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A Novel Computational Approach for the Management of Bioreactor LandfillsAbdallah, Mohamed E. S. M. 13 October 2011 (has links)
The bioreactor landfill is an emerging concept for solid waste management that has gained significant attention in the last decade. This technology employs specific operational practices to enhance the microbial decomposition processes in landfills. However, the unsupervised management and lack of operational guidelines for the bioreactor landfill, specifically leachate manipulation and recirculation processes, usually results in less than optimal system performance. Therefore, these limitations have led to the development of SMART (Sensor-based Monitoring and Remote-control Technology), an expert control system that utilizes real-time monitoring of key system parameters in the management of bioreactor landfills.
SMART replaces conventional open-loop control with a feedback control system that aids the human operator in making decisions and managing complex control issues. The target from this control system is to provide optimum conditions for the biodegradation of the refuse, and also, to enhance the performance of the bioreactor in terms of biogas generation. SMART includes multiple cascading logic controllers and mathematical calculations through which the quantity and quality of the recirculated solution are determined. The expert system computes the required quantities of leachate, buffer, supplemental water, and nutritional amendments in order to provide the bioreactor landfill microbial consortia with their optimum growth requirements.
Soft computational methods, particularly fuzzy logic, were incorporated in the logic controllers of SMART so as to accommodate the uncertainty, complexity, and nonlinearity of the bioreactor landfill processes. Fuzzy logic was used to solve complex operational issues in the control program of SMART including: (1) identify the current operational phase of the bioreactor landfill based on quantifiable parameters of the leachate generated and biogas produced, (2) evaluate the toxicological status of the leachate based on certain parameters that directly contribute to or indirectly indicates bacterial inhibition, and (3) predict biogas generation rates based on the operational phase, leachate recirculation, and sludge addition. The later fuzzy logic model was upgraded to a hybrid model that employed the learning algorithm of artificial neural networks to optimize the model parameters.
SMART was applied to a pilot-scale bioreactor landfill prototype that incorporated the hardware components (sensors, communication devices, and control elements) and the software components (user interface and control program) of the system. During a one-year monitoring period, the feasibility and effectiveness of the SMART system were evaluated in terms of multiple leachate, biogas, and waste parameters. In addition, leachate heating was evaluated as a potential temperature control tool in bioreactor landfills.
The pilot-scale implementation of SMART demonstrated the applicability of the system. SMART led to a significant improvement in the overall performance of the BL in terms of methane production and leachate stabilization. Temperature control via recirculation of heated leachate achieved high degradation rates of organic matter and improved the methanogenic activity.
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Tungmetaller i lakvatten : avskiljning med mineraliska filtermaterialHjelm, Veronica January 2005 (has links)
Four different kinds of filter-materials with reactive surfaces have been studied concerning their capacity to absorb heavy metals in leachate from a municipal waste deposit. The heavy metals studied were: lead, cadmium, copper, mercury, chromium, nickel and zinc. The leachate contains high levels of dissolved organic carbon (DOC) and has a high pH-value along with a high buffer capacity. These characteristics of the leachate make it difficult to remove pollutants and require efficient filters. The filters that were examined in the report are blast-furnace slag with CaO, sand covered with iron oxides, olivine and nepheline. The experiment was carried out in two sets, starting with batch experiments followed by a column study. The objective of the batch experiments was to find out how variations in pH affected the sorption capacity of the materials. The interval used during the test was from pH 5 to pH 10. The computer program VisualMinteq was used to evaluate the dominating sorption processes when the materials interacted with the solutions. Two different kinds of solutions were used in the batch experiment. One of them was the leachate, to which known concentrations of heavy metals were added (about 1 μM) and the other consisted of sodium nitrate, a solution without organic compounds, which was used as a reference. The sodium nitrate solution was also spiked with the same concentration of heavy metals as the leachate. The results from the batch experiment showed that the sorption of heavy metals was lowered if the DOC level was high. No relation between pH and sorption ability could be found for the leachate, but for some metals in the sodium solution a higher pH improved the removal of heavy metals. The two materials that showed best results in the batch experiment were the blast-furnace slag and the sand with iron oxides. These materials were used in the column study. The olivine material was somewhat better than the nepheline in the batch experiment. Four columns were used in the column study, two for each material. Leachate with heavy metals was pumped into the columns with a specific flow rate; at first a low flow rate was used and when half the experiment time had passed the flow rate was increased. The flow rates used were 0.12 m/24 h and 0.62 m/24 h. The outcome of the column experiment showed that the slag had the highest ability to adsorb metals. The metal sorption was over 60 percent for lead, cadmium and zinc, where the highest sorption was obtained for lead. No affects were noticed when the flow rate was increased. / I detta examensarbete har fyra olika reaktiva filtermaterials kapaciteter att ur deponilakvatten avskilja tungmetallerna bly, kadmium, koppar, kvicksilver, krom, nickel och zink testats. Deponilakvatten är ett avloppsvatten med höga halter organiskt material (DOC), högt pH och en hög buffertkapacitet. Dess sammansättning ställer stora krav på ett filter och närvaron av ligander påverkar sorptionprocesserna. Filtren som ingått i studien är kalciumoxiddopad masugnsslagg, järnoxidtäckt sand, olivin och nefelin. Försöken utfördes i två delar, med inledande skakförsök och därefter kolonnförsök. I båda försöken användes lakvatten med en extra tillsats av tungmetaller. I skakförsöken användes även natriumnitrat; ett referensvatten utan organiska ligander, även det spikat med tungmetaller. I skakförsöken studerades avskiljningens pH-beroende för de olika filtren, med ett pH-intervall på ca 5 – 10. Därefter modellerades resultaten i jämviktsprogrammet VisualMinteq för att fastställa vilka processer som styr avskiljningen. Skakförsöken och kolonnförsöken utfördes båda i klimatrum vid 8 ºC, för att efterlikna markens naturliga temperatur. Resultaten från skakförsöken visade att masugnsslagg och järnoxidsand gav bäst avskiljning för de flesta tungmetaller. Olivin och nefelin var sämre metallsorbenter, där olivin uppvisade något bättre resultat än nefelin. Inget tydligt pH-beroende för lakvattnet kunde utläsas, men för natriumnitratlösningen gav en pH-höjning en ökad sorption för vissa metaller. Den kemiska jämviktsmodelleringen visade att den dominerande processen i filtermaterialen var sorption på filterytorna. De två filtermaterialen som visade bäst resultat i skakförsöken (slagg och järnoxid) studerades vidare i kolonnförsök, där ett bestämt lakvattenflöde pumpades genom kolonner packade med materialen. Två olika flödeshastigheter testades (0,12 m/dygn och 0,62 m/dygn) och sorptionen av metaller analyserades. Slaggen uppvisade högst avskiljningskapacitet av de studerade filtren. De metaller som sorberades bäst var bly, kadmium och zink (över 60 % avskiljning), där den högsta sorptionen erhölls för bly. Gemensamt för både skak- och kolonnförsöken var att sorptionen försvårades då halten organiskt material (DOC) ökade, vilket beror på att DOC konkurrerar med de reaktiva ytorna på filtren om att binda den fria metalljonen. Ingen minskande avskiljningseffekt av en flödesökning kunde utläsas.
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Bottensubstrat och dess inverkan på reducering av BOD5, COD och TKN i lakvatten genom konstruerade rotzonsanläggningar : En pilotstudie vid Univates, Lajeado – RS BrasilienEkholm, Emy January 2010 (has links)
Treatment wetlands been showed efficient for reducing pollutant in waste water. In Lajeado – RS, Brazil the landfill has poor leachate water treatment. It is necessary to supplement the treatment plant because they need to reduce BOD, COD and nitrate of the water going to recipient. A subsurface flow wetland (SSF) can be a good choice. In order to be able to design an efficient SSF it is important to understand how the grain sizes of a substrate affect the reducing of pollutants in waste water. This study focus on two substrate, sand with grain size of 0 - 3 mm and gravel with the grain size of 10 - 20 mm. To see the grain size reduces BOD, COD and nitrate best, the experiment used eight pilot scales SSF for leachate water treatment, four filled with sand and four filled with gravel. Two different flows, four with batch and four used continuous flow; two of each was planted with Thypa angustifolia L. Samples were taken from each wetland every week during a four week period. The results showed that the wetlands with the fine- grained substrate; sand gave the better reduction of BOD, COD and TKN (total kjeldahl kväve). It also showed great reduction in color. Important to notice in this study is the lack of time; more samples are required to be able to establish a pattern.
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Leachate treatment and anaerobic digestion using aquatic plants and algaeStröm, Emma January 2010 (has links)
Phytoremediation as a way to control and lessen nutrient concentrations in landfill leachate is a cheap and environmentally sustainable method. Accumulated nutrients in the plants can then be removed by harvesting and anaerobically digesting the biomass. This study presents two aquatic plants (L. minor (L.) and P. stratiotes (L.)) and one microalgae species (C. vulgaris (L.)), their capacities for growth and nutrient removal in leachate from Häradsudden landfill, Sweden, are investigated. The biogas potential of the two plants is determined via anaerobic digestion in a batch run, followed by a lab-scale reactor run for L. minor only. Results show that growth in leachate directly from the landfill is not possible for the selected species, but at a leachate dilution of 50% or more. Nutrients are removed in leachates with plants to a higher extent than in leachates without, yet the actual amounts do not differ notably between plant species. L. minor proves a better choice than P. stratiotes despite this as growth is superior for L. minor under the experimental conditions of this study. Considering biogas production, L. minor gives more methane than P. stratiotes according to the results from the batch run. The former is however not suitable for large-scale anaerobic digestion unless as an additional feedstock due to practical cultivation issues.
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Treatment of leachate by combining PAC and UV/O3 processes / Kết hợp keo tụ với PAC và quá trình UV/O3 để xử lý nước rỉ rác phát sinh từ bãi chôn lấp chất thải rắnVan, Huu Tap, Trinh, Van Tuyen, Dang, Xuan Hien 15 November 2012 (has links) (PDF)
The landfill leachate is commonly treated for non-biodegradable organic matters, ammonia and colour. Experimental investigations using polyaluminium chlorite (PAC) and UV/O3 have been conducted for the determination of optimal pH value, reaction time and PAC concentration for the removal of chemical oxygen demand (COD) and colour. In pre-treatment coagulation stages, the highest COD and colour removal efficiencies were observed at the concentration of PAC ≥ 3,000 mglG1 and pH values between 7 and 8. However, these experiments also indicated significant removal efficiency for PAC starting with concentrations of 1,500 mglG1. The efficiency of COD and colour removal were approximately 30% and 70%, respectively. Similar efficiencies have been observed also during the second treatment stage where UV/O3 processes were used to treat coagulated leachate. After UV/O3 application, the pH of leachate reached the optimum value of 7.5 whereas the highest COD and colour removal efficiency was 55% and 72%, respectively, and the optimal reaction time was achieved after 80 min. / Nước rỉ rác sinh ra từ bãi chôn lấp chất thải rắn cần được xử lý các thành phần chất hữu cơ khó phân hủy sinh học, xử lí amoni và độ màu. Một số kết quả thử nghiệm về xử lý COD và màu của nước rỉ rác bằng việc sử dụng phương pháp keo tụ với PAC và quá trình UV/O3 đã được thực hiện cùng với việc xác định các giá trị pH tối ưu, thời gian phản ứng và nồng độ PAC tối ưu. Hiệu suất xử lý cao nhất đạt được khi nồng độ của PAC ≥ 3.000 mg/l, pH trong khoảng từ 7 đển 8 trong giai đoạn tiền xử lý. Tuy nhiên, hiệu quả loại bỏ COD và màu bắt đầu tăng rõ khi nồng độ PAC từ 1.500 mg/l trở lên. Hiệu quả loại bỏ COD và màu tương ứng là khoảng 30% và 70%. Các giá trị pH này phù hợp cho quá trình phản ứng UV/O3 được sử dụng sau giai đoạn keo tụ. Sau quá trình xử lý bằng hệ UV/O3, pH của nước rỉ rác tối ưu được xác định là 7,5 (hiệu suất xử lý COD và màu cao nhất tương ứng là 55% và 72%), thời gian phản ứng tối ưu là 80 phút.
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The Study of Treating Leachate in Landfill by Constructed WetlandsTsai, Kai-yuan 04 September 2004 (has links)
In this research, we dealt with the original landfill leachate, and put the flowing water by a traditional second one that has dealt with of sewage factory directly with the constructed wetland systems in lab-scale. The purpose the study is to compare the experimental results after dealing the leachate by different constructed wetland process systems which was judge at if the, can reach the water quality standard. The constructed wetland systems in the study were set up in a greenhouse on campus, which were operated between May 2004 and July 2003. Constructed wetland systematic attitude, include Free Water Surface System (FWS) and Subsurface Flow System (SSF) contact but two type constructed wetland system that become mainly. In the test use different waste water sources to feet the systems, and we planted different species of plants, and add extra carbon source. Thus, we can divide the experiment into two stages: (1) From July of 2003 to December of 2003. (2) From February of 2004 to May of 2004. In each stage, we tested three experiment group P1, P2 and total nitrogen of P3 except that the rate is 37¡Ó20%, 50¡Ó11% respectively and -6.1¡Ó37.8%. The removal efficiencies of ammonia nitrogen were estimated equal to 84¡Ó11%, 94¡Ó4% and 60¡Ó24% respectively. For total phosphorus and phosphate, P1 , P2 and P3 system were measured equal to 37¡Ó14%, 68¡Ó16% and 77¡Ó16%and 44¡Ó16%, 75¡Ó8% and 80¡Ó17% respectively . The ammonia nitrogen temperature correction coefficient (£c value) were calculated to be 0.921, 0.949, 0.926, 1.043 and 0.785 for P1, P2, P3, U1 and U2 system, respectively. The phosphate temperature correction coefficients (£c values ) were measured to be 1.006, 0.981, 1.070, 1.235 and 0.843 respectively for P1, P2, P3, U1 and U2 system. In order to increase the removal efficiencies of total nitrogen, it was always by adding carbon source. The experimental result showed, that it is add carbon source wait by system to SSF , its President nitrogen is it flow water pollution load leave 2g/m2/day nearly to put to have, and has not added water pollution load has flowed in the putting of the carbon source nearly has 6g/m2/day left, And nearly remain under 1g/m2/day in the putting and flowing water pollution load of TON, and not added the putting of the carbon source water pollution load has flowed nearly has 2-3g/m2/day left. This result showed that high total nitrogen removal efficiencies were by adding carbon source.
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Inorganic Sorption in Polymer Modified Bentonite ClaysNocon, Melody Schwartz 19 May 2006 (has links)
In 1986, geosynthetic clay liners (GCLs) were invented and successfully used as a replacement for the soil layer in composite lining systems. In some applications an additive (polymer) is mixed with the bentonite to increase performance, especially in those that have low concentrations of sodium bentonite (EPA 2001).Studies showing significant increases in hydraulic conductivity values for bentonite in the presence of high salt concentrations are frequently documented and there is a risk of early breakthrough due to performance failure of the GCL clay component. (Ashmawy et al, 2002). It has also been stated that sodium, potassium, calcium, and magnesium have such a high affinity for the clay's surface other chemical species have little chance of attenuation (EPA 2001).
For these reasons, researching sorption in the presence of major salt cations and polymers gains great importance.Distribution coefficients were extrapolated from Linear, Freundlich and Langmuir sorption isotherms for sodium and calcium cations modeled from data collected from batch tests of sodium bentonite and various manufactured and custom mixed polymer modified bentonites. Surface characterization before and after calcium or sodium solution exposure of all tested media was accomplished by use of scanning electron microscopy and energy dispersive x-ray analysis.
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Environmental Risk Assessment and Leachate Evolution at the Ekebyboda Landfill Site, Uppland, Sweden. / Miljöriskbedömning och lakvattenutvecklingvid Ekebyboda deponi, Uppland, SverigeFors, Jonas January 2015 (has links)
Ekebyboda is a closed landfill north west of Uppsala where the leachate water was studied to evaluate the status of the landfill for, the Uppsala environmental office. Leachate water has been measured continually for different parameters, during the period 1959-2007. The landfill differs from other landfills because of its mixed content of domestic and industrial waste. Close to the investigated area is a small number of residents with private water wells which enhances the importance of the investigation of the landfill. In this thesis, the period 1990 -2007 is the investigated with an additional measurement during 2014. Precipitation data is compared with leachate water composition, to evaluate correlations between leachate water and precipitation. Correlation between leachate water, and precipitation also gave an indication of the status of the cover on the landfill during, 2014. Due to the problematic history of Ekebyboda the aim of the thesis is to evaluate the status of the landfill and do a risk classification according to MIFO, which is a classification system for polluted sites from the Swedish Environmental Protection Agency. Measurements were made in four wells in the spring of 2014. These results showed a decline in eight analysed parameters and increase or stagnant trend for pH, NO3- + NO2 and SO42-. Most of the parameters have large fluctuations during the period. A field investigation is also show a non-functional culvert system with stagnant water and indications of water running in the direction of the residential area due to malfunctioning pumps. The stagnant water and problematic culvert system raised concerns for the quality of the datasets. The data analysis shows that the correlation between precipitation and leachate water was non-existent and the second covering of the landfill has reduced the amount of infiltrating water. Analysis of the leachate water and problematic management of leachate water were two major causes for a high risk classification. The MIFO classification of this landfill was set as one which is the highest possible which mainly was due to a possible risk for contamination for private water wells. / Ekebyboda är en nedlagd deponi nordväst om Uppsala vars lakvatten studeras för att utvärdera deponins tillstånd på uppdrag av Miljökontoret i Uppsala. Lakvattenkvalitén från deponin har kontinuerligt blivit mätt under perioden, 1959-2007. Deponin skiljer sig från andra deponier främst på grund av sitt blandade innehåll av både hushållsavfall och industriavfall. Deponin stängdes 1970 och täcktes över i två omgångar först 1970 och senare även 1994. Ekebyboda har under den aktiva fasen orsakat en rad olika problem så som förorenade brunnar och vattendrag samt åkrar inom närliggande område. I när-heten av det undersökta området finns ett mindre antal privata hushåll med egna dricksvattenbrunnar vilket är en förhöjande faktor till att deponin undersöks. Uppsatsen fokuserar på åren 1990-2007 med en ytterligare mätning under 2014 som ligger till grund för utvärderingen av deponins rådande tillstånd. Lakvattensammansättningsmätningar under denna tidsperiod jämförs med nederbörden för att utvärdera hur stor inverkan nederbörden har på lakvatten sammansättning och uppmätta ämnen i det. Korrelationer mellan lakvattensammansättning och nederbörd gav även en indikation om deponins tillstånd idag. På grund av deponins problematiska historia ska detta området studerats men även riskklassificera enligt Naturvårdsverkets MIFO modell. MIFO står för Metod för Inventering av Förorenade Områden och är en mall för att riskklassificera förorenade områden. Fältprovtagning visade även ett ickefungerande kulvertsystem för transport av lakvatten. Stillastående vatten och problematiska lakvatten kulvertar ledde till att datasetets kvalité ifrågasätts. Korrelationen mellan nederbörden och lakvattenkvalitén var låg och en andra täckning av deponin minskade mängd infiltrerat vatten. Analysen av lakvattnet sammansättning och problematisk hantering av lakvatten var ytterligare två anledning för en hög risk-klassificering av deponin. För den fyrskaliga klassificeringen som metoden består av fick Ekebyboda den högsta riskklassificeringen som mestadels beror på en viss risk för spridning av lakvatten till när-liggande privata brunnar. Mätserien är en generell nedåtgående trend där ett flertal näringsämnen m.m. har minskat i koncentration samt att mängden infiltrerat vatten i deponin har minskat i mängd.
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The applicability of boron isotopes in determining fate and transport of leachate from electric utility solid wasteDavidson, Gregg Randall, 1963- January 1989 (has links)
The boron isotopic ratios of three contaminated ground water samples and of leachate from four fly ash samples are shown to be significantly different than the isotopic ratio of naturally occurring boron in a selected ground water. Analysis is performed using thermal ionization mass spectrometry with a precision of less than 1 per mil. Inductively coupled plasma mass spectrometry is shown to be ineffective for this application. Boron is isolated from solution and concentrated using Amberlite IRA-743 resin with no isotopic fractionation observed. Boron desorption from fly ash is shown to be rapid. Boron isotopic analysis is shown to be a superior method to boron concentration analysis for identifying leachate in a ground water, (1) at the outer limits of a leachate plume, and, (2) when the difference between the boron concentration of the leachate and background water is small. The degree of contamination can be determined if both end members are known.
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MAS-sten som ett möjligt material för att minska föroreningarna från deponin i Karlslund : Undersökning av nya tillämpningsområden för slagg från stålindustrinBodén, Birgitta January 2013 (has links)
Till följd av den hårda utbyggnaden av vårt samhälle åtgår en stor mängd naturresurser. Samtidigt som samhället utveckas sätts allt högre krav på att det ska hushållas med naturresurser, inte minst genom införandet av miljömålet en ”God bebyggd miljö”. Enligt detta miljömål ska användningen av såväl mark som naturresurser ske på ett så hållbart sätt som möjligt. Samtidigt med detta skapar den svenska stålindustrin över en miljon ton slagg årligen, varav en stor del deponeras. Då deponeringen är kostsam pågår i dagsläget många försök att finna användningsområden för slagg så att deponeringskostanden ska undvikas. Bland annat har slagg i vissa fall visats kunna ersätta naturmaterial i formav sten, grus och sand vid till exempel vägbyggen och deponitäckningar. Slagg har i försök även visat på en vattenrenande förmåga då föroreningar fastläggs på slaggen.Detta har lett till försök med att använda slagg som filtermaterial i reaktiva filter där sand traditionellt sett ofta används. Denna studie avsåg att undersöka om slagg från Avesta Jernverk fungerar att använda i reaktiva filter för att rena vatten som är förorenade med till exempel tungmetaller och fosfor. Detta gjordes genom att skaka tre olika slagger som produceras i Avesta med ett lakvatten från en kommunal deponi med behov att renas från bland annat fluor, fosfor, kadmium, klorider, nickel och zink. I försöken användes en EAF-slagg och två AOD-slagger. För att undersöka om upptaget av föroreningar påverkades av om slaggen hade nya ytor användes både nykrossad slagg och slagg som varit placerade utomhus under 71 dagar. Resultaten visade att slaggerna som bildas vid Avesta Jernverk har en vattenrenandeförmåga och att samtliga av de testade slaggerna sorberade aluminium, fosfor och zink. Utöver dessa ämnen sorberade flera av slaggerna även arsenik, fluor, koppar och magnesium. Resultatet visade också på ett tidsberoende hos sorptionen, då mer föroreningar togs upp om slagg och lakvatten skakades i fem dygn jämfört med om skaktiden var femton minuter eller fyra timmar. Det gick dock inte att avgöra om slagg med nygjorda ytor sorberade bättre än slaggen som stått utomhus. Beräkningar utfördes och visade att ett slaggfilter dimensionsmässigt skulle vara möjligt att bygga och att en kornstorlek som sand skulle behövas för att få rätt uppehållstid. / As a result of the harsh exploitation and development of our society, a large amount of natural resources are needed. While the society develops it becomes more and more important to economise natural resources. This is also stated as an environmental objective, called "A good built environment" by the Swedish government. According to this environmental objective, the use of both land and natural resources should be undertaken in the most sustainable way possible. At the same time the Swedish steel industry produces over one million tonnes of slag annually, which is mostly deposited. Slag is a byproduct from the steel manufacturing process. When disposal is costly, many attempts to find usage for slag are done instead. For instance, in some cases it has been as replacement for natural resources as stone, gravel and sand in road constructions and landfill covers. Slag has also been shown to have a water-purifying capacity since contamination can be sorbed by the slag. This has led to attempts to use slag as filter material in reactive filters where sand traditionally is widely used. This study aimed to investigate whether slag from Avesta Works could be used in reactivefilters to purify water that is contaminated with heavy metals and phosphorus. This was done by shaking a number of slags produced in Avesta with leachate water from amunicipal landfill, Karlslund. The leachate water needs to be purified from i.a. fluorine, phosphorus, cadmium, chloride, nickel and zinc. In the experiments an EAF slag and two AOD slags were used. To investigate if the sorption of contamination in the leachate water was affected by the age of the particle surfaces, both slags with freshly made and aged surfaces were tested. The aging of the slags was created by placing the slags outdoor for 71 days. The results showed that the slags formed at Avesta Works has a water-purifying ability and that all of the tested slags sorbed aluminum, phosphorus and zinc. Beside these substances, some of the slags also sorbed arsenic, fluorine, copper and magnesium. Theresults also showed a time-dependence of the sorption: More contaminations were sorbed after reacting slag and leachate water for five days compared with fifteen minutes or four hours. It was not possible to determine if the slags with fresh surfaces sorbed better than the slags with aged surfaces. Calculations showed that a filter containing slag should be possible to build dimensionally and that the grain size of sand would be needed to get the required retention time.
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