Rapid, Simple, and Accurate Method for Measurement of VFA and Carbonate Alkalinity in Anaerobic Reactors ORI LAHAV,* ,† BARAK E. MORGAN, ‡ AND RICHARD E. LOEWENTHAL ‡ Faculty of Agricultural Engineering, Technion sIsrael Institute of Technology, Haifa, 32000, Israel, and Department of Civil Engineering, University of Cape Town, Rondebosch 7700, South Africa This paper presents a new simple, rapid, and accurate method suitable for on-site measurement of volatile fatty acids (VFA) and carbonate alkalinity in anaerobic reactors. This titrimetric method involves eight pH observations, and typically, the full procedure takes approximately 15 min. An important feature of the method is a built-in quality control mechanism allowing the user a rapid means of assessing the reliability of the experimental procedure. To evaluate the accuracy of the method, both laboratory- made waters and industrial UASB effluent were tested. High accuracy for both VFA and carbonate alkalinity measure- ments (error within 2% and 1%, respectively) plus good repetition (average standard deviation of 6.7% and 1.45%, respectively) was obtained. The method takes into account the effects of the phosphate, ammonium, and sulfide weak acid subsystems. Appraisal of the effect of an input error in these subsystems revealed that VFA measurement is fairly insensitive to phosphate and ammonium concentrations. It is, however, sensitive to H 2 S loss during titration where the sulfide concentration is higher than approximately 100mg/Las S.With regard to the carbonate alkalinity measurement, error in concentration of either phosphate or sulfide or H 2 S loss might result in a significant error. Short guidelines for correct execution of the method are given in an appendix. Introduction Startup and successful control of anaerobic treatment facilities are notoriously difficult and delicate processes, requiring both patience and accurate and rapid monitoring techniques.The controlstrategyhinges around maintaining a low concentration of volatile fatty acids (VFA) and a pH in the range of 6.6 < pH < 7.4. Normally, in such reactors, the carbonate system forms the main weak acid system respon- sible for maintaining the pH around neutrality, while the VFA systems (acetic, propionic, and butyric acids) are the major cause for pH decline. Under stable operatingconditions,the H2 and acetic acid formed due to acidogenic and acetogenic bacterial activity are utilized immediatelybythe methanogens and converted to methane.Consequently,the VFAconcentration istypically very low, carbonate alkalinity is not consumed, and the pH is stable. Conversely, under overload conditions or in the presence of toxins or inhibitory substances, the activity of the methanogenic and acetogenic populations is reduced causing an accumulation of VFA which, in turn, increases the total acidity in the water, reducing the pH. The extent of the pH drop dependson the H2CO3*alkalinityconcentration (the term H2CO3*alkalinity is used here to define the proton accepting capacity of the carbonate weak acid subsystem). In medium and well-buffered waters,high concentrations of VFAwould have to form in order to cause a detectable pH drop, by which time reactor failure would have occurred. Therefore, pH measurement cannot form the sole control means, and direct measurement of either (or both) VFA or H2CO3* alkalinity concentrations is necessary. Measurement of H2CO3* alkalinity in a mixture of weak acid subsystems cannot be executed via direct titration to the H2CO3* equivalence point (around pH 4.5) because at that pH the VFAweakacid subsystem has significant buffering capacity and no clear end point can be defined (1). Characterization of the carbonate subsystem can be carried out using an inorganic carbon analyzer; however, this instrument,apart from not beinggenerallyavailable on-site, is prone to gross inaccuracy due to CO2 loss. Therefore, VFA concentration is the most practical measurement for moni- toring any indication of stress in an anaerobic treatment system.Ifthe system is not rectified at this earlystage,failure is likely. This control problem has gained importance in recent years due to the introduction and wide use of high-rate anaerobic treatment processes, where control needs to be fine-tuned. In addition to conventionalanaerobic digesters, other treatment systems such as biological sulfate removal reactors and hydrolysis reactors (prefermenters) depend on VFAmeasurementasa principalmeansofmonitoringreactor performance. Therefore, there is a need for a simple, rapid, and accurate on-site means of measuring VFAand H2CO3* alkalinity. Currently, VFA can be measured by straight distillation, steam distillation, a colorimetric technique (2, 3) or using gas chromatography. However, all of these methods are time-consuming, the latter three require specialized equipment and a dedicated operator, and gener- ally the equipment will not be available on-site. Conversely, in terms of simplicity, speed, and cost-effectiveness, it is generally accepted that titrative methods are superior for the purpose of routine monitoring and control. During the last 4 decades, a considerable number of quantitative and semiquantitative titrimetric methods have been proposed for the measurement ofeither VFAor H2CO3*concentrations or both. Most ofthese methods were reviewed elsewhere (4) and found to be either too elaborate or too approximate for general practical application. Recently, Munch and Greenfield (5), using a number of simplifying assumptions, suggested an approximate rela- tionship between VFA,carbonate species alkalinity,and pH, allowing VFA estimation based only on pH readings. This method was developed based on criteria confined to pre- fermenters working at low pH and high VFAconcentrations and,therefore,cannot be generalized to most other anaerobic reactors. The most comprehensive workon the subject to-date was presented by Moosbrugger and co-workers (6, 7). They devised a rapid and simple strong acid titration technique forboth VFAand alkalinitymeasurement termed “The 5-Point Titration Method”. When applied within limitations, the *Correspondingauthorphone: 97248292191;fax: 97248221529; e-mail: agori@techunix.technion.ac.il. † Technion sIsrael Institute of Technology. ‡ University of Cape Town. Environ. Sci. Technol. 2002, 36, 2736-2741 2736 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 12, 2002 10.1021/es011169v CCC: $22.00  2002 American Chemical Society Published on Web 05/10/2002