Head losses were continuously monitored over the 21 days of experimental period for both processes (Figure 7.10a). The fixed bed contactor exhibited increase in the head loss over the filtration period which varied largely depending on the influent water characteristics. On the other hand, the head loss across the PFB was stable (33 kPa) and independent of the influent water characteristics. As expected, the total head loss across the fluidized bed was comprised of the apparent weight of the pyrolucite media per cross-section unit of the column and the water head in the column, which are independent of the superficial fluid velocity and change in influent water quality conditions.
Under very low iron concentration, the very low turbidity of the influent water (Figure 7.10b) allowed for the operation of the fixed bed contactor at a high HLR (20 m/h) with no concern about excessive head loss build-up. As a result, when the manganese concentration was 0.090.01 or
03 . 0 35 .
0 mg/L in the influent water, the fixed bed contactor was required to be backwashed once or twice over the 21 days. The increased head loss is most likely due to the accumulation of precipitated MnOx(s) (less than 10% solution oxidation of manganese was observed) on the contactor media and partial blockage of the void spaces. Under an elevated iron concentration in the influent water, a substantial head loss developed across the fixed bed and, thus, the filtration cycle was significantly shortened (around 3 days) owing to the retention of more particulate materials (60-90% of iron were oxidized before reaching the filter media) in the pyrolucite filter media. The higher turbidity (0.3 NTU) of the influent groundwater spiking with iron compare to the assays without spiking (0.1 NTU) also have contributed in shortening the filtration cycles.
Figure 7.10. Evolution of (a) head loss and (b) turbidity in fixed and fluidized bed contactors under different experimental conditions. T= 9oC.
Figure 7.10b illustrates that the fixed bed contactor was able to maintain the turbidity of the treated water at 0.1 NTU while the turbidity of the PFB effluent water varied between 0.1 and 0.5 NTU. However, even
the turbidity of 0.5 NTU, which was only experienced under an elevated iron concentration, is still satisfactory for the water treatment utilities.
In conclusion, although the pyrolucite fixed bed contactor controls efficiently the concentration of iron and manganese in the treated water under an elevated iron concentration, it gives rise to a considerably larger energy consumption and wastewater production (with a typical turbidity
0 3 6 9 12 15 18 21
between 40 NTU and 70 NTU measured during this assay). On the other hand, the PFB only needs to be backwashed prior to a regeneration. No regeneration was needed during the 21 days period investigated. Combining a PFB contactor with a downstream process with the capability of operating at very high loading rate would however be required under influent conditions with elevated iron concentrations. In addition, despite the fact that both contactors were highly efficient to remove manganese under very low iron concentrations, the PFB offers the important advantage of operating at high HLR values allowing for the use of smaller contactor footprints and, thus, a decreases in capital costs. As stated earlier, the PFB also greatly reduces the production of wastewaters which is an important design constraint due to the manganese discharge limits imposed in many regulations. On a final note, the adsorbed manganese accumulating on the PFB media is expected to increase the pyrolucite media size over time. However, this phenomenon is expected to be marginal as an influent manganese concentration (0.1 mg/L) that would be removed entirely in a 45 m/h PFB would only increase by 3% the mass content of manganese dioxide in the reactor after one year of operation. This would represent a theoretical increase in media size of 7 µm.
7.4 Conclusions
The main purpose of this research was to assess the long-term fate of iron and manganese in a PFB contactor supplied by natural groundwater. Accordingly, the results of a pilot-scale experiments under different operating conditions with varying iron and manganese concentrations were described here. The following conclusions were derived from this research study:
Selection of an appropriate distributor plate for the PFB contactor was crucial for an efficient process performance. Among different configurations of distributor plate that were tested, a shrouded perforated distributor plate with 15 cm of gravel on top of it proved to be optimal.
The PFB contactor has a strong adsorption capacity for dissolved iron and manganese.
However, the adsorption capacity and long-term stability of the process were influenced by the initial concentration of these species.
The PFB contactor proved effective at promoting a high degree of total manganese removal under a wide range of influent manganese concentrations as long as iron levels were low.
It also minimized wastewater production while providing a high treatment performance and a small footprint. Consequently, this process is recommended for water treatment facilities
with manganese treatment objectives and negligible iron in the influent water. Under elevated iron concentrations in the influent water, a downstream process after the PFB contactor is required to achieve acceptable iron and manganese removal.
The pyrolucite fixed bed contactor demonstrated its ability to effectively control iron and manganese under all conditions tested. However, fairly rapid head loss build-up was observed across the pyrolucite fixed bed media operated at 20 m/h under elevated iron concentrations (3 days filtration cycle).
The concentration of iron in the groundwater mainly controlled the head loss build-up rate through the fixed bed. On the other hand, the head loss in the PFB contactor remained relatively constant, regardless of the influent water quality.
In both fixed and fluidized bed contactors, manganese was principally removed by the sorption uptake of dissolved manganese onto the pyrolucite media followed by the catalytic oxidation of the adsorbed species on the surface of pyrolucite particles. A similar mechanism could be inferred for a part of iron removal in the PFB contactor while the role of pyrolucite fixed bed in the iron removal was mainly via the capture of particulate and/or colloidal ferric hydroxide that formed following solution oxidation of soluble iron by chlorine.
Future studies should focus on the development of a high rate post-filtration technique that is stable and cost effective for the removal of the residual particles from the effluent of the PFB contactor for waters containing both iron and manganese species.
Acknowledgments
This work has been financially supported by RES’EAU-WATERNET, a NSERC (Natural Sciences and Engineering Research Council of Canada) collaborative strategic network (Grant No. 364635-07), and the NSERC Discovery Grant Program (RGPIN-2015-04920). The authors wish to thank the Sainte-Marthe-sur-le-Lac water treatment plant personnel and Mireille Blais, Yves Fontaine, and Valentin Pfeiffer from Polytechnique Montréal for their assistance during this study.