The global population, currently around 7.8 billion, is continually rising and is expected to reach 10 billion by 2050. This puts pressure on limited freshwater supplies, which in the era of climate change are dwindling in many areas. More people equates to an increase in demand for both water and food, which in turn requires water to grow (crops) or sustain (livestock). As water scarcity becomes the norm in many highly populated regions of the world, we are going to have to come up with sustainable solutions to ensure both food and water security in the future.

Drought proofing for future sustainability

Many of the world’s most populated cities and agricultural hubs lie in drought-prone regions. In the U.S., for example, the agricultural hub of California regularly faces the reality of an unstable water supply. This ultimately affects the rest of the nation, as more than half of all the fruit, vegetables and nuts supplied across the U.S. are produced in California, while 20% of the country's milk comes from Californian dairy cows. During the 2014 drought, the California agriculture sector suffered a loss of $2 billion in lost crops alone.

Recycling wastewater is one solution that has been employed by many water-stressed regions, mainly to A lavender-colored non-potable water pipeline in Mountain View, California. Source: Grendelkhan via Wikimedia/CC BY-SA 3.0A lavender-colored non-potable water pipeline in Mountain View, California. Source: Grendelkhan via Wikimedia/CC BY-SA 3.0provide a source of non-potable water for irrigating crops, recreation facilities and landscaping, as well as other non-potable uses. This is supplied in a separate non-potable water pipeline and is usually identified by a unique color coding.

In California and several other severely water-stressed regions around the world, wastewater is also recycled into drinking water. Yet, while recycling wastewater offers the perfect drought-proofing solution, the general public seems to be rather skeptical when it comes to actually drinking what is essentially recycled toilet water. Consumers are understandably concerned that recycled wastewater may contain pathogenic contaminants that could pose a public health risk. But are these fears warranted considering the vigorous water treatment processes that wastewater and drinking water has to undergo, and the stringent drinking water quality standards that have to be met; especially when weighed up against the risk of potentially not having any water at all?

With the introduction of new innovative advanced wastewater treatment technologies, it is possible to recycle wastewater to a standard that meets health and safety regulations for drinking water. While many cities deliberate whether or not to go this route, the drought-prone desert nation of Namibia in southern Africa took the initiative more than 50 years ago. The Goreangab wastewater treatment plant located in the country’s capital, Windhoek, was opened in 1968 to convert wastewater generated by the city’s residents back into drinking water. It now provides 35% of the city’s water and has been instrumental in providing water security during dry periods.

Wastewater treatment process

Before any wastewater effluent can be discharged from a wastewater treatment plant into the environment it has to meet strict environmental health standards. The level of treatment (secondary, tertiary or advanced) will depend on what the recycled water will be used for. Recycled wastewater that is intended to be used as a source of drinking water will undergo treatment at secondary and tertiary levels, followed by an additional stage of treatment using advanced water treatment technologies thereafter to ensure water quality of the highest standard.

Indirect potable reuse

After advanced treatment, wastewater can be used to replenish natural freshwater sources that supply drinking water, for example, to recharge underground aquifers or replenish surface reservoirs, where it is diluted with naturally occurring freshwater in these systems. Drinking water obtained from these water sources undergoes further treatment at the drinking water treatment plant to ensure it meets drinking water safety standards before it is supplied to consumers. As wastewater is a resource that is constantly being generated, utilizing this resource to replenish natural freshwater systems offers a sustainable solution for addressing water scarcity and building climate resilience in water-stressed cities and drought-prone regions.

Direct potable reuse

Recycled wastewater can also be treated further, either on site at the wastewater treatment facility or at a water treatment plant, using the most appropriate advanced wastewater treatment technologies (biological, chemical and physical) available to ensure that it meets drinking water quality standards before it is distributed directly to the consumer. In this case, the recycled drinking water is not discharged back into the environment but serves as a direct source of potable water.

Any water that enters a drinking water treatment facility, regardless of the source, can contain a wide range of undesirable and potentially harmful pollutants such as animal waste, nitrates, dead and decaying organic matter, heavy metals, pesticides and other hazardous chemicals that often originate upstream. These contaminants will need to be removed before the water can be supplied to consumers as drinking water. The type of water treatment technologies employed will depend on the contaminants present. In most cases, water treatment systems are specifically designed to address the contaminants found in the source water. Recycled wastewater is no exception.

Technologies and applications

Several different types of technologies can be employed during the advanced wastewater treatment stage. Advanced wastewater treatment technologies such as ozonation, reverse osmosis, ultra membrane filtration and ultraviolet disinfection are able to remove pathogenic microorganisms and chemical pollutants commonly found in wastewater. When used in combination, these technologies can meet the stringent Reverse osmosis system for drinking water treatment plant. Source: Navintar via Adobe StockReverse osmosis system for drinking water treatment plant. Source: Navintar via Adobe Stockstandards required for drinking water, ensuring that the water is safe to drink.

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As the global pioneer of wastewater recycling systems and the world’s first direct potable reuse wastewater recycling project, Windhoek’s Goreangab Water Reclamation Plant, often referred to as the ‘cradle of water reclamation,’ originally relied on hardworking bacteria to recycle wastewater. The New Goreangab Water Reclamation Plant, built in 2002, was upgraded to include state-of-the-art technology that incorporates pre-ozonation, enhanced coagulation, dissolved air flotation, sand filtration, ozonation, biologically activated carbon filtration, granular activated carbon filtration, and ultra membrane filtration followed by residual chlorination into the advanced treatment process. The wastewater treatment plant recycles wastewater generated by the city’s 400,000 residents, converting it to drinking water that is supplied back to those residents directly through the water distribution network. As the world’s oldest toilet-to-tap project, Goreangab water reclamation plant serves as a global benchmark for other cities interested in recycling wastewater to secure their water supply.

Wastewater to drinking water schemes are starting to pop up in other cities around the world, as city managers begin to realize that this offers a safe and sustainable solution for ensuring the water security of populated urban areas. Israel currently recycles around 90% of its wastewater, most of which is used for agriculture or for indirect potable use (to replenish aquifers). In Australia, wastewater recycling projects have been implemented in several major cities, as well as the use of modular small-scale portable wastewater treatment plants to treat and recycle wastewater for reuse onsite.

As the global population together with its demand for water continues to increase, more and more urban areas are likely to become water-stressed in the future. Recycling wastewater to drinking water offers a safe, environmentally sound and sustainable solution to the world’s water woes, and may be the only option to ensure future water security in drought-prone regions that may be heavily impacted by the effects of climate change.

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