| Q1. |
What
is thermal desorption? How could the process destroy dioxins in
contaminated soil? |
| A1.
|
Thermal desorption
is a process in which heat is used to evaporate the contaminants
from the soil into gaseous phase, which is subsequently condensed
for phased extraction. In a thermal desorption plant such as the
one to be used for decontamination of the soil at the Cheoy Lee
Shipyard, the dioxin contaminated soil will be heated up to 540°C
in an enclosed chamber. The process will produce an oil-water mixture
with the dioxins dissolved in the oil. The oily residue will then
be collected for incineration at the Tsing Yi Chemical Waste Treatment
Centre. |
| Q2. |
What
are the alternatives to thermal desorption process and their drawbacks? |
| A2.
|
Direct incineration
of soil is an alternative method to treat dioxin contaminated soil
but this option would cost 50% more than the thermal desorption
process, consume more energy and may create more greenhouse gases.
Thermal desorption, in comparison, is the preferred method since
Hong Kong has an established and well-managed centralised chemical
waste incinerator for treating the condensate from the thermal desorption
plant. |
| |
The use
of chemical dechlorination for treating the residue is not recommended
because of the chemical waste by-products, which require further
treatment and disposal.
|
| Q3. |
The
Base-catalysed decomposition technology has been adopted for the
soil de-contamination project by the Sydney Olympic coordinating
authority. Why didn't the Government adopt this technology for the
Cheoy Lee shipyard ? |
| A3.
|
The technology
adopted by the Sydney Olympic coordinating authority was a 2 stage
on-site process. About 400 tonnes of contaminated soil were treated
initially by thermal desorption to extract the contaminants for
further treatment by based-catalysed decomposition (BCD). |
| |
The BCD,
which is a chemical de-chlorination process, has the following
shortcomings :
a) the destruction efficiency of 99.999% is lower than that of
incineration (99.9999%) and in the event of incomplete de-chlorination,
other forms of dioxins may be generated;
b) chemical
de-chlorination would generate 5 times more oily residue as a
by-product which requires further treatment, and this would result
in double handling and potentially more secondary environmental
impacts;
c) chemical
de-chlorination is highly dependent on the characteristics of
the feed waste, and treatability tests are required to determine
the design parameters for the plant. |
| |
In light
of the shortcomings of the BCD process, the Government has adopted
the most proven technology of high temperature incineration to
treat the contaminated materials, after they have been extracted
from the contaminated soil by thermal desorption. The Tsing Yi
Chemical Waste Treatment Centre is fully equipped with a high
temperature incinerator to completely destroy the dioxins and
other organic contaminants, and its emissions can meet the stringent
standard as laid down in the licence.
|
