BAUER Environment Group

Bioleaching – Biological Treatment of Heavy Metal Contaminated Sediments

The problem

Fig.: SECON pilot plant for sediment conditioning close to the Wei ße-Elster river (Kleindalzig, Germany)

Subaqueous sediments deposited in rivers and lakes of industrial and mining areas as well as in ports remain an unsolved ecological problem. Frequently, these sediments are contaminated with heavy metals which, in case of exposure to oxygen, are mobilized by microbial oxidation and acidification processes. In Saxony (Germany) alone, 6 million tons of sediments had to be removed due to contamination with heavy metals originating mainly from mining and industrial activities. Currently, excavated sediments are mainly disposed of in landfills. However, this is ecologically unsatisfactory and cannot be regarded as a sustainable solution. Given the high disposal costs and the residual hazard of deposited contaminated sediments, alternative solutions to this problem are urgently required.

Thus, the Centre for Environmental Research (UFZ) in Halle – Leipzig (Germany) is developing a new method for biological treatment of heavy-metal contaminated sediments. The aim of the research project is the controlled intensification of oxidation and acidification processes to separate the heavy metals.

 

 

Fig. : The bioleaching process providing cleaned sediments  

The solution

In a first step, the raw sediment is split into two fractions: a negligibly contaminated sand fraction, and an organic-rich fine fraction which is highly contaminated. Since freshly dredged sludge is impermeable for water, this fine fraction is then conditioned by reed plants and deeply rooted weeds which provide a soil-like structure permeable for water and air.

The subsequent bioleaching is carried out using the percolation technique. During the solubilisation step, sulphur is added and the conditioned sediment is sprinkled with process water and aerated over a period of several weeks. On optimised conditions, naturally occurring thio-bacteria produce acids thus contributing to a dissolution of heavy metals such as zinc, cadmium, nickel, cobalt, manganese, and copper. During the washing step, the solubilised heavy metals are washed from the solid material, precipitated from the solution and concentrated in a heavy-metal gypsum sludge with a mass of just 5-7% of the original sediment. Field experiments showed, that up to 80 % of the heavy metals can be removed by the bioleaching process.

The cleaned sediment is revitalised by liming and the addition of mineral nutrients and compost. Then, it can be used as soil-like substrate in construction or other fields.

Development status

BIOLEA pilot plant

The bioleaching technique is being optimised together with commercial partners experienced in the area of remediation and soil treatment. The process is up-scaled in two pilot plants:

The SECON pilot plant for sediment conditioning by plants is situated close to the Weiße-Elbe river in Kleindalzig near Leipzig (Germany). The plant has 6 chambers and a total capacity of 300 m³ of dredged material.

The BIOLEA pilot plant is part of the soil treatment centre in Hirschfeld operated by our commercial partner BAUER und MOURIK Umwelttechnik GmbH. It constitutes a bioleaching percolation facility with integrated process water treatment. The fixed-bed reactor is capable of leaching up to 2 m of sediment in depth. In the BIOLEA pilot plant the bioleaching technique is being advanced to commercial application.

 

Bioleaching