Analysis of nutrient requirements for the anaerobic digestion of Fischer-Tropsch reaction water.
Nutrients play an important role in the functioning of microorganisms during anaerobic digestion. The anaerobic treatment of industrial wastewaters, such as Fischer-Tropsch Reaction Water (FTRW), requires the addition of nutrients suitable for micro-organisms (micronutrients) since these wastewaters are devoid of essential metals. However, the dosing of nutrients is only effective if the metals are in a bioavailable form which in turn is dependent on the chemical speciation of the system. This study aimed to investigate and model the influence of precipitation on bioavailability by considering the extent to which precipitation can sequester metals into forms that are not bioavailable and the extent to which this sequestration can describe biological effects in an anaerobic system. Visual MINTEQ and Excel were used to develop a combined mass balance and chemical-equilibrium speciation model that considered the soluble and the precipitate metal phases. The model was compared to two sets of experimental analysis. Experiment A included metal analysis on the sludge and supernatant from glucose and ethanol fed ASBRs while Experiment B included similar analysis on FTRW fed ASBRs while biological parameters were monitored during a micro-metal washout experiment. Precipitation was found to sequester Al, Zn and Fe to a large extent making them non-bioavailable in Experiment A, while sulphide precipitates were predicted to dominate the metal speciation in Experiment B. In Experiment A, the organically bound metals phase was also a significant phase that sequestered metals. Furthermore, the rates of washout of most of the metals (excluding Mg) were over-predicted, which may have been due to the absence of other solid related phases in the model. This may also be attributed to kinetic effects in the system. Although there were reasonable correlations between the model predicted and the experimentally determined concentrations, it is recommended that the model should include the organically bound phase and consider mass transfer effects in the system. After 12 cycles without dosing micro-metals in Experiment B, the biogas production decreased by 43%. A decline in the predicted and determined soluble concentrations of a variety of metals were observed during this time, suggesting that there may be an agreement between predicted metals washout and reduction in anaerobic activity. Since the soluble metal concentrations did not decrease as rapidly as predicted by the model, a lag period between the two parameters was observed. Therefore, although the model provides an improved understanding of metal speciation and bioavailability such that recommendations may be made for prudent micro-metal dosing, further development is required for more accurate representations of the system.