PERSISTENCE OF PATHOGENS IN SOIL, CROPS, MANURE, AND SLUDGE
According to Feachem et al. (1980), the persistence of fecal pathogens in the environment can be summarized as follows:
IN SOIL
Survival times of pathogens in soil are affected by soil moisture, pH, type of soil, temperature, sunlight, and organic matter. Although fecal coliforms can survive for several years under optimum conditions, a 99% reduction is likely within 25 days in warm climates (see Figure 7.1). Salmonella bacteria may survive for a year in rich, moist, organic soil, although 50 days would be a more typical survival time. Viruses can survive up to three months in warm weather, and up to six months in cold. Protozoan cysts are unlikely to survive for more than ten days. Roundworm eggs can survive for several years.
The viruses, bacteria, protozoa, and worms that can be passed in human excrement all have limited survival times outside of the human body. Let's take a look at their survival times when deposited raw into soil (refer to Tables 7.8 through 7.12).
SURVIVAL OF PATHOGENS ON CROPS
Bacteria and viruses cannot penetrate undamaged vegetable skins. Furthermore, pathogens are not taken up in the roots of plants and transported to other portions of the plant.30 However, pathogens can survive on the surfaces of vegetables, especially root vegetables. Sunshine and low air humidity will promote the death of pathogens. Viruses can survive up to two months on crops but usually live less than one month. Indicator bacteria may persist several months, but usually only last less than one month. Protozoan cysts usually survive less than two days, and worm eggs usually last less than one month. In studies of the survival of Ascaris eggs on lettuce and tomatoes during a hot, dry summer, all eggs degenerated enough after 27 to 35 days to be incapable of infection.31
Lettuce and radishes in Ohio sprayed with sewage inoculated with Poliovirus I showed a 99% reduction in pathogens after six days; 100% were eliminated after 36 days. Radishes grown outdoors in soil fertilized with fresh typhoid-contaminated feces four days after planting showed a pathogen survival period of less than 24 days. Tomatoes and lettuce contaminated with a suspension of roundworm eggs showed a 99% reduction in eggs in 19 days and a 100% reduction in four weeks. These tests indicate that if there is any doubt about pathogen contamination of compost, the compost should be applied to long-season crops at the time of planting so that sufficient time ensues for the pathogens to die before harvest.
PATHOGEN SURVIVAL IN SLUDGE AND FECES/URINE
Viruses can survive up to five months, but usually less than three months in sludge and night soil. Indicator bacteria can survive up to five months, but usually less than four months. Salmonellae survive up to five months, but usually less than one month. Tubercle bacilli survive up to two years, but usually less than five months. Protozoan cysts survive up to one month, but usually less than ten days. Worm eggs vary depending on species, but roundworm eggs may survive for many months.
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Table 7.8 SURVIVAL OF ENTEROVIRUSES IN SOIL
Source: Feachem et al., 1980 |
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Table 7.9 SURVIVAL TIME OF E. HISTOLYTICA PROTOZOA IN SOIL
Source: Feachem et al., 1980 |
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Table 7.10 SURVIVAL TIME OF SOME BACTERIA IN SOIL
Source: Feachem et al., 1980 |
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Table 7.11 SURVIVAL TIME OF POLIOVIRUSES IN SOIL
Source: Feachem et al., 1980 |
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Table 7.12 SURVIVAL TIME OF SOME PATHOGENIC WORMS IN SOIL
Source: Feachem et al., 1980 |
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Table 7.13 PARASITIC WORM EGG DEATH
Source: Compost, Fertilizer, and Biogas Production from Human and Farm Wastes in the People's Republic of China, (1978), |
Source: The Humanure Handbook. Jenkins Publishing, PO Box 607, Grove City, PA 16127. To order, phone: 1-800-639-4099.
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