3 years ago

Influence of hydraulic loading rate on ion precipitations and performance of a vertical-flow soil filter treating contaminated groundwater under cold climatic conditions

Influence of hydraulic loading rate on ion precipitations and performance of a vertical-flow soil filter treating contaminated groundwater under cold climatic conditions
Long-term investigations were carried out in a pilot-scale vertical-flow soil filter (VFSF) system to characterize the fate of different precipitating ions (Fe2+, Mn2+, Ca2+) during the remediation of heavily contaminated groundwater under different hydraulic loading rates (HLRs) and to assess MTBE and BTEX removal efficiencies under low winter temperatures. Two specially designed VFSF, a roughing filter (RF) packed with expanded clay material (grain size: 3–6mm) and a polishing filter (PF) with zeolite material (grain size: 0–5mm) were operated in both single-stage and two-stage configuration. During single-stage operation, a stepwise increase of HLRs from 60 to 120, 240, 480Lm−2 d−1 in both RF and PF were applied. During two-stage operation, a highly efficient Fe2+ removal with a mean removal efficiency of 99% was already observed in the effluent of the first treatment step (RF; HLR=960Lm−2 d−1) and protected the second filter (PF; HLR=480Lm−2 d−1) from potential clogging due to iron hydroxide precipitates. The removal of Ca2+ decreased remarkably from 28% to 7% in the RF and from 54% to 10% in the PF as the HLR increased stepwise in single-stage operation. During two-stage operation with a high HLR in the RF resulted in only 3% removal of the mean Ca2+ inflow concentration (211±18mgL−1). This clearly showed that at high HLR, Ca2+ was rapidly flushing out of the VFSF system instead of carbonate precipitations and proved to be very effective to protect the filter from potential clogging. During winter, when daily mean outside air temperature was as low as −20°C, a relatively low water temperature in the range of 5.1–8°C in the effluent and a low MTBE and benzene mean removal rate of 0.41±0.07 and 2.08±0.33gm−2 d−1, respectively, was observed at a low HLR (120Lm−2 d−1). Whereas, at a high HLR (960Lm−2 d−1), a mean temperature of 13.0±1.3°C was maintained inside the filter resulting in a high MTBE and benzene removal rate of 2.37±0.02 and 14.41±0.65gm−2 d−1, respectively. Our results with high removal efficiencies for MTBE and benzene together with highly efficient Fe2+ and Mn2+ removal suggest that a two-stage VFSF can be a very promising technology for the remediation of heavily contaminated groundwater in oil and gas industries. Furthermore, it was shown that at high HLRs, filter clogging due to Ca-precipitation was prevented and treatment efficiency was not affected even under cold climate conditions with atmospheric air temperature as low as −20°C.

Publisher URL: www.sciencedirect.com/science

DOI: S0925857417304597

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