Mechanisms of microwave irradiation involved in the destruction of fecal coliforms from biosolids

Microwaves have been found to be effective in destructing pathogens in sewage sludge (biosolids) (75th Annual Water Environment Federation Conference, Chicago, September 29–October 2, 2002; Third World Water Congress, International Water Association, Melbourne, Australia, April 7–12, 2002). Mechanisms and roles of microwaves on fecal coliform destruction were investigated using bacterial viability tests, electron transport system (ETS) and β-galactosidase activity assays, gel electrophoresis, and genomic deoxyribonucleic acid (DNA) optical density (OD) measurements with fecal coliforms isolated from biosolids. Bacterial viability tests demonstrated cell membrane damage as microwave irradiation intensity and temperature increased. Above 60±3°C, viable cells were rarely found when pure fecal coliforms were irradiated with microwaves. ETS and β-galactosidase activity assays revealed increased activity for externally heated samples due to fecal coliform growth but decreased activity for microwave-irradiated samples as temperature was increased from 20°C to 57°C, indicating other destruction mechanisms besides heating. Between 57°C and 68°C, microwave irradiation led to a more rapid decrease in activity than external heating by convection. Above 68°C, bacterial activity almost ceased for both pretreatments. DNA bands in gel electrophoresis tests and OD of genomic DNA decreased more rapidly for microwave-irradiated samples than for externally heated samples, implying that microwaves disrupted DNA in fecal coliform cells at lower temperatures than external heating. Microwave irradiation of sludge appears to be a viable and economical method of destructing pathogens and generating environmentally safe sludge.