13C NMR studies on vinasses effluent treated with iron

Controlling the concentration and toxicity of propionic acid in upflow anaerobic sludge blanket (UASB) reactors is a challenging problem in biological wastewater treatment. Addition of FeSO4•7H2O and FeCl3•6H2O to vinasses effluent yields a precipitate and considerably reduction of propionic acid in solution. This is due to the coordination of volatile fatty acids (VFAs) to metal ions, according to 13C NMR techniques. 13C NMR broadening of signals was carried out in vinasses effluent as well as in pure propionic acid (5.0 g/l) with various concentrations of Fe ions. The results show that Fe(II) is one of the micronutrients in anaerobic digestion, forms less stable complexes with propionic acid, and leads to a higher degree of bioassimilation of the dissolved organic compounds to produce biogas. This is one of the main reasons why FeSO4•7H2O facilitates the anaerobic digestion. 13C NMR signals of propionic acid and vinasses effluent with Fe(III) were considerably more broadened as compared to Fe(II), suggesting that chelation of VFAs with Fe(III) is stronger than with Fe(II) and, however, Fe(III) reduced to Fe(II) at anaerobic conditions. Metal complexes were not readily degraded by bacteria as the biodegradability of the metal complexes depends upon the stability and chemical nature of the complexes, and not on the toxicity of the metal ions towards the bacteria.