Desalination

From Wikipedia, the free encyclopedia

Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture.

Saltwater is desalinated to produce water suitable for human consumption or irrigation. One by-product of desalination is salt. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few rainfall-independent water sources.

Due to its energy consumption, desalinating sea water is generally more costly than fresh water from rivers or groundwater, water recycling and water conservation. However, these alternatives are not always available and depletion of reserves is a critical problem worldwide. Currently, approximately 1% of the world's population is dependent on desalinated water to meet daily needs, but the UN expects that 14% of the world's population will encounter water scarcity by 2025.

Desalination is particularly relevant in dry countries such as Australia, which traditionally have relied on collecting rainfall behind dams for water.

Desalinated water is usually healthier than water from rivers and ground water, and there is less salt and limescale in it.

According to the International Desalination Association, in June 2015, 18,426 desalination plants operated worldwide, producing 86.8 million cubic meters per day, providing water for 300 million people. This number increased from 78.4 million cubic meters in 2013, a 10.71% increase in 2 years. The single largest desalination project is Ras Al-Khair in Saudi Arabia, which produced 1,025,000 cubic meters per day in 2014. Kuwait produces a higher proportion of its water than any other country, totaling 100% of its water use.

Schematic of a multistage flash desalinator
A – steam in
B – seawater in
C – potable water out
D – waste out
E – steam out
F – heat exchange
G – condensation collection
H – brine heater

Plan of a typical reverse osmosis desalination plant

Methods

Reverse osmosis desalination plant in Barcelona, Spain

There are several methods. Each has advantages and disadvantages.

Vacuum distillation

The traditional process used in these operations is vacuum distillation—essentially boiling it to leave impurities behind. In desalination, atmospheric pressure is reduced, thus lowering the required temperature needed. Liquids boil when the vapor pressure equals the ambient pressure and vapor pressure increases with temperature. Effectively, liquids boil at a lower temperature, when the ambient atmospheric pressure is less than usual atmospheric pressure. Thus, because of the reduced pressure, low-temperature "waste" heat from electrical power generation or industrial processes can be employed.

Multi-stage flash distillation

Water is evaporated and separated from sea water through multi-stage flash distillation, which is a series of flash evaporations. Each subsequent flash process utilizes energy released from the condensation of the water vapor from the previous step.

Multiple-effect distillation

Multiple-effect distillation (MED) works through a series of steps called "effects". Incoming water is sprayed onto pipes which are then heated to generate steam. The steam is then used to heat the next batch of incoming sea water. To increase efficiency, the steam used to heat the sea water can be taken from nearby power plants. Although this method is the most thermodynamically efficient among methods powered by heat, a few limitations exist such as a max temperature and max number of effects.

Vapor-compression distillation

Vapor-compression evaporation involves using either a mechanical compressor or a jet stream to compress the vapor present above the liquid. The compressed vapor is then used to provide the heat needed for the evaporation of the rest of the sea water. Since this system only requires power, it is more cost effective if kept at a small scale.

Reverse osmosis

The leading process for desalination in terms of installed capacity and yearly growth is reverse osmosis (RO). The RO membrane processes use semipermeable membranes and applied pressure (on the membrane feed side) to preferentially induce water permeation through the membrane while rejecting salts. Reverse osmosis plant membrane systems typically use less energy than thermal desalination processes. Desalination processes are driven by either thermal (e.g., distillation) or electrical (e.g., RO) as the primary energy types. Energy cost in desalination processes varies considerably depending on water salinity, plant size and process type. At present the cost of seawater desalination, for example, is higher than traditional water sources, but it is expected that costs will continue to decrease with technology improvements that include, but are not limited to, improved efficiency, reduction in plants footprint, improvements to plant operation and optimization, more effective feed pretreatment, and lower cost energy sources.

Reverse osmosis uses a thin-film composite membrane, which comprises of an ultra-thin, aromatic polyamide thin-film. This polyamide film gives the membrane its transport properties, whereas the remainder of the thin-film composite membrane provides mechanical support. The polyamide film is a dense, void-free polymer with a high surface area, allowing for its high water permeability.

The Reverse Osmosis process is not maintenance free. Various factors interfere with efficiency: ionic contamination (calcium, magnesium etc.); DOC; bacteria; viruses; colloids & insoluble particulates; biofouling and scaling. In extreme cases the RO membranes are destroyed. To mitigate damage, various pretreatment stages are introduced. Anti-scaling inhibitors include acids and other agents like the organic polymers Polyacrylamide and Polymaleic Acid), Phosphonates and Polyphosphates. Inhibitors for fouling are biocides (as oxidants against bacteria and viruses), like chlorine, ozone, sodium or calcium hypochlorite. At regular intervals, depending on the membrane contamination; fluctuating seawater conditions; or when prompted by monitoring processes, the membranes need to be cleaned, known as emergency or shock-flushing. Flushing is done with inhibitors in a fresh water solution and the system must go offline. This procedure is environmental risky, since contaminated water is diverted into the ocean without treatment. Sensitive marine habitats can be irreversibly damaged.

Freeze-thaw

Freeze-thaw desalination uses freezing to remove fresh water from salt water. Salt water is sprayed during freezing conditions into a pad where an ice-pile builds up. When seasonal conditions warm, naturally desalinated melt water is recovered. This technique relies on extended periods of natural sub-freezing conditions.

A different freeze-thaw method, not weather dependent and invented by Alexander Zarchin, freezes seawater in a vacuum. Under vacuum conditions the ice, desalinated, is melted and diverted for collection and the salt is collected.

Solar evaporation

Solar evaporation mimics the natural water cycle, in which the sun heats the sea water enough for evaporation to occur. After evaporation, the water vapor is condensed onto a cool surface.

Electrodialysis reversal

Electrodialysis utilizes electric potential to move the salts through pairs of charged membranes, which trap salt in alternating channels.

Membrane distillation

Membrane distillation uses a temperature difference across a membrane to evaporate vapor from a salty brine solution and condense pure condensate on the colder side. 

Wave-powered desalination

CETO is a wave power technology that desalinates seawater using submerged buoys. Wave-powered desalination plants began operating on Garden Island in Western Australia in 2013 and in Perth in 2015.

Considerations and criticism

Energy consumption

Energy consumption of seawater desalination has reached as low as 3 kWh/m³, including pre-filtering and ancillaries, similar to the energy consumption of other fresh water supplies transported over large distances, but much higher than local fresh water supplies that use 0.2 kWh/m³ or less.

A minimum energy consumption for seawater desalination of around 1 kWh/m³ has been determined, excluding prefiltering and intake/outfall pumping. Under 2 kWh/m³ has been achieved with reverse osmosis membrane technology, leaving limited scope for further energy reductions.

Supplying all US domestic water by desalination would increase domestic energy consumption by around 10%, about the amount of energy used by domestic refrigerators. Domestic consumption is a relatively small fraction of the total water usage.

Energy consumption of seawater desalination methods.

Desalination Method >>	Multi-stage Flash MSF	Multi-Effect Distillation MED	Mechanical Vapor Compression MVC	Reverse Osmosis RO
Electrical energy (kWh/m3)	4–6	1.5–2.5	7–12	3–5.5
Thermal energy (kWh/m3)	50–110	60–110	None	None
Electrical equivalent of thermal energy (kWh/m3)	9.5–19.5	5–8.5	None	None
Total equivalent electrical energy (kWh/m3)	13.5–25.5	6.5–11	7–12	3–5.5

Note: "Electrical equivalent" refers to the amount of electrical energy that could be generated using a given quantity of thermal energy and appropriate turbine generator. These calculations do not include the energy required to construct or refurbish items consumed in the process.

Cogeneration

Cogeneration is generating excess heat and electricity generation from a single process. Cogeneration can provide usable heat for desalination in an integrated, or "dual-purpose", facility where a power plant provides the energy for desalination. Alternatively, the facility's energy production may be dedicated to the production of potable water (a stand-alone facility), or excess energy may be produced and incorporated into the energy grid. Cogeneration takes various forms, and theoretically any form of energy production could be used. However, the majority of current and planned cogeneration desalination plants use either fossil fuels or nuclear power as their source of energy. Most plants are located in the Middle East or North Africa, which use their petroleum resources to offset limited water resources. The advantage of dual-purpose facilities is they can be more efficient in energy consumption, thus making desalination more viable.

The Shevchenko BN350, a nuclear-heated desalination unit

The current trend in dual-purpose facilities is hybrid configurations, in which the permeate from reverse osmosis desalination is mixed with distillate from thermal desalination. Basically, two or more desalination processes are combined along with power production. Such facilities have been implemented in Saudi Arabia at Jeddah and Yanbu.

A typical Supercarrier in the US military uses nuclear power to desalinate 1,500,000 L of water per day.

Economics

Costs of desalinating sea water (infrastructure, energy, and maintenance) are generally higher than fresh water from rivers or groundwater, water recycling, and water conservation, but alternatives are not always available. Desalination costs in 2013 ranged from US$0.45 to $1.00/cubic metre. More than half of the cost comes directly from energy cost, and since energy prices are very volatile, actual costs can vary substantially.

The cost of untreated fresh water in the developing world can reach US$5/cubic metre.

Average water consumption and cost of supply by sea water desalination at US$1 per cubic metre(±50%)

Area	Consumption Litre/person/day	Desalinated Water Cost US$/person/day
USA	378	0.38
Europe	189	0.19
Africa	57	0.06
UN recommended minimum	49	0.05

Factors that determine the costs for desalination include capacity and type of facility, location, feed water, labor, energy, financing and concentrate disposal. Desalination stills control pressure, temperature and brine concentrations to optimize efficiency. Nuclear-powered desalination might be economical on a large scale.

While noting costs are falling, and generally positive about the technology for affluent areas in proximity to oceans, a 2004 study argued, "Desalinated water may be a solution for some water-stress regions, but not for places that are poor, deep in the interior of a continent, or at high elevation. Unfortunately, that includes some of the places with biggest water problems.", and, "Indeed, one needs to lift the water by 2000 m, or transport it over more than 1600 km to get transport costs equal to the desalination costs. Thus, it may be more economical to transport fresh water from somewhere else than to desalinate it. In places far from the sea, like New Delhi, or in high places, like Mexico City, transport costs could match desalination costs. Desalinated water is also expensive in places that are both somewhat far from the sea and somewhat high, such as Riyadh and Harare. By contrast in other locations transport costs are much less, such as Beijing, Bangkok, Zaragoza, Phoenix, and, of course, coastal cities like Tripoli." After desalination at Jubail, Saudi Arabia, water is pumped 320 km inland to Riyadh. For coastal cities, desalination is increasingly viewed as a competitive choice.
In 2014, the Israeli facilities of Hadera, Palmahim, Ashkelon, and Sorek were desalinizing water for less than US$0.40 per cubic meter. As of 2006, Singapore was desalinating water for US$0.49 per cubic meter. The city of Perth began operating a reverse osmosis seawater desalination plant in 2006. A desalination plant now operates in Sydney, and the Wonthaggi desalination plant was under construction in Wonthaggi, Victoria.

The Perth desalination plant is powered partially by renewable energy from the Emu Downs Wind Farm. A wind farm at Bungendore in New South Wales was purpose-built to generate enough renewable energy to offset the Sydney plant's energy use, mitigating concerns about harmful greenhouse gas emissions.

In December 2007, the South Australian government announced it would build a seawater desalination plant for the city of Adelaide, Australia, located at Port Stanvac. The desalination plant was to be funded by raising water rates to achieve full cost recovery.

A January 17, 2008, article in the Wall Street Journal stated, "In November, Connecticut-based Poseidon Resources Corp. won a key regulatory approval to build the $300 million water-desalination plant in Carlsbad, north of San Diego. The facility would produce 190,000 cubic metres of drinking water per day, enough to supply about 100,000 homes. As of June 2012, the cost for the desalinated water had risen to $2,329 per acre-foot. Each $1,000 per acre-foot works out to $3.06 for 1,000 gallons, or $.81 per cubic meter.

Poseidon Resources made an unsuccessful attempt to construct a desalination plant in Tampa Bay, FL, in 2001. The board of directors of Tampa Bay Water was forced to buy the plant from Poseidon in 2001 to prevent a third failure of the project. Tampa Bay Water faced five years of engineering problems and operation at 20% capacity to protect marine life. The facility reached capacity only in 2007.

In 2008, a Energy Recovery Inc. was desalinating water for $0.46 per cubic meter.

Environmental

Factors that determine the costs for desalination include capacity and type of facility, location, feed water, labor, energy, financing and concentrate disposal.

Intake

In the United States, cooling water intake structures are regulated by the Environmental Protection Agency (EPA). These structures can have the same impacts to the environment as desalination facility intakes. According to EPA, water intake structures cause adverse environmental impact by sucking fish and shellfish or their eggs into an industrial system. There, the organisms may be killed or injured by heat, physical stress, or chemicals. Larger organisms may be killed or injured when they become trapped against screens at the front of an intake structure. Alternative intake types that mitigate these impacts include beach wells, but they require more energy and higher costs.

The Kwinana Desalination Plant opened in Perth in 2007. Water there and at Queensland's Gold Coast Desalination Plant and Sydney's Kurnell Desalination Plant is withdrawn at 0.1 m/s (0.33 ft/s), which is slow enough to let fish escape. The plant provides nearly 140,000 m³ (4,900,000 cu ft) of clean water per day.

Outflow

Desalination processes produce large quantities of brine, possibly at above ambient temperature, and contain residues of pretreatment and cleaning chemicals, their reaction byproducts and heavy metals due to corrosion. Chemical pretreatment and cleaning are a necessity in most desalination plants, which typically includes prevention of biofouling, scaling, foaming and corrosion in thermal plants, and of biofouling, suspended solids and scale deposits in membrane plants.

To limit the environmental impact of returning the brine to the ocean, it can be diluted with another stream of water entering the ocean, such as the outfall of a wastewater treatment or power plant. With medium to large power plant and desalination plants, the power plant's cooling water flow is likely to be several times larger than that of the desalination plant, reducing the salinity of the combination. Another method to dilute the brine is to mix it via a diffuser in a mixing zone. For example, once a pipeline containing the brine reaches the sea floor, it can split into many branches, each releasing brine gradually through small holes along its length. Mixing can be combined with power plant or wastewater plant dilution.

Brine is denser than seawater and therefore sinks to the ocean bottom and can damage the ecosystem. Careful reintroduction can minimize this problem. Typical ocean conditions allow for rapid dilution, thereby minimizing harm.

Alternatives without pollution

Some methods of desalination, particularly in combination with evaporation ponds, solar stills, and condensation trap (solar desalination), do not discharge brine. They do not use chemicals or burn fossil fuels. They do not work with membranes or other critical parts, such as components that include heavy metals, thus do not produce toxic waste (and high maintenance).

A new approach that works like a solar still, but on the scale of industrial evaporation ponds is the integrated biotectural system. It can be considered "full desalination" because it converts the entire amount of saltwater intake into distilled water. One of the advantages of this system is the feasibility for inland operation. Standard advantages also include no air pollution and no temperature increase of endangered natural water bodies from power plant cooling-water discharge. Another important advantage is the production of sea salt for industrial and other uses. As of 2015, 50% of the world's sea salt production relies on fossil energy sources.

Alternatives to desalination

Increased water conservation and efficiency remain the most cost-effective approaches in areas with a large potential to improve the efficiency of water use practices. Wastewater reclamation provides multiple benefits over desalination. Urban runoff and storm water capture also provide benefits in treating, restoring and recharging groundwater.

A proposed alternative to desalination in the American Southwest is the commercial importation of bulk water from water-rich areas either by oil tankers converted to water carriers, or pipelines. The idea is politically unpopular in Canada, where governments imposed trade barriers to bulk water exports as a result of a North American Free Trade Agreement (NAFTA) claim.

Public health concerns

Desalination removes iodine from water and could increase the risk of iodine deficiency disorders. Israeli researchers claimed a possible link between seawater desalination and iodine deficiency, finding deficits among euthyroid adults exposed to iodine-poor water concurrently with an increasing proportion of their area's drinking water from seawater reverse osmosis (SWRO). They later found probable iodine deficiency disorders in a population reliant on desalinated seawater. A possible link of heavy desalinated water use and national iodine deficiency was suggested by Israeli researchers. They found a high burden of iodine deficiency in the general population of Israel: 62% of school-age children and 85% of pregnant women fall below the WHO’s adequacy range. They also pointed out the national reliance on iodine-depleted desalinated water, the absence of a universal salt iodization program and reports of increased use of thyroid medication in Israel as a possible reasons that the population’s iodine intake is low. In the year that the survey was conducted, the amount of water produced from the desalination plants constitutes about 50% of the quantity of fresh water supplied for all needs and about 80% of the water supplied for domestic and industrial needs in Israel.

Other issues

Due to the nature of the process, there is a need to place the plants on approximately 25 acres of land on or near the shoreline. In the case a plant is built inland, pipes have to be laid into the ground to allow for easy intake and outtake. However, once the pipes are laid into the ground, they have a possibility of leaking into and contaminating nearby aquifers. Aside from environmental risks, the noise generated by certain types of desalination plants can be loud.

Experimental techniques

Other desalination techniques include:

Waste heat

Thermally-driven desalination technologies are frequently suggested for use with low-temperature waste heat sources, as the low temperatures are not useful for many industrial processes, but ideal for the lower temperatures found in desalaination. In fact, such pairing with waste heat can even improve electrical process: Diesel generators commonly provide electricity in remote areas. About 40%–50% of the energy output is low-grade heat that leaves the engine via the exhaust. Connecting a thermal desalination technology such as membrane distillation system to the diesel engine exhaust repurposes this low-grade heat for desalination. The system actively cools the diesel generator, improving its efficiency and increasing its electricity output. This results in an energy-neutral desalination solution. An example plant was commissioned by Dutch company Aquaver in March 2014 for Gulhi, Maldives.

Low-temperature thermal

Originally stemming from ocean thermal energy conversion research, low-temperature thermal desalination (LTTD) takes advantage of water boiling at low pressure, even at ambient temperature. The system uses pumps to create a low-pressure, low-temperature environment in which water boils at a temperature gradient of 8–10 °C (46–50 °F) between two volumes of water. Cool ocean water is supplied from depths of up to 600 m (2,000 ft). This water is pumped through coils to condense the water vapor. The resulting condensate is purified water. LTTD may take advantage of the temperature gradient available at power plants, where large quantities of warm wastewater are discharged from the plant, reducing the energy input needed to create a temperature gradient.

Experiments were conducted in the US and Japan to test the approach. In Japan, a spray-flash evaporation system was tested by Saga University. In Hawaii, the National Energy Laboratory tested an open-cycle OTEC plant with fresh water and power production using a temperature difference of 20 C° between surface water and water at a depth of around 500 m (1,600 ft). LTTD was studied by India's National Institute of Ocean Technology (NIOT) in 2004. Their first LTTD plant opened in 2005 at Kavaratti in the Lakshadweep islands. The plant's capacity is 100,000 L (22,000 imp gal; 26,000 US gal)/day, at a capital cost of INR 50 million (€922,000). The plant uses deep water at a temperature of 10 to 12 °C (50 to 54 °F). In 2007, NIOT opened an experimental, floating LTTD plant off the coast of Chennai, with a capacity of 1,000,000 L (220,000 imp gal; 260,000 US gal)/day. A smaller plant was established in 2009 at the North Chennai Thermal Power Station to prove the LTTD application where power plant cooling water is available.

Thermoionic process

In October 2009, Saltworks Technologies announced a process that uses solar or other thermal heat to drive an ionic current that removes all sodium and chlorine ions from the water using ion-exchange membranes.

Evaporation and condensation for crops

The Seawater greenhouse uses natural evaporation and condensation processes inside a greenhouse powered by solar energy to grow crops in arid coastal land.

Other approaches

Adsorption-based desalination (AD) relies on the moisture absorption properties of certain materials such as Silica Gel.

Forward osmosis

One process was commercialized by Modern Water PLC using forward osmosis, with a number of plants reported to be in operation.

Hydrogel based desalination

Scheme of the desalination machine: the desalination box of volume V__box contains a gel of volume V__gel which is separated by a sieve from the outer solution volume V__out =V__box- V__gel. The box is connected to two big tanks with high and low salinity by two taps which can be opened and closed as desired. The chain of buckets expresses the fresh water consumption followed by refilling the low-salinity reservoir by salt water.

The idea of the method is in the fact that when the hydrogel is put into contact with aqueous salt solution, it swells absorbing a solution with the ion composition different from the original one. This solution can be easily squeezed out from the gel by means of sieve or microfiltration membrane. The compression of the gel in closed system lead to change in salt concentration, whereas the compression in open system, while the gel is exchanging ions with bulk, lead to the change in the number of ions. The consequence of the compression and swelling in open and closed system conditions mimics the reverse Carnot Cycle of refrigerator machine. The only difference is that instead of heat this cycle transfers salt ions from the bulk of low salinity to a bulk of high salinity. Similarly to the Carnot cycle this cycle is fully reversible, so can in principle work with an ideal thermodynamic efficiency. Because the method is free from the use of osmotic membranes it can compete with reverse osmosis method. In addition, unlike the reverse osmosis, the approach is not sensitive to the quality of feed water and its seasonal changes, and allows the production of water of any desired concentration.

Small-scale solar

The United States, France and the United Arab Emirates are working to develop practical solar desalination. AquaDania's WaterStillar has been installed at Dahab, Egypt, and in Playa del Carmen, Mexico. In this approach, a solar thermal collector measuring two square metres can distill from 40 to 60 litres per day from any local water source – five times more than conventional stills. It eliminates the need for plastic PET bottles or energy-consuming water transport. In Central California, a startup company WaterFX is developing a solar-powered method of desalination that can enable the use of local water, including runoff water that can be treated and used again. Salty groundwater in the region would be treated to become freshwater, and in areas near the ocean, seawater could be treated.

Passarell

The Passarell process uses reduced atmospheric pressure rather than heat to drive evaporative desalination. The pure water vapor generated by distillation is then compressed and condensed using an advanced compressor. The compression process improves distillation efficiency by creating the reduced pressure in the evaporation chamber. The compressor centrifuges the pure water vapor after it is drawn through a demister (removing residual impurities) causing it to compress against tubes in the collection chamber. The compression of the vapor increases its temperature. The heat is transferred to the input water falling in the tubes, vaporizing the water in the tubes. Water vapor condenses on the outside of the tubes as product water. By combining several physical processes, Passarell enables most of the system's energy to be recycled through its evaporation, demisting, vapor compression, condensation, and water movement processes.

Geothermal

Geothermal energy can drive desalination. In most locations, geothermal desalination beats using scarce groundwater or surface water, environmentally and economically.

Nanotechnology

Nanotube membranes of higher permeability than current generation of membranes may lead to eventual reduction in the footprint of RO desalination plants. It has also been suggested that the use of such membranes will lead to reduction in the energy needed for desalination.

Hermetic, sulphonated nano-composite membranes have shown to be capable of removing a various contaminants to the parts per billion level. s, have little or no susceptibility to high salt concentration levels.

Electrochemical

In 2008, Siemens Water Technologies announced technology that applied electric fields to desalinate one cubic meter of water while using only a purported 1.5 kWh of energy. If accurate, this process would consume one-half the energy of other processes. As of 2012 a demonstration plant was operating in Singapore. Researchers at the University of Texas at Austin and the University of Marburg are developing more efficient methods of electrochemically mediated seawater desalination.

Electrokinetic shocks

A process employing electrokinetic shocks waves can be used to accomplish membraneless desalination at ambient temperature and pressure. In this process, anions and cations in salt water are exchanged for carbonate anions and calcium cations, respectively using electrokinetic shockwaves. Calcium and carbonate ions react to form calcium carbonate, which precipitates, leaving fresh water. The theoretical energy efficiency of this method is on par with electrodialysis and reverse osmosis.

Facilities

Estimates vary widely between 15,000–20,000 desalination plants producing more than 20,000 m³/day. Micro desalination plants operate near almost every natural gas or fracking facility found in the United States.

In nature

Mangrove leaf with salt crystals

Evaporation of water over the oceans in the water cycle is a natural desalination process.
The formation of sea ice produces ice with little salt, much lower than in seawater.

Seabirds distill seawater using countercurrent exchange in a gland with a rete mirabile. The gland secretes highly concentrated brine stored near the nostrils above the beak. The bird then "sneezes" the brine out. As freshwater is not usually available in their environments, some seabirds, such as pelicans, petrels, albatrosses, gulls and terns, possess this gland, which allows them to drink the salty water from their environments while they are far from land.

Mangrove trees grow in seawater; they secrete salt by trapping it in parts of the root, which are then eaten by animals (usually crabs). Additional salt is removed by storing it in leaves that fall off. Some types of mangroves have glands on their leaves, which work in a similar way to the seabird desalination gland. Salt is extracted to the leaf exterior as small crystals, which then fall off the leaf.
Willow trees and reeds absorb salt and other contaminants, effectively desalinating the water. This is used in artificial constructed wetlands, for treating sewage.

History

Desalination has been known to history for millennia as both a concept, and later practice, though in a limited form. The ancient Greek philosopher Aristotle observed in his work Meteorology that “salt water, when it turns into vapour, becomes sweet and the vapour does not form salt water again when it condenses,” and also noticed that a fine wax vessel would hold potable water after being submerged long enough in seawater, having acted as a membrane to filter the salt. There are numerous other examples of experimentation in desalination throughout Antiquity and the Middle Ages,^[104] but desalination was never feasible on a large scale until the modern era.

Before the Industrial Revolution, desalination was primarily of concern to oceangoing ships, which otherwise needed to keep on board supplies of fresh water. When Protector (1779 frigate) was sold to Denmark in the 1780s (as the ship Hussaren) the desalination plant was studied and recorded in great detail.

In the newly formed United States, Thomas Jefferson catalogued heat-based methods going back to the 1500s, and formulated practical advice that was publicized to all U.S. ships on the backs of sailing clearance permits.

Significant research into improved desalination methods occurred in the United States after World War II. The Office of Saline Water was created in the United States Department of the Interior by the Saline Water Conversion Act of 1952. It was merged into the Office of Water Resources Research in 1974.

Research also took place at state universities in California, followed by development at the Dow Chemical Company and DuPont. Many studies focus on ways to optimize desalination systems.

Dryland farming

From Wikipedia, the free encyclopedia

 

Dryland farming in the Granada region in Spain

Dryland farming and dry farming encompass specific agricultural techniques for the non-irrigated cultivation of crops. Dryland farming is associated with drylands, areas characterized by a cool wet season that is followed by a warm dry season. They are also associated with arid conditions or areas prone to drought or having scarce water resources. Additionally, arid-zone agriculture is being developed for this purpose.

Dryland farming

Locations

Fields in the Palouse, Washington State

Dryland farming is used in the Great Plains, the Palouse plateau of Eastern Washington, and other arid regions of North America such as in the Southwestern United States and Mexico, the Middle East and in other grain growing regions such as the steppes of Eurasia and Argentina. Dryland farming was introduced to southern Russia and Ukraine by Ukrainian Mennonites under the influence of Johann Cornies, making the region the breadbasket of Europe. In Australia, it is widely practiced in all states but the Northern Territory.

Crops

Dryland farmed crops may include winter wheat, corn, beans, sunflowers or even watermelon. Successful dryland farming is possible with as little as 230 millimetres (9 in) of precipitation a year; higher rainfall increases the variety of crops. Native American tribes in the arid Southwest survived for hundreds of years on dryland farming in areas with less than 250 millimetres (10 in) of rain. The choice of crop is influenced by the timing of the predominant rainfall in relation to the seasons. For example, winter wheat is more suited to regions with higher winter rainfall while areas with summer wet seasons may be more suited to summer growing crops such as sorghum, sunflowers or cotton.

Process

Dryland farming has evolved as a set of techniques and management practices used by farmers to continually adapt to the presence or lack of moisture in a given crop cycle. In marginal regions, a farmer should be financially able to survive occasional crop failures, perhaps for several years in succession. Survival as a dryland farmer requires careful husbandry of the moisture available for the crop and aggressive management of expenses to minimize losses in poor years. Dryland farming involves the constant assessing of the amount of moisture present or lacking for any given crop cycle and planning accordingly. Dryland farmers know that to be financially successful they have to be aggressive during the good years in order to offset the dry years.

Dryland farming caused a large dust storm in parts of Eastern Washington on October 4, 2009. Courtesy: NASA/GSFC, MODIS Rapid Response

 

Dryland farming is dependent on natural rainfall, which can leave the ground vulnerable to dust storms, particularly if poor farming techniques are used or if the storms strike at a particularly vulnerable time. The fact that a fallow period must be included in the crop rotation means that fields cannot always be protected by a cover crop, which might otherwise offer protection against erosion.

Some of the theories of dryland farming developed in the late 19th and early 20th centuries claimed to be scientific but were in reality pseudoscientific and did not stand up to empirical testing. For example, it was alleged that tillage would seal in moisture, but such "dust mulching" ideas are based on what people imagine should happen, or have been told, rather than what testing actually confirms. The book Bad Land: An American Romance explores the effects that this had on people who were encouraged to homestead in an area with little rainfall; most smallholdings failed after working miserably to cling on.

Key elements of dryland farming

Capturing and conservation of moisture – In regions such as Eastern Washington, the average annual precipitation available to a dryland farm may be as little as 220 millimetres (8.5 in). Consequently, moisture must be captured until the crop can utilize it. Techniques include summer fallow rotation (in which one crop is grown on two seasons' precipitation, leaving standing stubble and crop residue to trap snow), and preventing runoff by terracing fields.

"Terracing" is also practiced by farmers on a smaller scale by laying out the direction of furrows to slow water runoff downhill, usually by plowing along either contours or keylines. Moisture can be conserved by eliminating weeds and leaving crop residue to shade the soil.

Effective use of available moisture – Once moisture is available for the crop to use, it must be used as effectively as possible. Seed planting depth and timing are carefully considered to place the seed at a depth at which sufficient moisture exists, or where it will exist when seasonal precipitation falls. Farmers tend to use crop varieties which are drought and heat-stress tolerant (even lower-yielding varieties). Thus the likelihood of a successful crop is hedged if seasonal precipitation fails.

Soil conservation – The nature of dryland farming makes it particularly susceptible to erosion, especially wind erosion. Some techniques for conserving soil moisture (such as frequent tillage to kill weeds) are at odds with techniques for conserving topsoil. Since healthy topsoil is critical to sustainable dryland agriculture, its preservation is generally considered the most important long-term goal of a dryland farming operation. Erosion control techniques such as windbreaks, reduced tillage or no-till, spreading straw (or other mulch on particularly susceptible ground), and strip farming are used to minimize topsoil loss.

Control of input costs – Dryland farming is practiced in regions inherently marginal for non-irrigated agriculture. Because of this, there is an increased risk of crop failure and poor yields which may occur in a dry year (regardless of money or effort expended). Dryland farmers must evaluate the potential yield of a crop constantly throughout the growing season and be prepared to decrease inputs to the crop such as fertilizer and weed control if it appears that it is likely to have a poor yield due to insufficient moisture. Conversely, in years when moisture is abundant, farmers may increase their input efforts and budget to maximize yields and to offset poor harvests.

Dry farming

Locations

Dry farming may be practiced in areas that have significant annual rainfall during a wet season, often in the winter. Crops are cultivated during the subsequent dry season, using practices that make use of the stored moisture in the soil. California, Colorado and Oregon, in the United States, are three states where dry farming is practiced for a variety of crops.

Crops

Dry farmed crops may include grapes, tomatoes, pumpkins, beans, and other summer crops. These crops grow using the winter water stored in the soil, rather than depending on rainfall during the growing season.

Process

Dry farming depends on making the best use of the "bank" of soil moisture that was created by winter rainfall. Some dry farming practices include:

• Wider than normal spacing, to provide a larger bank of moisture for each plant.
• Controlled Traffic.
• Minimal tilling of land.
• Strict weed control, to ensure that weeds do not consume soil moisture needed by the cultivated plants.
• Cultivation of soil to produce a "dust mulch", thought to prevent the loss of water through capillary action. This practice is controversial, and is not universally advocated.
• Selection of crops and cultivars suited for dry farming practices.

Arid-zone agriculture

An example of a dryland farming paddock

As an area of research and development, arid-zone agriculture, or desert agriculture, includes studies of how to increase the agricultural productivity of lands dominated by lack of freshwater, an abundance of heat and sunlight, and usually one or more of extreme winter cold, short rainy season, saline soil or water, strong dry winds, poor soil structure, over-grazing, limited technological development, poverty, political instability.
The two basic approaches are:

• view the given environmental and socioeconomic characteristics as negative obstacles to be overcome.
• view as many as possible of them as positive resources to be used.

Sustainable agriculture

From Wikipedia, the free encyclopedia

Sustainable agriculture is farming in sustainable ways based on an understanding of ecosystem services, the study of relationships between organisms and their environment.

History of the term

The phrase 'sustainable agriculture' was reportedly coined by the Australian agricultural scientist Gordon McClymont. Wes Jackson is credited with the first publication of the expression in his 1980 book New Roots for Agriculture. The term became popular in the late 1980s.

It has been defined as "an integrated system of plant and animal production practices having a site-specific application that will last over the long term", for example to satisfy human food and fiber needs, to enhance environmental quality and the natural resource base upon which the agricultural economy depends, to make the most efficient use of non-renewable and on-farm resources and integrate natural biological cycles and controls, to sustain the economic viability of farm operations, and to enhance the quality of life for farmers and society as a whole.

Key principles

There are several key principles associated with sustainability in agriculture:

1. The incorporation of biological and ecological processes into agricultural and food production practices. For example, these processes could include nutrient cycling, soil regeneration, and nitrogen fixation.
2. Using decreased amounts of non-renewable and unsustainable inputs, particularly the ones that are environmentally harmful.
3. Using the expertise of farmers to both productively work the land as well as to promote the self-reliance and self-sufficiency of farmers.
4. Solving agricultural and natural resource problems through the cooperation and collaboration of people with different skills. The problems tackled include pest management and irrigation.

Farming and natural resources

Traditional farming methods had a low carbon footprint.

Sustainable agriculture can be understood as an ecosystem approach to agriculture. Practices that can cause long-term damage to soil include excessive tilling of the soil (leading to erosion) and irrigation without adequate drainage (leading to salinization). Long-term experiments have provided some of the best data on how various practices affect soil properties essential to sustainability. In the United States a federal agency, USDA-Natural Resources Conservation Service, specializes in providing technical and financial assistance for those interested in pursuing natural resource conservation and production agriculture as compatible goals.

Conservation farming in Zambia

The most important factors for an individual site are sun, air, soil, nutrients, and water. Of the five, water and soil quality and quantity are most amenable to human intervention through time and labor.

Although air and sunlight are available everywhere on Earth, crops also depend on soil nutrients and the availability of water. When farmers grow and harvest crops, they remove some of these nutrients from the soil. Without replenishment, land suffers from nutrient depletion and becomes either unusable or suffers from reduced yields. Sustainable agriculture depends on replenishing the soil while minimizing the use or need of non-renewable resources, such as natural gas (used in converting atmospheric nitrogen into synthetic fertilizer), or mineral ores (e.g., phosphate). Possible sources of nitrogen that would, in principle, be available indefinitely, include:

1. recycling crop waste and livestock or treated human manure
2. growing legume crops and forages such as peanuts or alfalfa that form symbioses with nitrogen-fixing bacteria called rhizobia
3. industrial production of nitrogen by the Haber process uses hydrogen, which is currently derived from natural gas (but this hydrogen could instead be made by electrolysis of water using electricity (perhaps from solar cells or windmills)) or
4. genetically engineering (non-legume) crops to form nitrogen-fixing symbioses or fix nitrogen without microbial symbionts.

The last option was proposed in the 1970s, but is only gradually becoming feasible. Sustainable options for replacing other nutrient inputs such as phosphorus and potassium are more limited.
More realistic, and often overlooked, options include long-term crop rotations, returning to natural cycles that annually flood cultivated lands (returning lost nutrients indefinitely) such as the flooding of the Nile, the long-term use of biochar, and use of crop and livestock landraces that are adapted to less than ideal conditions such as pests, drought, or lack of nutrients. Crops that require high levels of soil nutrients can be cultivated in a more sustainable manner with appropriate fertilizer management practices.

Water

In some areas sufficient rainfall is available for crop growth, but many other areas require irrigation. For irrigation systems to be sustainable, they require proper management (to avoid salinization) and must not use more water from their source than is naturally replenishable. Otherwise, the water source effectively becomes a non-renewable resource. Improvements in water well drilling technology and submersible pumps, combined with the development of drip irrigation and low-pressure pivots, have made it possible to regularly achieve high crop yields in areas where reliance on rainfall alone had previously made successful agriculture unpredictable. However, this progress has come at a price. In many areas, such as the Ogallala Aquifer, the water is being used faster than it can be replenished.

Several steps must be taken to develop drought-resistant farming systems even in "normal" years with average rainfall. These measures include both policy and management actions:

1. improving water conservation and storage measures,
2. providing incentives for selection of drought-tolerant crop species,
3. using reduced-volume irrigation systems,
4. managing crops to reduce water loss, and
5. not planting crops at all.

Indicators for sustainable water resource development are:

• Internal renewable water resources. This is the average annual flow of rivers and groundwater generated from endogenous precipitation, after ensuring that there is no double counting. It represents the maximum amount of water resource produced within the boundaries of a country. This value, which is expressed as an average on a yearly basis, is invariant in time (except in the case of proved climate change). The indicator can be expressed in three different units: in absolute terms (km³/yr), in mm/yr (it is a measure of the humidity of the country), and as a function of population (m³/person per year).
• Global renewable water resources. This is the sum of internal renewable water resources and incoming flow originating outside the country. Unlike internal resources, this value can vary with time if upstream development reduces water availability at the border. Treaties ensuring a specific flow to be reserved from upstream to downstream countries may be taken into account in the computation of global water resources in both countries.
• Dependency ratio. This is the proportion of the global renewable water resources originating outside the country, expressed in percentage. It is an expression of the level to which the water resources of a country depend on neighbouring countries.
• Water withdrawal. In view of the limitations described above, only gross water withdrawal can be computed systematically on a country basis as a measure of water use. Absolute or per-person value of yearly water withdrawal gives a measure of the importance of water in the country's economy. When expressed in percentage of water resources, it shows the degree of pressure on water resources. A rough estimate shows that if water withdrawal exceeds a quarter of global renewable water resources of a country, water can be considered a limiting factor to development and, reciprocally, the pressure on water resources can affect all sectors, from agriculture to environment and fisheries.

Soil

Walls built to avoid water run-off

Soil erosion is fast becoming one of the world's severe problems. It is estimated that "more than a thousand million tonnes of southern Africa's soil are eroded every year. Experts predict that crop yields will be halved within thirty to fifty years if erosion continues at present rates." Soil erosion is occurring worldwide. The phenomenon is being called peak soil as present large-scale factory farming techniques are jeopardizing humanity's ability to grow food in the present and in the future. Without efforts to improve soil management practices, the availability of arable soil will become increasingly problematic. Intensive agriculture reduces the carbon level in soil, impairing soil structure, crop growth and ecosystem functioning, and accelerating climate change. Soil management techniques include no-till farming, keyline design, windbreaks to reduce wind erosion, incorporating carbon-containing organic matter back into fields, reducing chemical fertilizers, and protecting soil from water run-off.

Phosphate

Phosphate is a primary component in chemical fertilizer. It is the second most important nutrient for plant after nitrogen, and is often a limiting factor. It is important for sustainable agriculture as it can improve soil fertility and crop yields. Phosphorus is involved in all major metabolic processes including photosynthesis, energy transfer, signal transduction, macromolecular biosynthesis, and respiration. It is needed for root ramification and strength and seed formation, and can increase disease resistance.

Phosphorus is found in the soil in both inorganic and organic forms and makes up approximately 0.05% of soil biomass. However, only 0.1% of that phosphorus present can be absorbed by plants. This is due to poor solubility and phosphorus' high reactivity with elements in the soil such as aluminum, calcium, and iron, causing the phosphorus to be fixed. Long-term use of phosphate-containing chemical fertilizers cause eutrophication and deplete soil fertility, so people have looked to other sources.

An alternative is rock phosphate, a natural source already in some soils. In India, there are almost 260 million tons of rock phosphate. However, rock phosphate is a non-renewable resource and it is being depleted by mining for agricultural use: reserves are expected to be exhausted in 50–100 years; peak phosphorus will occur in about 2030. This is expected to increase food prices as phosphate fertilizer costs increase.

A way to make rock phosphate more effective and last longer is to implement microbial inoculants such as phosphate-solubilizing microorganisms, known as PSMs. A source of these PSMs is compost or the recycling of human and animal waste. Specific PSMs can be added to soil. These solubilize phosphorus already in the soil and use processes like organic acid production and ion exchange reactions to make that phosphorus available for plants. When these PSMs are present, there has been an increase in crop growth, particularly in terms of shoot height, dry biomass, and grain yield.

Phosphorus uptake is even more efficient with the presence of mycorrhizae in the soil. Mycorrhiza is a type of mutualistic symbiotic association between plants and fungi, which are well-equipped to absorb nutrients, including phosphorus, in soil. These fungi can increase nutrient uptake in soil where phosphorus has been fixed by aluminum, calcium, and iron. Mycorrhizae can also release organic acids that solubilize otherwise unavailable phosphorus.

Land

As the global population increases and demand for food increases, there is pressure on land resources. In land use planning and management, considering the impacts of land use changes on factors such as soil erosion can support long-term agricultural sustainability, as shown by a study of Wadi Ziqlab, a dry area in the Middle East where farmers graze livestock and grow olives, vegetables, and grains.

Looking back over the 20th century shows that for people in poverty, following environmentally sound land practices has not always been a viable option due to many complex and challenging life circumstances. Currently, increased land degradation in developing countries may be connected with rural poverty among smallholder farmers when forced into unsustainable agricultural practices out of necessity.

Land is a finite resource on Earth. And although expansion of agricultural land can decrease biodiversity and contribute to deforestation, the picture is complex; for instance, a study examining the introduction of sheep by Norse settlers (Vikings) to the Faroe Islands of the North Atlantic concluded that, over time, the fine partitioning of land plots contributed more to soil erosion and degradation than grazing itself.

The Food and Agriculture Organisation of the United Nations estimates that in coming decades, cropland will continue to be lost to industrial and urban development, along with reclamation of wetlands, and conversion of forest to cultivation, resulting in the loss of biodiversity and increased soil erosion. Many tools will be called upon to offset these projections. In Europe, one such tool is a geo-spatial data system called SoilConsWeb which is being developed to inform soil conservation minded decision making within agricultural sectors and other areas of land management.

Energy

Energy is used all the way down the food chain from farm to fork. In industrial agriculture, energy is used in on-farm mechanisation, food processing, storage, and transportation processes. It has therefore been found that energy prices are closely linked to food prices. Oil is also used as an input in agricultural chemicals. The International Energy Agency projects higher prices of non-renewable energy resources as a result of fossil fuel resources being depleted. It may therefore decrease global food security unless action is taken to 'decouple' fossil fuel energy from food production, with a move towards 'energy-smart' agricultural systems including renewable energy. The use of solar powered irrigation in Pakistan has come to be recognized as a leading example of energy use in creating a closed system for water irrigation in agricultural activity.

Economics

Socioeconomic aspects of sustainability are also partly understood. Regarding less concentrated farming, the best known analysis is Netting's study on smallholder systems through history.

Given the finite supply of natural resources at any specific cost and location, agriculture that is inefficient or damaging to needed resources may eventually exhaust the available resources or the ability to afford and acquire them. It may also generate negative externality, such as pollution as well as financial and production costs. There are several studies incorporating these negative externalities in an economic analysis concerning ecosystem services, biodiversity, land degradation and sustainable land management. These include The Economics of Ecosystems and Biodiversity study led by Pavan Sukhdev and the Economics of Land Degradation Initiative which seeks to establish an economic cost benefit analysis on the practice of sustainable land management and sustainable agriculture.

The way that crops are sold must be accounted for in the sustainability equation. Food sold locally does not require additional energy for transportation (including consumers). Food sold at a remote location, whether at a farmers' market or the supermarket, incurs a different set of energy cost for materials, labour, and transport.

Pursuing sustainable agriculture results in many localized benefits. Having the opportunities to sell products directly to consumers, rather than at wholesale or commodity prices, allows farmers to bring in optimal profit.

Triple bottom line frameworks (including social and environmental aspects alongside the financial) show that a sustainable company can be technologically and economically feasible. For this to happen, growth in material consumption and population need to be slowed down and there has to be a drastic increase in the efficiency of material and energy use. To make that transition, long- and short-term goals will need to be balanced enhancing equity and quality of life.

Methods

Polyculture in Andhra Pradesh

What grows where and how it is grown are a matter of choice. Two of the many possible practices of sustainable agriculture are crop rotation and soil amendment, both designed to ensure that crops being cultivated can obtain the necessary nutrients for healthy growth. Soil amendments would include using locally available compost from community recycling centers. These community recycling centers help produce the compost needed by the local organic farms.

Using community recycling from yard and kitchen waste utilizes a local area's commonly available resources. These resources in the past were thrown away into large waste disposal sites, are now used to produce low cost organic compost for organic farming. Other practices includes growing a diverse number of perennial crops in a single field, each of which would grow in separate season so as not to compete with each other for natural resources. This system would result in increased resistance to diseases and decreased effects of erosion and loss of nutrients in soil. Nitrogen fixation from legumes, for example, used in conjunction with plants that rely on nitrate from soil for growth, helps to allow the land to be reused annually. Legumes will grow for a season and replenish the soil with ammonium and nitrate, and the next season other plants can be seeded and grown in the field in preparation for harvest.

Rotational grazing with pasture divided into paddocks

Monoculture, a method of growing only one crop at a time in a given field, is a very widespread practice, but there are questions about its sustainability, especially if the same crop is grown every year. Today it is realized to get around this problem local cities and farms can work together to produce the needed compost for the farmers around them. This combined with growing a mixture of crops (polyculture) sometimes reduces disease or pest problems but polyculture has rarely, if ever, been compared to the more widespread practice of growing different crops in successive years (crop rotation) with the same overall crop diversity. Such methods may also support sustainable weed management in that the development of herbicide-resistant weeds is reduced. Cropping systems that include a variety of crops (polyculture and/or rotation) may also replenish nitrogen (if legumes are included) and may also use resources such as sunlight, water, or nutrients more efficiently.

Replacing a natural ecosystem with a few specifically chosen plant varieties reduces the genetic diversity found in wildlife and makes the organisms susceptible to widespread disease. The Great Irish Famine (1845–1849) is a well-known example of the dangers of monoculture. In practice, there is no single approach to sustainable agriculture, as the precise goals and methods must be adapted to each individual case. There may be some techniques of farming that are inherently in conflict with the concept of sustainability, but there is widespread misunderstanding on effects of some practices. Today the growth of local farmers' markets offer small farms the ability to sell the products that they have grown back to the cities that they got the recycled compost from. This will help move people away from the slash-and-burn or slash-and-char techniques that are the characteristic feature of shifting cultivation. These are often cited as inherently destructive, yet slash-and-burn cultivation has been practiced in the Amazon for at least 6000 years. Serious deforestation did not begin until the 1970s, largely as the result of Brazilian government programs and policies.

There are also many ways to practice sustainable animal husbandry. Some of the key tools to grazing management include fencing off the grazing area into smaller areas called paddocks, lowering stock density, and moving the stock between paddocks frequently.

Sustainable intensification

In light of concerns about food security, human population growth and dwindling land suitable for agriculture, sustainable intensive farming practises are needed to maintain high crop yields, while maintaining soil health and ecosystem services. The capacity for ecosystem services to be strong enough to allow a reduction in use of synthetic, non renewable inputs whilst maintaining or even boosting yields has been the subject of much debate. Recent work in the globally important irrigated rice production system of east Asia has suggested that - in relation to pest management at least - promoting the ecosystem service of biological control using nectar plants can reduce the need for insecticides by 70% whilst delivering a 5% yield advantage compared with standard practice.

Soil treatment

Sheet steaming with a MSD/moeschle steam boiler (left side)

Soil steaming can be used as an ecological alternative to chemicals for soil sterilization. Different methods are available to induce steam into the soil in order to kill pests and increase soil health.
Solarizing is based on the same principle, used to increase the temperature of the soil to kill pathogens and pests.

Certain crops act as natural biofumigants, releasing pest suppressing compounds. Mustard, radishes, and other plants in the brassica family are best known for this effect. There exist varieties of mustard shown to be almost as effective as synthetic fumigants at a similar or lesser cost.

Off-farm impacts

A farm that is able to "produce perpetually", yet has negative effects on environmental quality elsewhere is not sustainable agriculture. An example of a case in which a global view may be warranted is over-application of synthetic fertilizer or animal manures, which can improve productivity of a farm but can pollute nearby rivers and coastal waters (eutrophication). The other extreme can also be undesirable, as the problem of low crop yields due to exhaustion of nutrients in the soil has been related to rainforest destruction, as in the case of slash and burn farming for livestock feed. In Asia, specific land for sustainable farming is about 12.5 acres which includes land for animal fodder, cereals productions lands for some cash crops and even recycling of related food crops. In some cases even a small unit of aquaculture is also included in this number (AARI-1996).
Sustainability affects overall production, which must increase to meet the increasing food and fiber requirements as the world's human population expands to a projected 9.3 billion people by 2050. Increased production may come from creating new farmland, which may ameliorate carbon dioxide emissions if done through reclamation of desert as in Israel and Palestine, or may worsen emissions if done through slash and burn farming, as in Brazil.

Anthropogenic changes

As the Earth is entering the Anthropocene, an epoch characterized by human impacts such as climate change, agriculture and agricultural development are at risk. Agriculture has an enormous environmental footprint, and is simultaneously leading to huge amounts of environmental changes globally and being hugely impacted by these global changes. Additionally, the human population is continuing to grow rapidly at a rate which will require an increase in food production globally. This is complicated by the fact that the Earth is undergoing rising amounts of environmental risks. Sustainable agriculture provides a potential solution to enable agricultural systems to feed a growing population while successfully operating within the changing environmental conditions.

Social

Development

In 2007, the United Nations reported on "Organic Agriculture and Food Security", stating that using organic and sustainable agriculture could be used as a tool to reach global food security without expanding land usage and reducing environmental impacts. Another way to define sustainable agriculture is to give attention to the "human and environmental aspects," because of the turn to a more unsustainable way of farming in U.S. agriculture. During the Great Depression in the United States many farming families were living in subhuman and hungry conditions and treated "sustainability as a resource-input and food-output equation." Though conditions have improved, the farming has not as much done so. There has been evidence provided by developing nations from the early 2000s stating that when people in their communities are not factored into the agricultural process that serious harm is done. Although global food security would most likely not drastically fall, these practices would impact, first hand, local, rural farming communities, making them unable to feed themselves and their families. The social scientist Charles Kellogg has stated that, "In a final effort, exploited people pass their suffering to the land." This turn to more unsustainable farming has seen suffering for many people. For if something is sustainable, it should be that way in all aspects of it, not just the crop yield or soil health. It has been seen in the developing country of Bangladesh, the starving of rural farming communities due to their unsustainable farming methods. Sustainable agriculture mean the ability to permanently and continuously "feed its constituent populations."

There are a lot of opportunities that can increase farmers’ profits, better communities, and continue sustainable practices. For example, in Uganda Genetically Modified Organisms (GMOs) were originally illegal, however, under stressful circumstances where Banana Bacterial Wilt (BBW) has the potential to wipe out 90% of yield they decided to explore GMOs as a possible solution. Therefore, as a result of the banana crisis in Uganda caused by the BBW, the government issued the National Biotechnology and Biosafety bill which will allow scientists that are part of the National Banana Research Program to start experimenting with genetically modified organisms. This effort has the potential to help local communities because a significant portion live off the food they grow themselves and it will keep their economy in check because their main sources of produce will remain stable.

Women

Woman at an American farmers market

In the past 30 years (1978-2007) in the United States the number of women farm operators has tripled. Today, women operate 14 percent of farms, compared to five percent in 1978. Much of the growth is due to women farming outside the "male dominated field of conventional agriculture". In community supported agriculture women represent 40 percent of farm operators, and 21 percent of organic farmers. With the change of laws in land ownership over the past century, women are now allowed all the same freedom of land ownership that men have.

International policy

Sustainable agriculture has become a topic of interest in the international policy arena, especially with regards to its potential to reduce the risks associated with a changing climate and growing human population.

The Commission on Sustainable Agriculture and Climate Change, as part of its recommendations for policy makers on achieving food security in the face of climate change, urged that sustainable agriculture must be integrated into national and international policy. The Commission stressed that increasing weather variability and climate shocks will negatively affect agricultural yields, necessitating early action to drive change in agricultural production systems towards increasing resilience. It also called for dramatically increased investments in sustainable agriculture in the next decade, including in national research and development budgets, land rehabilitation, economic incentives, and infrastructure improvement.

Policy ethics

Most agricultural professionals agree that there is a "moral obligation to pursue [the] goal [of] sustainability." The major debate comes from what system will provide a path to that goal. Because if an unsustainable method is used on a large scale it will have a massive negative effect on the environment and human population. The best way to create policy for agriculture is to be free of any bias. A good review would be done with "practical wisdom," a virtue identified by Aristotle, distinguishing practical wisdom from scientific knowledge, this coming from Nichomachean Ethics. The science of agriculture is called "agronomy", the root of this word relating to scientific law. Although agriculture may not fit well under scientific law, and may not be designed to be treated as an Aristotelian scientific knowledge, but more practical wisdom. Practical wisdom requires recognition of past failures in agriculture to better attain a more sustainable agricultural system.

Urban planning

The use of available city space (e.g., rooftop gardens, community gardens, garden sharing, and other forms of urban agriculture) for cooperative food production may be able to contribute to sustainability. A recent idea (2014) is to create large, urban, technical facilities for Vertical farming. Potential advantages include year-round production, isolation from pests and diseases, controllable resource recycling, and reduced transportation costs.

Increasing threats of climate change have influenced cities and public officials are thinking more proactively about the ways they can deliver services and food more efficiently. The environmental cost of transportation could be avoided if people take back their connection to fresh food. This raises questions; however, about the excess environmental costs associated with local farming vs more large scale operations which offer food security around the world.

Key debates

There are several key debates involving sustainable agriculture:

Ecocentric vs technocentric

The main debate on how sustainable agriculture might be achieved centers around two different approaches: an ecocentric approach and a technocentric approach. The ecocentric approach emphasizes no- or low-growth levels of human development, and focuses on organic and biodynamic farming techniques with the goal of changing consumption patterns, and resource allocation and usage. The technocentric approach argues that sustainability can be attained through a variety of strategies, from the view that state-led modification of the industrial system like conservation-oriented farming systems should be implemented, to the argument that biotechnology is the best way to meet the increasing demand for food.

Multifunctional agriculture vs ecosystem services

There are different scientific communities that are looking at the topic of sustainable agriculture through two separate lenses: multifunctional agriculture (MFA) and ecosystem services (ES). While both of these frameworks are similar, they look at the function of agriculture in different lights. Those that employ the multifunctional agriculture philosophy focus on farm-centered approaches, and define function as being the outputs of agricultural activity. The central argument of MFA is that agriculture has other functions aside from the production of food and fiber, and therefore agriculture is a multifunctional enterprise. These additional functions include renewable natural resource management and conservation of landscape and biodiversity. On the other hand, ES focuses on service-centered approaches, and defines function as the provision of services to human beings.  Specifically, ES posits that individuals and society as a whole receive benefits from ecosystems, which are called ecosystem services. Within the field of sustainable agriculture, the services that ecosystems provide include pollination, soil formation, and nutrient cycling, all of which are necessary functions for the production of food.

Barriers

Since World War II, dominant models of agriculture in the United States and the entire national food system have been characterized by a focus on monetary profitability at the expense of social and environmental integrity.

In sustainable agriculture, changes in lower rates of soil and nutrient loss, improved soil structure, and higher levels of beneficial microorganisms are not quick. The changes are not immediately evident to the operate when using sustainable agriculture. In conventional agriculture the benefits are easily visible with no weeds, pests, etc. and the "process of externalization" hides the costs to soil and ecosystems around it. A major barrier to sustainable agriculture is the lack of knowledge of its benefits. Many benefits are not visible, so they are often unknown.

Criticism

Efforts toward more sustainable agriculture are supported in the sustainability community, however, these are often viewed only as incremental steps and not as an end. Some foresee a true sustainable steady state economy that may be very different from today's: greatly reduced energy usage, minimal ecological footprint, fewer consumer packaged goods, local purchasing with short food supply chains, little processed foods, more home and community gardens, etc.