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Naturliga system för rening av lakvatten i Ranstad: vilka är möjligheterna?

Börjesson, Erika

Sammanfattning

During the 1960s uranium was extracted from alum slate that was mined in the Ranstad area south of the town of Skövde in the southwest part of Sweden. Uranium was leached from the alum slate, tailings were created and deposited in a tailingstock. Inside the tailings metal containing mine drainage is formed due to infiltration of precipitation and weathering processes. The drainage water is collected in a ditch surronding the tailings. From the ditch the drainage water is pumped into a chemical treatment plant where it is treated by liming. Finally the cleaned water is released into a small storage lake. When the extraction of uranium stopped a reconditioning of the landscape was started. During 1990-1992 a thorough restoration of the Ranstad area was done. The tailings were covered with different soil layers to stop infiltration, perculation of water and weathering and to decrease the amount of drainage water. One of the last steps in the reconditioning of the Ranstad area is to replace the chemical treatment plant with a natural treatment method to treat the drainage water. The aim of this study was to identify the problems with the drainage water in Ranstad and to investigate available alternatives to chemical treatment. The work has been carried out through studies of literature and communication with researchers at Universities and departements. Over the last years the metal content in the drainage water has gradually decreased. There are environmental goals for the metal content in the drainage water and in a comparison with theese, the iron, manganese, nickel and uranium content in the drainage water are too high. When considering alternative treatment of the drainage water, mainly theese metals should be taken into consideration. Natural treatment systems, for example constructed wetlands, has become popular solutions when dealing with stormwater runoff and sewage water. Recently such methods have been considered also for treating mine drainage water and drainage water from industrial sewage. So far, there are few studies with parameters comparable to the situation in Ranstad. A comparison with metal content in stormwater runoff and treatment methods for this was made in this study. The comparison showed that a constructed wetland can be one component in a natural treatment system but not the on ly one. One solution, accordning to this study, is to combine different treatment methods to get the best result when treating metal containing drainage water. Thereby not only one treatment process is used but several ones since such a system becomes less vulnerable. A suggestion for treatment of the drainage in Ranstad is a combined treatment system where one or a couple of ponds are connected with a filter (constructed of gravel and peat) and, finally, a constructed wetland. The drainage water in Ranstad has a pH of about 7. In a pond ferrous iron can be oxidised to ferric iron and precipitation of Fe(OHh can occur. At pH>6 manganese may be coprecipitated. Nickel can be caught in a peat filter. Peat has high adsorption capacity and can be readely found in the area. AIso uranium can be adsorbed in peat. In the wetland the remaining metal ions and compounds can be removed through metal sulfide precipitations and/or sedimentation in a reducing environment where microorganisms playan important role. Advantages with natural treatment systems compared to chemical treatment are that they do not need energy supply, has low operation costs and are more or less self running. The metal content in the drainage water in Ranstad will most probably decrease with time. With such a developement a natural system, that lasts for a long time can be created. The disadvantages with a natural system compared with chemical treatment is that it needs a thorough control programm in the beginning of operation. It is hard to foresee how a natural system is going to work throughout the year with differing weather conditions. Further, it may need maintenance, at least the first years, when for example sediment in ponds must be removed. To be able to construct a natural treatment system in Ranstad, further studies and research are needed. The effect of different steps should be tested in field and laboratory studies

Nyckelord

lakvatten; reningsprocess; dammar; uran; torv; våtmark

Publicerad i

Avdelningsmeddelande / Sveriges lantbruksuniversitet, Institutionen för markvetenskap, Avdelningen för lantbrukets hydroteknik
1999, nummer: 99:2Utgivare: Institutionen för markvetenskap, Avdelningen för lantbrukets hydroteknik, Sveriges lantbruksuniversitet

UKÄ forskningsämne

Fisk- och akvakulturforskning
Miljö- och naturvårdsvetenskap
Jordbruksvetenskap

Permanent länk till denna sida (URI)

https://res.slu.se/id/publ/29892