Development and Evaluation of a Method to Determine Indicator Microorganisms in Air Emissions and Residue from Medical Waste Incinerators

Abstract

To allow testing of microbial destruction in medical waste incinerators, methods were developed to determine indicator microorganisms (Bacillus Stearothermophilus spores) in incinerator air emissions and residue. The emission trapping train consisted of a water cooled glass probe and impingers containing a neutral phosphate buffer. In field tests, spores were injected directly into the probe, and results showed that approximately 60 percent of the spores were recovered. Spores were analyzed with adequate precision using a microbial membrane filter unit. Lab experiments indicated that spores were stable in neutral pH phosphate buffer for up to 20 days, and heat shocking samples (heating to 80°C for 20 minutes) reduced spore numbers in acidic or basic buffer. Laboratory tests also showed that 60 to 70 percent of spores initially added to ash were recovered up to 22 days after addition of the spores. In addition, lab tests showed that spores can be effectively recovered from residue test pipes spiked with indicator spores. ImplicationsThis paper presents a method for testing the microbial destruction efficiency of medical waste incinerators. The method includes evaluation of indicator spore survival in both air emissions and incinerator residue (ash). This method could be used to test microbial survival following waste incineration in existing waste incinerators or in new incinerators. The effect of incinerator operating conditions on microbial survival could also be evaluated. Tests results could have implications for medical waste treatment by incineration.Certain components of medical waste incinerator emissions, if released, might reasonably be expected to contribute to the endangerment of public health and welfare. One pollutant of potential concern is pathogenic microorganisms (pathogens). However, there is currently no accepted method for measurement of microorganisms in incinerator air emissions. This study was undertaken to develop a test method to evaluate the pathogen destruction efficiency of medical waste incinerators.The small amount of research conducted to date has demonstrated two general approaches for measuring microbial emissions from incinerators. The first is to collect, culture, and identify the native species present in the stack gas.1 -2 The second is to spike the incinerator waste feed with an "indicator" organism, which is typically highly heat resistant bacterial spores. These indicator spores are then collected, and selectively cultured and quantified.3 ' 4The second approach was selected to provide the basis for a microbial destruction efficacy test method for several reasons. The incinerator can be challenged with heat resistant spores simulating worst case conditions, and enough indicator spores can be spiked to ensure that the destruction efficiency of the incinerator can be demonstrated to 99.9999 percent efficiency. In addition, the analysis of the indicator spores does not require culturing and identification of multiple species, some of which may be pathogenic. Rather, the spore...