Wastewater has been widely used for irrigation of agricultural crops as the nutrients contained in the wastewater are considered beneficial to plant growth. According to several estimates, about 80 per cent of water used in towns and cities ultimately adopts the form of Sewage Water. The majority of the farmers in the developing world prefer wastewater water irrigation as it is a rich source of nutrients while municipal authorities consider this practice as a viable option for disposal. 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 (FAO, 2002). 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 (Ensink et al., 2004). 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 (Ensink et al., 2002). 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 build up of metals in several organs of the body.
Treatment cost of polluted wastewater
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.
Economics of untreated wastewater use
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, 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.
The review work summarized has been published and can be accessed from the following links: http://www.sciencedirect.com/science/article/pii/S1002016009602794
For further details, please contact Dr M. H. Zia, E-Mail: firstname.lastname@example.org
Ensink, J.H.J, T. Mahmood, W.van der Hoek, L. Raschid-Sally and F.P. Amerasinghe. 2004. A nationwide assessment of wastewater use in Pakistan: an obscure activity or a vitally important one? Water Policy 6: 197-206.
Ensink, J.H.J., W. van der Hoek, Y. Matsuno, S. Munir, M.R. Aslam. 2002. Use of untreated wastewater in peri-urban agriculture in Pakistan: Risks and opportunities. IWMI Research Report No. 64. International Water Management Institute (IWMI), Colombo, Sri Lanka.
Food and Agriculture Organization of the United Nations (FAO). 2002. Investment in Land and Water. In Proceedings of the regional consultation Bangkok, Thailand 3-5 October, 2001. Publication 2002/09. FAO Regional Office for Asia and the Pacific, Bangkok.
Hussain, S.I. (2000). Irrigation of crops with sewage effluent. Ph.D. Thesis, University of Agriculture, Faisalabad, Pakistan.
Murtaza, G., A. Ghafoor, M. Qadir, G. Owens, M.A. Aziz, M.H. Zia and Saifullah. Disposal and Use of Sewage on Agricultural Lands in Pakistan: A Review. Pedosphere 20:23-34.