In this study it was hypothesized that a combined mass balance and chemical speciation model could be used to predict the soluble metal ion concentration such that changes in methanogenic activity, in response to changes in metal ion concentration, could be predicted. This was undertaken by investigating the influence of precipitation on bioavailability and examining 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.
The sequential extraction of the sludge in Experiment A showed that for Al, Zn and Fe, majority of the metals occurred within two significant phases, namely, the precipitates and the organically bound phase. The fraction of metals in the adsorbed phase was small. Mg was found to have a large portion in the bioavailable phase. For trace metals Cu, Cr and Ni, the sequential extraction analysis was unsuccessful since these metals occur in such small concentrations that when separated into the different metal phases, the concentrations are difficult to detect during the ICP-AES analysis.
Nonetheless, the results suggested that a significant portion of Cr lies within the organically bound phase. Precipitation was found to sequester metals to a large extent making them non-bioavailable.
However, the organically bound metals were a significant portion of sequestered metals.
For the anaerobic system treating FTRW (Experiment B), a large degree of sequestration of Co, Cu, Ni, Zn, Fe and Mo by sulphide precipitates was predicted by the model. This was not unexpected since these metals have small Ksp values for sulphide precipitates. This was supported by experimental analysis as the metals Co, Cr, Cu, Mn and Zn were too small to be detected in the soluble metal analysis, but were found in the solid phase analysis. This is problematic in terms of metal bioavailability since only a small soluble concentration will be in equilibrium with the predicted precipitates. This is regardless of the presence of other metal phases since the Ksp value controls the soluble concentration which in turn dictates the amount of metals sequestered by the different phases. Since the sludge is the chief source of metals, it is recommended that, for future experimental work, a more accurate method should be developed for sampling and analysing the sludge, and direct measurements should be used where possible.
At the current stage of the model development, only the precipitate phase was considered as a form of metal sequestration. In Experiment A, although there were reasonable correlations observed, deviations between the model predicted and the experimentally determined metal soluble concentrations were found; the experimentally determined soluble concentrations were higher than the predicted values. There are three possible reasons for these differences. The first is that there
128 was an experimental error with the analysis. The second is that the assumption that the system was at equilibrium does not hold, and as such, there were kinetic effects in the system that needed to be accounted for. The third reason could be that the two phase (soluble and precipitate) model was insufficient to accurately predict the soluble concentrations and the inclusion of an additional phase such as the organically bound phase would provide a better correlation. Since the soluble concentration of a metal in the reactor will dictate the amount of metal ions in the other phases (according to equilibrium/formation constants), the model should predict the soluble concentrations fairly well if the system is at equilibrium. Since the soluble concentrations were higher than the model predicted values, the deviations were most likely due to kinetic effects in the system that hindered complete precipitation from occurring. It is also possible that a portion of the metal ions in the aqueous phase were present as chelates. Attempts should thus be made to further develop and improve the model by revisiting these assumptions in the next layer of development.
The washout experiment and monitoring of biological processes in Experiment B allowed testing whether changes in biological activity in response to changes in the soluble metal concentrations could be predicted by the mass-balance equilibrium speciation model. The deviations during the washout experiment between the soluble concentrations determined experimentally (Mg, Ca and Fe) and the model predicted concentrations may also be attributed to mass transfer effects, to the absence of other phases that sequester metals in the model or to a combination of these effects. If mass transfer effects were at play in the system, sufficient dissolution of precipitates to replenish the lost soluble concentrations from the washout would not occur, causing the model to over- predict the rate at which metals were washed out of the system. The presence of other metal solid phases such as organically bound metals would also cause the model to predict a different rate of soluble metal washout since the movement of metals from these phases to the soluble phase is dependent on a number of other effects. In the case of organically bound metals, which was found to be a significant phase that sequesters metals, the formation of stable organic substances that do not easily release metals would cause the model to over-predict the rate of soluble metal washout.
However, at this stage, it is not clear which of the two effects are responsible for the differences or whether it is a combination of these effects. It is also interesting to note that from the three metal ions (Mg, Ca and Fe) that were determined experimentally during the washout experiment, Mg, which has majority of its ions in the soluble phase, had the best correlations while Fe, which has majority of its ions in solid related phases, did not correlate as well as Mg or Ca.
When comparing the biogas production from before the washout experiment to 12 cycles after, the biogas production decreased by 43%. In between these 12 cycles there was a recovery period
129 observed where the alkalinity dosage was increased to overcome acid accumulation and the biogas production recovered to about 90% of the production prior to the metal washout experiment. This type of response has been observed before in similar micronutrient limiting studies where after the recovery period, the measure of biological activity decreased further than the initial period of decline.
The varying dosage strategy used for pH control for some measure of pH stability consequently allowed for other effects such as micronutrient limitation to dictate effects for digestion stability.
Although, this approach indicates that the commencement of severe instability and digestion failure from stable operating conditions may not always be clearly seen, resulting changes in biogas production and composition are more directly linked to limitations in biological processes due to micronutrient limitation.
A lag between the soluble metal washout and a possible decrease in biological activity is expected since the metals already within the microorganisms will persist for some time. Additionally, the inclusion of other phases such as the organically bound phase may result in a slower reduction in the soluble concentration than predicted by the model. When comparing the cycles where a decreased biogas production was observed to the decreases in soluble metal concentration obtained experimentally as well as those predicted by the model, and concurrently taking into account a lag period, certain correlations were found. From the first cycle without micro-metal dosage, the soluble concentrations determined experimentally for Ca and Mg started decreasing while the Cu and Mo soluble concentrations were predicted to decrease. Therefore, a decrease in one or a combination of these metals may have resulted in the initial decline in biogas production. During the recovery period, the soluble concentrations of Zn, Co and Ni were predicted to start decreasing while the sludge metal concentration for Mn was also found to decrease, suggesting that the soluble concentration was decreasing. A decrease in one or a combination of these metals most likely resulted in the second decline observed in the biogas production.
Since there was a correlation between the times at which metals washed out of the reactor and the observed changes in the biogas production, it provided strong evidence that washout of metals had a direct negative influence on anaerobic activity. Although the model formulation proposed made use of significant simplifications, an agreement between predicted metals washout and reduction in anaerobic activity was observed, indicating that this is a promising new approach for understanding and modelling bioavailability of metals in anaerobic digestion. Furthermore, the model together
130 with the experimental analysis was able to provide an understanding of the speciation of the metals in the system such that recommendations to improve the micronutrient dosing were made.
The high soluble metal washout rate for Ca prior to the actual washout experiment suggests that there is a large reservoir of Ca in the reactor and that the Ca dosage to the system may be reduced.
Zn was found to have a high sludge retention capacity. Furthermore, Co, Fe, and Cu also displayed minimal changes in the sludge concentration during the washout experiment. Therefore, it is recommended that for Zn, the dosage concentration should be reduced and for all the metals including Zn, an intermittent dosing strategy may be employed. Co, Cu, Fe and Ni were found or predicted to occur in very small soluble concentrations most likely due to the small Ksp values of their sulphide precipitates. For these metals, an increase in the yeast extract dosage concentration or dosing another weak chelating agent should be investigated to try and increase their soluble concentration.
The recommendations above will mostly be applicable for this system under investigation or similar systems treating FTRW anaerobically. However, the mass-balance speciation model together with discerning experimental analysis would provide valuable information on the speciation of metals in any other system which may be used to develop a prudent micronutrient dosing strategy.
131