Control of VFAs in anaerobic reactors: Is it an easy task? What do we know about off-line analytical performance?

Anaerobic digestion (AD) is sensitive to many environmental factors and, therefore, a robust control of the process is essential to avoid possible instability due to metabolic disturbances. One of the analytical measurements considered as a “crucial key parameter” are the volatile fatty acids (VFAs). They are very important intermediates metabolites that in stable AD systems are produced by hydrolytic-acidogenic bacteria and utilized by methanogenic microorganisms to produce methane. Thus, the VFAs imbalance and following accumulation is interpreted as a process inhibition. Total amount of organic acids (TVFA) to control the relative changes over time has been suggested as useful tool, and a variety of techniques (titrimetric, chromatography, infrared) for online monitoring has been assayed. However, TVFA are not sufficient information to reveal the overall reactor status and individual components have been put forward as being of particular interest. Although some tentatives have been described as on-line measurements, normally the different individuals VFAs are determined by off-line chromatographic techniques such as GC and HPLC. Considering that no standard method is available, the majority of research groups on this field use different analytical procedures characterized by diverse factors such as instrument, column, injector, experimental conditions, preparation of sample and calibration. No previous information was reported on the literature about the harmonization of analytical results relating to individual VFAs. Therefore an interlaboratory comparison was organised using a diluted standard sample. The general performance of data proportioned in this interlaboratory comparison was assessed using the z-score value. The overall performance at starting target error (7.5% of CV) was 45.6% of satisfactory results. In addition, a linear model was obtained to relate the general performance against the fit for purpose criterion to analyze the data. To achieve an overall 75% of satisfactory results the target error should be almost three times higher, from the initial 7.5% to 20.9%. Further evaluation of the experimental results will include an in-depths analysis of the laboratories individual methods.