The four incinerators for treating municipal solid waste (MSW), namely the Lai Chi Kok Incineration Plant, Kennedy Town Incineration Plant, Mui Wo Incineration Plant and Kwai Chung Incineration Plant, were closed in 1990, 1993, 1994 and 1997 respectively.

Some small incineration facilities are still in use : four hospital pathological incinerators at Tuen Mun Hospital and Yan Chai Hospital for treating body parts and human organs, and the six crematoria managed by the Food & Environmental Hygiene Department for dead bodies. In addition, the Chemical Waste Treatment Centre (CWTC) on Tsing Yi Island is equipped with a high temperature incinerator for the treatment of organic chemical waste.

Dioxins are unintended by-products of the incineration process. It has been reported that they are formed from the thermal breakdown of organic materials, combined with transition metals and chlorinated compounds. It is also known that dioxins are formed at temperature above 200°C but completely destroyed at 800°C. Reformation of dioxins occurs when the temperature is between 200°C and 400°C.

Modern incineration plants can be designed and operated to achieve nearly complete destruction of the combustible portion of the waste with very low emission of air pollutants under normal operating conditions. The following are examples of practicable measures to reduce and control emission of dioxins:

• Good combustion chamber design to optimize the supply of air for achieving more complete destruction of waste.
• The flue gas resulting from the combustion process is raised to a temperature of 850 °C for at least 2 seconds in municipal waste incinerators or to a temperature of 1,100 °C for at least 2 seconds in hazardous waste incinerators for destruction of dioxins in the flue gas.
• Quick cooling of flue gas to minimize dioxin reformation between 200 °C to 400 °C.
• Regular cleaning of boiler tubes to prevent the build up of fly ash which can serve as a catalyst for dioxin reformation.
• Injection of activated carbon by a powered injection system which is operated in parallel with the alarm warning system to capture any dioxins, if reformed, for treatment.
• Regular monitoring of combustion products including dioxin emission.
• Suspension of the waste feeding operation to allow urgent trouble-shooting and problem-fixing, when abnormal monitoring readings of air emissions or incineration temperature are detected.

The CWTC has extensive controls on its emissions which are regularly monitored by the Environmental Protection Department. To ensure proper control of dioxins, the operator of the Centre has to meet the following operational conditions:

• The rotary kiln operates at temperatures above 1,000°C
• The secondary chamber operates at 1,100°C to 1,250°C which can retain the flue gas for more than 2 seconds
• The temperature at the exit of waste heat boiler is about 400°C
• Efficient functioning of the air scrubbing system to remove hydrogen chloride
• The flue gas is cooled abruptly to below 200°C to reduce dioxin reformation
• Daily manual checking of the two independent activated carbon injection systems which are operated in parallel with an alarm warning system
• A fabric filter system to capture the fly ash

In terms of dioxin emission, the operator of the Centre is required to meet an emission standard of 0.1 ng I-TEQ/m³ . This standard is more stringent than the standards adopted in most advanced countries, as set out in the following table :

It is very unlikely that the operation of the high temperature incinerator at the CWTC will release dioxins at a level that endangers human health. The CWTC has adopted the best practicable technology to control emission of dioxins to meet the stringent dioxin emission standard of 0.1 ng I-TEQ/m³ (i.e. 100 pg I-TEQ/m³). Apart from monitoring of dioxin emissions at the stack, dioxins in the ambient air are monitored by the operator of the Centre every month at Cheung Ching Estate, which is in the vicinity of the CWTC. Since 1999, the annual average concentration of ambient dioxins level is in the range of 0.044 pg/m³ to 0.077pg/m³. The readings are comparable to those in the ambient air measured at the other two air monitoring stations at Central/Western and Tsuen Wan, and are lower than or comparable to levels observed in other large cities in the world. Moreover, there is no scientific consensus that dioxins cause adverse health effects in people at today's low levels of ambient dioxin concentrations.

The results of the dioxin measurements are available at the EPD's website : [http://www.epd.gov.hk/epd/english/environmentinhk/waste/data/data_cwtc.html]

According to the findings of the study "An Assessment of Dioxin Emissions in Hong Kong" completed in March 2000, the total inventory in 1997, i.e. adding up the emissions from all known potential sources, was estimated to be between 23 to 33 g I-TEQ with 21 to 27 g I-TEQ attributed to the old municipal solid waste incineration plants. It was far less than comparable inventories elsewhere in the world. As the municipal solid waste incineration in old incineration systems contributed to most of the emissions of dioxins, the level of dioxin emissions has been significantly reduced after the old incineration plants at Lai Chi Kok, Kennedy Town and Kwai Chung had stopped operation in 1990, 1993 and 1997 respectively.

EPD monitors ambient dioxins at two stations in the territory, one at Tsuen Wan and the other at Central/Western. Average concentration over the past seven years (1998 to 2004) ranged from 0.051 pg/m3 to 0.120 pg/m3. These levels are lower than or comparable to levels in large cities in the world.

It has also been reported that the concentration of dioxins in cigarette smoke (around 1.81 ng I-TEQ/m³, which is 18 times higher than the emission standard of 0.1 ng I-TEQ/m³ for modern incinerators) would have more significant impact on human health because cigarette smoke is inhaled directly into the lungs without diffusion and/or dilution.

While the number of incinerators has decreased over the years, it is not true that the US and European countries have banned the construction of new incinerators.

In Europe, small facilities have been replaced by larger and more economic incineration plants with better environmental performance, improved energy efficiency and lower unit operating costs. In light of the new EU landfill directive which stipulates a requirement to decrease biodegradable waste going to landfill to 35 % of 1995 level within 15 years, the use of incineration in the future should increase, as it is one of the main resource recovery options available.

In the US, while most solid waste is being sent to large landfill sites for disposal, around 14 % of the solid waste is sent to some 100 incinerators, the majority of which are equipped with energy recovery facilities.

In the Mainland, in order to cope with the increasing waste problem associated with the population and economic growth, many cities such as Shanghai and Guangzhou have plans to install new incineration facilities, and several new facilities have been commissioned recently.

In addressing the problem of increasing quantity of municipal solid waste (MSW), the Government's priority action is to promote waste reduction, reuse and recovery. However, even with the best effort, we still have a substantial quantity of solid waste which could not be recycled or avoided, and would require further treatment and disposal. Since the three strategic landfills which currently receive the solid waste will soon be filled up, in anticipation of the shortage of disposal capacity, we need to consider other treatment technologies for bulk waste reduction.

In April 2002, the EPD invited local and international companies to propose waste management technologies for the Integrated Waste Management Facilities (IWMF). An Advisory Group (AG), chaired by the Permanent Secretary (Environment) and made up of non-officials, including academics and professionals, has been set up to assist and advise the Government in selecting the most appropriate technologies based on environmental, technological, social, economical as well as consumer consideration. The AG has recommended a multi-technology approach so that the most suitable technology may be applied to deal with different waste streams of the mixed unavoidable waste. The technological combination will include biological treatment, mechanical-biological treatment and thermal technologies.

Among the various thermal technologies, incineration is a well-proven method adopted by many advanced countries in Europe and Asia. Modern incinerators adopt advanced process control measures to ensure complete combustion, and to destroy all organic pollutants and prevent the production of new pollutants. They can meet the most stringent international emission standards by using advanced gas cleaning and pollution abatement equipment.