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Online monitoring of concentration and dynamics of volatile fatty acids in anaerobic digestion processes with mid-infrared spectroscopy.

Bioprocess and biosystems engineering (2014-08-22)
Harry Michael Falk, Peter Reichling, Christian Andersen, Roland Benz
ABSTRACT

An ATR-MIR-FTIR spectrometer was integrated into a laboratory scale anaerobic digestion setup. Automatically, a sludge sample from the digester was transferred to a measurement cell; an IR spectrum was recorded and evaluated by chemometric models to estimate the concentration of the individual volatile fatty acids (VFA). The calibration set included semi-artificial samples spiked with known concentrations of the VFA as well as original samples from a continuous fermentation. High-performance liquid chromatography (HPLC) was used as a reference analysis of the samples. The models were optimized for a low root mean square error of prediction (RMSEP). R(2) for acetic acid, propionic acid, isobutyric acid, butyric acid, valeric acid, and isovaleric acid were 0.94, 0.88, 0.83, 0.75, 0.59, and 0.90, respectively. The accuracy of the models was validated in a second experiment. Considering the complex and heterogeneous sludge composition and the chemical similarity of VFA, absolute concentration and dynamic (increasing and decreasing concentration of VFA) was predicted well for acetic, propionic, isobutyric, and isovaleric acid (in their respective concentration range); Butyric acid could not be detected. The installed setup was able to gather and measure native samples from the digester (every 2 h) automatically over a period of 6 months without problems of clogging or biofouling. The instant and continuous analysis of the concentration of the VFA made it possible to evaluate the current bioprocess status and adjust the organic loading rate accordingly.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Acetic acid, glacial, ReagentPlus®, ≥99%
Sigma-Aldrich
Acetic acid, glacial, ACS reagent, ≥99.7%
Sigma-Aldrich
Acetic acid, glacial, puriss., meets analytical specification of Ph. Eur., BP, USP, FCC, 99.8-100.5%
Sigma-Aldrich
Propionic acid, ACS reagent, ≥99.5%
Sigma-Aldrich
Butyric acid, ≥99%
Sigma-Aldrich
Acetic acid, glacial, ≥99.99% trace metals basis
Sigma-Aldrich
Acetic acid, natural, ≥99.5%, FG
Sigma-Aldrich
Acetic acid solution, suitable for HPLC
Sigma-Aldrich
Propionic acid, ≥99.5%
Sigma-Aldrich
Valeric acid, ≥99%
Sigma-Aldrich
Acetic acid, glacial, puriss. p.a., ACS reagent, reag. ISO, reag. Ph. Eur., ≥99.8%
Sigma-Aldrich
Isovaleric acid, 99%
Sigma-Aldrich
Butyric acid, natural, ≥99%, FCC, FG
Supelco
Propionic acid, analytical standard
Sigma-Aldrich
Isobutyric acid, 99%
Supelco
Butyric acid, analytical standard
USP
Glacial acetic acid, United States Pharmacopeia (USP) Reference Standard
Sigma-Aldrich
Propionic acid, ≥99.5%, FCC, FG
Sigma-Aldrich
Propionic acid, natural, 99%, FG
Sigma-Aldrich
Acetic acid, ≥99.5%, FCC, FG
Sigma-Aldrich
Butyric acid, ≥99%, FG
Sigma-Aldrich
5α-Androstan-17β-ol-3-one, ≥97.5%
Supelco
Valeric acid, analytical standard
Sigma-Aldrich
Valeric acid, ≥99%, FCC, FG
Sigma-Aldrich
Propionic acid, puriss. p.a., ≥99.5% (GC)
Sigma-Aldrich
Acetic acid, for luminescence, BioUltra, ≥99.5% (GC)
Supelco
5α-Androstan-17β-ol-3-one, VETRANAL®, analytical standard
Sigma-Aldrich
Isobutyric acid, puriss. p.a., ≥99.5%
Sigma-Aldrich
Isovaleric acid, ≥99%, FCC, FG
Supelco
Isovaleric acid, analytical standard