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Entfernung natürlicher organischer Stoffe durch die Verfahrenskombination Flockung-Ultrafiltration bei der Aufbereitung reduzierter Grundwässer
Citation Link: https://doi.org/10.15480/882.2687
Publikationstyp
Doctoral Thesis
Date Issued
2020
Sprache
German
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2019-12-13
TORE-DOI
TORE-URI
Citation
Technische Universität Hamburg (2020)
Anoxic groundwaters, which are the main drinking water source in northern Germany, often contain high concentrations of natural organic matter (NOM), which is insufficiently removed in conventional treatment by aeration and rapid sand filtration. In order to comply with the German drinking water limit for color (SAC436 = 0.5 m-1) or to minimize undesirable effects of high NOM concentrations, numerous water suppliers are facing the necessity to supplement the existing treatment chains with additional processes for NOM removal. A promising option for this treatment objective is the combination of inline coagulation and ultrafiltration. The thesis investigates the potential of this hybrid process regarding its performance for NOM removal during drinking water treatment from anoxic groundwaters using a total of nine groundwaters and a surface water from northern Germany as well as various model waters.
The NOM characterization revealed large variations between the investigated groundwaters in quantitative (e. g. TOC, UVA254, SAC436) and qualitative terms (e. g. size distribution, aromaticity, fluorescence). Differences were mainly attributed to the origin and the genesis conditions of the NOM. Properties and behavior of NOM are dominated by the fraction of humic substances (70–86 % of DOC). Due to the small average molecule sizes (d50DOC = 500–750 g·mol-1), the NOM is insignificantly rejected by UF alone. Applying an inline coagulation prior to the UF, NOM removal can be significantly increased, whereby the fraction of humic substances is preferably removed due to its high-molecular structure and the high negative charge density. The extent of NOM removal depends on the coagulant concentration, the NOM properties and concentration and the pH value, while the coagulant type (FeCl3, AlCl3) and the operating conditions of the UF (flux, filtration time, backwash duration) showed only a minor impact on removal efficiency. Based on the experimental data, an empirical model was developed which accurately predicts the quantity of NOM removal of the hybrid process with respect to TOC, UVA254 and SAC436 in the pH range from 5.5 to 8.0 based on a few easily determined input parameters.
Regarding the filtration behavior, UF of the groundwaters without coagulation showed that the NOM causes a relatively low permeability loss, which is, however, to a large extent hy-draulically irreversible. Inline coagulation significantly reduces hydraulically irreversible fouling. However, the additional solid load in the system leads to a considerable increase in the total filtration resistance, which depends linearly on the solid mass on the membrane surface resulting from applied coagulant concentration, flux and filtration time. Based on a derived mass-dependent cake layer resistance, the pressure required for filtration can be esti-mated. A novel membrane module design developed as part of this thesis has shown that the filtration performance of in-out capillary membranes is additionally affected by inhomogeneously distributed cake layers. An approach to interpret flux and pressure data was devel-oped based on the modelling of hypothetical fouling mechanisms in in-out capillary membranes. Based on the model, important findings were derived for the assessment of filtration data and recommendations for the sustainable operation of UF plants.
In conclusion, this study shows that the hybrid process coagulation-UF is an efficient, compact, energy- and cost-saving option for the removal of macromolecular NOM components during the treatment of anoxic groundwaters. The findings of this work contribute to a pro-found understanding of the underlying mechanisms and provide a valuable basis for the design and optimization of this membrane hybrid process.
The NOM characterization revealed large variations between the investigated groundwaters in quantitative (e. g. TOC, UVA254, SAC436) and qualitative terms (e. g. size distribution, aromaticity, fluorescence). Differences were mainly attributed to the origin and the genesis conditions of the NOM. Properties and behavior of NOM are dominated by the fraction of humic substances (70–86 % of DOC). Due to the small average molecule sizes (d50DOC = 500–750 g·mol-1), the NOM is insignificantly rejected by UF alone. Applying an inline coagulation prior to the UF, NOM removal can be significantly increased, whereby the fraction of humic substances is preferably removed due to its high-molecular structure and the high negative charge density. The extent of NOM removal depends on the coagulant concentration, the NOM properties and concentration and the pH value, while the coagulant type (FeCl3, AlCl3) and the operating conditions of the UF (flux, filtration time, backwash duration) showed only a minor impact on removal efficiency. Based on the experimental data, an empirical model was developed which accurately predicts the quantity of NOM removal of the hybrid process with respect to TOC, UVA254 and SAC436 in the pH range from 5.5 to 8.0 based on a few easily determined input parameters.
Regarding the filtration behavior, UF of the groundwaters without coagulation showed that the NOM causes a relatively low permeability loss, which is, however, to a large extent hy-draulically irreversible. Inline coagulation significantly reduces hydraulically irreversible fouling. However, the additional solid load in the system leads to a considerable increase in the total filtration resistance, which depends linearly on the solid mass on the membrane surface resulting from applied coagulant concentration, flux and filtration time. Based on a derived mass-dependent cake layer resistance, the pressure required for filtration can be esti-mated. A novel membrane module design developed as part of this thesis has shown that the filtration performance of in-out capillary membranes is additionally affected by inhomogeneously distributed cake layers. An approach to interpret flux and pressure data was devel-oped based on the modelling of hypothetical fouling mechanisms in in-out capillary membranes. Based on the model, important findings were derived for the assessment of filtration data and recommendations for the sustainable operation of UF plants.
In conclusion, this study shows that the hybrid process coagulation-UF is an efficient, compact, energy- and cost-saving option for the removal of macromolecular NOM components during the treatment of anoxic groundwaters. The findings of this work contribute to a pro-found understanding of the underlying mechanisms and provide a valuable basis for the design and optimization of this membrane hybrid process.
Subjects
Huminstoffe
Flockung
Ultrafiltration
Membranfouling
Trinkwasseraufbereitung
Grundwasser
DDC Class
570: Biowissenschaften, Biologie
620: Ingenieurwissenschaften
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