Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB
Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB

Biotechnical precipitation of metals in a fixed-bed circulation reactor

The processing of metallic materials unavoidably involves the production of waste water from cooling lubricants. The waste water is mostly a mixture of aqueous, oily and particulate components. Before disposal of used cooling lubricants conventionally they are processed by ultrafiltration. The filtrate which is the aqueous phase, contains dissolved toxic metals that might be present in too high concentrations.

Objective

Therefore, before the aqueous phase can be discharged into the drainage ditch, concentrations of dissolved metals have to be reduced below the legal limits. To date this has been realized by chemical precipitation, which is disadvantageous from the economical and ecological point of view. Under contract to DaimlerChrysler AG, Fraunhofer IGB is currently developing a biotechnical process as an alternative for this conventional separation technique.

Solution

Scanning electron micrograph of a particle overgrown with sulfate reducing bacteria (3000-fold).
Fig. 1: Scanning electron micrograph of a particle overgrown with sulfate reducing bacteria (3000-fold).

The process utilizes the ability of sulfate reducing microorganisms to reduce sulfate to sulfide for their own metabolism. Because of the low solubility product of metal sulfides precipitation occurs and dissolved metal ions are converted into insoluble metal sulfides. For the development of the biotechnical process the principle of a fixed-bed circulation reactor is used; on its fixed-bed particles microorganisms are immobilized, settling out of the waste water flux to be treated in an anaerobic environment (Fig. 1).

Metal precipitation in the fixed-bed circulation reactor

Waste water is fed into the bottom of the reactor and passes through the fixed bed while the bacteria convert dissolved toxic metals into insoluble metal sulfides. During the process the fixed bed is loaded with these crystallized metal sulfides. To prevent blocking of the fixed bed which would occur in conventional fixed-bed reactors, a pump periodically drives water from the top of the reactor into the inner conveying pipe (Fig. 2). The particles of the fixed bed are dragged along the water stream and washed up to the top of the reactor. The resulting turbulence cleans the particles. Particle circulation allows a continuous and trouble-free operation of the reactor. During the cleaning process of the particles the metal sulfides are removed by means of a hydrocyclone. With regard to the hydraulic residence time (HRT) the fixed-bed circulation reactor can be operated both as an ideal stirred tank reactor as well as an ideal tubular flow reactor.

Results

Photograph of the pilot plant.
Fig. 2b: Photograph of the pilot plant.

At DaimlerChrysler, a 20-liter pilot plant was operated for about one year with used up cooling lubricants. The hydraulic residence time and loading were varied. The concentration of the dissolved metal ions in the microfiltrated drainage was always below the legal limits. Fig. 3 exemplifies the percentage reduction of individual metal ions for a hydraulic residence time of one hour.

To describe the dynamic behavior of the plant and thereby the stability of the process with a very high feed concentration of metal ions, studies of the load characteristics were carried out in a 35-liter pilot plant at the Fraunhofer IGB. For this, the concentration of various metal ions in the reactor feed was increased for a short period of time to five times the legal limits. Under these extreme loading conditions the retention of metals in the fixed-bed loop reactor was at least 80 percent without affecting the biological activity. During the cleaning process of the fixed-bed the metal sulfides are removed from the circulation flow by a hydrocyclone (Fig. 2). The concentration of metals in the removed solid matter is at least 5 g/kg.