With the continuously growing population, the gap between the demand and supply of acceptable water quality for irrigation constantly increases. Driven by the need for food security, small-scale farmers from developing countries around the world use more and more marginal-quality water for food production. At the down of an era where climate change, land and water degradation and groundwater depletion are the major obstacles in meeting the Millennium Development Goals, marginal water brings new opportunities, but also lots of challenges to overcome.
Irrigation was and remains the key component which enables the production of enough food to feed the planet. Despite the fact that the per capita water availability in many developing countries drops much below the limit that triggers the water crisis (>500 m3/person/year), irrigation accounts for over 90% of water withdrawal from available surface and subsurface sources (UN, 2006).
Most recent data on per capita water availability by country can be monitored here.
Also, an up to date baseline water stress map (the ratio of total annual freshwater withdrawals) is available here with projected change in water stress scenarios for the years 2025, 2050 and 2095. It provides an assessment of the demand for freshwater from households, industry, and irrigated agriculture relative to freshwater availability in a typical year (ISciences L.L.C. , 2011).
The constantly growing demand, especially in the arid, semi-arid and hyper-arid regions, has “encouraged” professionals, researchers and scientist into exploring new sources of water. The demand and usage of marginal-quality water is rising.
The concept of marginal water
The concept of marginal water defines two major types of marginal-quality water
- Wastewater from urban and peri-urban areas
- Saline and sodic agricultural drainage water and groundwater (Manzoor Qadir, 2007).
Wastewater from urban and peri-urban areas
With the high rate of the population growth the volume of wastewater has been exponentially increasing over the years. Unfortunately, the wastewater supply expands faster than the ability and possibility of developing nations to build wastewater treatment facilities. Most of the wastewater from urban areas is being discharged in waterways and afterwards used untreated, partially treated, diluted or treated, by small scale farmers for irrigation purposes. Many other times, wastewater, for its nutrient content (nitrates and phosphorus), is used for food production by farmers that cannot afford fertilizers.
Often, wastewater contains a high variety of pollutants, such as pathogens, nutrients, metals, salts, residual drugs etc. The use of wastewater in agriculture along with poor irrigation practices is as harmful for human health as it is for the environment. Farmers and costumers that purchase the irrigated crops are endangered in an equal manner; farmers by getting in contact with the pollutants, and the consumers by ingesting the final product. Unfortunately, some farmers are not aware of this risk. On the other hand, deep percolation of marginal-quality water used for irrigation leads to groundwater degradation.
Wastewater reuse represents a promising resource for water scarce countries, but a very careful approach is needed when managing it. In order to protect public and environmental health, good policies and regulations are needed to be implemented, good irrigation technics and practices, along with raised awareness among the stakeholders and the final consumer. In developed countries, treated wastewater is only used for irrigating non-agricultural crops, such as fodder or for greening purposes, like parks, on road planting or sport fields.
Apart from the agricultural use, “treated wastewater is also used for:
- environmental purposes (wetlands, wildlife refuges, riparian habitats, urban lakes and ponds),
- industrial functions (cooling, boiling, processing)
- non-potable applications (firefighting, air conditioning, dust control, toilet flushing), and
- groundwater recharge” (Manzoor Qadir, 2007)
Wastewaters effective and efficient reuse can “help to close the loop between water supply and wastewater disposal” (P. Nila Rekha, 2012) which has been proved to be an additional challenge of urbanization, improved lifestyle conditions and economic development.
Saline and sodic agricultural drainage water and groundwater
In contrast to the wastewater, saline and sodic water rarely contains pathogens and heavy metals (that may penetrate the soil from land-based activities), but it contains a high level of salts (that occur from reactions that take place when the water percolates the soil profile and the deeper groundwater layers). Using this type of marginal water for irrigation requires a careful management and a good salinity control (e.g. sprinkling at night), otherwise it may lead to salinization (accumulation of salts in the plant root zone), waterlogging and eventually - the loss of agricultural land (Manzoor Qadir, 2007).
Besides salts, agricultural drainage water, presented by the surface runoff and the subsurface drainage water, it is also rich in pesticides, nutrients, sediment, bacteria, toxic trace elements etc. (FAO, 1997) making its use even more dangerous for the public and the environment than saline/sodic groundwater.
Marginal water reuse - Challenges and opportunities
Challenges - Human health, environmental and economic impacts:
- Untreated wastewater disposal pollutes freshwater that can lead to:
- Waterborne diseases, such as diarrhea, typhoid fever, ascariasis, hookworm disease, lymphatic filariasis, hepatitis A (in poor countries untreated freshwater is used for drinking, cooking and hygiene purposes (women and children are most vulnerable)),
- Impacts biodiversity (change or loss of biodiversity),
- Reuse in agriculture threatens human health, reduces yields, and leads to loss of agricultural land due to salinization,
- Groundwater contamination (leaching pathogens, metals, salts, etc.).
- Soil salinization impacts soil, crop choices and yields,
- Water quality degradation (disposal of saline and sodic water into freshwater ways leads to environmental and economic losses) (Manzoor Qadir, 2007).
Opportunities for marginal water reuse
- High quality treated wastewater can be used for groundwater recharge (important for regions with a high rate of groundwater depletion),
- Municipal wastewater is rich in nutrients, with a good management and periodic monitoring. The reuse of this type of marginal water in agriculture can contribute to crop growth and economical savings due to the decreased need of fertilizers,
- Use of untreated wastewater and poorly treated wastewater for non-agricultural crops (that are likely to transmit contaminates and pathogens), such as for greening purposes and/or fodder.
- In water scarce countries, where alternative water sources for irrigation are not available, but irrigation is essential (arid regions), this type of marginal-quality water represents the only resource. With good maintaining salt balance practices (e.g. drainage systems) drainage water is a resource that can extend farm-level and regional water supplies,
- Can support wildlife habitat and wetlands (Manzoor Qadir, 2007),
- Can be used to irrigate certain plant species (Anon., n.d.).
How to reach sustainable use of marginal water – possible solutions
- Risk management strategies (e.g. policies on improved management of marginal water use in agriculture),
- Establish property rights to wastewater,
- Implement economic incentives for reusing treated wastewater (in regions where other sources are available),
- Conduct public awareness programs (on possible risks, risk management, proper usage, food hygiene practices etc.),
- Strengthen political will, regional policies and institutions,
- Support research,
- Invest in infrastructure and institutional capacity, etc. (Manzoor Qadir, 2007).
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Manzoor Qadir, D. W. L. R.-S. P. S. M., 2007. Agricultural use f marginal-quality water- opportinities and challenges. In: C. T. Bruce Ross-Larson, ed. Water for Food, Water for Life: A Comprehensive Assessment of Water Management in Agriculture. London: Earthscan, and Colombo: International Water Management Institute: Earthscan Uk and USA, pp. 425-457.
P. Nila Rekha, N. A., 2012. Marginal Waters for Agriculture - Characteristics and Suitability Analysis of Treated Paper Mill Effluent, Irrigation - Water Management, Pollution and Alternative Strategies. [Online]
Available at: http://www.intechopen.com/books/irrigation-water-management-pollution-and-alternative-strategies/marginal-waters-for-agriculture-characteristics-and-suitability-analysis-of-treated-paper-mill-efflu
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