Incineration at the Tsing Yi Chemical Waste Treatment Centre is both a safe and effective way to dispose of dioxin-containing residues, imposing no additional loading to the environment, Independent Expert Assessor Prof Jonathan Wong says.

Prof Wong today submitted his report to the Kwai Tsing District Council on monitoring the last round of incineration of about 23 tonnes of dioxin-containing residues at the centre from March 26 to 30.

Analysis of samples taken on March 27 and 29 showed the concentration of dioxins in stack emission was 0.00004 nanograms per cubic metre in average, well below the prevailing international standard for incinerator dioxin emissions of 0.1 ng per cubic metre.

A nanogram is one-billionth of a gram.

The monitoring result on the concentration of dioxins in kiln bottom ash was 1.495 ng per kg in average, far below the most commonly acceptable soil contamination criteria of 1,000 ng per kg.

The monitoring results on average ambient dioxin concentration at five locations in Kwai Tsing areas ranged from 0.039 to 0.057 picograms per cubic metre. All were below the alert indicator of 0.1pg per cubic metre .

A picogram is one-trillionth of a gram.

Given the very low concentrations of dioxins in the stack emission and kiln bottom ash, as well as the low ambient dioxin level recorded during the incineration period, Prof Wong concluded the incineration at the centre is safe and effective.