Otimization of Volatile Fatty Acid Formation by Fermentation of Primary Sludge
Creators: name: Baise, John W.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 2002

description: The Orange County (North Carolina) Water and Sewer Authority (OWASA) operates the Mason Farm Wastewater Treatment Plant to achieve the removal of biochemical oxygen demand (BOD), nitrification of ammonium-nitrogen, and biological phosphorus removal (BPR). Future discharge permits may require total nitrogen removal, which would be accomplished by incorporating denitrification into the biological treatment process. One means of providing the carbon sources necessary for denitrification would be to provide volatile fatty acids (VFAs) from the fermentation of primary sludge. Although OWASA already ferments primary sludge at the Mason Farm Plant, the purpose of this study was to evaluate the conditions under which VFA production could be maximized during fermentation. Fermentation involves the conversion of organic compounds to volatile fatty acids (VFAs) by microorganisms under anaerobic conditions. When primary sludge is fermented, the principal compounds available for fermentation are solids such as cellulose and other polysaccharides, proteins, and lipids or fats. Before these solids can be fermented, however, they are first broken down to low-molecular-weight products that are then converted to VFAs by fermentative bacteria. Anaerobic bioreactors generally contain complex communities of microorganisms. If the residence time of the organisms in such reactors is long enough, microorganisms that convert acetic acid and other compounds to methane (methanogens) will normally be present. Thus, the presence of methanogens can serve as a sink for the VFAs that are produced by the fermentative bacteria. Optimizing the production of VFAs in a fermenter therefore involves an optimum balance of conditions that maximize hydrolysis and fermentation reactions with those that minimize methanogenesis. The most common means of accomplishing this in the field is to operate sludge fermenters at residence times that are too short to permit significant growth of the slow-growing methanogens. In addition to residence time, other factors such as pH might influence the rate and extent of fermentation. For example, the optimum pH for growth of methanogens is near neutral, so that the acidic conditions often associated with fermentation could help minimize methanogenesis. Methanogens are also known to be very sensitive to oxygen, so that intermittent exposure to air might also reduce methanogenesis in fermentation processes. Our review of the literature indicates that the effects of pH and oxygen on the fermentation of primary sludge are not well understood. In this study the effects of pH and oxygen addition on VFA formation from primary sludge were evaluated in batch anaerobic incubations. In addition, the effect of hydraulic residence time in a continuous-flow reactor was also evaluated. The combined results suggested that pH is the most important variable influencing VFA formation. In general, pH near or below 5.0 led to maximum VFA formation (between 0.4 and 0.5g VFAs as chemical oxygen demand per gram volatile solids fed).