Jökull - 01.12.2007, Side 81

A hot water drill with built-in sterilization
Figure 4. Calculated decrease in drilling water tem-
perature with depth. The measured mass flow rate of
450 l/hr is assumed. – Reiknað hitafall í borslöngu
með dýpi. Gert er ráð fyrir að vatnsmagnið 450 l/klst
streymi út um spjótið.
Figure 5. Calculated borehole diameter as a func-
tion of depth, for three different values of the drilling
speed. – Reiknað þvermál borholu sem fall af dýpi,
fyrir þrjá mismunandi borhraða.
For the fastest drilling rate (60 m/hr), the calcu-
lations suggest a hole diameter of 6.6 cm at 300 m
depth, but at 600 m depth the available heat energy
only suffices to melt a hole with a diameter equal to
the diameter of the drill stem (4 cm). At 300 m depth,
the two other curves shown in Figure 5 indicate a
borehole diameter of 9.3 cm and 14.7 cm for drilling
rates of v=30 m/hr and v=12 m/hr, respectively. For
reasons mentioned above, these are minimum diame-
ters, and in addition the operator will normally slow
the drilling speed with depth, which will contribute to
the production of a larger hole diameter.
SUMMARY AND OUTLOOK
A hot water drilling system with built-in sterilization
has been built and tested with success on a temper-
ate ice cap. The meltwater used for drilling is filtered,
exposed to UV-radiation and heated to high tempera-
tures in order to minimize the risk of biological con-
tamination. Filtering of the drilling water prevents
larger particles from entering the high pressure pump;
a valid concern for drilling in regions where the snow
is likely to contain windblown dust and deposits from
nearby volcanoes. The filter pore-size used in the tests
(50 µm) will, however, not block individual microbes
or small dust particles from passing through the sys-
tem. All three tests conducted with the new system
indicate that the UV water treatment system is very
effective in reducing the number of viable cells in the
meltwater. Some microorganisms do survive the UV
radiation in the tests, probably because the exposure
time was not long enough. Analysis of samples col-
lected during a test on the Langjökull ice cap indi-
cated a significant drop in the number of viable cells
when the drilling water was heated to 95!C. Reduc-
tion to undetectable levels of microbial colony form-
ing units was achieved in later tests by running the
meltwater heater at maximum temperature and clean-
ing with 70% ethanol.
Some limitations are inherent in the laboratory
methods used to investigate samples from the drilling
system. The 0.45 µm filters are not likely to collect all
bacterial cells and the plate count agar (PCA) used for
cultivation is a high organic content medium on which
cells that survive in oligotrophic (nutrient-poor) en-
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