Effect of pretreatments on biogas production from microalgae biomass grown in pig manure treatment plants

Approaches to mitigation of hydrogen sulfide during anaerobic digestion process - A review

Anaerobic digestion (AD) is the primary technology for energy production from wet biomass under a limited oxygen supply. Various wastes rich in organic content have been renowned for enhancing the process of biogas production. However, several other intermediate unwanted products such as hydrogen sulfide, ammonia, carbon dioxide, siloxanes and halogens have been generated during the process, which tends to lower the quality and quantity of the harvested biogas. The removal of hydrogen sulfide from wastewater, a potential substrate for anaerobic digestion, using various technologies is covered in this study. It is recommended that microaeration would increase the higher removal efficiency of hydrogen sulfide based on a number of benefits for the specific method. The process is primarily accomplished by dosing smaller amounts of oxygen in the digester, which increases the system's oxidizing capacity by rendering the sulfate reducing bacteria responsible for converting sulfate ions to hydrogen sulfide inactive. This paper reviews physicochemical and biological methods that have been in place to eliminate the effects of hydrogen sulfide from wastewater treated anaerobically and future direction to remove hydrogen sulfide from biogas produced.

The role of pretreatments in handling antibiotic resistance genes in anaerobic sludge digestion - A review

Sludge is among the most important reservoirs of antibiotic resistance genes (ARGs), which would cause potential environmental risks with the sludge utilization. Currently, anaerobic digestion (AD) is effective to simultaneously realize the resource recovery and pollutants removal, including antibiotic resistance genes (ARGs), and various pretreatments are used to enhance the performance. Recently, plentiful publications have focused on the effects of pretreatment on ARGs removal, but the contradictory results are often obtained, and a comprehensive understanding of the research progress and mechanisms is essential. This study summarizes various pretreatment techniques for improving AD efficiency and ARGs reduction, investigates promising performance in ARGs removal when pretreatments combined with AD, and analyzes the potential mechanisms accounting for ARGs fates. The results showed that although thermal hydrolysis pretreatment showed the best performance in ARGs reduction during the pretreatment process, the significant rebound of ARGs would occur in the subsequent AD process. Conversely, ozone pretreatment and alkali pretreatment had no significant effect on ARGs abundance in the pretreatment stage, but could enhance ARGs removal by 15.6-24.3 % in the subsequent AD. Considering the efficiency and economic effectiveness, free nitrous acid pretreatment would be a promising and feasible option, which could enhance methane yield and ARGs removal by up to 27 % and 74.5 %, respectively. Currently, the factors determining ARGs fates during pretreatment and AD processes included the shift of microbial community, mobile genetic elements (MGEs), and environmental factors. A comprehensive understanding of the relationship between the fate of ARGs and pretreatment technologies could be helpful for systematically evaluating various pretreatments and facilitating the development of emerging and effective pretreatment techniques. Moreover, given the effectiveness, economic efficiency and environmental safety, we called for the applications of modern analysis approaches such as metagenomic and machine learning on the optimization of pretreatment conditions and revealing underlying mechanisms.

Improvement of Asterarcys quadricellulare biomass solubilization and subsequent biogas production via pretreatment approaches: structural changes and kinetic modeling evaluation

This study investigated the effect of enzymatic and hydrothermal pretreatment approaches on the solubilization of organic matter, structure, and biogas yield from microalgal biomass. The soluble chemical oxygen demand (sCOD) concentration increased by 1.21-3.30- and 5.54-6.60-fold compared to control by enzymatic and hydrothermal pretreatments respectively. The hydrothermal pretreatment affected the structural changes in the microalgal biomass markedly; nonetheless, increased enzymatic concentration also had a definite effect on it as determined by qualitative approaches like scanning electron microscopy and Fourier transform infrared spectroscopy. Also, the hydrothermal pretreatment (100 °C for 30 min) resulted in the highest biogas production potential (P) of 765.37 mLg^-1 VS at a maximum biogas production rate (R_m) of 22.66 mLg^-1 day^-1 with a very short lag phase (λ) of 0.07 days. The biogas production of pretreated microalgal biomass particularly at higher enzyme dose (20%, 24 h) and higher hydrothermal pretreatment temperature (120 °C, 30 min) showed a significant but weak correlation (R = 0.53) with sCOD, thus demonstrating that the less organic matter was used up for the biogas production. The modified Gompertz model explained the anaerobic digestion of microalgal biomass more accurately and had a better fit to the experimental data comparatively because of the low root mean square error (3.259-16.728), residual sum of squares (78.887-177.025), and Akaike's Information Criterion (38.605-62.853).

Ultrasonic Disintegration to Improve Anaerobic Digestion of Microalgae with Hard Cell Walls-Scenedesmus sp. and Pinnularia sp

Microalgae are considered to be very promising feedstocks for biomethane production. It has been shown that the structure of microalgal cell walls can be highly detrimental to the anaerobic digestibility of biomass. Therefore, there is a real need to seek ways to eliminate this problem. The aim of the present study was to assess the effect of ultrasonic disintegration of Scenedesmus sp. and Pinnularia sp. microalgal biomass on the performance and energy efficiency of anaerobic digestion. The pretreatment was successful in significantly increasing dissolved COD and TOC in the system. The highest CH₄ yields were noted for Scenedesmus sp. sonicated for 150 s and 200 s, which produced 309 ± 13 cm³/gVS and 313 ± 15 cm³/gVS, respectively. The 50 s group performed the best in terms of net energy efficiency at 1.909 ± 0.20 Wh/gVS. Considerably poorer performance was noted for Pinnularia sp., with biomass yields and net energy gains peaking at CH₄ 250 ± 21 cm³/gVS and 0.943 ± 0.22 Wh/gVS, respectively. Notably, the latter value was inferior to even the non-pretreated biomass (which generated 1.394 ± 0.19 Wh/gVS).

Kinetic analysis of microalgae cultivation utilizing 3D-printed real-time monitoring system reveals potential of biological CO2 conversion

The microalgae-based bioconversion process is a promising carbon utilization technology because it can upgrade CO2 into valuable substances, but a multiplex monitoring system required for process control to maximize biomass productivity has not been well established. Herein, a 3D printed real-time optical density monitoring device (RTOMD) combined platform was presented. This platform enables precise kinetics analysis by maintaining high accuracy (over 95 %) under raucous outdoor conditions. Through RTOMD-based high-frequency measurements, it was observed that maximum biomass productivity of 4.497 g L-1 d-1 was reached, which greatly exceeds the requirements for a feasible microalgae process. We discovered that the CO2 fixation efficiency could be achieved to 70.75 %, indicating the potential of a bioconversion process to realize a carbon-neutral society. Consequently, the RTOMD system can contribute to promoting microalgae cultivation as an attractive carbon mitigation technology based on an improved understanding of the photosynthetic CO2 fixation kinetics.

Microalgae and Cyanobacteria Biomass Pretreatment Methods: A Comparative Analysis of Chemical and Thermochemical Pretreatment Methods Aimed at Methane Production

Anaerobic digestion of microalgae and cyanobacteria was first proposed as a destination for algal biomass accumulated on stabilization ponds since it could not be disposed of directly in the environment. Now, the versatility of algal biomass makes them a suitable candidate to produce biofuels and other biomolecules in biorefineries. Anaerobic digestion of biomass is advantageous because it does not require the extraction of specific cellular constituents or drying of the biomass. Nevertheless, challenges remain regarding biomass concentration and their resistant cell walls, which are factors that could hamper methane yield. Many pretreatment methods, including chemical and thermochemical, have been proposed to break down the complex polymers present on the cell wall into smaller molecules. Unfortunately, the relationship between biomass solubilization and methane yield is not well defined. This article intends to review the anaerobic digestion of algal biomass and the role of chemical and thermochemical pretreatments in enhancing methane production. Several pretreatment conditions selected from the scientific literature were compared to verify which conditions actually improve methane yield. The severity of the selected pretreatments was also assessed using the combined severity factor. Results suggest that thermochemical pretreatment in less severe conditions is the most efficient, leading to a greater increase in methane yield. Only enzymatic pretreatments and some thermal pretreatments result in a positive energy balance. The large-scale implementation of pretreatment methods requires technological innovations to reduce energy consumption and its integration with other processes in wastewater treatment plants.

Strategies and advances in the pretreatment of microalgal biomass

Modification of structural components, especially the cell wall, through adequate pretreatment strategies is critical to the bioconversion efficiency of algal biomass to biorefinery products. Over the years, several physical, physicochemical, chemical and green pretreatment methods have been developed to achieve maximum productivity of desirable by-products to sustain a circular bioeconomy. The effectiveness of the pretreatment methods is however, species specific due to diversity in the innate nature of the microalgal cell wall. This review provides a comprehensive overview of the most notable and promising pretreatment strategies for several microalgae species. Methods including the application of stress, ultrasound, electromagnetic fields, pressure, heat as well as chemical solvents (ionic liquids, supercritical fluids, deep eutectic solvents etc.) have been detailed and analyzed. Enzyme and hydrolytic microorganism based green pretreatment methods have also been reviewed. Metabolic engineering of microorganisms for product specificity and lower inhibitors can be a future breakthrough in microalgal pretreatment.

Effects of pyrolysis parameters on physicochemical properties of biochar and bio-oil and application in asphalt

Adoption of renewable energy sources such as biomass has been increasing worldwide. In this study, fast pyrolysis as an acceptable and viable method to get renewable bio-oil and biochar is used. Different temperatures and N₂ flow velocities were used in the fast pyrolysis process to evaluate the pyrolysis yield of biochar and bio-oil. The waste wood and pig manure were utilized to prepare biochar and bio-oil. X-ray fluorescence, X-ray diffraction, high-pressure liquid chromatograph, Micro confocal laser Raman spectrometer, Fourier transform infrared spectrometer, and dynamic shear rheometer were used to measure the chemical compositions, structure, and pyrolysis yield of biochar and bio-oil. The obtained results indicate that pyrolysis temperature increases the purity of inorganic oxide in biochar and N₂ flow velocity promotes the yield of carbon in biochar. The increase of N₂ flow velocity would increase the acid property of bio-oil and damage the products yield of bio-oil. It was also observed that biochar could remarkably alter the fundamental performances of petroleum asphalt including penetration, softening point, ductility, viscosity, and complex modulus. The most important is that the upgraded bio-oil can be used to replace partly or fully the petroleum asphalt which is a promising biomass application.

Evaluation of pretreatments for solubilisation of components and recovery of fermentable monosaccharides from microalgae biomass grown in piggery wastewater

Microalgae-bacteria biomass cultured in wastewater is an interesting renewable material capable of metabolising nutrients from wastes into carbohydrates, proteins, and lipids through photosynthesis. Despite the interest in the valorisation of this biomass to improve the viability of microalgae-based wastewater treatment processes, very scarce research has been devoted to the fractional recovery of its components. This work evaluates the effect of different pretreatments coupled with enzymatic hydrolysis on the solubilisation of biomass components and on the recovery of fermentable monosaccharides (glucose and xylose) from Scenedesmaceae based biomass grown in a thin layer reactor feed with piggery wastewater. Chemical pretreatments generated high concentrations of byproducts, mainly organic acids. No bacterial DNA was found in these pretreated biomasses. The acid pretreatment provided the highest carbohydrate solubilisation (98%) and monosaccharide recovery (81%). Enzymatic hydrolysis coupled with alkaline NaOH 2 M pretreatment achieved almost complete solubilisation of the biomass components, but high carbohydrate losses. Physical pretreatments remarkably increased the solubilisation of the biomass components during the enzymatic hydrolysis step, especially bead milling, which achieved solubilisation yields of 83% of carbohydrates, 43% of proteins, and 60% of lipids. The presence of viable bacteria in these pretreated biomasses could be related to the high carbohydrate losses and the generation of methanol and ethanol in addition to organic acids as byproducts.

Integration of enzymatic pretreatment and sludge co-digestion in biogas production from microalgae

Integration of microalgae-based systems with conventional wastewater treatment plants provides an effective alternative to waste stream management. In this work, alkaline and enzymatic pretreatments of a microalgal culture mainly constituted by Chlorella sp. and Scenedesmus sp. and cultivated in wastewater from an industrial winery wastewater treatment plant were assessed. Microalgal enzymatic pretreatments were expected to overcome algal recalcitrancy before anaerobic digestion. pH-induced flocculation at pH 10 and 11 did not enhance microalgal harvesting and solubilisation, achieving a performance similar to that of natural sedimentation. Enzymatic hydrolysis of algal biomass was carried out using three commercial enzymatic cocktails (A, B and C) at two enzymatic doses (1% and 2% (v/v)) over 3 h of exposure time at 37 °C. Since pretreatments at a 1% dose for 0.5 h and 2% dose for 2 h achieved higher solubilisation, they were selected to evaluate the influence of the pretreatment on microalgal anaerobic digestibility. Biochemical methane potential tests showed that the pretreatments increased the methane production of the raw algal biomass 3.6- to 5.3-fold. The methane yield was 9-27% higher at the lower enzyme dose. Hence, microalgae pretreated with enzymes B and C at a 1% dose were co-digested with waste activated sludge (WAS). Even when the enzyme increased the methane yield of the inoculum and the WAS, the methane yield of the raw microalgae and WAS mixture was not significantly different from that obtained when algae were enzymatically pretreated. Nonetheless, co-digestion may achieve the goals of a waste recycled bio-circular economy.

Synergistic effect of growth conditions and organic carbon sources for improving biomass production and biodiesel quality by the microalga Choricystis minor var. minor

In the search for microalgae species with potential for biodiesel production, Choricystis minor var. minor has been seen as a promising source of biomass due to its high lipid content and the satisfactory characteristics of its fatty acid methyl esters (FAMEs). For this reason, the objective of this study was to investigate the synergistic effect of growth conditions and organic carbon sources on cultivation of this microalga. To do so, experimental cultivations were conducted in photoautotrophic, heterotrophic and mixotrophic metabolisms using glucose, fructose, glycerol or sucrose - in growth conditions that use organic carbon. Thus, growth parameters of the cultures were evaluated and at the end of the cultivations, FAMEs yield and profile were determined by gas chromatography, the efficiency of carbon conversion into biomass was evaluated and a microbial analysis was conducted. Regarding growth conditions, the findings have confirmed that, regardless of the organic carbon source used, the heterotrophic and mixotrophic metabolisms can present advantages over the photoautotrophic one. In addition, biomass production was higher with the use of glucose than with other organic carbon sources, regardless of growth condition (heterotrophic or mixotrophic). Moreover, cultivations with the addition of CO₂ have converted carbon into biomass less efficiently. On the other hand, photoautotrophic cultures presented the lowest bacterial load. In comparison to photoautotrophic and mixotrophic, heterotrophic cultures have led to lower FAMEs content and higher yields of unsaturated fatty acids. The most satisfactory FAMEs profile for biodiesel production was obtained with mixotrophic growth using fructose.

Phyco-remediation of swine wastewater as a sustainable model based on circular economy

Pork production has expanded in the world in recent years. This growth has caused a significant increase in waste from this industry, especially of wastewater. Although there has been an increase in wastewater treatment, there is a lack of useful technologies for the treatment of wastewater from the pork industry. Swine farms generate high amounts of organic pollution, with large amounts of nitrogen and phosphorus with final destination into water bodies. Sadly, little attention has been devoted to animal wastes, which are currently treated in simple systems, such as stabilization ponds or just discharged to the environment without previous treatment. This uncontrolled release of swine wastewater is a major cause of eutrophication processes. Among the possible treatments, phyco-remediation seems to be a sustainable and environmentally friendly option of removing compounds from wastewater such as nitrogen, phosphorus, and some metal ions. Several studies have demonstrated the feasibility of treating swine wastewater using different microalgae species. Nevertheless, the practicability of applying this procedure at pilot-scale has not been explored before as an integrated process. This work presents an overview of the technological applications of microalgae for the treatment of wastewater from swine farms and the by-products (pigments, polysaccharides, lipids, proteins) and services of commercial interest (biodiesel, biohydrogen, bioelectricity, biogas) generated during this process. Furthermore, the environmental benefits while applying microalgae technologies are discussed.

Wastewater based microalgal biorefinery for bioenergy production: Progress and challenges

Treatment of industrial and domestic wastewater is very important to protect downstream users from health risks and meet the freshwater demand of the ever-increasing world population. Different types of wastewater (textile, dairy, pharmaceutical, swine, municipal, etc.) vary in composition and require different treatment strategies. Wastewater management and treatment is an expensive process; hence, it is important to integrate relevant technology into this process to make it more feasible and cost-effective. Wastewater treatment using microalgae-based technology could be a global solution for resource recovery from wastewater and to provide affordable feedstock for bioenergy (biodiesel, biohydrogen, bio-alcohol, methane, and bioelectricity) production. Various microalgal cultivation systems (open or closed photobioreactors), turf scrubber, and hybrid systems have been developed. Although many algal biomass harvesting methods (physical, chemical, biological, and electromagnetic) have been reported, it is still an expensive process. In this review article, resource recovery from wastewater using algal cultivation, biomass harvesting, and various technologies applied in converting algal biomass into bioenergy, along with the various challenges that are encountered are discussed in brief.

Current and novel approaches to downstream processing of microalgae: A review

Biotechnological application of microalgae cultures at large scale has significant potential in the various fields of biofuels, food and feed, cosmetic, pharmaceutic, environmental remediation and water treatment. Despite this great potential application, industrialisation of microalgae culture and valorisation is still faced with serious remaining challenges in culture scale-up, harvesting and extraction of target molecules. This review presents a general summary of current techniques for harvesting and extraction of biomolecules from microalgae, their relative merits and potential for industrial application. The cell wall composition and its impact on microalgae cell disruption is discussed. Additionally, more recent progress and promising experimental methods and studies are summarised that would allow the reader to further investigate the state of the art. A final survey of energetic assessments of the different techniques is also made. Bead milling and high-pressure homogenisation seem to give clear advantages in terms of target high value compounds extraction from microalgae, with enzyme hydrolysis as a promising emerging technique. Future industrialisation of microalgae for high scale biotechnological processing will require the establishment of universal comparison-standards that would enable easy assessment of one technique against another.

Efficient superantioxidant and biofuel production from microalga Haematococcus pluvialis via a biorefinery approach

A biorefinery approach was implemented to produce a superantixoident, i.e., astaxanthin, and biofuels, i.e., ethanol and biogas, from the biomass of microalga Haematococcus pluvialis. The hydrolysis of residual biomass obtained from astaxanthin extraction was conducted using α-amylase and glucoamylase for hydrolysis of α-glucans and a mixture of cellulases for β-glucan hydrolysis. Four different hyudrolysis processes were employed and the efficiency of 97.2% over the total residual glucan was obtained, which was then fermented to produce 0.21 g ethanol/g residual biomass. The residuals obtained from astaxanthin extraction and fermentation were anaerobically digested to produce biomethane. The yield of biomethane was 264.8 ml/g volatile solids, 2.9 fold greater than methane yield from raw microalgal biomass. Overall, the process of astaxanthin extraction and consecutive production of ethanol and biogas from H. pluvialis biomass was recognized as a promising process to produce 45.8 g astaxanthin and 7095.3 KJ energy per Kg of raw biomass.

Optimization of alginate bead size immobilized with Chlorella vulgaris and Chlamydomonas reinhardtii for nutrient removal

Photo-bioreactor experiments using three different size beads (2.0, 3.5, and 5.0 mm) immobilized with two different types of microalgae namely Chlorella vulgaris and Chlamydomonas reinhardtii were conducted to evaluate the nutrient removal efficiency. The highest nutrient removal was obtained at gel bead pore size of 3.5 mm for both species of C. vulgaris and Ch. reinhardtii. 95% removal of T-N and complete reduction of T-P were achieved within 3 stages of treatment in photo-bioreactors containing 20% algal bead volume fraction. Moreover, the results observed by confocal laser scanning microscopy (CLSM) using SYTOX red dye and SYTOX green dye in alginate beads indicated that the effective depth of C. vulgaris and Ch. reinhardtii was about 3.6 mm and 3.0 mm, respectively. This optimized cell immobilization technology would accelerate the nutrient uptake rate of microalgae for improving efficiency of wastewater treatment systems.

Nanoparticle induced biological disintegration: A new phase separated pretreatment strategy on microalgal biomass for profitable biomethane recovery

This study involves the application of new phase separated biological pretreatment (PSBP) strategy on microalgal biomass using the nickel nanoparticle induced cellulase secreting bacterial disintegration. Particularly, interest was focussed on cell wall weakening (CWW) of microalgae biomass besides the cell disintegration (CD) and release of organics. During CWW, protein, carbohydrate, cellulose, hemicellulose and DNA were used as evaluation indexes. Similarly, during CD, soluble chemical oxygen demand was used as evaluation index to assess the disintegration effect. A higher CWW was achieved at nickel nanoparticle (Np) dosage of 0.004 g/g SS. During CD, a clear demarcation in biomass solubilisation was achieved by PSBP (36%) than the sole biological pretreatment -BP (24%). The biomethanogenesis test results showed that enhanced methane production of 411 mL/g COD was achieved by PSBP than BP. Energy analysis showed that a higher net energy production of 6.467 GJ/d was achieved by PSBP.

Optimisation of the production of fermentable monosaccharides from algal biomass grown in photobioreactors treating wastewater

Biomass grown in wastewater treatment photobioreactors is a cheap raw material with high contents of carbohydrates, proteins and lipids. This work studies the production of fermentable monosaccharides from three biomasses grown in piggery wastewater (P), domestic wastewater (W) and synthetic medium (S) by applying chemical pretreatment and enzymatic hydrolysis, using a Taguchi design. ANOVA identified temperature, chemical reagent type and chemical reagent concentration as significant operational parameters. However, the biomass concentration, pretreatment time, enzyme dosage and enzymatic hydrolysis time had no remarkable effect. The bacterial content of the biomass had no relevant impact on carbohydrate and protein solubilisation but had a remarkable effect on the degradation of the released carbohydrates (57, 60 and 37% for P, W and S), while also affecting lipid solubilisation. Pretreatment with HCl 2 M at 120 °C resulted the optimal conditions, achieving a monosaccharide recovery of 53, 59 and 80% for P, W and S biomasses, respectively.

Energy balance and life cycle assessment of a microalgae-based wastewater treatment plant: A focus on alternative biogas uses

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.