Biodiesel Industry Waste: Bane or Boon
Authors: Vipin Chandra Kaliaa,b*, Subhasree Raya,b, Ravi Kumar a, Shikha Koula,b, Jyotsana Prakasha,b
aMicrobial Biotechnology and Genomics, CSIR - Institute of Genomics and Integrative Biology (IGIB), Delhi University Campus, Mall Road, Delhi-110007.
bAcademy of Scientific & Innovative Research (AcSIR), 2, Rafi Marg, Anusandhan Bhawan, New Delhi- 110001.

Introduction

Biodiesel is an energy source, which can be used as a substitute for conventional and non-renewable fossil fuels. It can be categorized as renewable in nature, since it is produced from plant oils (canola, sunflower, soybean, etc.), and fats of animal origin, tallow and waste cooking oil. The transesterification of these natural oils results in biodiesel production. During this chemical process carboxylic acid ester is converted into different carboxylic acid esters. In general, this reaction involves ester and alcohol (methanol or ethanol) along with an acid catalyst. The process leads to the generation of methyl or ethyl esters and glycerol also called as glycerin. The process of biodiesel production needs treatment to remove dirt and non-oil material. These is also a necessity to remove water, which may otherwise lead to free fatty acids. The effluent thus contains largely glycerin (i.e. soapy water) and organic residues such as free fatty acids, esters, soaps, inorganic acids and salts, traces of methanol, etc. The effluent has high biochemical oxygen demand (BOD) (4,500-37,000 mg/L), total suspended solids, oil and grease, and other pollutants. With rapid increase in biodiesel production, markets for glycerin are becoming non-existent. Biodiesel production worldwide has been increasing quite steadily from around 10 million tonnes per year in 2000 to 60 million tonnes per year in 2014.

Biodiesel as fuel has quite a few good characteristics especially that it emits: (i) 45-60% less green-house gases, (ii) 50% lower particulate emissions, in comparison to petroleum diesel, and (iii) it gets degraded at the rate equal to that of sugars and 2-5 times faster than petroleum diesel. There are a few limiting factors associated with biodiesel production: (i) it is about one and a half times costlier than petroleum diesel, (ii) the energy input is still substantial, on account of the energy needed to produce it from agricultural crops, including sowing, and harvesting, and (iii) it does show some toxicity (to tested animals) at very high concentration, and (iv) for every 10 tonnes of diesel produced, there is an associated 1 tonne of glycerol rich effluent to be disposed. A few proposals have been made to produce valued added products - hydrogen, polymers, methane, ethanol, and propanediols rather than simply disposing it off.



Propanediols

1,3- and 2,3-propanediol (PDO) produced by anaerobic fermentation of crude glycerol (CG) by Klebsiella, Clostridium, Citrobacter, Lactobacillus, mixed cultures (Enterobacter aerogenes and Clostridium butyricum) and can act as good substrates for production of polyurethanes. And polyesters. Genetically modified Saccharomyces cerevisiae with genes (i) gldA (glycerol dehydrogenase), and (ii) mgsA (for methylglyoxal synthase) of E. coli helped in producing 1,2-PDO.

Ethanol

Clostridium pasteurianum can convert algal biomass and CG to ethanol, 1,3-PDO, and n-butanol (~16 g/L of culture). E. aerogenes could use CG to produce ethanol. Upscaling to 3.6 L continuous culture resulted in producing 0.75 mol ethanol/ mole CG. E. coli produced ethanol and succinate from CG, and these compounds constituted 93% of the products. Genetically modified E. coli produced 1 mol ethanol/mol CG. Fermentation of CG by Klebsiella planticola, resulted in ethanol and formate - 30-32 mmol/L CG.

Hydrogen

Bacteria such as Enterobacter, Clostridium, Klebsiella, Thermotoga and Citrobacter have been reported to have an ability to convert CG to H2 (up to 1.23 mol/mol). Continuous culture H2 yield of 0.86 mol/ mole feed have been observed. Immobilized Bacillus produced 0.39 mol H2/mol CG, a 2.3-fold enhancement in comparison to control. In a biofilm reactor, H 2 productivity of 107 L/kg CG has been achieved.

Methane

Anaerobic digestion (AD) of CG produced 74 mL CH4/L/d in stirred tank, whereas 993 mL CH4/L/d was produced in a baffled reactor. Digestion of 3-6% glycerin along with biowastes produced up to 680 LCH4/kg volatile solids. Co-digestion of macroalgae and CG resulted in 18% enhancement in CH4 productivity. AD of CG and canned seafood wastewater produced 5.8 m3 CH4/m3 or 207 MJ or 58 kWh of electricity.

Polyhydroxyalkanoates (PHAs)

Bacteria can produce PHAs also called as bioplastics. Cupriavidus necator, Zobellella denitrificans, and Pseudomonas oleovorans produce PHA from CG. 20.9 ton PHB can be produced from 10 million gallon per year capacity biodiesel plant. CG and biowastes can be exploited to produce co-polymers of PHA. Bacillus species can produce PHA from CG under conditions which are not influenced by carbon : nitrogen ratios. Bacillus thuringiensis, utilize CG to produce about 1.83 g/L of PHA, containing 13 mol% 3-Hydroxyvalerate.

An integrated approach to combine different processes is necessary to make best use of this biological waste.

References:

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About Author / Additional Info:
Researchers in Microbial Biotechnology and Genomics at CSIR-IGIB, Delhi.