The dry rot (biobed) method
operated at SGDA sites biologically cleans mineral material
(demolition rubble, soil, material excavated in ports) and wood
material with organic contamination such as by mineral oil, BTX
aromatics, polycyclic aromatic hydrocarbons (PAH) and others.
Compared with other methods, it has the advantage of high
environmental compatibility by the conserving technical support of
natural processes. Unlike many other methods, especially
physical/chemical methods, it does not shift the ecological problem
to another site but eliminates it genuinely. If necessary, it can be
combined with physical and chemical methods, such as soil atmosphere
evacuation (for
easily
volatile aromatic hydrocarbons,
easily volatile chlorinated hydrocarbons), soil washing, etc.
Preliminary studies
Before the redevelopment work
begins, chemical and microbiological studies are made by independent
laboratories in which the biological degradability of a concrete
pollution case is studied and the optimal process parameters are
defined.
If redevelopment is possible, the next step is to find if the
microorganisms’ autochtonous potential is sufficient or adapted strains
must be brought in.
Pit operation
Once positive results of the
preliminary studies are on hand, the contaminated material is
conditioned for placement in the biobed facilities, treated
professionally and can then be used again. Treatment usually includes a
phase of intensive mixing the contaminated material with different
structural and nutritional additions. If indicated by the results of the
preparatory treatment, special microbial cultures can be added.
The biobeds in the resting bed
phase (here is an aerial picture of the biobed facility in
Großbreitenbach), in which the pretreated material is kept, are lined
with HDPE film to provide a seal to the soil below. The film is secured
by two layers of geotextile. A gravel layer (approx. 0.3 m) takes up and
evenly distributes the moisture in the beds. Perforated pipes are
embedded in the gravel layer through which optimal oxygen supply to the
microorganisms can be provided by forced ventilation.
Prewashed material from sand
catchers and material excavated in ports is treated preferably in
turn-over pits. Nutrients are added and necessary seeding introduced
with liquid. The material is turned over on an area in a hall whose base
is sealed and macadamized and where the treated substrate is turned over
in short time intervals by suitable transfer machinery.
The resting-phase biobeds are
covered with PE film on top and the hall kept closed to prevent
evaporation and emission. Generally, the exhaust air of the beds and the
hall is treated by activated charcoal filters. Depending on the level of
contamination, chemical and microbiological examinations are made at
four to eight week intervals and at the end of the biological cleaning
process.
After release by the competent
authority, all treated material is used for building or landfill
construction purposes.
