Pavlo Bohutskyi

The effects of alternative pretreatment strategies on anaerobic digestion and methane production from different algal strains

The effect of various pretreatment strategies on methane yields following anaerobic digestion (AD) of five different microalgal strains was investigated. Pavlova_cf sp., Tetraselmis sp. and Thalassiosira weissflogii exhibited substantial methane yields of 0.4-0.5L/g volatile solids (VS) without pretreatment, providing up to 75-80% of theoretical values. In contrast, methane yields from Chlorella sp. and Nannochloropsis sp. were around 0.35L/g VS, or 55-60% of the theoretical values, respectively. Alkali treatment was not effective and thermal pretreatment only enhanced Nannochloropsis methane yields. Thermochemical pretreatment had the strongest impact on biomass solubilization with methane yields increasing by 30% and 40% for Chlorella and Nannochloropsis, respectively. The lipid content had a strong beneficial impact on the theoretical and observed methane yields as compared to protein and carbohydrate content. Other features such as cell-wall composition are also likely to be important factors dictating algal biodegradability and methane yields addressed in part by thermochemical pretreatment.

Biogas Production from Algae and Cyanobacteria Through Anaerobic Digestion: A Review, Analysis, and Research Needs

Anaerobic digestion is a common process for the treatment of a variety of organic wastes and biogas production. Both, macro- and microalgae are suitable renewable substrates for the anaerobic digestion process. The process of biogas production from algal biomass is an alternative technology that has larger potential energy output compared to green diesel, biodiesel, bioethanol, and hydrogen production processes. Moreover, anaerobic digestion can be integrated into other conversion processes and, as a result, improve their sustainability and energy balance. Several techno-economic constraints need to be overcome before the production of biogas from algal biomass becomes economically feasible. These constraints include a high cost of biomass production, limited biodegradability of algal cells, a slow rate of biological conversion of biomass to biogas, and high sensitivity of methanogenic microorganisms. The research opportunities include a variety of engineering and scientific tasks, such as design of systems for algae cultivation and anaerobic digestion; optimization of algae cultivation in wastewater, nutrients recycling and algal concentration; enhancement of algal biomass digestibility and conversion rate by pretreatment; deep integration with other technological processes (e.g., wastewater treatment, co-digestion with other substrates, carbon dioxide sequestration); development and adaptation of molecular biology tools for the improvement of algae and anaerobic microorganisms; application of information technologies; and estimation of the environmental impact, energy and economical balance by performing a life cycle analysis.

Anaerobic digestion of lipid-extracted Auxenochlorella protothecoides biomass for methane generation and nutrient recovery

This study evaluated methane production and nutrient recovery from industrially produced, lipid extracted algal biomass (LEA) of Auxenochlorella protothecoides using semi-continuous anaerobic digestion (AD) at different organic loading rates (OLRs) and hydraulic retention times (HRTs). It was shown, that AD can improve biofuel production efficiency and sustainability, especially for scaled processes, through up to 30% increase in energy generation (up to 0.25 L of methane per g of LEA volatile solids) and partial nutrient recovery and recycling. The nutrient recycling with the AD effluent may reduce the cost of the supplied fertilizers by up to 45%. However, methane production was limited to nearly 50% of theoretical maxima potentially due to biomass recalcitrance and inhibition effects from the residual solvent in the LEA. Therefore, further AD optimization is required to maximize methane yield and nutrient recovery as well as investigation and elimination of inhibition from solvent residues.

Phytoremediation of agriculture runoff by filamentous algae poly-culture for biomethane production, and nutrient recovery for secondary cultivation of lipid generating microalgae

An integrated system was implemented for water phytoremediation and biofuel production through sequential cultivation of filamentous algae followed by cultivation of lipid-producing microalgae Chlorella sorokiniana. Natural poly-culture of filamentous algae was grown in agricultural stormwater using the Algal Turf Scrubber®, harvested and subjected for lipid extraction and/or methane production using anaerobic digestion (AD). While filamentous algae lipid content was too low for feasible biodiesel production (<2%), both whole biomass and lipid-extracted algal residues (LEA) yielded ∼0.2LmethanepergVS at loading rates up to 5gVS/L-day. Importantly, essential macro-nutrients and trace elements captured from stormwater were released into the AD effluent as soluble nutrients and were successfully tested as fertilizer replacement for cultivation of lipid-accumulating C. sorokiniana in a subsequent stage. Accordingly, filamentous algae poly-culture was exploited for waste nutrient capturing and biofuel feedstock generation. These nutrients were recovered and reused as a concentrated supplement for potentially high-value microalgae.

Synergistic co-digestion of wastewater grown algae-bacteria polyculture biomass and cellulose to optimize carbon-to-nitrogen ratio and application of kinetic models to predict anaerobic digestion energy balance

This study investigated enhancing methane production from algal-bacteria biomass by adjusting the C/N ratio through co-digestion with a nitrogen-poor co-substrate - cellulose. A biomethane potential test was used to determine cumulative biogas and methane production for pure and co-digested substrates. Four kinetic models were evaluated for their accuracy describing experimental data. These models were used to estimate the total energy output and net energy ratio (NER) for a scaled AD system. Increasing the algal C/N ratio from 5.7 to 20-30 (optimal algae:cellulose feedstock ratios of 35%:65% and 20%:80%) improved the ultimate methane yield by >10% and the first ten days production by >100%. The modified Gompertz kinetic model demonstrated highest accuracy, predicting that co-digestion improved methane production by reducing the time-lag by ∼50% and increasing rate by ∼35%. The synergistic effects increase the AD system energy efficiency and NER by 30-45%, suggesting potential for substantial enhancements from co-digestion at scale.

Conversion of stranded waste-stream carbon and nutrients into value-added products via metabolically coupled binary heterotroph-photoautotroph system

Growth of heterotrophic bacterium Bacillus subtilis was metabolically coupled with the photosynthetic activity of an astaxanthin-producing alga Haematococcus pluvialis for conversion of starch-containing waste stream into carotenoid-enriched biomass. The H. pluvialis accounted for 63% of the produced co-culture biomass of 2.2 g/L. Importantly, the binary system requires neither exogenous supply of gaseous substrates nor application of energy-intensive mass transfer technologies due to in-situ exchange in CO2 and O2. The maximum reduction in COD, total nitrogen and phosphorus reached 65%, 55% and 30%, respectively. Conducted techno-economic assessment suggested that the astaxanthin-rich biomass may potentially offset the costs of waste treatment, and, with specific productivity enhancements (induction of astaxanthin to 2% and increase H. pluvialis fraction to 80%), provide and additional revenue stream. The outcome of this study demonstrates a successful proof-of-principle for conversion of waste carbon and nutrients into value-added products through metabolic coupling of heterotrophic and phototrophic metabolisms.