Process Biochemistry 39 (2004) 1491–1494 Volatile fatty acid formation in an anaerobic hybrid reactor Nurdan Buyukkamaci ∗ , Ayse Filibeli Department of Environmental Engineering, Kaynaklar Campus, Dokuz Eylul University, Buca, Izmir 35160, Turkey Received 28 January 2003; received in revised form 15 May 2003; accepted 3 July 2003 Abstract A laboratory scale model reactor was used to investigate volatile fatty acid (VFA) formation during anaerobic degradation in an anaerobic hybrid reactor. The purpose of the study was to determine the role of upflow anaerobic sludge blanket (UASB) and anaerobic filter (AF) regions of the hybrid reactor on bio-degradation of organic compounds using VFA as an operational parameter. Acetic, propionic, butyric, and valeric acid concentrations of samples taken from the reactor effluent, fixed bed region and the upper part of the sludge bed region were analysed. VFA formation was monitored at two phases. In phase-I, 10kg COD m -3 per day organic loading rate (OLR) (C o = 10,000 mg COD l -1 and θ H = 1 day) and in phase-II 7.6 kg COD m -3 per day OLR (C o = 15,000 mg COD l -1 and θ H = 2 days) was applied. The average acetic acid concentrations of the upper end of the reactor and sludge bed region were 150 and 354.9 mg l -1 , respectively. These results showed that VFA concentrations increase from the top to the bottom of the reactor, indicating higher methanogenic activity in the upper part and higher acetogenic activity in the bottom of the reactor. pH measurements confirmed this result. © 2003 Elsevier Ltd. All rights reserved. Keywords: Volatile fatty acids; Anaerobic; Hybrid; pH 1. Introduction Volatile fatty acid (VFAs) are important intermediate com- pounds in the metabolic pathway of methane fermentation and cause microbial stress if present in high concentrations. This results in a decrease of pH, ultimately leading to fail- ure of the digester. Therefore, the monitoring of VFA con- centrations is very important for the operation performance of an anaerobic digester. It is therefore necessary to inves- tigate the optimum conditions and efficiencies of digesters by examining VFA concentrations. The intermediates produced during the anaerobic bio-degradation of an organic compound are mainly acetic acid, propionic acid, butyric acid, and valeric acid. Amongst these, acetic and propionic acids are the major VFAs present during anaerobic bio-degradation and their concentrations provide a useful measure of digester performance. VFAs indicate the metabolic state of the obligate hydrogen pro- ducing acetogens and the acetoclastic methanogens, which are the most delicate microbial groups. Therefore, the con- ∗ Corresponding author. Fax: +90-232-453-1153. E-mail address: nurdan.buyukkamaci@deu.edu.tr (N. Buyukkamaci). centrations of individual volatile acids (especially acetic, propionic and butyric acid) can be considered as the best control parameters in the liquid phase. Instruments, that can be utilized to monitor total volatile acids on-line or by semi-continuous sampling are available, based either on GC or on titration and back titration methods, which also allow the measurement of bicarbonate alkalinity [1]. It has been suggested that the propionic acid to acetic acid ratio can be used as an indicator of digester imbalance [2,3]. Hill et al. proposed that acetic acid level in excess of 800 mg l -1 or a propionic acid to acetic acid ratio greater than 1.4 indicated digester failure [4]. Most anaerobic hybrid reactors are a combination of up- flow anaerobic sludge blanket reactor (UASB) and anaerobic filter (AF). Several studies have been carried out with hybrid reactors [5–8], but the role of each region (UASB and AF) of the hybrid reactor has not been defined clearly. The aim of this study was to determine the behaviour of UASB and AF regions of the hybrid reactor through monitoring VFAs. The performance of the model hybrid reactor at different operational conditions has already been evaluated [9]. This paper presents the results of experimental studies on volatile fatty acids formation along the hybrid model reactor. 0032-9592/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0032-9592(03)00295-4