Agriculture Reference
In-Depth Information
within 24 hours of the last irrigation event allowing little time for pathogen die-off
(Tyrell and others 2006).
Standards for Irrigation Water Quality
Most standards for the microbial quality of irrigation water have been developed for
the use of treated wastewater. Few standards have been suggested for water derived
from other sources. Based on results of a study of irrigation waters in the western
United States in the late 1960s Geldreich and Bordner (1971) suggested an irrigation
standard of 1,000 fecal coliforms/100 ml based on the absence of
Salmonella
in irriga-
tion waters, which had values below this level. Several states in the United States have
standards for irrigation of food crops if reclaimed wastewater is used; however, irriga-
tion with reclaimed water is seldom practiced. The state of California has a treatment
requirement with a coliform standard of
2 coliforms/100 ml for the treated water.
The WHO (2006) recently revised its recommendations for the safe reuse of waste-
water using a risk-based approach for development of treatment and microbial stan-
dards. The WHO guidelines for the safe use of wastewater in agriculture are based on
a risk analysis approach and recommends treatment requirements for pathogen reduc-
tion, monitoring (verifi cation of treatment performance), and management strategies.
The level of protection goal is defi ned in terms of disability adjusted life years
(DALY), a measure that combines years of life lost by premature mortality with years
lived with a disability, standardized or weighted by severity of illness (Pruss and
Havelaar 2001). The acceptable level of risk from consumption of pathogens on food
is defi ned as 10
− 6
DALY (WHO 2006). Based on this analysis the minimum require-
ments for irrigation water for use on root crops are equal or less than 1,000
E. coli
per 100 ml and zero helminth eggs per liter. This guideline is based upon a wastewater
treatment process that provides a 4-log (99.99%) reduction in pathogens (approxi-
mately equivalent to an
E. coli
of 1,000/100 ml in unchlorinated effl uents), a 2-log
pathogen reduction due to die-off between the last irrigation and consumption, and a
1-log reduction by washing of the salad crops or vegetables with water prior to con-
sumption. The WHO believes this option provides the needed 7-log pathogen reduc-
tion for crops eaten uncooked. For options totally dependent on the treatment to
remove pathogens (again a 7-log reduction) to the required level of acceptable risk,
the
E. coli
level in irrigation water for crops eaten uncooked should be equal to or
less than one
E. coli
per 100 ml.
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Process Waters
Feces-contaminated water postharvest can also result in the contamination of produce.
In Ghana it was found that water was an important source of bacterial contamination
(Keratia and Drechsel 2004). Contamination of wash water is most likely to occur if
tanks of water are used or if the water is recirculated during the process. Chaidez and
others (2005) in Mexico found that 16% of the wash water tanks used in the fresh
produce industry contained
Cryptosporidium
oocysts and 83%
Giardia
cysts. Levels
of chlorine normally used in the wash water tanks (200 mg/l) would expect to have
little impact against the
Cryptosporidium
oocysts. Castillo and others (2004) docu-
mented the occurrence of
Salmonella
and
E. coli
in wash water used to process can-
