Direct use of wastewater in Mexico

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Direct use of wastewater in Mexico

• Dr Ursula Blumenthal, LSHTM

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Organisation

• Ministry of Agriculture and Water Resources runs
  irrigation districts
• Infrastructure material human
• Payment for wastewater
• Policy - Untreated wastewater plus crop
  restrictions
• Enforcement - fines and confiscation crops

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Crop restrictions

• Depends on water quality
• Untreated cereal (maize, wheat), fodder
  (alfalfa), chillies
• Improved (dilution, treatment) some vegetable
  crops
• River water with effluent not controlled,
  vegetables eaten raw grown

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How well does it work?

• Agriculture
• Allows for cash crops not just subsistence
  farming
• Farmers prefer to grow vegetables
• Many plots are small leased, p/t work
• Management issues water demand varies, storage
  issues, XS to river
• Water table rising run-off issues salinity

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Health effects ?

• Studies in Irrigation Districts 03 and 100
• Evaluate policy
• Reservoirs used to store water ref irrigation
  demand ? water quality improvements
• Studies to find out risks of untreated wastewater
  use and improvements necessary

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Implications

• Restricted irrigation - workers health-
• (a) Nematode egg guideline
• ? 1 egg/litre ? 0.1 egg/litre as children of
  farmworkers have increased prevalence and
  intensity of Ascaris infection at ? 1 egg/litre -
  little excess at 0 egg/litre (Cifuentes, 1998
  Blumenthal 2000, Peasey 2000)

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Implications

• (b) Faecal coliform guideline Need to add FC
  guideline of
• (ii) 103 FC/100ml where use flood/furrow
  irrigation and/or children exposed
• - as 2x increase in diarrhoea and 4x inc. in
  Norwalk-like virus in children 5-14 yrs at 104
  FC/100ml (Blumenthal et al, 2000)

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Indirect use of wastewater

• Part flow from one reservoir diverted to river
  authorities lose control of it
• Unrestricted irrigation - Used to irrigate large
  area of vegetable and salad crops including
  tomato, onion, lettuce, cucumber, parsley,
  cauliflower etc
• Sold in local wholesale and retail markets
• Studied isolated population eating vegetables
  originating in wholesale market

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Effect of total consumption of raw vegetables on
prevalence of diarrhoea ()
Age Group No. of days vegetables eaten during
(years) last two weeks
0 1-2 3-5 6 p value
1-4 20.7 13.8 14.5 16.8
0.407
5-14 9.0 5.6 4.9
4.2 0.279
15 5.5 3.6 3.8 4.7
0.047
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Implications international

• Faecal coliform guideline for unrestricted
• increase in diarrhoeal disease when guideline
  exceeded by factor of 10 (10,000 FC/100ml)
  (Blumenthal et al, 2000) so guideline needs to be
  below 104 FC/100ml
• ? 1000 FC/100ml adequate in endemic area

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Mexico guidelines 1996

• Unrestricted irrigation 103 FC/100ml (monthly
  mean) and ? 1 egg/litre
• Restricted irrigation - 103 FC/100ml and 5
  eggs/litre (1998 internal 105 FC only
  semi-restricted 103 105 FC only ?!)
• Achievable by secondary treatment and
  chlorination added filtration too costly ref
  restricted irrigation

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Reality

• Most STPs inefficient due to operation and
  maintenance problems much wastewater not
  treated
• Time limits for compliance yr 2000-2010 but who
  pays ?
• Public health anthelminthic campaigns in areas
  high helminths also w/w areas?
• Keep restricted irrigation where possible