Risks and opportunities in sewage irrigation
Using treated wastewater for agriculture, provides a means through which sewage effluent can safely be re-used. To ensure sustainable and safe use of wastewater for food production in urban and peri-urban areas, there is a need to explore management options.
In Pakistan, sewage irrigation is an informal and unregulated activity, making it difficult to define the pattern and extent to which it is practiced. In the case of informal irrigation, wastewater is accessed directly or indirectly.
Indirect use occurs when domestic wastewater and in some instances, industrial wastewater is discharged directly into watercourses within and around the urban areas without treatment.
The sewage disposal system entails discharging wastewater into surface water bodies which are often irrigation channels and canals. In this case, there is no control over the subsequent use of water for crop production.
Direct use occurs mainly when farmers deliberately divert sewage effluent from drain to their fields. Thus, under conditions of water scarcity and weak enforcement, wastewater irrigation has thrived as an unplanned and spontaneous activity.
In many developing urban centres, wastewater is generally a mixture of domestic and untreated industrial effluent. The uncontrolled and varied nature of wastewater sources used for irrigation makes it difficult to define, monitor and control the practice.
There are no comprehensive figures of the extent of wastewater used for irrigation but the available estimates indicate that about 900,000 hectares of farmland in developing countries are irrigated with wastewater.
Globally 20 million hectares producing nearly four per cent of food are irrigated with wastewater. The estimated sewage production of cities like Karachi, Lahore, Peshawar, Hyderabad, Faisalabad, Multan, Rawalpindi/Islamabad, Kasur, Sukkur, Quetta, etc., range from 32 to 68 gallons per capita per day.
On the basis of population census of 1998, 1,16,590 million gallons per day of sewage is generated from large industrial-urban centres. Nearly 10 per cent of the municipal and industrial effluent is treated only to primary level and the rest is disposed off into irrigation system or left astray without any treatment.
A major proportion of wastewater is confined to the vicinity of cities. It is estimated that about 95,000 acres (32,000 hectares) in Pakistan are irrigated with municipal effluent.
The main reasons of sewage irrigation are drought, high content of plant nutrients and year-round access of farmers to this source. The effluent is mainly used for raising vegetables and fodder crops because of the nearby markets. It is estimated that the application of 40cm of sewage effluent can add 100-200kg N, 6-20kg phosphorus and 100-250kg potassium, eliminating the need for artificial fertilizers.
However, the use of raw sewage in agriculture is not without danger. The major risk in utilizing raw city effluent is food contamination by pathogenic micro-organisms and outbreak of water-borne diseases.
Untreated city effluent contains full spectrum of pathogens (helminthes, protozoa, enteric bacteria and viruses) found in urban population and many of these can survive for several weeks when discharged into the fields. There are public health concerns for producers, handlers, consumers and communities using wastewater around production areas. These potential health risks are a major constraint of current wastewater use practices, and can possibly limit its long-term sustainability.
Irrigation with raw sewage containing high level of trace elements and heavy metals is likely to be toxic to plants and also poses risk to human health. Heavy metal in sewage effluent for most developing countries is mainly related to the mixing of domestic and industrial wastewater in the same sewage system.
Examples of potentially toxic trace elements include mercury, lead, arsenic, copper, cadmium, manganese. Generally farmers grow leafy vegetables and those vegetables whose edible portions are root that flourish under or near the ground e.g., spinach, lettuce, radish, carrot, sugar beet, cabbage, cauliflower, etc. These accumulate higher amounts of heavy metals like cadmium, lead, zinc, copper, nickel, manganese.
Studies in China, Japan and Taiwan indicate that the rice accumulated high concentrations of cadmium and other heavy metals when grown in soils contaminated with irrigation water containing substantial industrial discharges. These examples indicate that certain food crops have a higher possibility of transferring heavy metals to humans. Most heavy metals are carcinogenic and cause mental disorder, respiratory problems and hormonal imbalance. A more dangerous consequence of raw municipal effluent is transmission of heavy metals through animal milk into human as fodder grown by polluted water accumulates higher quantities of heavy metals in animals.
Heavy metals, therefore, remains concern especially in instances where industrial effluent is an important factor. In addition, health risks of heavy metals can be looked at from an occupational hazard point of view where chemical pollutants in wastewater can cause harm to farmers as a result of direct contact with water during farming.
The lack of awareness among farmers on health hazards of untreated effluent also constitutes a major problem. Training and awareness to build skills and knowledge on using raw sewage for irrigation should be considered in planning process of re-use projects so as to protect all risk groups.
Awareness campaigns and programmes should be conducted to educate and orient farmers on precautions of raw effluent use and inform consumers about the safety of agricultural products irrigated with sewage.
The absence of strict regulations is resulting in damage to health and environment. The long-term use of saline and sodium-rich water tends to destroy soil structure and reduces productivity.
Moreover, increasing industrialization is changing the composition of wastewater, raising the level of heavy metals, acids and the like which impacts soil and crops. Groundwater contamination from nitrates and other pollutants including heavy metals is another potential danger and many such problems are irreversible. Regulations are also needed to control the re-use of treated sewage effluent.
The environmental impact of sewage irrigation varies considerably from city to city depending on industrialization, type of industry, nature of water distribution and the degree of treatment and dilution if any.
Environmental impact of sewage irrigation also raises doubts about its long-term sustainability and includes visual untidiness, soil erosion, and destruction of vegetation, silting, depletion and pollution of land and water resources. However, the use of sewage in agriculture helps water conservation.
Sewage use scheme, if properly planned and managed can have positive environmental impacts. Some degree of treatment must normally be provided to raw municipal effluent before it can be used for irrigation. For local governments, using treated wastewater for irrigation can be beneficial, as an economically feasible and environmentally sound method of disposing municipal effluent.
The management of wastewater through treatment has two major objectives. The first is to protect the environment by reducing the pollution of freshwater resources and productive lands and hence reducing health hazards. The second is to mobilize this available water resource for mitigating water scarcity and improving crop production.
Putting restrictions on the type of crop that can be grown with raw effluent is another way of reducing risks. Instead of growing vegetables and food and fodder crops, grain crops like wheat, maize, sorghum and ornamental crops and woody trees.
Other simple options for farmers include changes in irrigation methods which can have low levels of risks. Using micro irrigation techniques to irrigate vegetables, for example, can greatly reduce the risk of pathogenic contamination compared to flood or furrow irrigation. The use of sewage in urban and peri-urban agriculture can also improve use efficiency of water – especially in cities with limited water resources.
Properly treated sewage effluent can be used for groundwater recharge, aquaculture and irrigation of lawns and urban recreational parks and other non-potable uses. The best approach will need to balance both farmers’ livelihood needs, and public health concerns.
In future, water shortage and wastewater handling would be a major challenge. The result of increased attention to this issue is expected to improve the health, economic and agricultural factors of developing community in relation to wastewater treatment and re-use in agriculture.