A quantitative assessment of the BSE risk associated with fly ash and slag from the incineration of meat-and-bone meal in a gas-fired power plant in Denmark

It has been recommended that meat-and-bone meal (MBM) be incinerated at 850 degrees C for at least 2 s and the ashes and slag disposed of in controlled landfills, to dispose of animal-derived proteins. Most commonly, the MBM is incinerated in cement works or coal-fired power plants and the ashes and slag are incorporated into the cement or concrete. Our goal was to assess with a Monte Carlo simulation model the bovine spongiform encephalopathy (BSE) risk to cattle and humans posed by the ash and slag. The results will be used by decision makers to evaluate the need for disposal of the fly ash in controlled landfills and the feasibility of use of the ash by the phosphate and fertilizer industries. We assumed that all specified risk material (SRM) and MBM produced in Denmark would be incinerated in this gas-fired power plant. Based on observations in 2001, we assumed that, on average, six (range: 0-15) clinical BSE cases each year were rendered into MBM and incinerated. In addition, SRM or carcasses from 0 to 31 (median = 10) BSE-infected-but-undetected animals/BSE case were also incinerated. The simulations were run on a 1-week basis. Our results suggest that if the slag is collected and re-incinerated the median BSE infectivity remaining in the fly ash per week would be 3.1E-11 cattle ID50. A cattle ID50 is the amount of infectivity that will cause infection in 50% of cattle exposed to it. During the weeks when BSE was infected in the SRM-MBM, the median infectivity in the fly ash was estimated as 8.7E-10 cattle ID50 and 2.9E-12 human ID50. The 95th percentiles were 2.1E-08 cattle ID50 and 5.8E-10 human ID50, respectively. One ton of fly ash would contain less than or equal to 1.8E-07 cattle ID50 95% of the time. These are the potential exposures of the cattle or human populations. The potential exposures of individuals are far less.