Abstract
Effluents of anaerobic processes still contain valuable components, among which volatile fatty acids (VFAs) can be regarded and should be recovered and/or used further in applications such as microbial electrochemical technology to generate energy/energy carriers. To accomplish the separation of VFAs from waste liquors, various membrane-based solutions applying different transport mechanisms and traits are available, including pressure-driven nanofiltration (NF) and reverse osmosis (RO) which are capable to clarify, fractionate and concentrate salts and organics. Besides, emerging techniques using a membrane such as forward osmosis (FO) and supported liquid membrane (SILM) technology can be taken into consideration for VFA separation. In this work, we evaluate these four various downstream methods (NF, RO, FO and SILM) to determine the best one, comparatively, for enriching VFAs from pH-varied model solutions composed of acetic, butyric and propionic acids in different concentrations. The assessment of the separation experiments was supported by statistical examination to draw more solid conclusions. Accordingly, it turned out that all methods can separate VFAs from the model solution. The highest average retention was achieved by RO (84% at the applied transmembrane pressure of 6 bar), while NF provided the highest permeance (6.5 L/m2hbar) and a high selectivity between different VFAs.
Highlights
Volatile fatty acids (VFAs) are carboxylic acids with six or fewer carbon atoms that can be saturated or unsaturated, the most common forms being acetic, formic, propionic and butyric acid [1].For industrial applications, volatile fatty acids (VFAs) are produced mostly via chemical routes [2], microbial fermentations offer an alternative [3]
All selected separation methods are capable of purifying VFAs from the model solution
VFA rejection can be achieved by reverse osmosis (RO) filtration, but this method has the highest energetic demand and intensive pretreatment requirements
Summary
Volatile fatty acids (VFAs) are carboxylic acids with six or fewer carbon atoms that can be saturated or unsaturated, the most common forms being acetic, formic, propionic and butyric acid [1]. Forward osmosis (FO) is an osmotically driven separation process where the higher osmotic potential of the draw solution governs the water through a tight (comparable to RO) semipermeable membrane. The concentration-driven transport of the VFAs is facilitated through the ionic liquid membrane by circulating the feed on one side of the membrane, and a stripping solution (pure water in our case) on the other side. The pure VFA solution obtained in this process can be concentrated by a second process step, e.g., by RO Another major difference of SILMs—compared to pressure- and osmotically driven membranes—is that the passage on SILMs depends mainly on polarity (partition coefficients) and much less on molecular size [55]. To study the effect of the varying feed solution, we have set up an appropriate experimental design to investigate the efficiency of each separation process
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