Water scarcity is a problem common to all continents. According to the United Nations (Coping with water scarcity. Challenge of the twenty-first century. UN-Water, FAO, 2007), water scarcity affects 1200 million people–almost a fifth of the world’s population–living in regions with physical water scarcity, with a further 500 million people close to this situation.
Water scarcity is one of the most important problems that will be faced by many societies in the 21st century. Although there is sufficient water on the planet to supply 7000 million people, it is not equally distributed and the majority is squandered, polluted or managed unsustainably.
Hydrology experts assess water scarcity on the basis of the ratio between the population and the water available. A region is considered to suffer from hydric stress when the annual quantity of water available is less than 1700 m3 per person. When this ratio reaches 1000 m3 per person, the situation is one of water scarcity. A figure of less than 500 m3 per person indicates absolute scarcity. The regions of the world suffering from water scarcity are shown in Figure 1. A difference can be made between physical scarcity, when the water available is less than the demand, and economic scarcity, in which human, institutional and financial factors limit access to water despite sufficient water being available.
Figure 1. Global physical and economic water scarcity. Source: World Water Development Report 4. World Water Assessment Program (WWAP), March 2012.
The management of water scarcity does not just involve major construction projects, such as interbasin transfers, to increase the availability of water resources; it is also essential to make better use of the water available without wasting or polluting it. Water reuse, especially by industries that require large amounts, is essential in order to move towards a solution to this problem. Thus, such industries must make rational use of their water and it is also essential that they treat their effluents so as not to contaminate public supplies, thereby wasting this precious resource. A contaminated public supply reduces ecosystem biodiversity, decreases the quality and yield of agricultural crops that require water for irrigation, and even poses a health risk to the population.
With regard to industries that consume high volumes of water and generate significant polluted effluent flows, a key tool against water scarcity in the region is the zero liquid discharge (ZLD) concept. This concept is based on the use of techniques and processes that make it possible to reuse all polluted effluents and achieve three key targets: (1) reduce the consumption of water taken from outside the company as far as possible, (2) correctly treat polluted effluents in order to prevent environmental pollution, and (3) minimize the quantity of waste generated. The advantages of applying this philosophy in comparison with consuming as much water as necessary and discharging the effluents produced are numerous, although the following are perhaps the most important:
- A dramatic saving in the consumption of water from outside the company, as the majority of water is reused.
- No contamination of natural supplies as a result of the discharge of waste effluents as no waste is discharged.
- Minimal waste production, with its inherent environmental and economic benefits.
- Economic savings due to the elimination of administrative fines resulting from environmental pollution.
- A high degree of self-sufficiency with regard to water consumption.
The implementation of a zero liquid discharge system based on the zero waste concept involves the treatment of all liquid waste to such an extent that its quality allows it to be reintroduced into the process, hence minimizing final rejection. The technologies used to concentrate and minimize the final effluent can be classified as either membrane or thermal separation technologies. The former, which generally involve micro- and ultrafiltration units as an initial pretreatment and processes such as nanofiltration and reverse osmosis to recover up to 80% of the water, allow the pollution to be concentrated into a liquid rejection flow while producing high quality water. Thermal separation technologies, normally vacuum evaporators and crystallizers, concentrate the rejection flow from membrane techniques to produce distilled water, which can be reused in the process, and a dry solid residue, thus allowing the goal of no final discharge to be achieved. Specifically, vacuum evaporation is a technology that combines the ability to reduce discharges as far as possible with efficacy, robustness, and sustainability. For this reason such systems are practically indispensable when implementing a zero waste management system. When the only waste should be a dry solid, vacuum evaporation is used in combination with a crystallizer, which crystallizes the residue from the evaporator.
The field of application of such management systems is as broad as the number of different activities that generate liquid effluents, with some minor limitations. The activities in which a zero waste management system is particularly useful and advantageous are summarized in Figure 2.
Figure 2. Industrial sectors in which a zero waste system is a key tool.
The virtuous circle representing the zero waste concept is shown in figure 3. Not all the processes shown are required in all cases. Membrane processes (MF, UF, RED, and RO) generate a large volume of water suitable for reuse. And thermal processes (evaporation and crystallization) treat the rejection flows produced by the former processes, producing yet more water suitable for reuse and a final dry solid residue.
Figure 3. Virtuous circle for the zero waste concept. MF: microfiltration; UF: ultrafiltration; RED: reversible electrodialysis.
In summary, a zero liquid discharge systems is a robust, useful, and effective system in all cases and is especially advantageous for treating complex liquid effluents by separating the water instead of separating the pollutants. As such systems are based on physical rather than chemical processes, their field of application is particularly broad. Pollutants are concentrated and finally reduced to a dry residue, thus making them cheaper and easier to manage, while the water can be reused in the process.