wastewater irrigation - hazard or lifeline? gez cornish & neeltje kielen hr wallingford ltd. uk
TRANSCRIPT
Wastewater irrigation - hazard or lifeline?
Based on field studies in Nairobi and Kumasi
• The need for a typology of wastewater irrigation
• Variations in microbiological water quality
• Positive impacts of wastewater irrigation
• What are the trade-offs?
• The need for “interim” water quality standards
Summary:
Wastewater irrigation - hazard or lifeline?
The need for a typology of wastewater irrigation :
Great variation exists in wastewater:
• sources
• conveyance systems
• treatment
• in-field management
A typology is essential to guide discussion of practice or the formulation of guidelines or regulations.
Wastewater irrigation - hazard or lifeline?
Source Conveyance Treatment Disposal
Formal collectionnetwork
Road tankers
Natural drainage None
Natural / Biological
Conventional
Irrigation
Groundwaterrecharge
River or surface water bodyIndustrial
Municipal
Info
rmal
Bac
kyar
d
Indi
rect
use
Dir
ect u
se
Dir
ect u
se
Indi
rect
use
On-farm conditionsType of irrigation Type of crop Crop destination Irrigators
SurfaceSub-surfaceOverheadDrip
Food crop eaten rawFood crop eaten cookedPastureTreesOrnamental plants
Home consumptionInternal marketExport market
MenWomenChildren
A proposed typology
Formal use = Use of wastewater with a certain level of permission and control by state agenciesInformal use = Use of wastewater lacking permission and control by state agencies
Direct use = Wastewater conveyed to a defined area for irrigationIndirect use = Wastewater discharged into water bodies with scattered and uncontrolled downstream abstraction
Informal use
Informal or formal
use
Wastewater irrigation - hazard or lifeline?
Examples - Mau Mau Bridge, Nairobi
Informal, indirect. Untreated municipal waste.
Natural drainage channels running to river.
Surface & overhead irrigation of vegetable crops sold in local Nairobi markets.
Wastewater irrigation - hazard or lifeline?
Examples - Maili Saba, Nairobi
Informal, indirect. Untreated municipal waste diverted from sewer to fields.
Surface irrigation of vegetable crops grown for home consumption and sold in local Nairobi markets.
Wastewater irrigation - hazard or lifeline?
Examples - Asago, Kumasi
Informal, indirect. Untreated municipal & industrial waste. Natural drainage to river + dumping by vault emptying tankers.
Overhead irrigation of vegetable crops, grown mainly for Kumasi market.
Wastewater irrigation - hazard or lifeline?
Variations in water quality - Nairobi
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
1.0E+07
1.0E+08
Thiboro (w ell) Mau MauBridge
Kimathi Njiru Bridge Maili Saba(Sew age)
Me
an
no
. fa
eca
l co
lifo
rm /
10
0m
l WHO guideline for unrestricted irrigation
Wastewater irrigation - hazard or lifeline?
Variations in water quality - Kumasi
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
Septimpon Wiw i Kaase Daban Asago Adw aden Ofaose Well 1 Well 2
Mea
n no
. fae
cal c
olifo
rm /
100
ml
WHO guideline for irrigation
Wastewater irrigation - hazard or lifeline?
Positive impacts Area Study
area irrigated (ha) No. of HHs (ha)
Kumasi 502,700 12,70011,900
Nairobi 125,700 3,700 2,220
Wastewater irrigation - hazard or lifeline?
Positive impacts
Main crops
Av. Revenue US$ / ha
Total value of production
Nairobi Kumasi
Kale, tomato, spinach, green maize, cabbage
Tomato, garden egg, okra, chilli
1,770 (annual) 544 (7 months)
US$ 3.9 million
US$ 6 million
Wastewater irrigation - hazard or lifeline?
Positive impacts - revenue + profit, Nairobi
FarmerCost$US
Revenue$US
Profit$US
Plot area ha
1
2
3
4
5
6
Overall
237 232 -4 0.109
417 1,802 1,385 0.143
60 68 8 0.041
33 196 162 0.088
1 63 62 0.032
0 60 60 0.069
124 403 279 0.080
Wastewater irrigation - hazard or lifeline?
Positive impacts - profits, KumasiFarm profit $US
-100
0
100
200
300
400
500
34 15 25 27 11 22 32 13 26 24 35 21 31 36 17 33 37 12 14 16 23
Average = $US 152
Wastewater irrigation - hazard or lifeline?
What are the trade-offs
• HH income
• Urban food supply
• Nutrient recycling
• Managed waste disposal
• Chronic ill-health
• Wide-scale disease outbreaks
• Damage to soils & groundwater
CostsBenefits
Where is the greatest ‘public good’ secured?
Wastewater irrigation - hazard or lifeline?
Interim water quality standardsCurrent WHO guidelines:
• Apply to TREATED wastewater
• Aim to secure “no measurable excess risk of infection”
Use of UNTREATED wastewater is widespread. Under these conditions:
• “No risk” standard seems unrealistic
• A single threshold is unhelpful - all wastewater is not equal
• Are there acceptable levels of risk & what do they imply?
• Can we provide greater guidance - risk assessment scales?
• Can quantitative microbiological risk assessment models provide these answers?