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Water
quality management is an expensive and time-consuming business. In order
to monitor water quality, samples must be taken regularly and chemically
analysed.
It
is decided, on the basis of these results, whether there is a worrying
level of contamination. Very often, the result of the analysis will
indicate that contamination thresholds have not been exceeded.
This
early warning system, capable of noting contamination and issuing an
alarm without external control and without an extensive analysis system,
was developed in order to monitor the water continuously.
The
early warning system is based on the behaviour of mussels. In clean
water, mussels move the two halves of their shells according to a
characteristic pattern. They remain open for the majority of the time,
and only close for short periods. A mussel in contaminated water behaves
differently.
Depending
on the type and level of contamination, mussels affect a movement
pattern which can differ greatly from the normal pattern.
The
variations on the normal movement pattern include a more rapid opening
and closing of the shells (flapping), keeping the shell closed for a
fixed period, opening the shell less and less far, and in the worst
case, if a period of (high) contamination continues for too long, death
of the mussels (gaping).
By
entering the movements of the shell into a microprocessor, and by
carrying out certain calculations on this data, it is possible to
ascertain the behaviour of the mussel. Currently, a memory is being used
to store the data for evaluation of trends in shell movement behaviour.
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By
fitting a miniature coil to each half of the shell, the distance between
the shell halves of a mussel can be measured. A high-frequency voltage
is passed through the first coil, such that a magnetic field is created.
This
magnetic field induces a voltage in the other coil. The strength of the
induced signal is dependent on the distance between the two coils, and
is therefore indicative of the shell position.
Tests
have shown that mussels react differently to different substances. Just
like all other biological sensors (including fish, daphnia and
bacteria), they are not equally sensitive to all substances.
For
many substances, detection levels have been laid down (and the list is
continuously being extended). Using the movement pattern recognition
system of mussels, it is perfectly possible to monitor water quality,
both in fresh and in salt water.
The
MOSSELMONITOR®
has been designed to operate on a stand-alone basis, using built-in,
rechargeable batteries. The housing for the electronics is a practical
field unit, which can be used in many locations.
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