Assessment of microbial health hazards associated with wastewater application to willow coppice, coniferous forest and wetland systems

Treatment and reuse of wastewater by irrigation of willow coppice, forest or wetlands may create new exposure routes for pathogens. This thesis summarises results from a series of field and laboratory studies aimed at identifying and quantifying the microbial health hazards associated with such alternative wastewater treatment systems. Leaching and retention of viruses in the soil-plant system were studied in a lysimeter experiment using a bacteriophage as model organism. The presence and die-off of pathogens was studied in three full-scale systems with wastewater irrigation of willow in southern Sweden. The reduction in pathogens was also studied in microcosms under controlled conditions. In addition, the presence and die-off of pathogens in two wetlands was studied. Finally, a risk assessment was made in order to identify and quantify the most important exposure routes of pathogens. In the Swedish full-scale systems, the average reduction in microorganisms in the wastewater treatment plants was in the range 1.3-2.5 log10. Analyses of faeces collected in the irrigated area did not indicate an increase in pathogens in mammals and birds, whereas indicator organisms were detected in foliage and in some groundwater samples in the fields. The results of the lysimeter study showed very high retention of viruses in sandy soils, whereas leaching to groundwater was substantial and extremely rapid in the clay soil. In the microcosm study Campylobacter were rapidly reduced (<3 h) while Salmonella bacteria were highly resistant. No single factor (light, temperature or radiation) was found to govern the reduction. In the wetlands studied, the reduction in suspended particles seemed to be the main factor controlling bacterial elimination from the water phase. In the sediment, survival of microorganisms was prolonged. The theoretical microbial risk assessment indicated a substantial risk of viral infections caused by direct contact with the wastewater, with aerosols from irrigation, or by drinking contaminated groundwater