In many former and present-day production sites soils and groundwater are often considerably polluted with halogeneous-organic contaminants.
A cleanup with pump-and-treat methods is only carried out in exceptional cases, since the funds for these technologies, which can be used in these cases, are often lacking. However, there is no sound alternative available for many of these cases so far – if a cleanup has to be carried out. On site procedures have been advertised for years as principally promising and cost-efficient – however, they are only rarely ready for application.
In addition, the acceptance on the part of the industry and the authorities in relation to biological methods for cleanups of contaminated groundwater was rather small; the doubts were mainly caused by a lack of respective practical applications. At the same time lacking reference applications thwarted the implementation of newly developed procedures.
The technology described in the following was developed between 1994 and 1997 within a special research field at the Technical University Berlin, which focused already at an early stage on its practical utilisability. This was achieved by the integration of a renowned plant builder and was documented by the successful operation of a pilot plant at a chemical industry site in Berlin.
The bioreth® procedure
The meanwhile wide acceptance of the procedure prompted also the experts of the German Rethmann Wasserwirtschaft GmbH & Co. KG to think twice and finally to further develop this technology by the end of 1999 into a treatment method for heavily polluted groundwater and wastewater. This is how the bioreth® procedure was created, by means of which the Rethmann water management company was commissioned already in 2000 by the Chemiepark Bitterfeld Wolfen GmbH to examine the treatment of local groundwaters.
The bioreth® procedure combines micro-biological and adsorptive processes and is particularly effective in groundwater polluted by BTXE (aromatic components) and chlorobenzene. In addition to this group of pollutants, halogenated aliphates with high degradation rates are eliminated. The majority of halogeneous-organic contaminants are mineralised by highly enriched mixed cultures in an aerobe fluidised bed reactor. The elimination of the non-degradable organic components from the water takes subsequently place in an adsorption step.
For biomass immobilisation, Rethmann uses polyurethane-foam cubes. At the beginning the reactor is inoculated with a highly enriched mixed culture, which is adapted for the degradation of different pollutants and immobilised on polyurethane-foam substrates.
The fluidised bed reactor is characterised by a very good mass transfer, which leads to an efficient biofilm and high degradation rates. The retention time behaviour of the reactor corresponds to that of an ideal agitating boiler. The turbulence is mostly effected via a proprotor integrated in a conduit tube. This leads to a downward current in the tube, which provides for the turbulence of the immobilised substances. For oxygen supply, technical oxygen or air is metered on the sieve bottom. The exhaust gas passes an adsorption level to avoid the emission of pollutants into the air in case of stripping effects (theoretically possible, but not quantifiable in practice). After the fluidised bed reactor the biologically pre-purified groundwater finally passes a sedimentation container for biomass and solids and finally an activated carbon adsorbent for final purification.
The substantial part of organic substances is eliminated in the microbial treatment step. The mixed culture is so durable that no failures in the degradation activity occur even in brief undersupply with oxygen or substrate.
In past applications the highest biomass concentrations in the reactor could be proven with the highest degradation rates. Nevertheless, it must be guaranteed that the service life of the adsorber is as high as possible. This requires – among other things – an optimal biomass separation, in order to optimally use the capacity of downstream adsorption steps.
For downstream adsorption, different adsorbents were successfully tested at the Rethmann Technikum. In addition, reliable biomass separation and feedback systems have been implemented in the meantime, which both increase the degradation performances of the microbial step and the service lives of the downstream adsorption levels.
The immobilisation of special mixed cultures on suitable substrates opens for the bioreth® procedure the way to the economically and ecologically sound – being largely free of chemicals and residues – elimination of materials difficult to degrade, as they are found in many groundwaters contaminated by refuse dumps.
(Source: aqua press Int. 03/02)
Dr. Ing. Lars Meierling
Contact & Information:
Rethmann Wasserwirtschaft GmbH & Co. KG
Dr.-Ing. Lars Meierling
(Bereichsleiter Industrie der Rethmann Wasserwirtschaft)
D-44 536 Lünen, Brunnenstr. 138
Phone: +49/23 06/106-692; Fax: DW -699