Biosorption of long-chain fatty acids in UASB treatment process

Biosorption of long-chain fatty acids (LCFA) in the upflow anaerobic sludge blanket (UASB) treatment process was investigated using batch tests and continuous reactor runs. Batch experiments were conducted, for characterization of the biosorption, with two active and one inactivated (autoclaved) sludge granules as sorbents and with a single (oleic acid) or a mixture of LCFA (LCFAm; 50% oleic, 35% palmitic and 15% stearic acid) as sorbates. The LCFAm showed a faster adsorption onto the granules than oleic acid. With the active sludge granules, adsorption was followed by desorption. Methane production increased significantly, either simultaneously (at lower LCFA concentrations) or succeedingly (at higher concentrations) with desorption. The desorption was mediated by biological activity, since it did not prevail with inactivated granules or with active granules inhibited at higher LCFAm concentrations. The inactivated granules had a slightly higher initial biosorption capacity. Increased LCFA concentrations resulted in more LCFA adsorbed and greater inhibition of their biodegradation. A hypothesis is proposed to explicate the relationship between biosorption, desorption and biodegradation of LCFA by sludge granules: adsorption is a prerequisite for biodegradation while desorption is a consequence of biodegradation. Isothermal studies with oleate showed that the apparent biosorption could be described by the physical multilayer adsorption theory and the sorption isotherm derived was consistent with the Freundlich model. The quantitative relation between LCFA biosorption and granular sludge flotation was investigated in a UASB reactor fed with LCFAm. Sludge flotation depended on the LCFAm loading rate rather than on their concentration. The higher the loading implemented, the more flotation occurred and the shorter time required for complete flotation of the sludge bed. Flotation started when the LCFAm loading rate exceeded 0.09 g COD/g VSS•d, while complete flotation occurred at the loading rates exceeding 0.2 g COD/g VSS•d. These results suggest that sludge bed wash-out is likely to be encountered before inhibition of methanogenesis during the treatment of LCFA-containing wastewaters by the UASB process.