Wastewater irrigation - hazard or lifeline?

Gez Cornish & Neeltje Kielen

HR Wallingford Ltd. UK

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?