Carbon balance of major volatile fatty acids (VFAs) in recycling algal residue via a VFA-platform for reproduction of algal biomass

Pathways, challenges, and strategies for enhancing anaerobic production of short-chain and medium-chain carboxylic acids from algal slurry derived from wastewater

Algal slurry (AS) generated from microalgae-based wastewater treatment processes holds significant potential for carboxylic acids production through anaerobic digestion (AD), which have emerged as promising products due to their high energy density, great economic value, and versatile applications. A comprehensive analysis of the pathways and optimization strategies for producing short-chain (SCCAs) and medium-chain (MCCAs) carboxylic acids using AS substrates is presented in this review. It begins by introducing and comparing two types of microalgae-based wastewater treatment processes: the microalgae process and the microalgal-bacterial consortia process. Afterwards, the review systematically examines the metabolic pathways involved in SCCAs and MCCAs production using AS substrates. Moreover, pretreatment strategies for enhancing the release of organic matter are critically discussed. Ultimately, specific emphasis is placed on addressing technical challenges and discussing future perspectives. This review provides a deeper understanding of the mechanisms and strategies involved in carboxylic acids production from wastewater-generated AS.

Dark fermentation and microalgae cultivation coupled systems: Outlook and challenges

The implementation of a sustainable bio-based economy is considered a top priority today. There is no doubt about the necessity to produce renewable bioenergy and bio-sourced chemicals to replace fossil-derived compounds. Under this scenario, strong efforts have been devoted to efficiently use organic waste as feedstock for biohydrogen production via dark fermentation. However, the technoeconomic viability of this process needs to be enhanced by the valorization of the residual streams generated. The use of dark fermentation effluents as low-cost carbon source for microalgae cultivation arises as an innovative approach for bioproducts generation (e.g., biodiesel, bioactive compounds, pigments) that maximizes the carbon recovery. In a biorefinery context, after value-added product extraction, the spent microalgae biomass can be further valorised as feedstock for biohydrogen production. This integrated process would play a key role in the transition towards a circular economy. This review covers recent advances in microalgal cultivation on dark fermentation effluents (DFE). BioH₂ via dark fermentation processes and the involved metabolic pathways are detailed with a special focus on the main aspects affecting the effluent composition. Interesting traits of microalgae and current approaches to solve the challenges associated to the integration of dark fermentation and microalgae cultivation are also discussed.

Enhancement of Carbon Conversion and Value-Added Compound Production in Heterotrophic Chlorella vulgaris Using Sweet Sorghum Extract

High-cost carbon sources are not economical or sustainable for the heterotrophic culture of Chlorella vulgaris. In order to reduce the cost, this study used sweet sorghum extract (SE) and its enzymatic hydrolysate (HSE) as alternative carbon sources for the heterotrophic culture of Chlorella vulgaris. Under the premise of the same total carbon concentration, the value-added product production performance of Chlorella vulgaris cultured in HSE (supplemented with nitrogen sources and minerals) was much better than that in the glucose medium. The conversion rate of the total organic carbon and the utilization rate of the total nitrogen were both improved in the HSE system. The biomass production and productivity using HSE reached 2.51 g/L and 0.42 g/L/d, respectively. The production of proteins and lipids using HSE reached 1.17 and 0.35 g/L, respectively, and the production of chlorophyll-a, carotenoid, and lutein using HSE reached 30.42, 10.99, and 0.88 mg/L, respectively. The medium cost using HSE decreased by 69.61% compared to glucose. This study proves the feasibility and practicability of using HSE as a carbon source for the low-cost heterotrophic culture of Chlorella vulgaris.

Tuning microbial community in non-conventional two-stage anaerobic bioprocess for microalgae biomass valorization into targeted bioproducts

Unspecific microorganisms consortia are normally used in anaerobic biodegradation of solid wastes. However, these consortia can be tuned to optimally obtain determined bioproducts. In this study, high value-added products and biogas were obtained via an innovative two-stage anaerobic bioprocess from microalgae biomass. The anaerobic fermentation (AF) entailed the production of short-chain fatty acids (SCFAs) and subsequently, only the solid spent of AF effluent was valorized for methane production via conventional anaerobic digestion (AD). Applied conditions in AF (25 °C, HRT 8 days) favored Firmicutes predominance (64%) enabling a conversion efficiency of 32.1% g SCFAs-COD/g COD_in. Opposite, a wider microbial biodiversity was determined in the AD reactor (35 °C, HRT 20 days), being mainly composed by Firmicutes (28.6%), Euryarchaeota (17.7%) and Proteobacteria (15.3%). AD of the AF-solid spent reached 168.9 mL CH₄ /g COD_in. Strikingly, operational conditions imposed mediated a microbial specialization that maximized product output.

Application of attached algae flow-ways for coupling biomass production with the utilization of dilute non-point source nutrients in the Upper Laguna Madre, TX

The purpose of this study is to determine the potential for an attached algae flow-way system to efficiently produce algal biomass in estuarine surface waters by utilizing dilute non-point source nutrients from local urban, industrial, and agricultural discharges into the Upper Laguna Madre, Corpus Christi, Texas. The study was conducted over the course of two years to establish seasonal base-line biomass productivity and composition for bioproducts applications, and to identify key environmental factors and flow-way cohorts impacting biomass production. For the entire cultivation period, continuous ash-free biomass production at 4 to 10 g/m²/day (corresponding to nutrient recovery at 300 to 500 mg of nitrogen/m²/day and 15 to 30 mg of phosphorus/m²/day) was successfully achieved without system restart. Upon start-up, a latency period was observed which indicates roles for species succession from relatively low productivity, high ash content pioneer periphytic culture composed primarily of benthic diatoms from the source waters to higher productivity, reduced ash content, and more resilient culture mainly composed of filamentous chlorophyta, Ulva lactuca. Principal Component Analysis (PCA) was used to identify environmental factors driving biomass production, and machine learning (ML) models were constructed to assess the predictive capability of the data set for system performance using the local multi-season environmental variations. Environmental datasets were segregated for ML training, validation, and testing using three methods: regression tree, ensemble regression, and Gaussian process regression (GPR). The predicted ash-free biomass productivity using ML models resulted in root-squared-mean-errors (RSME) from 1.78 to 1.86 g/m²/day, and R² values from 0.67 to 0.75 using different methods. The greatest contributor to net productivity was total solar irradiation, followed by air temperature, salinity, and pH. The results of the study should be useful as a decision-making tool to application of attached algae flow-ways for biomass production while preventing algal blooms in the environment.

Biodiesel synthesis from swine manure

This study demonstrates that the biodiesel (BD) from swine manure (SM) could be a promising way for large scale generation of biofuel. Also, the economic and environmental benefits of SM derived BD were evaluated. Transesterification of lipid contents extracted from the collected SM had low BD yield (14.2 wt%) using H₂SO₄ catalyst due to high acid value and impurities. However, thermo-chemical non-catalytic transesterification with a porous material showed 94.7 wt% yield of BD from the lipid in SM. Considering the current population of swine, the annual production of BD from SM was estimated. The SM derived BD could cover 19.7 and 46.8 wt% of BD currently produced in both Korea and the USA with the economic benefits of up to $96 million and $2.1 billion, respectively. The proposed approach also can save vast arable lands needed to cultivate oil-bearing feedstocks for BD production.

Anaerobic degradation of protein-rich biomass in an UASB reactor: Organic loading rate effect on product output and microbial communities dynamics

Anaerobic degradation of enzymatically pretreated Chlorella vulgaris was aimed in an upflow anaerobic sludge blanket reactor (UASB) to evaluate the organic loading rate (OLR) effect on biomass valorization. Low OLRs resulted in high methane yields (171 mL CH₄/g CODin) at low hydraulic retention time (HRT of 6 days). Firmicutes (35-43%), Bacteroidetes (17-18%) and Euryarchaeota (11%) dominated at low OLRs, promoting methanogenic activity. On the contrary, the highest OLRs resulted in low methane yield (86 mL CH₄/gCODin) with a concomitant short-chain fatty acids (SCFAs) accumulation of 37% SCFAs-COD/CODin. The highest OLR decreased UASB reactor biodiversity, hampering Euryarchaeota population development (2.5%) and boosting Firmicutes (55%) and Proteobacteria (14%). These results demonstrated the suitability of UASB reactor configuration to reach high bioprocess efficiency for both, biogas and SCFAs production, with lower energetic and area requirements than those normally needed in continuous stirred tank reactors.

Deoiled algal biomass derived renewable sugars for bioethanol and biopolymer production in biorefinery framework

The present study is designed to evaluate the potential of deoiled algal biomass (DAB) residue as an alternative resource for the production of bioethanol and biopolymers in a biorefinery approach. Hybrid pretreatment method resulted in higher sugar solubilization (0.590 g/g DAB) than the corresponding individual physicochemical (0.481 g/g DAB) and enzymatic methods (0.484 g/g DAB). Subsequent utilization of sugars from hybrid pretreatment for bioethanol using Saccharomyces cerevisiaeresulted in maximum bioethanol production at pH 5.5 (0.145 ± 0.008 g/g DAB) followed by pH 5.0 (0.122 ± 0.004 g/g DAB) and pH 6.0 (0.102 ± 0.002 g/g DAB). The experiments for biopolymer (PHB: polyhydroxybutyrate) production resulted in 0.43 ± 0.20 g PHB/g DCW. Extracted polymer on NMR and FT-IR analysis showed the presence of PHB. Exploration of DAB as an alternative renewable resource for multiple biobased products supports sustainability and also enables entirety use of DAB by addressing the DAB-residue allied disposal issues.

Algal biorefinery models with self-sustainable closed loop approach: Trends and prospective for blue-bioeconomy

Microalgae due to its metabolic versatility have received a focal attention in the biorefinery and bioeconomy context. Microalgae products have broad and promising application potential in the domain of renewable fuels/energy, nutraceutical, pharmaceuticals and cosmetics. Biorefining of microalgal biomass in a circular loop with an aim to maximize resource recovery is being considered as one of the sustainable option that will have both economical and environmental viability. The expansive scope of microalgae cultivation with self-sustainability approach was discussed in this communication in the framework of blue-bioeconomy. Microalgae based primary products, cultivation strategies, valorization of microalgae biomass for secondary products and integrated biorefinery models for the production of multi-based products were discussed. The need and prospect of self-sustainable models in closed loop format was also elaborated.

Volatile Fatty Acids Production from Microalgae Biomass: Anaerobic Digester Performance and Population Dynamics during Stable Conditions, Starvation, and Process Recovery

Disturbances in anaerobic digestion (AD) negatively impact the overall reactor performance. These adverse effects have been widely investigated for methane generation. However, AD recently appeared as a potential technology to obtain volatile fatty acids (VFAs) and thus, the impact of process disturbances must be evaluated. In this sense, microbial response towards a starvation period of two weeks was investigated resulting in a conversion of organic matter into VFAs of 0.39 ± 0.03 COD-VFAs/CODin. However, the lack of feeding reduced the yield to 0.30 ± 0.02 COD-VFAs/CODin. Microbial analysis revealed that the starvation period favored the syntrophic acetate-oxidizing bacteria coupled with hydrogenotrophic methanogens. Finally, the system was fed at 9 g COD/Ld resulting in process recovery (0.39 ± 0.04 COD-VFAs/CODin). The different microbiome obtained at the end of the process was proved to be functionally redundant, highlighting the AD robustness for VFAs production.

Microalgae Biomass as a Potential Feedstock for the Carboxylate Platform

Volatile fatty acids (VFAs) are chemical building blocks for industries, and are mainly produced via the petrochemical pathway. However, the anaerobic fermentation (AF) process gives a potential alternative to produce these organic acids using renewable resources. For this purpose, waste streams, such as microalgae biomass, might constitute a cost-effective feedstock to obtain VFAs. The present review is intended to summarize the inherent potential of microalgae biomass for VFA production. Different strategies, such as the use of pretreatments to the inoculum and the manipulation of operational conditions (pH, temperature, organic loading rate or hydraulic retention time) to promote VFA production from different microalgae strains, are discussed. Microbial structure analysis using microalgae biomass as a substrate is pointed out in order to further comprehend the roles of bacteria and archaea in the AF process. Finally, VFA applications in different industry fields are reviewed.