Value-addition in production of ethanol from macroalgae

As the quest for alternate sources of energy in the face of global warming due to fossil-fuel usage gains momentum, the focus has turned to macro algae (sea weeds) as a source of bio-fuel. A new study now demonstrates how macro algal bio mass from Gelidiella acerosa and Gracilaria dura collected from Adri and Veraval on west coast of India respectively and Gelidium pusillum collected from Valinokam on southeast coast of India could be used in a bio-refinery process not only to produce a substantial amount of bio-ethanol (fuel) but also a slew of valuable byproducts such as agar, pigments, lipids and fertilizer.

A ton of fresh biomass supplies several valuable extracts: 0.3–0.7 kg of R-phycoerythrin (R-PE, pigment), 0.1–0.3 kg of R-phycocyanin (R-PC, pigment), 1.2–4.8 kg of lipids, 28.4–94.4 kg of agar (polysaccharide), 4.4–41.9 kg of cellulose and 3.1–3.6 kilolitres of mineral solution (fertilizer). The enzymatic hydrolysis and fermentation of cellulose thus obtained would yield 1.8–17.4 kg of ethanol. A highlight of the process is sequential extraction of the derivatives leading to full utilisation of the feedstock. The process, developed by Dr. C. R. K. Reddy, Chief Scientist, Seaweed Biology and Cultivation Group, Discipline of Marine Biotechnology and Ecology, Central Salt and Marine Chemicals Research Institute, Bhavnagar, Gujarat, and others, is published in the journal Green Chemistry.

Biofuel production alone from seaweed resources is not cost effective if other components remain unutilized. Till date, the seaweed processing technologies allow extraction of one or the maximum of two products out of the three —agar, carrageenan, alginate which constitute only 15-30 per cent of total mass. This means larger proportion of biomass (70-85 per cent) remained unutilized and was drained off along with effluents.

Keeping this in mind, Dr. Reddy and others for the first time developed a biorefinery process enabling recovery of almost all primary constituents (water, pigments, lipids, polysaccharides and cellulose) of biomass. In this process, first biomass was crushed in a phosphate buffer and pigment was recovered using ultra membrane filtration. The buffer with minerals could be reused for extraction of pigments from fresh biomass. The residue remaining after extraction of pigments was subjected to solvent extraction for recovery of crude lipid and the solvents employed in lipid extraction could also be reusable as earlier.

The residue left after lipid extraction was cooked in water at 120ºC for 90 minutes, blended and then separated from the viscous solution by centrifugation. The viscous solution was cooled at room temperature to form a soft gel from which agar was prepared. The residue obtained during agar processing was finally used as a source for extraction of cellulose using different chemical treatments. Dr. Reddy, in an email to this Correspondent, noted: “The advantages of this process are that it is a high throughput and integrated process. The agar’s gel strength improves becoming higher by 1.5-3 folds and the process bypasses some pretreatments of sample such as defatting, decolourisation, acid and alkaline treatments. There is a reduction by up to 85 per cent in chemicals usage in cellulose extraction, and biomass is effectively utilised without any leftover solid waste. The solvents and water can be reused during the process.”

The continued diversion of seaweed polysaccharides, otherwise known as hydrocolloids, for biofuel may kill the billion dollar world hydrocolloid industry. Hydrocolloids have a number of industrial applications. In this study seaweed cellulose only was used for bioethanol production instead of converting entire polysaccharide content into ethanol.

To meet the bioethanol targets, a vast sea front has to be farmed with seaweeds for producing several hundred million tons of biomass for feeding biorefineries.

Seaweed farming is manpower intensive and thus creates new additional employment and sustainable income sources, improving the livelihoods and socio-economic status of economically underprivileged coastal communities. The large scale farming of macroalgae creates ocean-based industry. It mitigates coastal eutrophication minimising the formation of macroalgal blooms and also mitigates the global warming and climate change effects arising from green house gas emissions by burning of fossil fuels, in addition to freeing the dependency on terrestrial resources for food, feed, water, chemicals and energy.

Photo: Floating brown seaweed Sargassum and occasional red seaweeds Sarconima.

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