5. Discussion
5.2 Experiment B Discussion
5.2.3 Sludge Metal Analysis
It is expected that changes in the soluble concentrations will have a more direct effect on the biological activity, if any, than changes in the sludge metal concentrations. This is because the soluble concentrations represent that portion of the metals that may be absorbed by the microorganisms to perform their various biological functions and a lack thereof may have adverse effects on the microorganism functioning. However, a decrease in the sludge metal concentration indicates that soluble metal ions are being washed out of the system and to replenish these lost ions in an attempt to re-establish equilibrium, the metals in the solid phase move to the soluble phase.
The concentration of metals within the sludge samples was higher than the supernatant samples, and metals which were undetectable in the soluble phase, were detectable in the sludge phase. As shown in figure 26, Co, Cr, Cu, Mn and Zn were found exclusively within the solid phase.
116 Therefore, although these were not detectable in the soluble phase, there is a small soluble concentration present in the reactor in equilibrium with the solid phases. Therefore, a decrease in the sludge concentration shows that the soluble concentration is being reduced. This is observed for Ca, Mg and Mn as shown in Figure 26. For Ca and Mg, this is supported by the decrease in their soluble concentrations observed in Figure 25 as well as the predicted decrease in soluble concentrations from cycle 0 shown in Figure 20. In Figure 19, the model also predicts that both these metals have a large portion of their ions in the soluble phase (Mg is predicted to occur solely in the soluble phase).
For Mn, although there is no experimental data to support the predicted decrease in sludge metal ion concentration, the model predicts that the dissolution of Mn associated precipitates occurs earlier than most of the other metals (Figure 19) and that the predicted soluble concentration also starts decreasing a few cycles after the washout experiment (Figure 21). This may be compared to a study where trace metals were omitted from one reactor and dosed to another and all other conditions were identical. After 40 days of operation, a decrease in the Mn sludge concentration was observed in the former reactor whereas an increase in the Mn sludge concentration was observed in the latter (Osuna et al., 2003).
Figure 26 also shows that for Co, Cr, Cu, Fe and Zn, it is difficult to observe a definitive trend in the sludge concentrations. This is most likely because these metals have extremely low Ksp values with their precipitate salts. Accordingly, the soluble concentration with which the precipitates are in equilibrium is very small. Therefore, the concentration of metal ions that would move from the solid phase to the soluble phase to replace the lost soluble ions will also be very small. Since the change in concentration to the solid phase will be small relative to the total solid phase concentration, the resulting change to the solid phase would be difficult to observe. Similar results have been found in other studies considering the limitation of metals. In one study where the OLR was varied between 2 and 10 gCOD/l.d over 140 days of no trace metal operation, the sludge metal concentrations of Cu, Zn and Fe displayed fluctuating trends whereas for Co, a decline was observed (Osuna et al., 2003). However, other studies of Co limitation indicate a range of between 2 and 55 days of operation (without Co) before a noticeable change in the methanogenic activity was observed (Fermoso, 2008).
When comparing the fluctuating trends observed in Figure 26 to the precipitate washout as predicted by the model, the model predicts that by cycle 11 most of the metals (except Co and Mo) would have dissolved completely from the precipitate phase whereas the experimental analysis
117 shows that at cycle 14, there are still significant concentrations of metals in the sludge phase. As discussed earlier (section 5.2.1), this could be due to mass transfer effects in the system that prevent the full amount of dissolution of the precipitates required to replenish the lost soluble concentrations and re-establish equilibrium. The deviations could also exist due to the presence of other sludge associated metal phases, such as the adsorbed and organically bound metals, which, at this stage, have not been taken into account in the model. When taking these phases into account, the rate of washout of metals would now also be dependent on the effective equilibrium/formation constants for the organic bonding and adsorption reactions. Since the organically bound metals are not considered very mobile, the rate of washout of metals from this phase will most likely occur after metals sequestered by other phases have been released. This alone would result in metals being retained in the reactor for a longer time than what the model predicts. Furthermore, metal ions in the adsorbed and organically bound phase are in equilibrium with the soluble metal concentration, which is controlled by the Ksp value. Therefore, the amount of metals in these phases would remain the same since the precipitates exert the same equilibrium soluble ion concentration, and will only start depleting once the precipitates have completely dissolved.
In Figure 26 Ni does not appear in the results for the sludge metal concentration as it was not detected during the ICP-AES analysis. However, Ni was dosed to the system prior to the metal washout experiment at a concentration of 0.1 mg/l, close to the Cu dosing concentration (0.15 mg/l) and higher than the Co dosing concentration (0.02 mg/l). The model also predicted that Ni will occur as a sulphide precipitate at a concentration of 0.02 mg/l (Figure 18). The absence of Ni could have been due to a problem with the ICP-AES analysis. Although Ni was calibrated successfully, interferences with other metals in the sample that have similar wavelengths could have occurred during the analysis (Manning and Grow, 1997).
If the ICP-AES analysis was correct, this suggests that Ni did not accumulate in the solid phases like most of the other metals and was most likely washed out in small concentrations (which were too small to be detected in the ICP-AES analysis) in the supernatant. This could have been due to the presence of yeast extract that was added to the reactors as part of the nutrient recipe or to some other complexing agent present in the system. In previous studies the addition of yeast extract was found to increase the solubility of Ni due to its chelating properties (Gonzalez-Gil et al., 2003). It is also possible that contrary to the model prediction, the Ni ions are not at a stage where they will precipitate. The model assumes that the redox potential is very low since all S is converted to S2-, however, the redox potential in the system may be higher, which may result in Ni being in a soluble
118 state at equilibrium. It would therefore be recommended to revisit the assumption of not considering electrical potential and reduction/oxidation reactions of the system in the model.
Figure 28 shows the percentage metal retained within the reactor. This parameter provides an indication of the ability of a metal to collect within the solid phases compared to the concentration that is dosed in the feed. The metals display retentions over a very wide range. Those metals that have a high retention capacity, especially if it remains high even during the metal washout experiment, are most likely metals that are overdosed to the reactor. Zn has the highest % metal retained, over 1000%, which does not change much over the 14 cycles (equivalent to 28 days of operation) where Zn was not dosed to the system. This is similar to other studies where Zn displayed small or no changes in the sludge metal concentration when under anaerobic operation with Zn limiting conditions (Fermoso, 2008, Osuna et al., 2003, Zandvoort et al., 2003). Figure 28 also shows that Co and Mn have very high retention capacities prior to the washout experiment.
Once these metals are no longer dosed, Co is still retained at a concentration of eight times the feed concentration whereas for Mn, the sludge concentration decreases from approximately seven times the feed concentration to just three times after 14 cycles. This shows that for some metals (Mn), while the metals are dosed to the system, the amount of metal retained in the sludge is high but once metals are omitted, the sludge metal concentration depletes rapidly. Knowing this characteristic for the different metals can be used when developing a dosing strategy as metals like Mn, Ca and Mg would need to be dosed consistently while metals like Zn, Co, Fe and Cu can be dosed intermittently.
Figures 26 and 27 highlight the importance of sampling the sludge correctly. The sludge samples were removed from the reactor after decanting the supernatant. The same peristaltic pump and pipes used for decanting the supernatant were used to extract the sludge sample. Although the pump was run for some time to remove any supernatant remaining in the pipes, it is possible that for cycle 15, the sludge sample was diluted by the mistaken addition of supernatant. Although a decrease in metal content is expected from cycle 14 to 15, the observed experimental values seem unreasonable. This is supported by the data shown in Figure 27.