Enhancing methane yield from crude glycerol anaerobic digestion by coupling with ultrasound or A. niger/E. coli biodegradation.

Larissa O Paulista,Vítor J P Vilar,Rui A R Boaventura,Alexei L N Pinheiro,Ramiro J E Martins

doi:10.1007/s11356-019-06748-w

Abstract

Anaerobic digestion of crude glycerol from biodiesel production is a feasible way for methane production. However, crude glycerol (CG) contains impurities, such as long-chain fatty acids (LCFA) that can inhibit methanogenic microorganisms. Ultrasound promotes the hydrolysis of LCFA and deagglomerates the microorganisms in biological flocs. Furthermore, Aspergillus niger and Escherichia coli produce lipases capable of degrading LCFA. This study aims at improving the methane yield from anaerobic digestion by coupling with ultrasound or E. coli/A. niger biodegradation. The effect of the different treatments was first assessed in a perfectly mixed batch reactor (PMBR), using diluted CG at concentrations of 0.2%, 1.7%, and 3.2% (v/v). Later, the best conditions were replicated in an upflow anaerobic sludge blanket (UASB) reactor to simulate full-scale practical applications. Experiments in the PMBR showed that ultrasound or A. niger biodegradation steps improved methane yield up to 11% for 0.2% CG and 99% for 1.7% CG, respectively. CG biodegradation by E. coli inhibited the subsequent anaerobic digestion for all concentrations tested. Using a UASB digester, ultrasonic treatment of CG led to an average increase of 29% in methane production. The application of ultrasound led to a lower accumulation of propionic acid in the digested material and increased biogas production. On the other hand, an average 77% increase in methane production was achieved using a preliminary CG biodegradation step by A. niger, when operated at a loading rate of 2.9 kg COD m-3 day-1. Under these conditions, an energy gain of 0.48 kWh day-1, with the production of the 0.434 m3 CH4 kg-1 CODremoval and 0.573 m3 CH4 kg-1 VS, and a biogas quality of 73% in methane were obtained. The digested material was analyzed for the detection and quantification of added-value by-products in order to obtain a broad assessment of the CG valorization through anaerobic digestion. In some experiments, propionic and oxalic acid were detected. However, the accumulation of propionic caused the inhibition of the acetogenic and methanogenic microorganisms.

Highlights

The most common process of biodiesel production is transesterification: an alcohol is added to a vegetable or animal oil, which forms esters and glycerol by the action of a catalyst
The use of high glycerol loads may cause the accumulation of volatile fatty acids and a decrease in pH, which leads to the inhibition of methanogenic microorganisms (Razaviarani and Buchanan 2015; Silva et al 2018)
For the 1.7 and 3.2 % crude glycerol (CG) concentration (v/v), methane production was always below 16 mL, and the 9-h ultrasound treatment improves the methane yield for the 1.7 % CG concentration (v/v), its benign effect does not compensate for the inhibitory effect of the high CG load

Summary

Introduction

The most common process of biodiesel production is transesterification: an alcohol (methanol) is added to a vegetable or animal oil, which forms esters (methyl ester) and glycerol by the action of a catalyst (sodium hydroxide or potassium hydroxide). The use of high glycerol loads may cause the accumulation of volatile fatty acids and a decrease in pH, which leads to the inhibition of methanogenic microorganisms (Razaviarani and Buchanan 2015; Silva et al 2018). This limits the glycerol concentration in the feed to values below 3% (v/v). Two strategies have been devised to overcome this issue: the anaerobic co-digestion of CG with suitable amounts of agroindustry waste, urban solid wastes, or biological sludge from wastewater treatment plants (Pachapur et al 2016; Razaviarani and Buchanan 2015) and the use of two reactors in series, the first one providing the breakdown of the molecule in the hydrolysis and acidogenesis phases and the second one for biogas production, just as favorable conditions for acetogenic and methanogenic microorganisms (Luo et al 2011)

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