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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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2.3. Watering greywater

Greywater is wastewater generated from domestic activities such as laundry, dishwashing and bathing that can be recycled on-site for reuse in landscape irrigation and constructed wetlands -(Zuma and Tandlich, 2010). Greywater is thus domestic wastewater, without any input from toilet, which carries finite concentrations microorganisms such as faecal coliforms, E.coli and opportunistic pathogens (WHO, 2006a) and -(Zuma and Tandlich, 2010).In greywater system, microbial hazards emanate mainly from faecal cross-contamination (e.g. from anal cleansing, hygienic practices, contaminated laundry and other sources) (WHO, 2006a). Thus, farm workers and their families are at the highest risk when flood or furrow irrigation techniques are used, particularly when protective clothing is not worn and earth is moved by hand -(Carr, 2005).Farmers can be exposed by different pathways when they irrigate the field with greywater according to Maimon et al., 2010as shown inthe exposure scenario in the table 1.

Table 1: Different routes of exposure of farmers by irrigation with greywater

Exposure type

Exposure scenario

Direct

Accidental ingestion of greywater

Ingestion of greywater from the irrigation system

Ingestion of soil contaminated with greywater

Inhalation of aerosols from spray irrigation system

Therefore, greywater is comprised of very diverse components, making the drafting creation of comprehension risk assessment, guidelines, and regulations a hard task (Maimon et al., 2010). Furthermore, according to same author, determining an acceptable risk for water reuse schemes will vary from place to place according to the severity of local water stress and the level of background risks as well as the existing `'governance'' in the water sphere and regulatory capacity (Maimon et al., 2010).Greywater used for irrigation may, depending on distribution practices, expose people via inhalation of aerosols as well as through consumption of irrigated contaminated crops, in a similar pathway as for wastewater (WHO, 2006b).

The faecal load in the greywater in the system was assessed on the basis of a range of microbial indicators (E.coli, enterococci, sulfite-reducing clostridia,coliphage) and chemical markers (faecal sterols) (WHO, 2006a).Furthermore the pathogen-related risks of greywater depend on the faecal load or faecal misplacement.According to WHO, 2006a, in all exposure scenarios, rotavirus posed the highest risk, partly due to its excretion in higher numbers, at least during the acute phase, compared to the other pathogens included in the study.Thus, different studies have tried to correlate the rotavirus load with faecal indicators such as E.coli (Maimon et al., 2010). The WHO guidelines suggest that there are between 0.1 to 1 rotavirus for every 105E.coli in 100 mL of domestic wastewater (WHO, 2006a) and (Mara et al., 2007a).Thus, the tolerable disease risks for these organisms (rotavirus, Campylobacter and Cryptosporidium)are in the range 10-3- 10-4 per person per year (pppy) according to WHO, 2006a.

Reliable epidemiological data relating to the safe use of greywater in agriculture are scarce. As an alternative, the range of tolerable disease risk can be deduced on the QMRA, for which the risks resulting from exposure to greywater, for both its final use and handling (WHO, 2006a).Furthermore, Ottosson & Strenström in 2003, suggested that guidelines for the safe use of greywater in agriculture should not be based on thermotolerant coliforms as a hygienic parameter, because of the large input of non-faecal coliforms and/or growth of coliforms, unless their concentrations are adjusted for false-positive levels (Ottosson & Strenström, 2003a in (WHO, 2006a)).Thus, the overestimation of the faecal load, and risk, resulting from these indicator bacteria is to some degree compensated for by the higher susceptibility to treatment and environmental die-off (WHO, 2006a and Mara et al., 2007).In greywater, a regrowth of E. coli sometimes occurs, which may lead to an overestimation of the risks if verification monitoring is based on this parameter. It is suggested that E. coli guideline values, which are applicable for wastewater use, be applied cautiously for greywater. If applied, they will give a level of additional safety in this application, since the faecal load is usually 100-1000 times less than wastewater (WHO, 2006a). Thus, a guideline value of <103E. coli per 100 mL is suggested for unrestricted irrigation with greywater by (WHO, 2006a).

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