Threats of Untreated Wastewater
Irrigation to Vegetable Crops in Pakistan
By: C.M.Ayub, M.Aslam Pervez, Waqas Ahmad & Abdul Manan
Pakistan’s population has increased from 65 to 161 million
over the past 3 decades and is forecast to increase to 234
million by 2025.
Due to population explosion, rural development, based
around productivity gains in food and livestock production,
is important to food security and crucial to poverty
It is widening the gap between supply and demand and
increasing food insecurity problem in the country.
To fill this gap, vegetable production is important to
squeeze this problem. Horticultural crops contribute about
6% of country’s GDP and 22% of national food production.
Pakistan annually produces about 12854.6 thousand tons of
fruits and vegetables in which annual vegetables production
is about 5675.8 thousand tons.
Vegetable crops are the minor
crops in the country but very important due to their higher
yield potential, low cost of production and higher
nutritional value being the added attributes. Vegetables are
the most commonly irrigated crops with wastewater, because
they fetch high prices in the nearby urban markets.
There is 21 and 13% increased
occur in area under vegetables and amount of vegetables
production (excluding Potato), respectively during the last
two decades (1990-1991 to 2007-2008) in Pakistan and similar
kind of increase is also observed in the Punjab province,
where area and production is increased by 23 and 18%,
In most cities of Pakistan,
the wastewater from municipal areas as well as from the
industries is disposed off untreated to the natural water
surface bodies. It has been reported that about 212.2
million gallon tones of sewage is being thrown in water
bodies and there is no proper mechanism for sewage
collection and treatment/safe disposal.
Such practices are causing
environmental pollution. The sewage water is used for
irrigating various crops in the surroundings of the cities.
The farmers also prefer to use this water because waste
water is extremely valuable source for farmers as pumping
cost from sewage nallah or drains is cheaper than a
borehole, which makes the practice more accessible to
farmers with fewer financial resources.
Sewage water creates crop management problems like weed
infestation and incidence of pests, however, these problems
are tackled with the use of plant protection measures.
Continuous use of waste water may lead to environmental
problems such as soil sickness, soil and groundwater
contamination and phytotoxicity.
The sewage water is primarily
used for raising vegetables in the vicinity of the cities.
This is one of the reasons that vegetables are mostly
concentrated around the big cities.
Leafy vegetables like radish,
cauliflower, cabbage, carrot and turnip etc. are root crops
and sensitive to the use of sewage water because sewage
water deteriorates the quality of such vegetables and
negatively affect the vegetable growth.
Concluded that poor quality
of ground water caused considerable losses to root crops.
Similarly, sewage water also affects badly root crops such
as vegetable (radish) during maturity stage and as a result
the production decreases substantially.
industrial effluent being discharged by factories, which
ultimately gets mixed with the urban sewage, may contain
excessive amounts of heavy metals/metalloids like cadmium,
chromium, nickel, manganese, zinc, lead, etc.
These metals/metalloids in
soil, if present in excessive quantities to permissible
levels, may enter the food chain thus becoming toxic to
plants and human beings.
At least one-tenth of the
world’s population is thought to consume foods produced by
irrigation with wastewater. It has been estimated that at
least 20 million hectares in 50 countries are irrigated with
raw or partially treated wastewater.
Wastewater is mainly used in
urban agriculture which often supplies a large proportion of
the fresh vegetables sold in many cities, particularly in
In India, irrigation with
untreated wastewater equivalent 7.5 cm per hectare has been
estimated to supply 36, 5 and 50 kg per hectare of nitrogen
(N), phosphorus (P) and potassium (K), respectively.
On an average, only about 10%
of all wastewater in developing countries receives
treatment. WHO/UNICEF has estimated the median percentage of
wastewater treated by effective treatment plants to be 35%
in Asia, 14% in Latin America and the Caribbean, 90% in
North America and 66% in Europe.
Poor quality groundwater and
lack of alternative water sources have resulted in the use
of untreated wastewater for local irrigation in urban, peri-urban
and even rural agriculture, all across Pakistan. It has been
estimated that in 2025, fifty per cent (100 million) of the
population of Pakistan will be residing in cities.
The growing population and
fresh water scarcity increase the scope of reuse of urban
wastewater in agriculture.Total discharge of wastewater for
14 major cities of Pakistan, computed on the basis of 1998
population census, is about 1.83 × 107 m3 h-1.
Latest estimates reveal that
total quantity of wastewater produced in Pakistan is 962,335
million gallons (4.369 x 109 m3/yr). A recent nationwide
wastewater assessment showed that total water supply is 4.6
x 106 m3/day, and about 30% of wastewater is used for
irrigating an area of 32,500 hectares.
It has also been estimated
that 64% of total wastewater of Pakistan is disposed of
either into rivers or into the Arabian Sea. Similarly
400,000 m3/ day wastewater is additionally added to canals.
Therefore, it is essential to look into prospects of sewage
irrigation to manage this nutrient rich water resource.
Farmers are getting good crop yields from sewage irrigated
crops without application of chemical fertilizers as
compared to those grown with freshwater. The yield
differences are attributed to the nutrients in wastewater,
but it is not often known which nutrients are most valuable
and what their optimum quantities are.
In general, the value of
wastewater as a source of nutrients for crops depends on
concentrations of nutrients, type of crop grown, and soil
fertility level. In case of wastewater, nutrient use
efficiency is almost 100% as the nutrients remain dissolved
in wastewater/ soil solution and thus more available to the
This is also because,
nutrient supply through irrigation water matches with crop
nutrient demand and given in increments with each irrigation
as compared to chemical fertilizers which are commonly
applied in two to three splits. The quantity of N, P and K
applied from sewage irrigations of 0.40 m in Faisalabad
(Pakistan) ranged from 116 to 195, 7 to 21 and 108 to 249 kg
These quantities of N and K
are quite sufficient for any crop while that of P is low and
would need to be supplemented. Since P applied through
sewage is 100% soluble, its availability is generally much
higher than P applied through fertilizers. In another study
conducted at Haroonabad (Pakistan), up to 2030, 1110 and
1580 kg ha-1 of N, P and K, respectively, per cropping
season were added to the soils when crops were irrigated
Efficiencies of nutrients
(excess of nutrient above the recommended rate) applied
through sewage irrigation ranged from 140 to 920 for N, 20
to 790 for P and 125 to 930% for K, depending upon the crop
type and amount of sewage.In most of the industrial cities
of Pakistan, domestic sewage is mixed with industrial
effluents, which contains very high concentrations of toxic
Therefore, domestic sewage
and industrial wastes should be treated separately and
different set of guidelines should be developed for their
use in agriculture. Moreover, the farmers using sewage for
irrigation should regularly monitor the concentration of
heavy metals and pathogens in sewage being used for
In this country it is common
practice to apply raw municipal sewage/industrial effluent
to grow vegetables especially, in the vicinity of big
cities. As a result of human intake of sewage irrigated
vegetables, there is a risk of various diseases in the long
run as a result of slow buildup of metals in several organs
of the body.
Assuming that all the installed treatment plants are working
at their full installed capacity, it is estimated that about
8% of urban wastewater is probably treated through
sedimentation pounds to a primary level only. There is no
prevailing concept of treatment at secondary and tertiary
level in this country.
Based on careful estimates,
to make raw effluent, germ free as per standards set by WHO
for a city with a population of 1 million, it will cost Pak
Rs. 750 million (90 Pak Rs = $1) annually. However, to bring
the same raw effluent up to standards set by United States
Environmental Protection Agency (USEPA) this cost will
exceed its double.
Only for disinfection of 100
cubic meter raw sewage the cost is about Pak. Rs. 700. The
use of major portion of sewage for irrigation without prior
treatment, especially in developing world, is largely blamed
to lack of funds and latest technology.
Currently, 1/4th of vegetables being grown in Pakistan are
irrigated with untreated sewage water.
According to a study
conducted by the International Water Management Institute,
Pakistan Chapter, farmers practicing sewage irrigation get
about Rs. 3000 per acre more income than that from fresh
irrigated water fields. This figure is quite close to the
estimated gross value (Pak Rs. 3278 per acre per annum) of
the nutrient load of wastewater assessed during a study in
Mexico by the same Institute.
Moreover, the net value of
produce from wastewater irrigated fields is calculated to be
about Pak Rs. 20,000 per acre. This is attributed to
essential nutrients present in sewage water, thus
eliminating the need for chemical fertilizers. Treatment of
sewage should be made mandatory prior to its use in
circumstances, wastewater should be diluted with freshwater.
But still, it should be got tested from some nearby
soil/water/food testing laboratory, especially for its heavy
To avoid metal entry in to food chain, untreated waste water
should preferably be used to irrigate non-vegetable crops
such as cotton, ornamental plants, green belts, urban forest
However, a gap of 1-2 years
should be given after every 3-4 years of sewage application
to allow the soil to come back to the equilibrium.
irrigation should be avoided to vegetable crops especially
leafy vegetables (spinach, coriander, fenugreek, lettuce,
cabbage, cauliflower etc.) as well as those ones which are
consumed raw/uncooked as a salad like cucumber, tomato,
carrot, radish etc.
Only those vegetable crops
should be irrigated that bear above-ground edible parts and
are cooked before consumption.