A New VFA Sensor Technique for Anaerobic Reactor Systems Peter F. Pind,* Irini Angelidaki,* Birgitte K. Ahring BioCentrum-DTU, Technical University of Denmark, Building 227, DK-2800 Lyngby, Denmark; telephone: +45 4525 6183 or +45 45 25 61 86; fax: +45 4588 3276; e-mail: birgitte.k.ahring@biocentrum.dtu.dk Received 24 April 2002; accepted 5 September 2002 DOI: 10.1002/bit.10537 Abstract: A key parameter for understanding and con- troling the anaerobic biogas process is the concentration of volatile fatty acids (VFA). However, this information has so far been limited to off-line measurements using labor-intensive methods. We have developed a new technique that has made it possible to monitor VFA on- line in one of the most difficult media: animal slurry or manure. A novel in situ filtration technique has made it possible to perform microfiltration inside a reactor sys- tem. This filter enables sampling from closed reactor sys- tems without large-scale pumping and filters. Further- more, due to its small size it can be placed in lab-scale reactors without disturbing the process. Using this filtra- tion technique together with commercially available membrane filters we have constructed a VFA sensor sys- tem that can perform automatic analysis of animal slurry at a frequency as high as every 15 minutes. Reproduci- bility and recovery factors of the entire system have been determined. The VFA sensor has been tested for a period of more than 60 days with more than 1000 samples on both a full-scale biogas plant and lab-scale reactors. The measuring range covers specific measurements of ac- etate, propionate, iso-/n-butyrate and iso-/n-valerate ranging from 0.1 to 50 mM (6–3000 mg). The measuring range could readily be expanded to more components and both lower and higher concentrations if desired. In addition to the new VFA sensor system, test results from development and testing of the in situ filtration tech- nique are being presented is this article. © 2003 Wiley Pe- riodicals, Inc. Biotechnol Bioeng 82: 54–61, 2003. Keywords: anaerobic manure treatment; in situ filtration; on-line measurement; VFA INTRODUCTION Volatile fatty acids (VFA) are some of the most important intermediates in the anaerobic biogas process. Conversion of the VFA through the acetogenic and acetoclastic step into methane and carbon dioxide is the most important conver- sion in the biogas process. It is well recognized that moni- toring the specific concentration of VFA can give vital in- formation on process status (Ahring et al., 1995; Alonso, 1992; Cobb and Hill, 1991; Hickey and Switzenbaum, 1991; Hill, 1982; Hill and Bolte, 1989; Hill and Holmberg, 1988; Mo ¨sche and Jo ¨rdening, 1999; O ¨ ztu ¨rk, 1991; Pind et al., 1999; Rozzi et al., 1997; Sørensen et al., 1991). Several authors have shown that especially the isoforms of butyrate and valerate are fast and reliable indicators of changes in the process balance (Ahring et al., 1995; Cobb and Hill, 1991; Hill and Bolte, 1989; Hill and Holmberg, 1988), while pro- pionate and butyrate levels are known to increase if the hydrogen level increases to inhibiting levels (Ahring et al., 1995; Mo ¨sche and Jo ¨rdening, 1999; O ¨ ztu ¨rk, 1991). The actual effect of VFA levels on the different microorganisms involved in the biogas process is quite complex and depen- dent on the actual species involved, e.g., methanogens (Mladenovska and Ahring, 2000; Mo ¨sche and Jo ¨rdening, 1998; Vavilin and Lokshina, 1996). Inhibition by increased VFA concentration is also known to be dependent on pH (Meyer and Heinzle, 1998; Mo ¨sche and Jo ¨rdening, 1999). Sudden increases in VFA concentration will cause pH to decrease if the alkalinity is low. This very complex inter- action of inhibition, substrate affinity, and pH dependency leads to the conclusion that no general assumptions on in- hibitory levels of VFA are possible. Instead, a more com- plex evaluation of the VFA concentration has to be con- ducted (Ahring et al., 1995; Bjo ¨rnsson et al., 1997; Pind et al., 1999; Pullammanappallil et al., 1998; Renard et al., 1988; 1991). Such a complex evaluation necessitates access to much and frequent information on VFA concentrations. VFA is easily measured using GC or HPLC, provided that all particulate matter has been removed from the sample. When dealing with anaerobic waste treatment, the presence of particulate matter is often high. Only a few reports exist on development of an on-line VFA monitoring systems. Slater et al. (1990) placed a filter on the recirculation loop of a fluidized-bed reactor and transferred the final permeate to a GC with a modified in- jection port allowing on-line analysis every 12 minutes. However, no data from these measurements and no valida- tion of the suitability of this procedure were ever published. Ryhiner et al. (1993) used a 0.45 m filter on a similar recirculation loop acidifying the permeate with 1% formic acid. The sample was transferred to a specially designed Correspondence to: Birgitte K. Ahring *Present address: Environment & Resources DTU, Technical Univer- sity of Denmark, Building 115, DK-2800 Lyngby, Denmark. Contract grant sponsor: Danish Energy Council Contract grant number: EFP no. 1383/98-0014 © 2003 Wiley Periodicals, Inc.