Anaerobic bioprocessing of wastewater-derived duckweed: Maximizing product yields in a biorefinery value cascade

Abstract

This study integrated the sugar and carboxylate platforms to enhance duckweed processing in biorefineries. Two or three bioprocesses (ethanol fermentation, acidogenic digestion, and methanogenic digestion) were sequentially integrated to maximize the carbon-to-carbon conversion of wastewater-derived duckweed into bioproducts, through a series of laboratory-scale experiments. Reactors were fed either raw (dried), liquid-hot-water-pretreated, or enzymatically-saccharified duckweed. Subsequently, the target bioproduct was separated from the reactor liquor and the residues further processed. The total bioproduct carbon yield of 0.69 ± 0.07 g per gram of duckweed-C was obtained by sequential acidogenic and methanogenic digestion. Three sequential bioprocesses revealed nearly as high yields (0.66 ± 0.08 g of bioproduct-C per duckweed-C), but caused more gaseous carbon (dioxide) loss. For this three-stage value cascade, yields of each process in conventional units were: 0.186 ± 0.001 g ethanol/g duckweed; 611 ± 64 mg volatile fatty acids as acetic acid/g VS; and 434 ± 0.2 ml methane/g VS.

Keywords: Bioethanol; Biomethane; Biorefinery; Carboxylate platform; Lemna obscura; Liquid hot water pretreatment; Volatile fatty acids.