Untreated Microalgae Research Articles

Effect of hydrothermal pretreatment on leachate fed Scenedesmus sp. biomass solubilization and biogas production

The aim of the present study was to assess the effect of hydrothermal pretreatment on the solubilization and anaerobic digestion (AD) of Scenedesmus sp. biomass. At first, the microalgae was cultivated in 5% fresh leachate (FL) to recover nutrients such as nitrogen and phosphorus. Scenedesmus sp. grown in 5% FL obtained 100%, 77% and 97% removal efficiency of ammonium nitrogen (NH4+ - N), total Kjeldahl nitrogen (TKN) and phosphate phosphorous (PO43− -P), respectively. In the following step, the hydrothermal pretreatment of Scenedesmus sp. biomass was carried out at 120, 150 and 170 °C and retention time of 0, 30 and 60 min to evaluate its solubilization and biogas production through AD in batch test. Soluble chemical oxygen demand (sCOD) increased by 260% compared to untreated microalgae at 170 °C for 60 min. In comparison to untreated microalgae, the highest increase in biogas (70%) and methane yield (100%) was observed for 150 °C and 60 min pretreated microalgae as a consequence of hydrothermal pretreatment. Hydrothermal pretreatment has shown effectiveness in enhancing biomass solubilization and increasing biogas yield. Nevertheless, further research at the pilot scale is necessary to thoroughly evaluate the potential and feasibility of hydrothermal pretreatment for full-scale implementation.

Enhancing Microalgae Content in Biocomposites through a Mechanical Grinding Method.

Microalgae-based biocomposites are gaining traction as ecofriendly and cost-effective alternatives to conventional petroleum-based plastics. However, achieving a homogeneous dispersion of microalgae within a biocomposite matrix remains a challenge. In this study, we investigated the effect of the size of dried microalgae (Chlorella sp.) on the quality of biocomposites. Ball milling, a mechanical grinding process, was used to control the size of the pretreated dried microalgae. Our results demonstrate that the microalgae size strongly depends on the total weight of the stainless-steel balls, rather than the number of balls used in the milling process. Poly(ethylene-vinyl acetate) (EVA), with functional groups resembling those of Chlorella sp., was incorporated into the ball-milled microalgae to produce homogeneous biocomposites. Smaller Chlorella sp. particles improved the ratio of microalgae and the mechanical properties of the biocomposites. Dried Chlorella sp. particles up to 161.43 μm, which were 72.84% smaller than the untreated microalgae, were obtained after 6 h of ball milling using 3/8-inch balls. This enabled the production of biocomposites with 60 wt.% microalgae and 61.02% of the tensile strength of pure EVA, comparable to traditional polymers. Our findings suggest that controlling the microalgae size through ball milling can improve the quality of microalgae-based biocomposites.

ANAEROBIC CO-DIGESTION OF MICROALGAE AND RESIDUAL GLYCEROL RECOVERED FROM BIODIESEL: EVALUATION OF PRETREATMENT AND COD/N RATIO

Anaerobic digestion can be a viable alternative for the destination of effluents from stabilization ponds rich in naturally produced microalgae, without the need for concentration processes, however, there are challenges related to the rigid cell wall of microalgae and the low carbon content in its composition, to be overcome. To improve these parameters, this study comparatively evaluated hydrolytic pretreatments in microalgae from effluents from stabilization ponds, aiming to hydrolyze the cell wall of these microorganisms, for co-digestion with residual glycerol from biodiesel. In this case, glycerol is a by-product with limited applicability, acting in this scenario as a carbon supplier, improving the C/N ratio, microalgae biodegradation and biomethane production. Effluents with microalgae submitted to thermal and ultrasonic hydrolysis (for 30 and 90 minutes) were tested to assess their potential in the production of methane-rich biogas, monitored by gauge measurements and gas chromatography, respectively, in co-digestion with residual glycerol from biodiesel. The heat treatment for 30 minutes showed more satisfactory results and was replicated in a benchtop anaerobic reactor (R2), in parallel with a reactor operating untreated microalgae (R1), in a continuous feed system. The effects of pretreatment and COD/N ratio were evaluated on organic matter removal and biomethane production. R2 showed the most satisfactory effect on COD removal, resulting in up to 90% COD removed, with a theoretical biogas production of 0.52 L g-1 COD removed. As for the methane content contained in biogas, R1 reached percentages of up to 84% against 73% in R2.

Development of a controlled release fertilizer by incorporating lauric acid into microalgal biomass: Dynamics on soil biological processes for efficient utilisation of waste resources

Utilisation of microalgae to extract nutrients from the effluent of anaerobic digestion of food waste is an emerging technology. A by-product of this process is the microalgal biomass which has potential to be used as an organic bio-fertilizer. However, microalgal biomass are rapidly mineralized when applied to soil which may result in N loss. One solution is to emulsify microalgal biomass with lauric acid (LA) to delay the release of mineral N. This study aimed to investigate whether combining LA with microalgae to develop a new fertilizer product with a controlled release function of mineral N when applied to soil, and any potential impacts the bacterial community structure and activity. The treatments were applied to soil emulsified with LA and were combined with either microalgae or urea at rates of 0%, 12.5%, 25% and 50% LA, untreated microalgae or urea and unamended control were incubated at 25 °C and 40% water holding capacity for 28 days. Quantification of soil chemistry (NH4+-N, NO3−-N, pH and EC), microbial biomass carbon, CO2 production and bacterial diversity were characterised at 0, 1, 3, 7, 14 and 28 days. The NH4+-N and NO3−-N concentration decreased with increasing rate of LA combined microalgae indicating that both N mineralization and nitrification were impacted. As a function of time, NH4+-N concentration increased up to 7 days for the microalgae at lower rates of LA, and then slowly decreased for 14 and 28 days, with an inverse relationship with soil NO3−N. Aligning with soil chemistry, an observed decrease in the predicted nitrification genes amoA·amoB and relative abundance of ammonia oxidizing bacteria (Nitrosomonadaceae) and nitrifying bacteria (Nitrospiraceae) with an increasing rate of LA with microalgae provides further support for possible inhibition of nitrification. The MBC and CO2 production was higher in the soil amended with increasing rates of LA combined microalgae and there was an increase in the relative abundance of fast-growing heterotrophs. Treating microalgae by emulsification with LA has the potential to control the release of N by increasing immobilization over nitrification and therefore it might be possible to engineer microalgae to match plant nutrient growth requirements whilst recovering waste from waste resources.

Effect of Electrofiltration on the Dewatering Kinetics of Arthrospira platensis and Biocompound Recovery

Arthrospira platensis (A. platensis) is a microalga with a wide range of commercial uses. One of the main concerns that needs to be addressed in microalgae biorefineries is the costs associated with the harvesting and concentration steps. Filtration has been shown to be an effective technique for concentrating microalgae and recent studies have attempted to enhance membrane filtration by applying an external electric field to the filtration cell. This study consisted of assessing the use of electrically assisted filtration (electrofiltration) at 60 A/m2 and 1 bar for the dewatering of A. platensis, as well as the effect of pretreating the microalgae with ultrasounds (US) on the filtration process. Untreated A. platensis exhibited better filtration kinetics than US-treated A. platensis, and electrofiltration was found to increase the cake dryness. More protein and pigments were present in the US-treated microalgae solution compared to the untreated microalgae, which led to the presence of higher concentrations of protein and pigments in the filtrate streams after pressure filtration at 1 bar without the application of an external electric field. Electrofiltration was found to consume less energy compared to traditional drying techniques used for A. platensis. However, electrofiltration degrades the biocompounds present in the filtrate and cake due to pH changes and other electrophoresis phenomena, which shows the need to optimize the process in future work.

A pilot study investigating the potential of antimicrobial photodynamic therapy (aPDT) to control Vibrio spp. development in microalgae and seawater

Antimicrobial photodynamic therapy (aPDT) uses light-specific wavelengths and nontoxic photosensitisers to eradicate target microbial cells. Bivalve hatchery systems are challenged with bacterial contamination of microalgae feed, which can be detrimental to all production life stages. The objectives of this study were to investigate the antimicrobial potential of blue LED and red laser aPDTs in a marine environment. The bacteria Vibrio spp. were targeted, as they have been associated with significant Pacific oyster Crassostrea gigas mortalities worldwide. The compartments tested were (a) microalgae feed (Tetraselmis sp., Isochrysis sp. and Chaetoceros muelleri) and (b) tank seawater. C. gigas seed-fed light-treated and untreated microalgae for 8 weeks were also screened for Vibrio spp. Molecular diagnostics (PCR, qPCR and Sanger sequencing) were used. The aPDTs were deemed non-destructive to the microalgae. V. splendidus was detected exclusively in C. muelleri growth cultures. Tank seawater samples were positive for V. splendidus pre-treatment. A complete eradication of V. splendidus RNA (indicative of pathogen viability) was observed in seawater samples treated by aPDTs, while no V. splendidus DNA (pathogen may be viable or not) was observed in seawater samples treated by red laser and a 33% reduction occurred when treated with LED. Of significance, a reduction of V. splendidus DNA and RNA was observed in oysters fed the aPDT-treated microalgae versus those oysters fed the untreated diet. This pilot study indicates that there is the potential to use aPDT to treat against pathogenic bacteria, in particular Vibrio spp., in hatchery microalgal feed and tank water.

Emulsifying properties of acid-hydrolyzed insoluble protein fraction from Chlorella protothecoides: Formation and storage stability of emulsions

Microalgae are promising alternative protein sources due to their high protein content. However, the techno-functional properties of microalgae proteins are limited by their high content of insoluble proteins. Recently though, it has been reported that hydrolysis with 0.5 M HCl improved their solubility and interfacial activity, which could enhance their ability to act as emulsifying agents. In this study, we, therefore, investigated the use of an insoluble microalgae protein fraction from Chlorella protothecoides as oil-in-water emulsifiers after hydrolysis at 65 °C and 85 °C for 4 h. Mean particle diameter, microscopic images, and ζ-potential of emulsions stabilized by untreated insoluble microalgae protein fraction and their hydrolysates at different protein concentrations (1%–5%) were recorded during storage for 14 days. Results showed that emulsions prepared by untreated and hydrolyzed protein fractions all exhibited droplet flocculation, but effective droplet size decreased with increasing protein concentrations and storage time. Moreover, emulsions droplets were stable against coalescence at protein concentrations ≥3%. Overall, hydrolysates showed higher interfacial activities compared to untreated microalgae proteins. Results were attributed to the presence of protein aggregates causing droplet flocculation, and mixed protein aggregate - hydrolyzed peptide interfaces preventing coalescence. Results indicated a potential application of hydrolysates of insoluble microalgae protein fraction in emulsion-based foods, especially concentrated emulsions, such as mayonnaise, dips or salad dressings.

Integrated protein extraction with bio-oil production for microalgal biorefinery

The economic feasibility of biofuel production from microalgae can benefit from the development of integrated processes that extract all valuable components from microalgal biomass. In this work, protein extraction of Chlorella vulgaris was integrated with bio-oil production using the protein-extracted microalgae. To extract the protein, the biomass was hydrolyzed at various pH, temperatures and times and then sonicated. This method recovered nearly 80% of the protein fraction from hydrolysis at pH 12 and 50 °C for 90 min. As a result, nitrogen content was 65.4% lower in protein-extracted microalgae and the bio-oil obtained from pyrolysis of the protein-extracted microalgae was of a higher quality than bio-oil from untreated microalgae. The bio-oil of protein-extracted microalgae had lower contents of acids, oxygenated, nitrogenated, and aromatic compounds but higher contents of alkanes and alkenes. While protein extraction improved the quality of the bio-oil, it did not increase the quantity produced. However, techno-economic analysis indicated that bio-oil production with an integrated hydrolysis process could produce profits from the extracted proteins. Therefore, the integration of effective and low-cost protein extraction into the bio-oil production process may contribute greatly to the development of the microalgal biorefinery concept.

Cultivation of the microalga Eustigmatos magnus in different photobioreactor geometries and subsequent anaerobic digestion of pre-treated biomass

Microalgal biomass as a feedstock for biogas production is linked to the parameters biomass productivity and biogas yield. Besides an easy-to-use strain for anaerobic digestion, the photobioreactor (PBR) design is important. A microalgae strain selection revealed Eustigmatos magnus (SAG 36.89) as the most promising strain yielding an average of 100 mg total suspended solids (TSS) L−1 day−1. The strain was tested in cost-effective sleevebag-PBR-systems of 10 cm, 20 cm and 30 cm diameter facing the light from the front or laterally. Highest mean productivity on a volumetric basis was measured in PBRs with the lowest diameter (104 and 117 mg L−1 day−1. The highest productivity per m−2 was achieved in 10 cm PBRs with front light configuration (9.35 g TSS m−2 day−1). The lateral light configuration of 10 cm PBRs had positive aspects such as the lowest mean water demand to produce 1 kg TSS (481 L−1 kg−1) and the lowest mean energy demand for medium separation of 1 kg TSS (106 Wh). The concentrated microalgal biomass was then subjected to ultrasonication and thermal pre-treatment (90 °C and 120 °C) and tested in BMP tests. Mesophilic anaerobic mono-digestion of untreated microalgae biomass led to a methane (CH4) yield of 343 L−1 kg−1 volatile solids (VS). Thermal pre-treatment at 120 °C resulted in significantly increased CH4 yields of 430 L−1 kg−1 VS. As thermal pre-treatment can be easily installed nearby a biogas plant it could be an interesting option for AD of microalgal biomass with only little investment.

Cell wall disruption: An effective strategy to improve the nutritive quality of microalgae in African catfish ( Clarias gariepinus )

The rigid cell walls of microalgae may hinder their utilization in fish feeds. The current experiment assessed the correlation between the accessibility of microalgae nutrients and their in vivo digestibility in African catfish. Nannochloropsis gaditana biomass was subjected to physical or mechanical treatments to weaken its cell wall; untreated—no disruption treatment (UNT), pasteurization (PAS), freezing (FRO), freeze-drying (FRD), cold pasteurization (L40) and bead milling (BEM). Six experimental diets formulated from differently treated and untreated microalgae (at 30% diet inclusion level) were tested on growth performance and apparent nutrient digestibility (ADCs) in juvenile African catfish. A basal diet (REF) containing no microalgae was used as reference diet. Results showed that biomass gain and feed conversion ratio of fish fed L40 and BEM diets increased by 13% and 11%, respectively, relative to the UNT diet. Additionally, FRD, FRO, L40 and BEM cell wall disruption treatments improved protein digestibility by 0.5%, 5.9%, 8.4% and 16.3%, respectively, compared to the UNT treatment. There was a positive correlation between accessibility of microalgal nutrients and their digestibility in African catfish. Nutrient digestibility of microalgae was dependent on extent of cell disruption. Also, the impact of cell disruption on nutrient digestibility of microalgae differs between African catfish and Nile tilapia.

Effects of partial maize silage substitution with microalgae on viscosity and biogas yields in continuous AD trials

The microalga Acutodesmus obliquus was investigated as a feedstock in semi-continuously fed anaerobic digestion trials, where A. obliquus was co-digested with pig slurry and maize silage. Maize silage was substituted by both 10% and 20% untreated, and 20% ultrasonicated microalgae biomass on a VS (volatile solids) basis. The substitution of maize silage with 20% of either ultrasonicated and untreated microalgae led to significantly lower biogas yields, i.e., 560 dm³ kg−1 VScorr in the reference compared to 516 and 509 dm³ kg-1VScorr for untreated and ultrasonicated microalgae substitution. Further, the viscosities in the different reactors were measured at an OLR of 3.5 g VS dm-3 d-1. However, all treatments with microalgae resulted in significantly lower viscosities. While the mean viscosity reached 0.503 Pa s in the reference reactor, mean viscosities were 53% lower in reactors where maize was substituted by 20% microalgae, i.e. 0.239 Pa s, at a constant rotation speed of 30 rpm. Reactors where maize was substituted by 20% ultrasonicated microalgae had a 32% lower viscosity, for 10% microalgae substitution a decrease of 8% was measured. Decreased viscosities have beneficial effect on the bioprocess and the economy in biogas plants. Nonetheless, with regard to other parameters, no positive effect on biogas yields by partial substitution with microalgae biomass was found. The application of microalgae may be an interesting option in anaerobic digestion when fibrous or lignocellulosic substances lead to high viscosities of the digested slurries. High production costs remain the bottleneck for making microalgae an interesting feedstock.

Microalgae cultivation for wastewater treatment and biogas production at Moscow wastewater treatment plant.

The process of cultivation of microalgae on purified and clarified wastewater of Kuryanovo wastewater treatment plants (KWWTP) was studied. The studies were conducted on monoculture (Scenedesmus quadricauda and Chlorella sorokiniana) and on polyculture, the composition of which was formed from microalgae present in the wastewater. The authors created and investigated the columnar photobioreactor (PBR), which acted as a pilot project on the purified and clarified water of KWWTP and allowed the removal of total nitrogen and phosphorus phosphates with an efficiency of up to 90%. The formation of a stable biocenosis from 22 species of algae (with 3-4 dominant species) and 31 species of zooplankton organisms belonging to six systematic subdivisions was recorded. The optimal retention time of the microalgae polyculture for the most effective wastewater treatment has been determined. The conducted studies have shown that the depth of decomposition of ashless matter and the ultimate biogas potential of untreated microalgae biomass is 15% lower than the corresponding values obtained with digestion of activated sludge, which necessitates studies in the field of pretreatment of algal biomass. The paper shows: connections between chlorophyll-a content, algal biomass and fluorescence index F0 and between biomass increment and Fv/Fm value.

Highly efficient methane generation from untreated microalgae biomass

BackgroundThe fact that microalgae perform very efficiently photosynthetic conversion of sunlight into chemical energy has moved them into the focus of regenerative fuel research. Especially, biogas generation via anaerobic digestion is economically attractive due to the comparably simple apparative process technology and the theoretical possibility of converting the entire algal biomass to biogas/methane. In the last 60 years, intensive research on biogas production from microalgae biomass has revealed the microalgae as a rather challenging substrate for anaerobic digestion due to its high cell wall recalcitrance and unfavorable protein content, which requires additional pretreatment and co-fermentation strategies for sufficient fermentation. However, sustainable fuel generation requires the avoidance of cost/energy intensive biomass pretreatments to achieve positive net-energy process balance.ResultsCultivation of microalgae in replete and limited nitrogen culture media conditions has led to the formation of protein-rich and low protein biomass, respectively, with the last being especially optimal for continuous fermentation. Anaerobic digestion of nitrogen limited biomass (low-N BM) was characterized by a stable process with low levels of inhibitory substances and resulted in extraordinary high biogas, and subsequently methane productivity [750 ± 15 and 462 ± 9 mLN g−1 volatile solids (VS) day−1, respectively], thus corresponding to biomass-to-methane energy conversion efficiency of up to 84%. The microbial community structure within this highly efficient digester revealed a clear predominance of the phyla Bacteroidetes and the family Methanosaetaceae among the Bacteria and Archaea, respectively. The fermentation of replete nitrogen biomass (replete-N BM), on the contrary, was demonstrated to be less productive (131 ± 33 mLN CH4 g−1VS day−1) and failed completely due to acidosis, caused through high ammonia/ammonium concentrations. The organization of the microbial community of the failed (replete-N) digester differed greatly compared to the stable low-N digester, presenting a clear shift to the phyla Firmicutes and Thermotogae, and the archaeal population shifted from acetoclastic to hydrogenotrophic methanogenesis.ConclusionsThe present study underlines the importance of cultivation conditions and shows the practicability of microalgae biomass usage as mono-substrate for highly efficient continuous fermentation to methane without any pretreatment with almost maximum practically achievable energy conversion efficiency (biomass to methane).Graphical abstractGrowth condition dependence of anaerobic conversion efficiency of microalgae biomass to methane

Enhancement of microalgae anaerobic digestion by thermo-alkaline pretreatment with lime (CaO)

The aim of this study was to evaluate for the first time the effect of a thermo-alkaline pretreatment with lime (CaO) on microalgae anaerobic digestion. The pretreatment was carried out by adding different CaO doses (4 and 10%) at different temperatures (room temperature (25°C), 55 and 72°C). The exposure time was 4days for pretreatments at 25°C, and 24h for pretreatments at 55 and 72°C. Following, a biochemical methane potential test was conducted with pretreated and untreated microalgae. According to the results, the pretreatment enhanced proteins solubilisation by 32.4% and carbohydrates solubilisation by 31.4% with the highest lime dose and temperature (10% CaO and 72°C). Furthermore, anaerobic digestion kinetics were improved in all cases (from 0.08 to 0.14day−1 for untreated and pretreated microalgae, respectively). The maximum biochemical methane potential increase (25%) was achieved with 10% CaO at 72°C, in accordance with the highest biomass solubilisation. Thus, lime pretreatment appears as a potential strategy to improve microalgae anaerobic digestion.

Assessing the agricultural reuse of the digestate from microalgae anaerobic digestion and co-digestion with sewage sludge

Microalgae anaerobic digestion produces biogas along with a digestate that may be reused in agriculture. However, the properties of this digestate for agricultural reuse have yet to be determined. The aim of this study was to characterise digestates from different microalgae anaerobic digestion processes (i.e. digestion of untreated microalgae, thermally pretreated microalgae and thermally pretreated microalgae in co-digestion with primary sludge). The main parameters evaluated were organic matter, macronutrients and heavy metals content, hygenisation, potential phytotoxicity and organic matter stabilisation. According to the results, all microalgae digestates presented suitable organic matter and macronutrients, especially organic and ammonium nitrogen, for agricultural soils amendment. However, the thermally pretreated microalgae digestate was the least stabilised digestate in comparison with untreated microalgae and co-digestion digestates. In vivo bioassays demonstrated that the digestates did not show residual phytotoxicity when properly diluted, being the co-digestion digestate the one which presented less phytotoxicity. Heavy metals contents resulted far below the threshold established by the European legislation on sludge spreading. Moreover, low presence of E. coli was observed in all digestates. Therefore, agricultural reuse of thermally pretreated microalgae and primary sludge co-digestate through irrigation emerges a suitable strategy to recycle nutrients from wastewater.

탈지미세조류로부터 폴리페놀 생산 증대를 위한 열수추출 조건 최적화

합성 항산화제에 대한 대체제로 천연 항산화제에 대한 연구가 활발히 진행되고 있으며 미세조류는 천연 항산화제의 원료로 많은 관심을 받고 있다. 본 연구에서는 탈지미세조류에서 총 폴리페놀(TPC) 추출증대를 위해 추출용매, 온도, 시간, 고액비율과 에탄올 첨가 농도 최적화를 수행하였다. 열수와 유기용매 추출성능을 비교했을 때, 열수추출이 유기용매 보다 우수한 성능을 보였으며 온도 증가에 따라 추출성능도 비례하여 증가함을 보였다. 열수에 의한 추출이 에탄올 용액 추출(>98%)에 비해 우수한 성능을 보였으며40% 에탄올 용액을 이용한 열수 추출이 가장 우수한 추출 효과를 보였다. 추출조건10 min, <TEX>$100^{\circ}C$</TEX>, 40% 에탄올 열수추출에서 최대 폴리페놀 농도인 3.35 mg GAE (gallic acid equivalent)/g DM을 얻을 수 있었다. 지질 추출을 위한 유기용매 전처리 공정이 선수행 되었음에도 불구하고 탈지미세조류(Tetraselmis KCTC 12236BP)의 폴리페놀 농도가 다른 탈지이전 미세조류와 동등한 수준임을 확인할 수 있어 탈지미세조류가 천연 폴리페놀의 원료로서 적합함을 확인 할 수 있었다. 또한, 고액추출을 모사하기 위해 Peleg 모델을 이용해 예측한 폴리페놀 농도가 실험에 의해 얻어진 값과 높은 일치도를 보임으로 모델을 이용한 모사가 폴리페놀 추출 모사에 유용함을 증명할 수 있었다. The search for natural antioxidants as alternatives to synthetic products is growing. Microalgae have emerged as a source of natural antioxidants with significant and diverse health-promoting properties. In this study, the effects of hot-water extraction conditions on total polyphenol compounds (TPC) production were investigated for lipid extracted microalgae (LEA). In order to enhance the polyphenol productivity, the extraction variables including solvents, temperature, time and ethanol concentration were optimized. The results showed hot-water extraction provided a higher extraction efficiency than the organic solvents and extraction at high temperatures showed a better extraction efficiency. While hot-water extract showed a higher extraction efficiency compared to 98% ethanol extraction, the mixture of water and ethanol (40:60 v/v) showed the highest production of polyphenols. The maximum polyphenols of 3.35 mg GAE (gallic acid equivalent)/g DM were obtained at the optimized extraction time of 10 min, <TEX>$100^{\circ}C$</TEX> and 40% ethanol, respectively. Although Tetraselmis KCTC 12236BP was preprocessed by hexane to remove lipid for bio-diesel production, the results showed LEA contains relatively high level of polyphenols compared to untreated microalgae which can be used in the production of value-added materials. The predictions obtained from the developed Peleg's model were compared with the experimental data under the same operating conditions. The predicted and experimental data were consistent, indicating the reliability of the model.

Thermal pretreatment of microalgae for biomethane production: experimental studies, kinetics and energy analysis

AbstractBACKGROUNDMicroalgae grown on wastewater are promising feedstocks for biomethane production by anaerobic digestion. However, the hemicellulose composition of the microalgae cell wall inhibits hydrolysis of intracellular substances and limits their anaerobic digestibility. This study investigated enhancement of biomethane production rates during anaerobic digestion of Chlorella sp. using thermal pretreatment at varying temperatures. Experimental data were fitted to three simplified kinetic models and an energy analysis was performed to gain insights into this potential application of thermal pretreatment.RESULTSMethane yields from untreated algae were 155 mL g−1 VSadd, while thermal pretreatment at 70 °C and 90 °C for 0.5 h increased the methane yield by 37% and 48%, respectively. Thermal pretreatment at 121 °C for 0.3 h resulted in the highest methane yield (322 mL g−1 VSadd), which is 108% higher than the untreated algae. Data from digestion of thermally pretreated microalgae were best described by a first‐order kinetic model. However, for untreated microalgae the Gompertz model, which includes a lag phase, provided the best fit to the methane production data.CONCLUSIONSThermal pretreatment improved the maximum methane production rate and shortened the lag period during anaerobic digestion. However, the energy balance indicated that pretreatment of microalgae could not achieve a positive energy balance compared with anaerobic digestion of untreated microalgae. Co‐digestion with other biomass or increasing the solids concentration of anaerobic digestion could be used to increase the overall energy efficiency. © 2016 Society of Chemical Industry

Biogas Production from Waste Microalgal Biomass Obtained from Nutrient Removal of Domestic Wastewater

In this study, a semi-continuous photobioreactor was operated for the investigation of nutrient removal efficiency of a unialgal culture, Chlorella vulgaris. Maximum nitrogen and phosphorous removal efficiencies of 99.6 and 91.2 % were achieved in the photobioreactor. The microalgal slurry obtained from the effluent of the photobioreactor was subjected to biochemical methane potential assay, after application of heat, autoclave, and thermochemical pretreatments to improve anaerobic digestibility and biogas production. Evaluation of pretreatment options indicated that heat pretreatment is the most efficient method in terms of enhancing anaerobic digestibility, at the chemical oxygen demand (COD) loading of 19 ± 0.5 g L−1. This method increased the methane yield by 83.0 %, from 223 to 408 mL CH4 g VS added −1 , compared to untreated microalgal slurry reactor with the same COD value. Among reactors with 35 ± 1.5 g L−1 initial COD concentration, autoclave-pretreated microalgal slurry was found to yield the highest methane value of 356 mL CH4 g VS added −1 , which was 43.0 % higher than the value observed in the reactor fed with untreated microalgal slurry. The thermochemical pretreatment caused production of inhibitory compounds and resulted in lower biomethane production and COD treatment values, compared to untreated microalgae. Outcomes of this study reveal that coupled micro-algal and anaerobic biotechnology could be a sustainable alternative for integrated nutrient removal and biofuel production applications.

Effect of high-temperature stress on microalgae at the end of the logarithmic phase for the efficient production of lipid

ABSTRACTEfficient production of microalgae lipid is significant for the production of renewable biodiesel. In the present study, the high temperature of 40°C as stress environment was tested for stimulating lipid accumulation after the microalgae (Scenedesmus quadricauda) cells in suitable conditions grew to the end of the logarithmic phase. Different stress cultivation times of 1, 2, 3, 4, 5 and 6 days were studied. Interestingly, the lipid content and productivity reached 33.5% and 23.2 mg/L d after one day stress cultivation, showing substantial improvements of 39.6% and 33.3% compared with that in the untreated (day 0) microalgae cells, respectively. Longer stress time led to the decrease of biomass and lipid content compared with the untreated microalgae. However, a maximum protein content of 58.7% was obtained after six days. The stress cultivation at the end of the microalgae exponential phase for one day at a high temperature of 40°C could be a very useful industrial approach for efficiently promoting lipid content and biodiesel production.

Steam explosion as a fractionation step in biofuel production from microalgae

In this study, various pretreatment methods (autoclaving, ultrasound, microwave and steam explosion) have been compared to determine the most efficient method for the extraction of lipids from three different samples of microalgae (Nannochloropsis gaditana, Chlorella sorokiniana and Phaeodactylum tricornutum). Among the studied methods, steam explosion gave the highest lipid extraction yields for the three microalgae species. Therefore, the method was further studied and the application of acid catalysed steam explosion pretreatment was investigated for simultaneous lipid extraction and sugar release from microalgae. The effect of different variables, including temperature and acid concentration, was analysed. The experimental results demonstrate the efficacy and feasibility of the acid catalysed steam explosion pretreatment, followed by n-hexane lipid extraction. Remarkable sugar yields up to 96% were achieved under the pretreatment conditions of 1.7% sulphuric acid concentration and a temperature of 150°C during steam explosion. Besides, this study verified high efficiencies in the extraction of lipid of exploded microalgae using n-hexane against the low efficiencies obtained for the untreated microalgae.