Obtaining added-value products through ammonium nitrogen recovery with gas-permeable membrane and chain elongation bioprocesses

Abstract

The organic waste generated in urban areas is considered a high-quality resource due to its biodegradability and potential nutrients and carbon recovery. The implementation of circular economy schemes in the urban sector requires innovative technologies that allow the combined production of platform biomolecules and ammoniacal nitrogen recovery. The use of food-waste or sewage sludge sources could represent a solution to mitigate the environmental impact, obtain renewable carbon-based products and recovery ammonium nitrogen in wastewaters while contributing to meet the growing demand for platform-molecules and fertilizers. Anaerobic digestion (AD) and fermentation processes coupled with ammonium recovery can sustain the renewable feedstock recovery by achieving ammonium removal and simultaneous added-value products synthesis (such as volatile fatty acids; VFAs). Gas-permeable membrane (GPM) technology can be utilized to recover the total ammonia nitrogen (TAN) from residual effluents in the form of ammonium sulphate ((NH4)2SO4) fertilisers and, at the same time, to retain the effluent as concentrated VFA-rich liquid solution. The VFAs are considered the “building blocks” to produce multiple bio-based products, such as Medium-Chain Fatty Acids (MCFAs), that have a wide range of applications in sectors like bioplastics, pharma and chemical industries. One promising and emerging biological technology to synthetize MCFAs is the Chain Elongation (CE), an anaerobic pathway in the fermentation process with the presence of electron donors, usually ethanol or lactic acid of biological origin, at relatively high concentration. In CE process, the VFAs are converted into longer fatty acids, mostly hexanoic and heptanoic acids. To explore new valorisation paths for urban organic waste, the focus of this study is to design an integrate different processes to obtain high-value products through an initial TAN removal and recovery of (NH4)2SO4 fertiliser from wastewater, in tandem with forward osmosis (FO) to concentrate the produced VFAs, subsequently used as building-blocks for MCFAs fatty acids in CE fed-batch process.