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

2.2. Comparison of the probabilistic values of different treatments related with the scenarios

The probabilistic values of all treatments compared with the WHO guideline values of risk for soil ingestion scenario are showed in the figure 9 above. Salmonella annual risk of infection from U+TW (9.55x10^-1) is higher than C+TW (3.87x10^-3), C+U+GW (1.44x10^-4) and GW (8.89x10^-6) for soil ingestion. The annual risk of infection in all treatment exceeded the tolerable risk of =10^-6 per person per year (WHO, 2006a). The recorded values were above the recommended annual risk of infection by a 5 order of magnitude (U+TW (9.55x10^-1)).

Ascaris annual risks of infection in worst case from C+TW and C+U+GW are equal to (4.67x10^-2) however;these probabilistic values are higher than WHO guideline values (2006).The recorded value was above the recommended annual risk of infection by a 4 order of magnitude. This is more than the range of annual risk of Ascaris infection of 10^-3 to 10^-4 reported by Seidu et al. (2008) who used data from studies in Ghana to assess the annual risk of infection associated with the reuse of diluted wastewater for irrigation (Nana O.B. Ackerson and Esi Awuah, 2012).

The probabilistic values of all treatments compared with the WHO guideline values of risk for ingestion of irrigated greywater scenario are showed in the figure 10 above.

Salmonella annual risk of infection in worst case from C+U+GW (1.53x10^-3) is higher than GW (1.02x10^-4). The annual risk of infection in all treatment exceeded the tolerable risk of =10^-6 per person per year (WHO, 2006a). The recorded value was above the recommended annual risk of infection by a 3 order of magnitude.

However, USEPA considers an annual risk of 10^-4 to be acceptable for microbial contamination of drinking water, therefore the annual risk of infection for C+U+GW is above this recommended annual risk of infection by a 1 order of magnitude (Shuval et al., 1997).

The probabilistic values of all treatments compared with the WHO guideline values of risk for lettuce consumption scenario are showed in the figure 11 above. Salmonella annual risk of infection in worst case from C+TW (1.54x10^-1) is higher than U+TW (1.30x10^-7), C+U+GW (5.00x10^-7) and GW (9.42x10^-4).

The annual risk of infection in all treatment exceeded the tolerable risk of =10^-6 per person per year (WHO, 2006a). Ascaris annual risk of infection from C+TW and C+U+GW is equal (4.67x10^-2), however, this probabilistic value is higher than WHO guideline values (2006). The recorded value was above the recommended annual risk of infection by a 4 order of magnitude. This is attributed to the relative low levels of Ascaris counts in lettuce. This is more than the range of annual risk of Ascaris infection of 10^-3 to 10^-4 reported by Seidu et al. (2008) who used data from studies in Ghana to assess the annual risk of infection associatedwith the reuse of diluted wastewater for irrigation (Nana O.B. Ackerson and Esi Awuah, 2012).

2.2.1. Risk assessment from farmers

The risk more proven meets at the time of the ingestion of the soil on which the urine wasspread urine (U+TW)is the Salmonella risk infection (9.55x10^-1).That could be explained by the fact why the urines used for the fertilization go back to less than one week of storage. With a significant load of the pathogens (Salmonellas) or indicator of the pathogen such as the enterococci (Annex xix). However, if the time of storage of urine is long, that contributes to the reduction of the risks of infection of the pathogen. That was proven by the works ofHöglund et al., (2002) which showed that for a time of storage of 4 weeks, the risk of infection of bacteria is at least of 10^-15. In combination of C+TW, C+U+GW and GW (figure 9), the annual risk of infection of Salmonella is higher than benchmark (10^-6 DALY) proposed by WHO, (2006a). Ascaris annual risk infection is high in C+TW and C+U+GW (4.67x10^-2)combination. However, compost which is used to spread in field, go back to less than 3 days (unstored) where 27 eggs/g dry weight of compost were fund. Risk could be reduced, if faeces were stored at least 12 months before its use for the fertilization of the cultures such as confirm by Schönning et al., (2007) who were determined values above 10^-4 recommended by Swedish EPA, (2007).

For ingestion of irrigated greywater, the risk of infection of Salmonellain both combinations (figure 10) is higher than the WHO guideline value. But value of C+U+GW (1.53x10^-3)is higher than GW (1.02x10^-4). Risk could be mitigated, if farmers use adequate equipment of protection before to irrigate the crops. In addition, if irrigated greywater amount of fecal indicators is below of 10³CFU/100 mL recommended by WHO, (2006a).