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

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.

Bio-electrolytic sensor for rapid monitoring of volatile fatty acids in anaerobic digestion process

“…Parameters like volatile fatty acid (VFA) concentrations, pH, biogas yield, biogas composition and alkalinity are commonly used as indicators of the complex biochemical process (Li et al, 2014). Among those indicators, it is widely acknowledged that the concentration of VFAs in the digester is prone to be a more meaningful indicator of the process status (Falk et al, 2015). Several off-line methods for VFA monitoring such as titration method (Purser et al, 2014), GC (Boe et al, 2007), high performance liquid chromatography (HPLC) and mid-infrared spectroscopy (Falk et al, 2015) have been developed.…”

Representative Sampling Implementation in Online VFA/TIC Monitoring for Anaerobic Digestion

“…There are publications on biogas process online measurements using different measurement principles [3,14]. Some of them describe indirect, non-invasive measurements implying an elaborate calibration and interpretation such as the use of Raman spectroscopy [15,16], Near Infrared Spectroscopy (NIRS) [17,18] and Mid Infrared Spectroscopy (MIR) [19]. Dissolved hydrogen in the liquid phase has been successfully measured using an extractor located in the substrate providing an interphase between extraction gas and aqueous medium [20,21] An alternative is using direct invasive measurement which requires taking and preparing samples and sending them to a measurement device.…”

Determination of alcohols and volatile organic acids in anaerobic bioreactors for H 2 production by near infrared spectroscopy

“…However, there is no defined threshold inhibitory level of any VFA species that is valid for all reactors [22]. In addition, a kinetic uncoupling between the syntrophic bacteria of acid producers and consumers in these different fermentation steps can be observed by an accumulation of the different VFA and they can reflect the metabolic state of the obligate H 2 -producing acetogens and the acetoclastic methanogens [23,24]. Several studies have pointed out that the determination of individual volatile fatty acids will also contribute towards improved understanding of anaerobic digestion and prevent a failure of the process [21,25].…”

“…Off-line determinations of the VFA and alcohols are usually performed with high performance liquid chromatography (HPLC) or gas chromatography (GC), however, these methods require sample preparation, expensive equipment, maintenance and skilled technicians [2,6,21]. A low cost and quick result of a VFA sum parameter can be achieved by online and off-line titrimetric methods, but these ignore the individual composition of the specific VFA [23]. In view of time delay between sampling and acquiring the analytical results as a limiting factor, there is a need for simple, rapid and costeffective methods that are able to determinate individual components for the routine monitoring and control of anaerobic reactors.…”

“…One way to reduce the amount of inserted errors in the model is by constructing a sample set with spiked samples, although this is not possible for all types of matrices due to their complexity [54]. Falk et al developed an online monitoring of concentration and dynamics of VFA in anaerobic digestion processes with mid-infrared spectroscopy using two calibration sets: one with spiked samples; and other with real samples with reference values determined by HPLC [23]. The most important step for using a spiked sample set is the model validation with real samples.…”