Threats of Untreated Wastewater Irrigation to Vegetable Crops in Pakistan
 By: C.M.Ayub, M.Aslam Pervez, Waqas Ahmad & Abdul Manan

The 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 alleviation.

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%, respectively.

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.

However, untreated/raw 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 developing countries.

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 crops.

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 ha-1, respectively.

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 with sewage.

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 metals.

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 irrigation.

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 agriculture.

Under unavoidable 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 metal contents.

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 plantations, etc.

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.

Untreated wastewater 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.

November, 2014

Source: Pakissan Report