Mining Wastewater Treatment by Slag Filters

RÉSUMÉ Les deux objectifs de ce projet étaient de proposer un système de traitement par filtres à scories du lixiviat de la mine Joplin, Missouri et de proposer un modèle préliminaire des mécanismes de rétention du phosphore dans les filtres à scories. Trois phases expérimentales ont été réalisées : essais individuels en flacon, essais séquentiels en flacon et essais en colonnes. Les essais individuels en flacon ont été utilisés pour comparer la performance de traitement de 10 scories et 1 apatite et pour choisir les meilleurs matériaux parmi ceux-ci. Les essais séquentiels en flacon ont été utilisés pour comparer les performances de traitement de quelques séquences de filtres. Finalement, les essais en colonnes ont été utilisés pour caractériser la performance d’enlèvement et la longévité d’un filtre soumis à des conditions d’alimentation contrôlées. Durant les essais en colonnes, l’effet de la composition de l’affluent, du temps de rétention hydraulique des vides et de la séquence de filtre a été mesuré. Certains essais en colonnes ont été opérés spécifiquement pour étudier les mécanismes d’enlèvement du phosphore. Le modèle proposé est défini par les étapes suivantes : 1) la dissolution cinétique des scories est représentée par la dissolution de CaO, 2) un fort pH dans le filtre cause la précipitation du phosphore et la croissance de cristaux, 3) la rétention des cristaux est causée par filtration, décantation et densification, 4) la diminution du volume de réaction (et du temps de réaction) disponible est causée par l’accumulation de cristaux et autres particules dans les vides, 5) le pH dans le filtre diminue avec le temps si le temps de réaction est trop court (ce qui résulte en une diminution de l’efficacité d’enlèvement). Une nouvelle approche pour définir les performances des filtres à scories est proposée. La capacité de rétention à saturation est exprimée en mg P / mL de vides. Le système proposé pour traiter le lixiviat de la mine Joplin est composé de deux filtres successifs de scories EAF Fort Smith opérés à un temps de rétention hydraulique des vides total de 34 h. Après 179 jours d’opération, les performances de traitement des deux filtres ont été de 99.9 %, 85.3 %, 98.0 % et 99.3 % pour le phosphore, le fluor, le manganèse et le zinc, respectivement.----------ABSTRACT The two objectives of this project were to propose a slag filters treatment system for the leachate of the Joplin mine, Missouri, and propose a preliminary model for phosphorus removal mechanisms in slag filters. Three experimental phases were realized: individual batch tests, sequential batch tests and column tests. Individual batch tests were used to compare the treatment performance of 10 slags and 1 apatite and to select the best materials from this list. Sequential batch tests were used to compare the treatment performance of several filter sequences. Finally, column tests were used to characterize the removal performance and longevity of controlled filters. During column tests, the effect of influent composition, void hydraulic retention time and filter sequences was measured. Some column tests were operated especially for the study of phosphorus removal mechanisms. The proposed model is expressed by the following steps: 1) the rate limiting dissolution of slag is represented by the dissolution of CaO, 2) a high pH in the slag filter results in phosphorus precipitation and crystal growth, 3) crystal retention takes place by filtration, settling and growth densification, 4) the decrease in available reaction volume is caused by crystal and other particulate matter accumulation (and an associated decrease in available reaction time), and 5) the pH decreases in the filter over time if the reaction time is too short (which results in a reduced removal efficiency). A new approach to define filter performance is proposed. Saturation retention capacity is expressed in units of mg P/ mL voids. The proposed system for the treatment of the Joplin mine leachate is composed of two successive Fort Smith EAF slag filters operated at a total void hydraulic retention time of 34 hours. After 179 days of operation, the treatment performance of this system was 99.9 %, 85.3 %, 98.0 % et 99.3 % for phosphorus, fluoride, manganese and zinc respectively

Phosphorus Treatment by Steel Slag Filters Modelling of Removal Mechanisms and Application for Phosphorus Removal from Septic Tank Effluents

RÉSUMÉ Le premier objectif de cette thèse était de proposer un système de traitement du P incluant un filtre à scories intégré à une chaîne de traitement de fosse septique et élément épurateur. L’utilisation d’un filtre à scories en recirculation dans la fosse septique a été étudiée (article #1). Deux modes de recirculation ont été testés. Le meilleur système a été la recirculation du 2e compartiment vers le 1er compartiment à 50% de recirculation, avec 4.2 et 1.9 mg P/L à l’effluent en P total et ortho-phosphates, respectivement. La cible de 1 mg /L à l’effluent n’a pas été atteinte, mais l’ajout du filtre à scories a permis la rétention de 79% du phosphore, comparativement à 29% sans filtre.----------ABSTRACT The first objective of this thesis was to propose a phosphorus treatment system based on a steel slag filter integrated to a septic tank-infiltration bed system. The use of a recirculating steel slag filter was studied in an experimental program (paper #1). Two recirculation modes with various recirculation ratios were tested. The best system was the one with a recirculation from the end to the inlet of the second compartment of a septic tank with a 50% recirculation ratio in the slag filter, achieving 4.2 and 1.9 mg P/L at the effluent for TP and o-PO4, respectively. The 1 mg P/L level goal was not reached, but 79% of phosphorus was kept in the system, compared to 29% without the slag filter

Chemical clogging and evolution of head losses in steel slag filters used for phosphorus removal

The objective of this study was to propose a conceptual model of clogging in alkaline granular filters. Two slag columns were operated for 600 days and monitored using piezometers and tracer tested at regular intervals. The type of influent (organic or inorganic) affected the loss of effective porosity in the filters. Well organized and loose crystal structures were observed by scanning electron microscopy in columns with inorganic and organic influents, respectively. It was postulated that the formation of crystals in unorganized structures results in confined voids that are not accessible for water flow, thus accelerating porosity loss. The effect of the combination of chemical clogging and biofilm on the porosity loss is higher than the effect of these two factors separately. The Kozeny-Carman equation for hydraulic conductivity could not efficiently predict the evolution of head losses in the column fed with an inorganic influent. The crystal structure and connectivity in the presence of homogeneous or heterogeneous precipitation are concepts that could improve predictions of hydraulic conductivity. The results of this study highlighted the importance of the inlet zone on the development of pressure head in alkaline granular filters. Future research on clogging should focus on precipitation mechanisms in the inlet zone and on the design of the feeding system

Numerical simulations with the P-Hydroslag model to predict phosphorus removal by steel slag filters

The first version of the P-Hydroslag model for numerical simulations of steel slag filters is presented. This model main original feature is the implementation of slag exhaustion behavior, crystal growth and crystal size effect on crystal solubility, and crystal accumulation effect on slag dissolution. The model includes four mineral phases: calcite, monetite, homogeneous hydroxyapatite (constant size and solubility) and heterogeneous hydroxyapatite (increasing size and decreasing solubility). In the proposed model, slag behavior is represented by CaO dissolution kinetic rate and exhaustion equations; while slag dissolution is limited by a diffusion rate through a crystal layer. An experimental test for measurement of exhaustion equations is provided. The model was calibrated with an experimental program made of three phases. Firstly, batch tests with 300 g slag sample in synthetic solutions were conducted for the determination of exhaustion equation. Secondly, a slag filter column test fed with synthetic solution was run for 623 days, divided into 9 cells and sampled at the end of the experiment. Finally, the column was dismantled, sampled and analyzed with XRD, TEM and SEM. Experimental column curves for pH, oPO4, Ca and inorganic carbon were well predicted by the model. Crystal sizes measured by XRD and TEM validated the hypothesis for homogeneous precipitation while SEM observations validated the thin crystal layer hypothesis. A preliminary validation of the model resulted in successful predictions of a steel slag filter longevity fed with real wastewater

Phosphorus removal by steel slag filters: modeling dissolution and precipitation kinetics to predict longevity

This article presents an original numerical model suitable for longevity prediction of alkaline steel slag filters used for phosphorus removal. The model includes kinetic rates for slag dissolution, hydroxyapatite and monetite precipitation and for the transformation of monetite into hydroxyapatite. The model includes equations for slag exhaustion. Short-term batch tests using slag and continuous pH monitoring were conducted. The model parameters were calibrated on these batch tests and experimental results were correctly reproduced. The model was then transposed to long-term continuous flow simulations using the software PHREEQC. Column simulations were run to test the effect of influent P concentration, influent inorganic C concentration and void hydraulic retention time on filter longevity and P retention capacity. High influent concentration of P and inorganic C, and low hydraulic retention time of voids reduced the filter longevity. The model provided realistic P breakthrough at the column outlet. Results were comparable to previous column experiments with the same slag regarding longevity and P retention capacity. A filter design methodology based on a simple batch test and numerical simulations is proposed

Improving phosphorus removal of conventional septic tanks by a recirculating steel slag filter

The objective of this project was to increase the phosphorus (P) retention capacity of a conventional septic tank by adding a recirculating slag filter. Two recirculation modes and recirculation ratios from 5 to 50% were tested in the laboratory with reconstituted domestic wastewater. The best system was recirculation from the end to the inlet of the second compartment of a septic tank with a 50% recirculation ratio in the slag filter, achieving 4.2 and 1.9 mg P/L at the effluent for total phosphorus (TP) and orthophosphate (o-PO4), respectively, and a pH of 8.8. The calculated size of the slag filter for a two-bedroom house application was 1,875 kg for an expected lifetime of 2 years. The 1 mg P/L level goal was not reached, but P precipitation may be favoured by the relatively high effluent pH reaching the infiltration bed

Model of phosphorus precipitation and crystal formation in Electric Arc Furnace steel slag filters

The objective of this study was to develop a phosphorus retention mechanisms model based on precipitation and crystallization in electric arc furnace (EAF) slag filters. Three slag columns were fed during 30 to 630 days with a reconstituted mining effluent at different void hydraulic retention times. Precipitates formed in columns were characterized by X-ray diffraction and transmission electronic microscopy. The proposed model is expressed in the following steps: 1) the rate limiting dissolution of slag is represented by the dissolution of CaO, 2) a high pH in the slag filter results in phosphorus precipitation and crystal growth, 3) crystal retention takes place by filtration, settling and growth densification, 4) the decrease in available reaction volume is caused by crystal and other particulate matter accumulation (and decrease in available reaction time), and 5) the pH decreases in the filter over time if the reaction time is too low (which results in a reduced removal efficiency). Crystal organization in a slag filter determines its phosphorus retention capacity. Supersaturation and water velocity affect crystal organization. A compact crystal organization enhances the phosphorus retention capacity of the filter. A new approach to define filter performance is proposed: saturation retention capacity is expressed in units of mg P/ mL voids

Phosphorus removal and carbon dioxide capture in a pilot conventional septic system upgraded with a sidestream steel slag filter

The objective of this work was to demonstrate the removal of the phosphorus and carbon dioxide capture potential of a conventional septic system upgraded with a sidestream steel slag filter used in recirculation mode. A pilot scale sidestream experiment was conducted with two septic tank and drainfield systems, one with and one without a sidestream slag filter. The experimental system was fed with real domestic wastewater. Recirculation ratios of 25%, 50% and 75% were tested. Limestone soils and non-calcareous soils were used as drainfield media. The tested system achieved a satisfactory compromise between phosphorus removal and pH at the effluent of the septic tank, thus eliminating the need for a neutralization step. The phosphorus removal efficiency observed in the second compartment of the septic tank was 30% in the slag filter upgraded system, compared to −3% in the control system. The slag filter reached a phosphorus retention of 105 mg/kg. The drainfield of non-calcareous soils achieved very high phosphorus removal in both control and upgraded systems. In the drainfield of limestone soil, the slag filtration reduced the groundwater phosphorus contamination load by up to 75%. The removal of chemical oxygen demand of the drainfields was not affected by the pH rise induced by the slag filter. Phosphorus removal in the septic tank with a slag filter was attributed to either sorption on newly precipitated calcium carbonate, or the precipitation of phosphate minerals, or both. Recirculation ratio design criteria were proposed based on simulations. Simulations showed that the steel slag filter partly inhibited the biological production of carbon dioxide in the septic tank. The influent alkalinity strongly influenced the recirculation ratio needed to raise the pH in the septic tank. The recirculation mode allowed clogging mitigation compared to a mainstream configuration, because an important part of chemical precipitation occurred in the septic tank. The control septic tank produced carbon dioxide, whereas the slag filter-upgraded septic tank was a carbon dioxide sink

Removal of phosphorus, fluoride and metals from a gypsum mining leachate using steel slag filters

The objective of this work was to evaluate the capacity of steel slag filters to treat a gypsum mining leachate containing 11-107 mg P/L ortho-phosphates, 9-37 mg/L fluoride, 0.24-0.83 mg/L manganese, 0.20-3.3 zinc and 1.7-8.2 mg/L aluminum. Column tests fed with reconstituted leachates were conducted for 145 to 222 days and sampled twice a week. Two types of electric arc furnace (EAF) slags and three filter sequences were tested. The voids hydraulic retention time (HRTv) of columns ranged between 4.3 and 19.2 h. Precipitates of contaminants present in columns were sampled and analyzed with X-ray diffraction at the end of tests. The best removal efficiencies over a period of 179 days were obtained with sequential filters that were composed of Fort Smith EAF slag operated at a total HRTv of 34 h which removed 99.9% of phosphorus, 85.3% of fluoride, 98.0% of manganese and 99.3% of zinc. Mean concentration at this system’s effluent was 0.04 mg P/L ortho-phosphates, 4 mg/L fluoride, 0.02 mg/L manganese, 0.02 zinc and 0.5 mg/L aluminum. Thus, slag filters are promising passive and economical systems for the remediation of mining effluents. Phosphorus was removed by the formation of apatite (hydroxyapatite, Ca5(PO4)3OH or fluoroapatite, Ca5(PO4)3F) as confirmed by visual and X-ray diffraction analyses. The growth rate of apatite was favored by a high phosphorus concentration. Calcite crystals were present in columns and appeared to be competing for calcium and volume needed for apatite formation. The calcite crystal growth rate was higher than that of apatite crystals. Fluoride was removed by precipitation of fluoroapatite and its removal was favored by a high ratio of phosphorus to fluoride in the wastewater

Steel slag filter design criteria for phosphorus removal from wastewater in decentralized applications

The objective of this project was to develop a novel phosphorus removal system using steel slag filters applicable in decentralized applications and to propose design criteria about maintenance needs. Slag exhaustion functions were measured on 2-3 mm, 3-5 mm, 5-10 mm and 16-23 mm slag. Three steel slag columns with particle size of 2-3 mm, 3-5 mm and 5-10 mm were fed with the effluent of an aerated lagoon during 589 days. A barrel reactor test was fed during 365 days with the effluent of an attached growth aerated biological reactor. The o-PO4 concentration at the effluent of the 2-3 mm and 3-5 mm columns and barrel reactor test was between 0.04 and 0.3 mg P/L. Particulate phosphorus, however, was removed by about 50%. The P-Hydroslag model implemented in PHREEQC was successfully calibrated with data from the column test, and validated with data from the barrel reactor test. The calibrated model was used to simulate long-term operation of a slag barrel reactor with two parallel streams of five replaceable steel slag barrels, with total hydraulic retention time of voids of 15 h. The system longevity was strongly influenced by the influent alkalinity. The simulated longevity was 7 years with an influent alkalinity of 50 mg CaCO₃/L and 2 years with an influent of 210 mg CaCO₃/L. The alkalinity of the steel slag filter influent was influenced by the type of aquifer supplying drinking water, the presence of nitrification activity and by the CO₂ concentration in the enriched air of the upstream biological process. Simulated scenarios with partial barrel replacement (e. g. barrels 1 and 2 out of 5 replaced at frequency of 0.5, 1, 1.5, 2, 2.5, 3, 3.5 or 4 years) increased the system longevity up to 14 years while slightly increasing the number of barrels needed