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BIOFILTRATION TREATMENT FOR IRON- AND MANGANESE-RICH GROUNDWATER AT LOW ON-SITE TEMPERATURES

Date

2018-01-11

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Thesis

Degree Level

Masters

Abstract

Manganese (Mn) is frequently detected in its reduced form, aqueous Mn(II), in groundwaters used as drinking water in Canada. Excess Mn(II) poses the potential risks of water discolouration, infrastructure corrosion, and health problems, as uncontrolled Mn(II) oxidation occurs and produces solid Mn(III/IV)-oxide precipitates in water supplies. As a solution for Mn(II) in groundwater, biofiltration technology has been considered globally to promote microbially mediated Mn(II) oxidation and produce solid Mn(III/IV) oxides that can be filtered out. However, previous studies have consistently reported that the biofiltration of cold groundwater with high Mn(II) and coexisting metal concentrations (typically Fe(II)) is challenging at low temperatures below 15 °C. Diverse cold-adapted manganese-oxidizing bacteria (MnOB) are ubiquitous in cold groundwater. The biofiltration of cold Mn(II)-rich groundwater, which relies on the onset, acclimation and acceleration of Mn(II) removal associated with the enrichment of cold-adapted MnOB and biofilter media ripening in the field, has not been extensively understood. The objectives of this research were therefore (1) to elucidate the onset, acclimation, and acceleration of Mn(II) oxidation (removal) from cold, natural Fe(II)- and Mn(II)-rich groundwater (4 to 8 °C) continuously fed into a two-stage pilot-scale biofiltration unit operated in the field at varying low on-site temperatures (8–14.8 °C) at the Langham Water Treatment Plant, Saskatchewan (Canada), (2) to characterize the microbial communities in the Mn and Fe biofilters and incoming groundwater, as well as in the surface coatings on field-ripened filter media, and (3) to explore the potential enhancement of biological and physicochemical Mn(II) oxidation in the field biofiltration unit. Over the course of the 183-day pilot-scale biofiltration experiment in the field, the onset of Mn(II) removal from the cold groundwater commenced at 8 °C in the Mn filter after 29 days elapsed and after complete Fe(II) removal through the Fe filter. The Mn filter (1.55 m high and 0.3 m in diameter) reached steady-state functioning after 97 days, consistently exhibiting a high Mn(II)-removal efficiency of 97±0.9%. A gradual shift in redox-pH conditions in the Mn filter, to oxidation-reduction potential (ORP) values over 300 mV, favoured biological Mn(II) oxidation, the growth of viable MnOB populations, and an increase in microbial metabolic activity estimated by the adenosine triphosphate (ATP) assay. These changes reflected enhanced biological Mn(II) oxidation at the low on-site temperatures. However, the empty bed contact time (EBCT) first-order rate constants (k) for Mn(II) removal were very low, in the range of 10-6 and 10-5 min-1, with a long half-life of 40 days, even though the Mn(II) removal efficiency was consistently at 97%. The Mn(II) removal rate constant accelerated to 0.21 min-1 with a very short half-life of 3.31 minutes at 11±0.6 °C, immediately after three consecutive backwashes and injections of backwash sludge slurry back into the filter. The substantial increase in k was correlated to the vertical progress of biofilter ripening from the bottom to the top of the Mn filter, which was not limited by the low on-site temperatures. Intermediate and end-product Mn(III/IV) oxides (birnessite and pyrolusite) were detected by synchrotron-based powder X-ray diffraction, confirming the occurrence of Mn(II) oxidation based on the known Mn(II)-oxidation pathway. High-throughput sequencing (16S rRNA genes, V4 region) of the microbial communities in the untreated incoming groundwater and filter media from the Fe and Mn filters revealed genus-level shifts in the bacterial community across the biofiltration unit. Previously known Mn(II)-oxidizing bacteria (MnOB) were minor members of the Mn-filter community. Betaproteobacteria including iron-oxidizing bacteria (FeOB) appeared in both the Fe and Mn filters. Hydrogenophaga sp., known FeOB, likely acted as a MnOB. Specifically, Hydrogenophaga strain CDMN isolated in this study can oxidize Mn(II) at the Mn-filter start-up temperature of 8 °C. Three new potential MnOB—Azospirillum sp. CDMB, Solimonas soli CDMK, and Paenibacillus sp. CDME—were also isolated from the Fe or Mn filters. A microbial consortium (51 genera including Pseudomonas, Leptothrix, Flavobacterium, and Zoogloea) was cultured from the field-aged biofilter and rapidly produced biogenic Mn oxides at 8 °C. Synchrotron-based X-ray absorption near-edge spectroscopy (XANES) coupled with electron paramagnetic resonance (EPR) suggested that biogenic birnessite was the dominant Mn oxide in the Mn-filter media surface coatings. The co-existence of amorphous and crystallized Mn-oxide surface morphologies on the Mn-filter media, observed using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX), suggested concomitant biological and autocatalytic (physicochemical) Mn(II) oxidation in the Mn filter. This study suggested that enhancing both biological and physicochemical Mn(II) oxidation is critical for the onset, acclimation and acceleration of Mn(II) removal during the biofiltration of cold groundwater. This study provides crucial insights for improving biofiltration performance in cold climates, representing a potential breakthrough for rapid biofiltration start-up, the biological acclimation of cold-adapted MnOB, and accelerated Mn(II) removal kinetics associated with the microbially mediated autocatalysis of Mn(II) oxidation at low temperatures.

Description

Keywords

Manganese, Biofiltration, Cold groundwaters, Microbial communities, Mn-oxides, Birnessite, Biological Mn(II) oxidation, Physico-chemical Mn(II) oxidation, Low temperatures, Mn-oxidizing bacteria, Accelerated Mn(II) kinetics, Filter ripening

Citation

Degree

Master of Science (M.Sc.)

Department

Civil and Geological Engineering

Program

Civil Engineering

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