Medical fly guys

Inductively coupled plasma-optical emission spectroscopy and X-ray fluorescence have been used to investigate the content of incinerated medical waste fly ash revealing important details about elemental concentrations in this problematic waste.

Lijuan Zhao, Fu-Shen Zhang, Mengjun Chen, Zhengang Liu, and Da Bo Jianzhi Wu of the Research Center for Eco-Environmental Sciences, at the Chinese Academy of Sciences, in Beijing, explain how the incineration of medical waste has become an important disposal route for this waste material.

Incineration reduces the weight of waste by more than 70 percent and dispels the possibility of pathogens surviving. However, it leaves behind large amounts of combustion residue. The team says that the further disposal of especially the bottom ash is fast becoming a serious problem in densely populated cities because of the high cost of disposal and diminishing landfill space. The team points out that rather worryingly, bottom ash is not included in the List of Hazardous Waste published by the Environmental Protection Agency of China, which means it is open for further disposal.

In terms of analysis of the final product, fly ash has been researched in detail but the bottom ash has not. Obviously, the disposal of the ash residue from medical waste incineration could itself represent a health or environmental hazard depending on its constituents.

Now, the Beijing team has collected samples from a typical medical waste incinerator and used various analytical techniques to study the constitutents. The likely materials present, they say, are heavy metals and polycyclic aromatic hydrocarbons (PAHs). They used X-ray fluorescence spectroscopy to analyse the bottom ash and found that calcium oxide, silicon dioxide, and aluminium oxide are the main components.

When they applied inductively coupled plasma-optical emission spectroscopy they could see that high concentrations of heavy metals, including barium, chromium copper, lead, manganese, nickel, tin, titanium, and zinc, are also present. Ba, Cr, Ni, and Sn are present in the residual fraction, but the Mn, Pb and Zn are present in the Fe-Mn oxides fraction, Cu is present in the organic-matter fraction.

The team has also carried out toxicity tests on the leachate from the bottom ash and demonstrated that the quantities of heavy metals that are released are below safety limits. However, the total PAH concentrations for 16 US EPA priority PAHs varied considerably, with levels of carcinogenic PAHs exceeding regulatory limits in several countries.

"The results of US EPA leaching tests verified that all the metals met the standard leaching limits set by the US EPA," the team explains. "The ash could be recyclable as construction material, but it must be treated at high temperature (850 to 1000 Celsius) so as to destroy polyaromatic hydrocarbons before or during the recycling process.

The team has thus provided baseline information for assessing medical waste incinerator bottom ash, which was not availably previously. "The properties of medical waste bottom ash must be extensively investigated before this type of special waste can be reused," the team says.

It is important that medical waste incinerators are operated at optimal maintenance levels and high enough temperatures to ensure hazardous substances are destroyed. However, even an optimal incininerator will not necessarily destroy all organics and the presence of heavy metals in the ash is inevitable. This factor must be considered in the further processing of the ash.