at different temperatures (35 °C and 55 °C), and sludge retention time - SRT (50 and 70 days). Mesophilic digestion was performed in a continuous stirred-tank reactor (CSTR) and in an anaerobic membrane bioreactor (AnMBR). Mesophilic CSTR operated at 50 days SRT only achieved 11.9% of anaerobic biodegradability whereas in the AnMBR at 70 days SRT and 50 days HRT reached 39.5%, which is even higher than the biodegradability achieved in the thermophilic CSTR at 50 days SRT (30.4%). Microbial analysis revealed a high abundance of cellulose-degraders in both reactors, AnMBR (mainly composed of 9.4% Bacteroidetes, 10.1% Chloroflexi, 8.0% Firmicutes and 13.2% Thermotogae) and thermophilic CSTR (dominated by 23.8% Chloroflexi and 12.9% Firmicutes). However, higher microbial diversity was found in the AnMBR compared to the thermophilic CSTR which is related to the SRT. since high SRT promoted low growth-rate microorganisms, increasing the hydrolytic potential of the system. These results present the membrane technology as a promising approach to revalue microalgal biomass, suggesting that microalgae biodegradability and consequently the methane production could be improved operating at higher SRT.

Microbial communities of biomethanization digesters fed with raw and heat pre-treated microalgae biomasses

Microalgae biomasses are considered promising feedstocks for biofuel and methane productions. Two Continuously Stirred Tank Reactors (CSTR), fed with fresh (CSTR-C) and heat pre-treated (CSTR-T) Chlorella biomass were run in parallel in order to determine methane productions. The methane yield was 1.5 times higher in CSTR-T with regard to CSTR-C. Aiming to understand the microorganism roles within of the reactors, the sludge used as an inoculum (I), plus raw (CSTR-C) and heat pre-treated (CSTR-T) samples were analyzed by high-throughput pyrosequencing. The bacterial communities were dominated by Proteobacteria, Bacteroidetes, Chloroflexi and Firmicutes. Spirochaetae and Actinobacteria were only detected in sample I. Proteobacteria, mainly Alfaproteobacteria, were by far the dominant phylum within of the CSTR-C bioreactor. Many of the sequences retrieved were related to bacteria present in activated sludge treatment plants and they were absent after thermal pre-treatment. Most of the sequences affiliated to the Bacteroidetes were related to uncultured groups. Anaerolineaceae was the sole family found of the Chloroflexi phylum. All of the genera identified of the Firmicutes phylum carried out macromolecule hydrolysis and by-product fermentation. The proteolytic bacteria were prevalent over the saccharolytic microbes. The percentage of the proteolytic genera increased from the inoculum to the CSTR-T sample in a parallel fashion with an available protein increase owing to the high protein content of Chlorella. To relate the taxa identified by high-throughput sequencing to their functional roles remains a future challenge.

Consolidated bioprocessing of microalgal biomass to carboxylates by a mixed culture of cow rumen bacteria using anaerobic sequencing batch reactor (ASBR)

This study employed mixed-culture consolidated bioprocessing (CBP) to digest microalgal biomass in an anaerobic sequencing batch reactor (ASBR). The primary objectives are to evaluate the impact of hydraulic residence time (HRT) on the productivity of carboxylic acids and to characterize the bacterial community. HRT affects the production rate and patterns of carboxylic acids. For the 5-L laboratory-scale fermentation, a 12-day HRT was selected because it offered the highest productivity of carboxylic acids and it synthesized longer chains. The variability of the bacterial community increased with longer HRT (R²=0.85). In the 5-L laboratory-scale fermentor, the most common phyla were Firmicutes (58.3%), Bacteroidetes (27.4%), and Proteobacteria (11.9%). The dominant bacterial classes were Clostridia (29.8%), Bacteroidia (27.4%), Tissierella (26.2%), and Betaproteobacteria (8.9%).

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.

Mechanism, kinetics and microbiology of inhibition caused by long-chain fatty acids in anaerobic digestion of algal biomass

BACKGROUND

Oleaginous microalgae contain a high level of lipids, which can be extracted and converted to biofuel. The lipid-extracted residue can then be further utilized through anaerobic digestion to produce biogas. However, long-chain fatty acids (LCFAs) have been identified as the main inhibitory factor on microbial activity of anaerobic consortium. In this study, the mechanism of LCFA inhibition on anaerobic digestion of whole and lipid-extracted algal biomass was investigated with a range of calcium concentrations against various inoculum to substrate ratios as a means to alleviate the LCFA inhibition.

RESULTS

Whole algal biomass of Nannochloropsis salina represents high lipid content algal biomass while lipid-extracted residue represents its low lipid counterpart. The anaerobic digestion experiments were conducted in a series of serum bottles at 35 °C for 20 days. A kinetic model, considering LCFA inhibition on hydrolysis, acidogenesis as well as methanogenesis steps, was developed from the observed phenomenon of inhibition factors as a function of the LCFA concentration and specific biomass content or calcium concentration. The results showed that inoculum to substrate ratio had a stronger effect on biogas production than calcium, and calcium had no effect on biogas production when inoculum concentration was extremely low. The microbial community analysis by high-throughput Illumina Miseq sequencing indicated that diversity of both bacterial and methanogenic communities decreased with elevation of lipid concentration. Hydrolytic bacteria and aceticlastic methanogens dominated bacterial and archaea communities, respectively, in both high and low LCFA concentration digesters.

CONCLUSIONS

This study demonstrated that inoculum concentration has a more significant effect on alleviating LCFA inhibition than calcium concentration, while calcium only played a role when inoculum concentration met a threshold level. The model revealed that each functional microbial group was subject to different levels of LCFA inhibition. Although methanogens were the most susceptible microbes to LCFA inhibition, the inhibition factor for hydrolytic bacteria was more highly affected by inoculum concentration. The microbial community analysis indicated that the bacterial community was affected more than the methanogenic community by high LCFAs concentration. Syntrophic acetogens were sensitive to high LCFA concentrations and thus showed a decreased abundance in such an environment. Graphical abstractProposed mechanism of calcium mitigated LCFA inhibition.

Development and testing of a fully gravitational submerged anaerobic membrane bioreactor for wastewater treatment

A gravity-operated submerged anaerobic membrane bioreactor (SAnMBR) was set up in order to test its principle of operation as an alternative to conventional pumped permeation of the membrane. This operating mode allowed the membrane flux rate to be measured accurately whilst maintaining a constant transmembrane pressure (TMP), and allowed small transient variations in the flux rate to be observed. The reactor was operated at 36°C for a period of 115 days using a nutrient-balanced synthetic substrate with a high suspended solids concentration. Membrane cleaning was in situ by a gas scouring system using recirculation of headspace biogas. With an initial TMP of 7.0 kPa, the membrane flux slowly decreased due to membrane fouling and had not reached a constant value by day 71. The results indicated that the system was still acclimatizing up to 50 days after start-up; but from that point onwards, performance parameters became much more stable. A constant flux of 2.2 L m(-2) h(-1) was achieved over the last 45 days after the TMP was reduced to 2.3 kPa. The stable flux was maintained over this period and the loading raised to 1 g COD L(-1) d(-1) by increasing the influent strength. Under these conditions, the average chemical oxygen demand removal efficiency was 96% and the specific methane potential was 0.31 L CH4 g(-1) COD removed.

Membrane applications for biogas production and purification processes: an overview on a smart alternative for process intensification

Biogas is the result of a complex conversion process that takes place because of the metabolic activity of various types of bacteria.

Biomass retention on electrodes rather than electrical current enhances stability in anaerobic digestion

Anaerobic digestion (AD) is a well-established technology for energy recovery from organic waste streams. Several studies noted that inserting a bioelectrochemical system (BES) inside an anaerobic digester can increase biogas output, however the mechanism behind this was not explored and primary controls were not executed. Here, we evaluated whether a BES could stabilize AD of molasses. Lab-scale digesters were operated in the presence or absence of electrodes, in open (no applied potential) and closed circuit conditions. In the control reactors without electrodes methane production decreased to 50% of the initial rate, while it remained stable in the reactors with electrodes, indicating a stabilizing effect. After 91 days of operation, the now colonized electrodes were introduced in the failing AD reactors to evaluate their remediating capacity. This resulted in an immediate increase in CH4 production and VFA removal. Although a current was generated in the BES operated in closed circuit, no direct effect of applied potential nor current was observed. A high abundance of Methanosaeta was detected on the electrodes, however irrespective of the applied cell potential. This study demonstrated that, in addition to other studies reporting only an increase in methane production, a BES can also remediate AD systems that exhibited process failure. However, the lack of difference between current driven and open circuit systems indicates that the key impact is through biomass retention, rather than electrochemical interaction with the electrodes.

Addressing the challenges for sustainable production of algal biofuels: II. Harvesting and conversion to biofuels

In order to ensure the sustainability of algal biofuel production, a number of issues need to be addressed. Previously, we reviewed some of the questions in this area involving algal species and the important challenges of nutrient supply and how these might be met. Here, we take up issues involving harvesting and the conversion ofbiomass to biofuels. Advances in both these areas are required if these third-generation fuels are to have a sufficiently high net energy ratio and a sustainable footprint. A variety of harvesting technologies are under investigation and recent studies in this area are presented and discussed. A number of different energy uses are available for algal biomass, each with their own advantages as well as challenges in terms of efficiencies and yields. Recent advances in these areas are presented and some of the especially promising conversion processes are highlighted.

Anaerobic conversion of microalgal biomass to sustainable energy carriers--a review

This review discusses anaerobic production of methane, hydrogen, ethanol, butanol and electricity from microalgal biomass. The amenability of microalgal biomass to these bioenergy conversion processes is compared with other aquatic and terrestrial biomass sources. The highest energy yields (kJ g(-1) dry wt. microalgal biomass) reported in the literature have been 14.8 as ethanol, 14.4 as methane, 6.6 as butanol and 1.2 as hydrogen. The highest power density reported from microalgal biomass in microbial fuel cells has been 980 mW m(-2). Sequential production of different energy carriers increases attainable energy yields, but also increases investment and maintenance costs. Microalgal biomass is a promising feedstock for anaerobic energy conversion processes, especially for methanogenic digestion and ethanol fermentation. The reviewed studies have mainly been based on laboratory scale experiments and thus scale-up of anaerobic utilization of microalgal biomass for production of energy carriers is now timely and required for cost-effectiveness comparisons.

Enhanced methane production from Taihu Lake blue algae by anaerobic co-digestion with corn straw in continuous feed digesters

Anaerobic digestion of Taihu blue algae was tested in laboratory scale, continuous feed digesters (hydraulic retention time 10 days) at 35°C and various organic loading rates (OLR). The methane production and biomass digestion performed well at OLR below 4.00 gVSL(-1)d(-1) but deteriorated as OLR increased due to the increased ammonia concentration, causing inhibition mainly to acetate and propionate degradation. Supplementing corn straw as co-feedstock significantly improved the digestion performance. The optimal C/N ratio for the co-digestion was 20:1 at OLR of 6.00 gVSL(-1) d(-1). Methane yield of 234 mL CH4 gVS(-1) and methane productivity of 1404 mL CH4 L(-1) d(-1) were achieved with solid removal of 63%. Compared with the algae alone, the methane productivity was increased by 46% with less accumulation of ammonia and fatty acids. The reactor rate-limiting step was acetate and propionate degradation.

Repeated pulse feeding induces functional stability in anaerobic digestion

Anaerobic digestion is an environmental key technology in the future bio-based economy. To achieve functional stability, a minimal microbial community diversity is required. This microbial community should also have a certain ‘elasticity’, i.e. the ability to rapidly adapt to suboptimal conditions or stress. In this study it was evaluated whether a higher degree of functional stability could be achieved by changing the feeding pattern, which can change the evenness, dynamics and richness of the bacterial community. The first reactor (CSTR_stable) was fed on daily basis, whereas the second reactor (CSTR_dynamic) was fed every 2 days. Average biogas production was 0.30 l CH₄ l⁻¹ day⁻¹ in both reactors, although daily variation was up to four times higher in the CSTR_dynamic compared with the CSTR_stable during the first 50 days. Bacterial analysis revealed that this CSTR_dynamic had a two times higher degree of bacterial community dynamics. The CSTR_dynamic also appeared to be more tolerant to an organic shock load of 8 g COD l⁻¹ and ammonium levels up to 8000 mg TAN l⁻¹. These results suggest that the regular application of a limited pulse of organic material and/or a variation in the substrate composition might promote higher functional stability in anaerobic digestion.