Capitalization of wastewater-grown algae in bioethanol production

Abstract

Global environmental policies to reduce the emissions of greenhouse gases and to increase the use of renewable energy have created premises for identification and exploitation of new, economic and non-polluting resources. Algae biomass has gained interest in the production of bioethanol because it is renewable, carbon-neutral, sustainable and can be cultivated in non-productive, non-arable land (without competing with production of food crops) or in fresh and grey water. Algae can grow in wastewater (in open ponds and photobioreactors), contributing to wastewater treatment, reducing nutrients (nitrogen and phosphorous), chemical oxygen demand, biochemical oxygen demand, suspended solids, heavy metals and coliforms. Algal biomass has fast growth-rate (12 days) and the doubling time during exponential growth is only 3.5 hours. Macroalgae contains carbohydrates (starch, cellulose and hemicellulose) and undergoes pretreatments (acidic, alkaline, or enzymatic hydrolysis) to obtain reducing sugars, which are fermented with yeast to obtain bioethanol. Many strains of algae grown in wastewater, including Chlorella, Chlamydomonas, Dunaliella, Porphyridium, Spirogyra and Spirulina contain up to 50 % of their dry weight carbohydrates (starch and glycogen) and are converted into bioethanol. Global production and consumption of bioethanol are expected to increase to 134.5 billion liters by 2024. This paper presents the importance of cultivating algae in wastewater, and reviews the most recent achievements in the production of third generation bioethanol from algae strains using acid, alkali and enzymatic pretreatments, respectively the importance of Saccharomyces cerevisiae for the fermentation of sugars extracted from algae. Introduction According to FAO, in 2016 the global bioethanol production was 100.2 billion litres and is expected to increase to nearly 134.5 billion litres by 2024. Shares of bioethanol production in 2024 refer to2 % Thailand, 2 % India, 7 % China, 7 % the European Union, 31 % Brazil, 42 % the USA and 9 % other countries. Small differences are given for bioethanol consumption: 2 % Thailand, 2 % India, 7 % China, 8 % the European Union, 29 % Brazil, 41 % the USA and 11 % other countries [1]. It is expected that biofuels will cover 30.7 % of total transport energy demand by 2060 [2]. Bioethanol feedstock includes: sugar-containing biomass (sugarcane, sugar beets, sweet sorghum, whey, molasses), starch-containing biomass (corn, wheat, whey, barley, grain sorghum, potato, cassava, Jerusalem artichoke, beverage residues) and lignocellulosic biomass (straws, corn stover, rice hulls, olive pulp, forestry residues, bagasse, switchgrass, alfalfa, respectively wood residues which contain 43 % cellulose, 27 % lignin, 20 % hemicellulose and 10 % other components. About 40 % of the global bioethanol production is mostly from sugarcane and sugar beet and 60 % is from starch-containing feedstock [3]. Although the lignocellulosic biomass is much cheaper for biofuel production than sugar and starch-based feedstock, the technology leading to its conversion into ethanol is currently under development worldwide [4]. Biofuels produced from crops cultivated in arable land are in competition with the food industries, which lately is rising strong opposition in Europe and globally. In this context, new resources like algae have gained increasing interest in biofuels production. Algae are renewable, sustainable, carbon-neutral, and can be cultivated in non-productive, non-arable land, reducing the threats on food security. Microalgae have fast growth rate and a very short harvesting cycle (1-10 days) compared to other biomass feedstocks, and their doubling time during exponential growth is 3.5 hours. Algae can grow in municipal, livestock, agricultural and industrial wastewater, consuming the macronutrients (C, N, P, S) and micronutrients (Co, Cu, Fe, Mn, Mg, Mo, Zn) [5] which otherwise would cause eutrophication. Growing in wastewater (in open ponds, and more recently in photobioreactors consisting of transparent plastic bags, flat and alveolar panels, Plexiglas, acrylic and glass tubes, or flexible plastic coils) [6], algae contribute to wastewater treatment without aeration by the symbiotic growth of photosynthetic algae and bacteria, reducing nutrients (N and P), organic ions,