In the face of growing demand, the use of non-conventional water sources, such as treated wastewater and desalinated water, is gaining momentum.
Most human water activities produce wastewater, potentially recoverable for secondary uses such as in agriculture. If all this water were recovered, it would substantially reduce pressures on freshwater and alleviate scarcity, provided accounting assessments ensured the return flow was not serving an environmental function.
WATER REUSE
Wastewater is predicted to increase considerably with population growth and urbanisation. On average, high-income countries treat about 73 per cent of their wastewater. The figure drops to 54 per cent in upper-middle-income countries and to 28 per cent in low-middle-income countries.
Globally, about 80 per cent of wastewater is released without adequate treatment. In 2019, 7.5 million m³/day of new water reuse capacity was forecast. China dominates this total (3.7 million m³/day), followed by the United States of America (880 000 m³/day) and India (680 000 m³/day).
Most is tertiary and/or advanced wastewater treatment. This is part of a broader trend towards advanced treatment driven by industrial demand for higher-quality water, and from agricultural users.
Although definitive numbers on water reuse in agriculture are hard to find, about 10 per cent of the total global irrigated land area receives untreated or partially treated wastewater, more than 30 million hectares in 50 countries. For decades, the most significant benefit of water reuse in agriculture has been that of decreased pressure on freshwater sources.
Waste water in agriculture
The Circular Economy has brought a different perspective on water reuse in agriculture, proposing a model where the value of products, materials and resources is sustained for as long as practical and waste reduced or even eliminated.
Treated wastewater is readily available for agriculture, including irrigation. Water reuse for irrigation brings more certainty that water will be available throughout the year, even during dry spells. Nutrients can be recovered from sewage sludge (biosolids) and reused as fertilizer, as widely practised in many countries.
In Europe, more than one-quarter of sewage sludge produced in 2017 was used in agriculture. A final benefit is energy recovery, such as biogas production from waste treatment at the farm level.
When treated according to the end users’ needs (fit for purpose), wastewater is a realistic option for non-conventional sources of water, nutrients and energy for agriculture. Reusing water in agriculture from fit-for-purpose treated wastewater is a “win-win” situation as it is based on improved sanitation (collection systems), treatment facilities, reuse of chemical elements (nitrogen and phosphorus) and making water available for higher-value uses.
Challenges
However, in some countries, using treated wastewater to irrigate food crops is still not culturally acceptable. With strong communication channels, government regulations and the involvement of stakeholders would help to change negative perspectives about the use of non-conventional waters for food production. Moreover, assessment of water quality criteria, potential environmental impacts and regulatory issues need to be resolved to foster best practices and implementation.
The current policies for using reclaimed water are highly fragmented, and in many countries incomplete, which tends to inhibit development. There is a need to develop both policy and planning frameworks for governments, municipalities and water resources groups to develop recycled wastewater as a future supply for irrigated agriculture.
Training and capacity-building programmes can promote technology uptake through local and international streams, taking into account local needs and conditions. Removing barriers and creating an enabling environment will require appropriate legislation and regulations to make finance available for their adoption.
DESALINATION
Desalination covers the removal of dissolved solids (predominantly inorganic salts) and other dissolved contaminants from several sources, including seawater, brackish water (surface water and groundwater), and irrigation drainage.
Aristotle, in his famous Meteorologica (written in about 350 BCE), described distillation to remove salts and other compounds to produce freshwater.
Since then, desalination has become a major option for urban water supply, especially in desert and drought-prone regions. Owing to almost unlimited seawater, desalination is an attractive solution to the age-old challenge of its abundance despite being undrinkable.
Different processes
There are approximately 16 000 desalination plants, producing about 100 million m3/day of drinking water for 5 per cent of the world’s population, of whom 48 per cent are in the Near East and North Africa. Since 2018, more than 400 desalination projects have been contracted worldwide, with 4 million m³/day in new capacity in the first half of 2019.
The main way to produce freshwater has been distillation, where saline water is distilled into steam and then condensed into pure water. The 1950s brought the development of membrane processes, such as electrodialysis and reverse osmosis.
In electrodialysis, an electric current separates salts in water. In reverse osmosis, pressure forces water through a semi-permeable membrane that extracts most of the salts. Unlike distillation, modern membranes use very little energy to produce freshwater, although a major environmental problem has been disposal of salts removed from water.
The cost
The main obstacle to desalination has always been the cost. Its application in agriculture has been limited to a small number of areas, for certain high-value crops, and needing government subsidies in capital costs.
Over the last decades, however, desalination has become much more efficient and cost-effective thanks to rising demand, technology improvements, reductions in costs and energy use, increase in plant size to large and mega capacity sizes, and more competitive project delivery.
A 2008 study showed a consistent reduction in desalination costs over nearly three decades, and estimates that large-scale desalination plants are capable of producing water in the range of USD 0.5–2.0/m3, depending on plant size. Similarly, a more recent study estimates that the cost of desalinated water varies between US$ 0.5–1.5/m3.160 In terms of cost, brackish-water desalination is more suitable for agricultural production than is seawater.
Membranes and renewable technologies such as solar power have made desalination more feasible, especially for high-value cash crops such as greenhouse vegetables. Farmers welcome it as the process removes salts (especially sodium and chloride) that damage soils, stunt plant growth and harm the environment.
High yield and salt-tolerant crops
Several countries, such as Australia, China, Mexico, Morocco and Spain, are now using desalinated water profitably for agriculture. Dévora-Isiordia et al. (2018) calculated the cost of desalination (US$ 0.338 /m3) and its economic use in agriculture in Sonora, Mexico. They concluded that, in order to ensure its viability, farmers should choose high-yield crops with profitable cost–benefit ratios, such as vegetables (e.g. tomatoes and chillies), and apply drip irrigation.
Integrated agri-aquaculture farms are testing the integration of saline water into farms using salt-tolerant crops. Policies and regulations have a powerful role in boosting both through public projects, enabling the private sector and knowledge exchange. Public–private partnerships also reduce investment risks.
Water desalination can have negative impacts on the environment (e.g. brine disposal of residues from desalination and GHG emissions). Although there are technology and management options to reduce such impacts, there is a need for standards and impact assessment studies (local and regional), as well as for brine disposal research and continuous monitoring of effluents.
Source: http://www.fao.org
