Anaerobic co-digestion of microalgae Chlorella vulgaris and potato processing waste: Effect of mixing ratio, waste type and substrate to inoculum ratio

Anaerobic co-digestion of microalgal biomass, sugarcane vinasse, and residual glycerol from biodiesel using simplex-centroid mixture design: methane potential, synergic effect, and microbial diversity

Microalgal biomass (MB) is a promising feedstock for bioenergy production. Nonetheless, the cell recalcitrance and the low C/N ratio limit the methane yield during anaerobic digestion. As an alternative to overcome these challenges, MB co-digestion with different feedstocks has been proposed. Thus, this study evaluated the anaerobic co-digestion (AcoD) of MB cultivated in wastewater with sugarcane vinasse (VIN) and residual glycerol from biodiesel production (GLY). Batch tests were conducted using augmented simplex-centroid mixture design to investigate the impact of AcoD on methane production (SMP), synergistic effects, and the influence on microbial community. When compared to MB digestion, 150 NmL CH4.g-1VS, binary and ternary AcoD achieved SMP increases from 120 to 337%. The combination of 16.7:16.7:66.7 (MB:VIN:GLY) showed the highest SMP for a ternary mixture (631 NmL CH4.g-1VS). Optimal synergies ranged from 1.3 to 1.4 and were primarily found for the MB:GLY AcoD. Acetoclastic Methanosaeta genus was predominant, regardless the combination between substrates. Despite the largest SMP obtained from the MB:GLY AcoD, other ternary mixtures were also highly synergetic and therefore had strong potential as a strategic renewable energy source.

Acidogenic fermentation of potato peel waste for volatile fatty acids production: Effect of initial organic load

As a renewable carbon source produced from organic wastes by acidogenic fermentation, volatile fatty acids (VFAs) are important intermediates in chemical and biological fields and beneficial to resource recovery and carbon neutrality. Maximizing VFA production by some strategies without additional chemicals is critical to increasing economic and environmental benefits. In this study, the effects of initial organic load (OL) on the performance of VFA production, variations of intermediate metabolites, and the thermogravimetric properties of potato peel waste (PPW) during batch acidogenic fermentation were studied. The results showed that the concentration of VFAs increased with the increase of initial OL, while the VFA yield decreased with the increase of initial OL. When the initial OL was in the range of 28.4 g VS/L-91.3 g VS/L, the fermentation type of PPW was butyric acid fermentation. The highest butyric acid proportion of 61.3% was achieved with the initial OL of 71.5 g VS/L. With the increase of initial OL, the proportion of acetic acid and the utilization rate of protein in the PPW decreased. VFAs were produced from proteins and carbohydrates in the early stage and mainly produced from carbohydrates in the later stage. The production efficiency of VFA was relatively high with the initial OL of 71.5 g VS/L, because more easily-biodegradable compounds were solubilized. The results showed that suitably increased initial OL could accelerate acidogenesis, reduce hydrolysis time, and increase the proportion of butyric acid. The findings in this work suggest that PPW is a promising feedstock for butyric acid biosynthesis and appropriate initial OL is beneficial to VFA production.

Anaerobic Co-Digestion of Pig Manure and Rice Straw: Optimization of Process Parameters for Enhancing Biogas Production and System Stability

The objective of this study was to optimize the process parameters of the anaerobic co-digestion of pig manure and rice straw to maximize methane production and system stability. In this study, batch experiments were conducted with different mixing ratios of pig manure and rice straw (1:0, 1:1, 1:5, 1:10, and 0:1), total solid concentrations (6%, 8%, 10%, 12%, and 14%), and inoculum accounts (5%, 10%, 15%, 20%, and 25%). The results show that a 1:5 mixing ratio of pig manure to rice straw, a 12% total solid content, and a 15% inoculum account yielded biogas up to 553.79 mL/g VS, which was a result of co-digestion increasing the cooperative index (CPI > 1). Likewise, the evolution of the pH and VFAs indicated that the co-digestion system was well-buffered and not easily inhibited by acidification or ammonia nitrogen. Moreover, the results of the Gompertz model's fitting showed that the cumulative methane production, delay period, effective methane production time, and methane production rate under optimal conditions were significantly superior compared to the other groups employed.

Sustainable microalgal biomass valorization to bioenergy: Key challenges and future perspectives

The global trend is shifting toward circular economy systems. It is a sustainable environmental approach that sustains economic growth from the use of resources while minimizing environmental impacts. The multiple industrial use of microalgal biomass has received great attention due to its high content of essential nutrients and elements. Nevertheless, low biomass productivity, unbalanced carbon to nitrogen (C/N) ratio, resistant cellular constituents, and the high cost of microalgal harvesting represent the major obstacles for valorization of algal biomass. In recent years, microalgae biomass has been a candidate as a potential feedstock for different bioenergy generation processes with simultaneous treating wastewater and CO₂ capture. An overview of the appealing features and needed advancements is urgently essential for microalgae-derived bioenergy generation. The present review provides a timely outlook and evaluation of biomethane production from microalgal biomass and related challenges. Moreover, the biogas recovery potential from microalgal biomass through different pretreatments and synergistic anaerobic co-digestion (AcoD) with other biowastes are evaluated. In addition, the removal of micropollutants and heavy metals by microalgal cells via adsorption and bioaccumulation in their biomass is discussed. Herein, a comprehensive review is presented about a successive high-throughput for anaerobic digestion (AD) of the microalgal biomass in order to achieve for sustainable energy source. Lastly, the valorization of the digestate from AD of microalgae for agricultural reuse is highlighted.

Integral Valorization of Two-Phase Olive Mill Solid Waste (OMSW) and Related Washing Waters by Anaerobic Co-digestion of OMSW and the Microalga Raphidocelis subcapitata Cultivated in These Effluents

This study evaluates the comprehensive valorization of the byproducts derived from the two-phase olive oil elaboration process [i.e., olive washing water (OWW), olive oil washing water (OOWW), and olive mill solid waste (OMSW)] in a closed-loop process. Initially, the microalga Raphidocelis subcapitata was grown using a mixture of OWW and OOWW as the culture medium, allowing phosphate, nitrate, sugars, and soluble chemical oxygen demand removal. In a second step, the microalgal biomass grown in the mixture of washing waters was used as a co-substrate together with OMSW for an anaerobic co-digestion process. The anaerobic co-digestion of the combination of 75% OMSW-25% R. subcapitata enhanced the methane yield by 7.0 and 64.5% compared to the anaerobic digestion of the OMSW and R. subcapitata individually. This schedule of operation allowed for integration of all of the byproducts generated from the two-phase olive oil elaboration process in a full valorization system and the establishment of a circular economy concept for the olive oil industry.

Influence of augmentation of biochar during anaerobic co-digestion of Chlorella vulgaris and cellulose

The anaerobic co-digestion (AcoD) of microalgae is a prospective option for generating biomethane from renewable sources. This study investigates the effects of inoculum-to-substrate ratio (ISR), C/N ratio and biochar (BC) load on the AcoD of Chlorella vulgaris and cellulose. An initial augmentation of BC at ISR 0.5-0.9 and C/N ratio 10-30 offered a pH buffering effect and resulted in biomethane yields of 233-241 mL CH₄/g VS, corresponding to 1.8-4.6 times the controls. BC addition ameliorated significantly AcoD, supporting the digestate stability at less favourable conditions. The effect of the process variables was further studied with a 2³ factorial design and response optimisation. Under the design conditions, the variables had less influence over methane production. Higher ISRs and C/N ratios favoured AcoD, whereas increasing amounts of BC reduced biomethane yield but enhanced production rate. The factorial design highlighted the importance of BC-load on AcoD, establishing an optimum of 0.58 % (w/v).

Anaerobic digestion and agronomic applications of microalgae for its sustainable valorization

Microalgae are considered potential candidates in biorefinery processes, and due to their biochemical properties, they can be used in the production of biofuels such as biogas, as well as for bioremediation of liquid effluents. The objective of this review is to study the current status of microalgae anaerobic digestion and agricultural uses (as bio-stimulants and biofertilizers), starting from microalgae cultivation. Indeed, the efficiency of these processes necessarily depends on the evaluation of different biotic and abiotic factors that affect the growth of microalgae. However, the adaptation and the optimization of process parameters on a large scale is also limited by energy and economic constraints. Moreover, the integration of biogas production processes with microalgae cultivation allows a nutrients and CO2 virtuous loop, thus promoting the sustainability of the process. Finally, this paper provides a general overview of biogas and biofertilizers production combination, as well as the related challenges and recommended future research perspectives to complement the gap in the literature.

Integrating anaerobic digestion of potato peels to methanol production by methanotrophs immobilized on banana leaves

In the present study, potato peels were subjected to anaerobic digestion (AD) to produce biogas (methane [CH₄] and carbon dioxide), which was subsequently used as a substrate for methanol production by methanotrophs. AD resulted in high yields of up to 170 L CH₄/kg total solids (TS) from 250 mL substrate (2% TS, w/v). Under optimized conditions, maximum methanol production of 4.97 and 3.36 mmol/L from raw biogas was observed in Methylocella tundrae and Methyloferula stellata, respectively. Immobilization of methanotrophs on banana leaves showed loading of up to 156 mg dry cell mass/g support. M. tundrae immobilized on banana leaves retained 31.6-fold higher methanol production stability, compared to non-immobilized cells. To the best of our knowledge, this is the first study on immobilization of methanotrophs on banana leaves for producing methanol from potato peels AD-derived biogas. Such integrative approaches may be improved through process up-scaling to achieve sustainable development.

Influence of temperature on enhancement of volatile fatty acids fermentation from organic fraction of municipal solid waste: Synergism between food and paper components

This study explores individual contributions and synergistic effects of food and paper, main components of organic fraction of municipal solid waste (OFMSW) towards volatile fatty acids (VFA) fermentation under different temperatures (25, 37, 42 and 52 °C). Thanks to the synergism of food and paper component (FC & PC), the results revealed that OFMSW is suitable for VFA production. Maximum VFA production was noticed to be 21.5 mg/L at 42 °C, ~2.1, and 1.42 times higher than fermentation of PC and FC. Enhanced hydrolysis of PC occurred at >37 °C, increasing alkalinity in leachate to 6.7 g/L at 42 °C, thus maintaining a stable pH (5.4-5.6) during acidogenic fermentation. Additionally, 74% of COD is hydrolyzed, of which 79% is converted to VFA based on biodegradable carbon at 42 °C. It is suggested that co-existence of FC and PC can enhance VFA production of OFMSW, and targeted VFA production can be maximized through process optimization.

Pretreatment and co-digestion of microalgae, sludge and fat oil and grease (FOG) from microalgae-based wastewater treatment plants

The aim of this study was to assess the co-digestion of residual biomass flows generated in microalgae-based wastewater treatment plants: microalgae, primary sludge and fat, oil and grease (FOG), with and without microalgae thermal pretreatment. The results evidenced the high methane yield of FOG (563 mL CH₄/g VS) as compared to microalgae (140 mL CH₄/gVS) and sludge (299 mL CH₄/g VS). The methane yield of microalgae and sludge co-digestion (50-50% VS) was increased by 25 and 42% by adding 10 and 20% VS of FOG, respectively. Moreover, co-digestion trials improved the anaerobic digestion first-order kinetics by up to 67%. Regarding the thermal pretreatment, it increased the methane yield of microalgae by 60%, and 15% upon co-digestion with sludge and FOG. Therefore, co-digestion of microalgae, primary sludge and FOG appears as a promising strategy to enhance the biogas production, hence bioenergy recovery from wastewater, even without pretreatment.

Co-digestion of microalgae with potato processing waste and glycerol: effect of glycerol addition on methane production and the microbial community

The production of methane-rich biogas from the anaerobic digestion (AD) of microalgae is limited by an unfavorable biomass carbon-to-nitrogen (C/N) ratio; however, this may be ameliorated using a co-digestion strategy with carbon-rich feedstocks. For reliable plant operation, and to improve the economics of the process, secure co-feedstock supply (ideally as a waste-stream) is important. To this end, this study investigated the feasibility of co-digesting microalgae (Chlorella vulgaris) with potato processing waste (potato discarded parts, PPW_dp; potato peel, PPW_p) and glycerol, while monitoring the response of the methanogenic community. In this semi-continuous study, glycerol (1 and 2% v/v) added to mixtures of C. vulgaris : PPW_dp enhanced the specific methane yields the most, by 53–128%, whilst co-digestion with mixtures of C. vulgaris : PPW_p enhanced the methane yields by 62–74%. The microbial communities diverged markedly over operational time, and to a lesser extent in response to glycerol addition. The acetoclast Methanosaeta was abundant in all treatments but was replaced by Methanosarcina in the potato peel with glycerol treatment due to volatile fatty acid (VFA) accumulation. Our findings demonstrate that the performance of microalgae co-digestion is substantially improved by the addition of glycerol as an additional co-feedstock. This should improve the economic case for anaerobically digesting microalgae as part of wastewater treatment processes and/or the terminal step of a microalgae biorefinery.

Glycerol as an additional co-substrate enhanced methane yields by up to 128% when co-digestion with microalgae and potato waste.

Evaluation of system stability and anaerobic conversion performance for corn stover using combined pretreatment

A novel pretreatment method combining freezing-thawing and ammonia was developed, and the system stability and anaerobic conversion performance of corn stover (CS) were investigated. The CS was pretreated by water freezing-thawing (water-FT) and ammonia freezing-thawing (ammonia-FT), and then pretreated and untreated CS were anaerobically digested in three continuously stirred tank biodigesters (CSTBRs) at three organic loading rate (OLR) of 1.6, 1.8 and 2.0 gVS L^-1 d^-1. The analyses showed that pH value and volatile fatty acids (VFAs) concentration of three systems were in reasonable ranges of 6.8-7.5 and 160-330 mg L^-1, respectively. Total ammonia nitrogen (TAN) in R3 system ranged from 600 mg L^-1 to 1300 mg L^-1, which was conducive to maintain system stability. The removal amount of soluble chemical oxygen demand (SCOD) of CS by ammonia-FT pretreatment was 17.3-20.5% higher than that of water-FT pretreatment. The results also showed that CS pretreated by ammonia-FT achieved 27.8-32.4% and 13.9-16.1% more methane yield (250-267 mL g_vs^-1) than those of untreated and water-FT pretreatment, respectively. Correspondingly, ammonia-FT pretreated CS in R3 obtained the highest conversion rates of 47.7%, 56.9%, 42.7%, and 48.8% for TS, VS, cellulose, and hemicellulose, respectively, and the lowest cost was of 0.0336 USD m³ CH₄^-1 and the highest energy recovery of 2428.1 kWh kg_vs^-1. The study demonstrated that ammonia-FT pretreatment is one of efficient and cost effective method for stable CS bioconversion.

The effect of ISR on OFMSW during acidogenic fermentation for the production of AD precursor: kinetics and synergies

Acidogenic fermentation of organic fraction of municipal solid waste (OFMSW) and it's components (food waste and paper wastes) was studied in batch percolator reactor without artificial pH adjustment.