The United Nations recognizes water as an essential human right, meaning that everyone has the right to have enough water for domestic and personal use. This amount is estimated at between 50 to 100 liters of water per person per day. Additionally, human consumption of water can be invisible; it can take about 3,000 liters to produce the daily food needs of one person.
By understanding types of water such as blue water, green water, gray water, black water and virtual water, one can gain insight into the hydrological cycle and how water is used and reused.
What is blue water?
Blue water is the water that is available in groundwater, rivers, lakes and aquifers. It is the basis for all aquatic ecosystems, including wetlands, and is also available for human use for consumption, irrigation and sanitation. Blue water can be defined as stocks and flows. Stocks are stored in lakes, behind dams, in aquifers and within glaciers or snow. In contrast, flows originate from river runoff and sub-surface recharge of water tables and groundwater. On global and annual scales, approximately 40% of precipitation that falls on land goes to blue water while the remaining 60% becomes green water.
Blue water is threatened by pollution and human overconsumption. An increase in blue water pollution can make it unfit for human use or reduce the ability of ecosystems to generate ecosystems services.
What is green water?
Green water is the water that is held in the soil and used by plants. Its usage is significant for the agricultural and forestry industries as well as supporting ecosystems. From the soil and transpiration from plants, green water eventually makes its way back into water systems, and with water from rivers, lakes and oceans creates precipitation. Its movement, also known as terrestrial moisture flows, connects source areas to sink areas.
Due to its key role in the hydrological system, disruptions to green water sources such as deforestation, increased agricultural area or urban expansion can disrupt rainfall in another area. Without a stable supply of green water, the loss of wetlands and soil moisture contribute to the reduction of the Earth’s carbon stores, exacerbating climate change.
Furthermore, in spite of its importance for ecosystems, green water is often overlooked in water policy, which usually only emphasizes blue water in view of crop water needs through irrigation.
What is gray water?
Gray water is wastewater that comes from household activities including laundry, showering, and cooking, but not toilets. While it might look dirty due to traces of hair, food, grease or oils, gray water is typically safe and beneficial for other uses such as irrigation of crops not eaten raw or flushing toilets. With some precautions, gray water can be safely collected and reused, increasing water availability for common tasks. For example, in water scarce regions such as refugee settlements in Ethiopia, Kenya and Uganda, graywater has been used for irrigation of home gardens. However, excess laundry detergents can affect water quality if gray water is released into natural water bodies.
What is black water?
Black water is water from toilets that includes fecal matter and/or urine. Black water, which includes fecal matter, contains pathogenic microorganisms such as E. coli, and can also include salmonella, shigella, enterovirus and Hepatitis A virus. As such, while black water can eventually be reused, it typically must undergo intensive biological or chemical treatment and disinfection before reuse. Urine is normally sterile but can contain pathogens if a person is sick.
Globally, over 3.5 billion people do not have access to safely managed sanitation services, and at least 2 billion people use water resources that are contaminated with feces. Thus, determining efficient and cost-effective ways to manage black water is essential for global health.
In most low-income countries, black water has been mainly treated on-site (at the household) using septic tanks (for septage) as a first (and often only) step, while in high income countries, sewer systems carry the black water to an off-site sewage treatment plant. Septic tanks are emptied by trucks that transport the excreta in an ideal situation to a septage treatment facility.
As globally only 55% of all wastewater is treated, irrigation with polluted water is a major health concern. Current estimates show that worldwide, the area under irrigation with poorly or untreated (but usually diluted) wastewater covers about 29 million ha while treated wastewater is at best irrigating some 1.5 million ha.
On the other hand, there have been significant advances in membrane treatments for wastewater to allow wastewater being treated up to the level of drinking water, like in water-scarce places such as Namibia, where about 30% of the wastewater is recycled into the freshwater supply. Even more famous is Singapore, where, however, public perception only supports a much smaller percentage to be injected into drinking water reservoirs while most of the reclaimed water is supporting water-intensive industries, like microchip manufacturing etc.
What is virtual water?
Virtual water comprises all of the water used in the production of a product or service. By importing water-intensive products, water-scarce regions are essentially importing virtual water. Recognizing this can help in making decisions about resource management and water allocation under water stress.