Experimental modelling and prevention of chemical Fe‐clogging in deep vertical wells for open‐pit dewatering
Aachen / Publikationsserver der RWTH Aachen University (2016) [Dissertation / PhD Thesis]
Chemical Fe-clogging is a major factor strongly interfering with the performance of vertical filter wells. All over the world, groundwater is pumped by means of vertical filter wells to locally dewater aquifers for mining or construction purposes or to reliably meet daily demand for drinking water. Especially considering the increase in global population, rising production of natural soil resources and rapidly increasing demand for high quality drinking water, a sustainably high performance of vertical filter wells is essential from an economic as well as ecological point of view. In this dissertation, the influence of different gravel materials and well operation concepts on the performance of open-pit dewatering wells in the Rhenish lignite mining district in Germany was assessed in an experimental laboratory approach. Special focus was laid on long-term elution and chemical Fe-clogging affinity of different filter pack materials and the distribution of chemical Fe-clogging products, when aeration of the screen pipe in a dewatering well is avoided. Long-term elution and release of different (semi-)metals from different gravel materials were evaluated with column experiments, whereas for evaluation of the affinity towards chemical Fe-clogging, a combined hydraulic and hydrochemical experimental model of a well filter section was developed and optimised. With the experimental Fe-clogging model, the relevant parameters that controlled the progression of chemical Fe-clogging were identified. The affinity towards chemical Fe-clogging of different gravel materials was evaluated in comparison to that of artificially produced glass beads. Apart from these material tests, the influence of screen pipe aeration was evaluated by application of a shortened screen pipe. For comparison, spatial distribution and temporal development of Fe(III)-precipitation was additionally calculated by a computed hydrochemical model of kinetic Fe(II)-oxidation with the hydrogeochemical modelling programme PhreeqC. Varying the input data of the computed model allowed for a sensitivity analysis of the hydrochemistry applied in the experimental model. Further comparison of type and spatial distribution of the Fe-clogging products produced in the experimental model was possible by samples taken from the gravel pack of wells in the field that had been excavated during succession of the open-pit Garzweiler. Chemical Fe-clogging in the model was found to exclusively develop in the unsaturated zone of the gravel pack and the screen pipe. In rough agreement with the hydrogeochemical PhreeqC-model, the highest Fe-clogging-intensity in the gravel pack was concentrated close to the boundary of the aquifer. This was due to unsaturated, vertical flow components in the vicinity of the material boundary. Similar distributions of the Fe-clogging zones were observed during field sampling. Intense Fe-clogging was found to be heterogeneously distributed in zones of unsaturated vertical flow that allowed mixing with the gaseous phase, as well as homogeneously distributed in zones of water level fluctuations. Fine-grained muddy or dusty plumes of Fe-precipitations on the inner surface of screen tubes in the field were found to be products of biological Fe-clogging that had been strictly excluded in the experimental model. Mineralogical analyses revealed Fe-encrustations produced in the experimental model to be of higher crystallinity than those sampled in the field. Whereas in the model goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) were primarily formed, the percentage of ferrihydrite (Fe5HO8∙4H2O) was higher in the field samples. Quickly precipitated feroxyhyte (δ’-FeOOH) was found in the laboratory samples only. With the XRD-measurements a gypsum phase was identified in the field samples, presumably originating from evaporation of residual mine water in the gravel pack of the sampled wells. The, however subtle, differences in mineralogy of the Fe-precipitates in the model occurred as a result of deviations in the hydrochemistry of the process water, especially the dominating anion (chloride in the model instead of sulphate or bicarbonate in the field) and the amount of dissolved silicon. As discovered with the computed hydrogeochemical model, apart from pH and temperature, the strongest effects on the reaction rate of Fe(II)-oxidation with Fe(III)-precipitation were the amount of dissolved oxygen in the process water as well as its pH-buffering content of bicarbonate. Microscopic analyses showed the surface structure of the gravel grains to be responsible for the thickness of Fe(III)-hydroxide coatings. Especially irregularly formed monocrystalline quartz grains, weathered feldspar grains and polycrystalline quartzite grains were found to develop a higher thickness of Fe(III)-hydroxide coatings, when their surface area was uneven. All of the different gravels and filter pack materials were found to perform similarly with respect to long-term elution as well as Fe-clogging affinity. Despite initial contents of ferric iron minerals in the Quaternary gravel materials, no significant autocatalytic effect of Fe(II)-oxidation and Fe(III)-precipitation was observed. Application of the alternative Quaternary gravel materials should therefore be possible without having to expect any substantial drawbacks or disadvantages when compared to the Tertiary Weilerswist gravel. However, application of the glass bead filter pack in the model did not result in a significantly better performance with respect to Fe-clogging-affinity, when compared to the natural gravel materials. Therefore, amortisation of the high investment cost of a glass bead filter pack during the relatively short lifetime of a dewatering well is uncertain and can thus not be recommended for the dewatering wells in the Rhenish lignite district, where no regeneration methods are applied. A new concept of multi-layered dewatering through the annular space of dewatering wells was implemented in three test wells by RWE Power AG, transferring the results of the filter aeration experiments to the field scale. The vertical hydraulic conductivity of the annular space and the functionality of the concept were then tested by tracer experiments. Improved hydraulic conductivities and flow rates in the annular space – filled with gravel pack material instead of the usual filling material – were confirmed by the field experiments. The decreased Fe-clogging affinity of such wells was confirmed in the Fe-clogging model. Avoiding aeration of the screen lead to preclusion of Fe(II)-clogging, so long as the filter pack stayed fully saturated and the screen pipe submerged. Further optimisation and automatisation of model operation could allow quicker evaluation of the Fe-clogging affinity of further well assembling materials and operating concepts. A more realistic situation could be simulated when considering sulphate or bicarbonate as the dominating anions. Matters that could be investigated by application of the proposed Fe-clogging model include optimising the screen pipe material and geometry, inert gas treatment and electrochemical methods for preventing chemical Fe-clogging, optimisation of regeneration measures, biological aspects of Fe-clogging and identification of relevant air-flow pathways.
Rüde, Thomas R.