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Health risk assessment associated with the reuse of compost, urine and greywater in agricultural field in sahelian climate.

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par Alexis Loukou BROU
Fondation 2iE - Master Environnement option Eau et Assainissement 2014

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2. Health risk assessment for farmers

Health risk can be localized in the different activities in the field when, the farm workers use compost, urine and greywater to amend the soil and the crops.

2.1. Spreading compost

When using fertilizer products containing human or animal excreta, the reduction of excreted pathogens is a critical step in minimizing the risk of further spreading of pathogens.Transmission of disease may occur if humans or animals come in contact with the excreta and accidentally ingest the pathogen-containing material before the pathogens have been inactivated (Schönning et al., 2007).According to WHO, (2006a) the variations in the risk for infection depend on the organism in question. Some Salmonella are able to regrow in stored but unstabilized materials, especially if the materials are partly moist. Viruses and parasites generally have longer survival in the environment as well as lower infectious doses, which resulted in high risks for rotavirus, the protozoa and Ascaris.For WHO, (2006a), in considering two mean scenarios which are unconditional (applying the incidence in the population) and conditional (assuming that one member of the family actually had an infection during period of collection). Thus in this situation, the difference in risk between the conditional and unconditional scenario was 1-4 orders of magnitude, and the difference between typical (50%) and worst case (95%) varied from none to five orders of magnitude, depending on the organism. For the unconditional scenario, the risk was never higher than 4x10-2 (rotavirus). Only after 12 months of storage and taking incidence into consideration were the risks <10-4 for all organisms, excluding Ascaris (Pinf = 8 x 10-4), when emptying the container and applying the material(WHO, 2006a); (Schönning et al., 2007).For -Carr, (2005), agricultural field workers are at high risk of parasitic infections because of the long survival of the protozoa and Ascaris in the compost because WHO guidelines recommend to reduce the helminth eggs in compost to = 1 egg/L (WHO, 2006a). But exposure to hookworm infection can be reduced, even eliminated, by the use of less contaminating irrigation methods and by the use of appropriate protective clothing (i.e. shoes for field workers and gloves for crop handlers).

2.2. Spreading urine

For the hygienic risks related to the handling and reuse of urine, temperature, dilution, pH ammonia and time are the mean determinants affecting the persistence of organisms in collected urine (WHO, 2006a). Urine contains the majority of plant macronutrients that originate from household wastewater (Swedish EPA, 2007). Furthermore, the potential pathogen content is low, especially compared to faeces. Therefore, separate collection of urine for later use as a fertilizer in agriculture has been promoted through the use of urine separating toilets and latrines (Höglund et al., 2002).The short survival of E. coli in urine makes it unsuitable as a general indicator for faecal contamination by, for example, viruses and protozoa(WHO, 2006a).According to WHO(2006a), the Gram-positive faecal streptococci has a longer survival process (normally a T90 value of 4-7 days at 20°C, but up to 30 days at 4°C), and spore-forming clostridia are not reduced at all during a period of 80 days. In general, lower temperature and higher dilution result in longer survival of most bacteria (Höglund et al., 1998; WHO, 2006). However, the urine is generally contaminated at the time of the micturition by germs coming from faeces, which increases the load of pathogenic and constitutes a health risk (Tagro, 2012). According to WHO (2006a), the pathogenic germs of bacterial, viral or parasitic origin are responsible for several diseases such as diarrhea, cholera, typhoid fever, salmonellosis, shigelloses, amoebiasis, bacterial dysentery, amoebic dysentery, and parasitism. But, urinary excretion of pathogens that can be transmitted through the environment are uncommon (Höglund et al., 2002). The use of non-treated urines as fertilizer in agriculture can contribute to the transmission of these diseases to the directly exposed field workers (Tagro, 2012). However if the farm workers are used the protective equipment before spreading of urine in the field, the risk of infection can be reduced(WHO, 2006a). Furthermore, Höglund et al., 1998 suggest that estimate the risk of pathogen transmission for handling, transportation and reuse of source separated urine that follow it is necessary to determine the exact amount of faecal material introduced in the urine fraction.

Therefore, the estimated risks of pathogens for different pathways were calculated by Höglund et al., 2002 for three indicator pathogens (C. jejuni, C. parvum and rotavirus). It arises that in the case of an epidemic, where no inactivation and accidental ingestion of 1 mL of unstored urine was assumed to occur in the collection tank and spreading in the field, viruses may pose an unacceptably high risk, and bacteria pose a greater risk than protozoa. The annual risk of viral infection at 4°C is 0.81, since very low inactivation of rotavirus occurs at this temperature and slightly lower at 20°C (Pinf = 0.55) (WHO, 2006a). The risk from exposure to aerosols when farm workers spread urine in the field depend, according to Höglund et al., 2002 and WHO, 2006 of the technique of spreading of the urine.

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