Alternatives to Methyl Bromide - DTIE

Sourcebook of Technologies for Protecting the Ozone Layer: Alternatives to Methyl Bromide
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underground to reduce gas losses and keep
termperatures stable.
Hermetic storage systems can include:
Concrete platforms, bunkers and silos.
Portable cocoons.
Vacuum-sealed retail packs; sealed packs
(up to 50kg) containing sachet of oxygen-remover
e.g. activated iron powder.
Nitrogen storage
Products are sealed in silos, containers or
inside well-sealed, gas-tight fumigation
sheets (Banks and Annis 1997, Cassells et al
1994, Hill 1997). Nitrogen, an inert gas, is
released into the container and pushes out
the air, with the aim of reducing oxygen levels
to less than 1%. The gas must be topped
up from time to time to ensure oxygen levels
remain at the desired level. Nitrogen can be
supplied as a liquefied gas in cylinders from
commercial suppliers or made on site with
machines that remove oxygen from the air
and deliver a gas stream containing about
0.5% oxygen.
The treatment time for total disinfestation
depends heavily on the temperature of the
commodity but is typically one to four weeks.
Nitrogen storage is most effective when grain
is more than 20°C; at lower temperatures a
very long treatment time is needed for complete
disinfestations if tolerant pests and
stages (such as Sitophilus pupae), are present
(Banks 1999). Nitrogen systems are effective
in reducing mould growth in higher storage
moistures (16 to 18% moisture), but anaerobic
fermentation can take place at moisture
levels above this. A major export terminal in
Australia regularly treats bins of grain (2,000-
tonne capacity) with nitrogen, requiring
about 1m 3 of nitrogen per tonne of grain
(Batchelor 1999).
Carbon dioxide storage or treatment
Effective treatments involve the release of carbon
dioxide gas into well-sealed enclosures.
The gas displaces the air, with a typical initial
target atmosphere of more than 60% carbon
dioxide. In some cases, 80% carbon dioxide is
required (Banks et al 1991). Depending upon
the target pest, carbon dioxide concentration
should not fall below 40 or 50% in the first
10 days of treatment. At 25°C the total treatment
period should be at least 15 days
(MBTOC 1998). Carbon dioxide works faster
than nitrogen because it has a direct toxic
effect on insects. The gas may have to be
topped up to keep carbon dioxide levels high.
The treatment time for disinfestation of grain
is typically two to three weeks. An in-transit
treatment is used for groundnuts shipped
from Australia.
Carbon dioxide and pressure
The combination of carbon dioxide and pressure
(e.g., about 25 bar) can reduce the disinfestation
time to less than 3 hours (Caliboso
et al 1994, Reichmuth and Wohlgemuth
1994, Prozell and Reichmuth 1991, Prozell et
al 1997). Treatments are typically conducted
in pressure-proof chambers with 20 mm steel
walls. The equipment has a high capital cost
but provides a very rapid quarantine treatment
for high value durable products.
For all of the modified atmosphere treatments
discussed above, the air-tightness of
stores or containers is an important factor for
effective control. Some existing structures can
be adapted. In the case of silo bins, the level
of sealing required for carbon dioxide or
nitrogen is greater than the level of sealing
typically used for MB fumigations in developing
countries but similar to the level of sealing
required for MB for safety reasons in a
number of developed countries.
Where systems provide a continuous flow of
gas, such as with a gas burner, the use of
somewhat less gas-tight enclosures is feasible
as well (Bell et al 1993, 1997a). Certain conditions,
such as a large difference between
the grain and ambient air temperatures, can
cause moisture to migrate to the grain surface.
Precautions to prevent or ameliorate