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    2.3 billion people live in water-stressed countries, meaning that their annual water availability is below 1 700 m3 per person per year. Yet, each year 380 billion m3 of municipal wastewater is generated globally. And the amount of wastewater is expected to increase by 24% by 2030 and 51% by 2050. Wastewater is not a problem, however. It’s an opportunity.

    Wastewater is a valuable and sustainable source of water, energy and nutrients. Some countries have recognized this. In recent years, the wastewater sector in developed nations has moved away from the idea of simply treating wastewater as a pollutant and has instead started seeing the potential of wastewater treatment plants to become water resource recovery facilities. These facilities can produce clean water, recover nutrients, and reduce carbon dioxide emissions through the production and use of renewable energy.

    That signals an important change from the current situation. Right now, over 80% of all wastewater produced globally is discharged into the environment without adequate treatment. According to UN Water, high-income countries treat on average about 70% of the wastewater they generate. This ratio drops to 38% in upper middle-income countries and to 28% in lower middle-income countries. In low-income countries, only 8% of wastewater generated undergoes treatment of any kind.

    We cannot afford to waste resources found in wastewater. Here’s how to turn this situation around.

    Water reuse

    The first and the most obvious resource we can obtain by treating wastewater is, of course, water. Although water covers 70% of the earth, less than 3% of the planet’s water resources are available as fresh water, of which only 1% is easily accessible. Despite the fact that fresh water is not abundant, only 3% is abstracted for drinking. The rest goes mainly to agriculture, which accounts for almost two thirds of fresh water use.

    Where freshwater resources are limited, reclaimed water can provide a sustainable and efficient solution. Reclaimed water is the wastewater that has been treated to meet a specific water quality standard for an intended use. This water can be reused for a variety of purposes, but we distinguish two main types of water reuse – non-potable and potable reuse.

    Recharging the environment

    Water reuse is the answer to water scarcity and the impacts of extreme climate events such as prolonged droughts. Non-potable reuse refers to the use of reclaimed water for non-drinking purposes, most notably agricultural irrigation. Reclaimed water is also used in industrial activities and the irrigation of urban green areas, as well as street cleaning.

    During hot periods and droughts, reclaimed water is used to restore or maintain river flows, augment lake levels, or restore wetlands, thus preserving biodiversity. The particular benefit of reclaimed water is that it is a reliable water supply source, independent of seasonal droughts and weather variability.

    A form of environmental enhancement gaining momentum is managed aquifer recharge. This involves the intentional injection of treated wastewater into groundwater aquifers for subsequent recovery.

    New water

    There are two types of potable water reuse. Indirect potable reuse refers to augmenting natural drinking water resources with recycled water. This can be either planned or unplanned. Planned reuse is becoming increasingly common in Australia and the United States, while there is only one full-time potable water reuse system in the European Union. The Torreele facility in Western Flanders, in northern Belgium, uses reclaimed water to artificially recharge the nearby dune aquifer of St-André, which in turn supplies water to 60 000 people in the local community.

    But, in reality, a large portion of treated, and even untreated, wastewater worldwide ends in the water supply systems as unplanned indirect potable reuse.

    On the other hand, direct potable reuse refers to the injection of reclaimed water directly into the water supply distribution system. But the use of reclaimed water for drinking is not common.

    In Namibia, the city of Windhoek has been doing it for 50 years. Reclaimed water makes up around 30% of the city’s current supply of drinking water, serving its 400 000 inhabitants. In Singapore, reclaimed water covers 40% of its water needs. The country’s NEWater process recycles treated water into an ultra-clean, high-quality reclaimed water. By 2060, NEWater is expected to meet up to 55% of Singapore’s water demand.

    Despite the increasing need, the use of reclaimed water remains controversial. The instinctive disgust associated with the idea of recycling sewage and the fear that reclaimed water is unsafe is known as the “yuck factor. This public perception, even though not necessarily based on the actual risks of recycled water, can create genuine challenges for the integration of water reuse in the water supply.

    Nutrient recovery

    Sewage sludge is the residual, semi-solid material produced as a by-product of wastewater treatment. This sludge contains metals and microplastics, as well as pathogenic organisms such as viruses and bacteria. Sludge is, however, also rich in nutrients, such as nitrogen and phosphorus, which originate from human waste, food and certain soaps and detergents. And both of these nutrients are valuable in agriculture as components of fertilisers.

    In urban areas with a lack of advanced wastewater treatment services, the high concentration of these nutrients causes pollution. Excess nitrogen and phosphorus are still a leading cause of water quality degradation in Europe. This makes an even stronger case for nutrient recovery from wastewater.

    The phosphorus crisis

    You might not hear about it a lot, but phosphorus is an element essential to sustaining life on the planet. Why? Because it’s necessary to produce our food. Phosphorus, in its phosphate form, is needed to fertilise the soil. But we are running out of it. Extractable phosphorus mineral resources are predicted to become scarce or even exhausted in the next 50 to 100 years. At the same time, we are wasting so much of it that it pollutes our waters.

    This is why phosphorus recovery is so important. Just by recycling the world’s domestic wastewater, we could meet 22% of the global phosphorus demand. Driven by the expected future shortages, an interest in phosphorus recovery from wastewater has led to extensive research in the field. These technologies are already being used at several locations. The world’s largest nutrient recovery facility operates close to Chicago in the United States. Its phosphorus recovery system can recover more than 85% of the phosphorus and up to 15% of the nitrogen from wastewater streams.

    The nitrogen cycle

    Unlike phosphorus, which is a limited and non-renewable resource, nitrogen is abundantly present in the atmosphere. Ever since the invention of the Haber-Bosch process in 1909, which managed to convert atmospheric nitrogen to ammonia, nitrogen-based fertilisers have supported the largest historical increase in food production capacity.

    The increased food production achieved by the use of these fertilisers has resulted in nitrogen finding its way into our wastewater in record amounts. Which is why wastewater treatment plants need to employ energy intensive methods to remove nitrogen.

    The method used to remove nitrogen from wastewater also results in the formation of nitrous oxide, which is a greenhouse gas. However, recovering nitrogen fully instead of just removing it takes care of that problem – not only contributing to the circular economy, but also reducing greenhouse gas emissions. Unfortunately, current technologies only allow for 5-15% of that nitrogen to be recovered. This alongsideits abundance in the atmosphere also poses a commercial challenge.

    Making reclamation profitable

    Technology is advancing, but not enough to be profitable and business opportunities remain limited. The low nutrient content in biosolids, in particular nitrogen, does not allow for profitable sale on the market. Only 5–15% of the available nitrogen in wastewater can be recovered, while it is possible to capture 45–90% of phosphorus.

    Scientists and engineers are also considering a list of other under-exploited resources in wastewater, including bioplastics, enzymes, metals, and minerals, but more work is needed to make their reclamation economically viable.

    Energy recovery

    The wastewater treatment industry consumes large amounts of energy, accounting for approximately 0.8% of the entire electricity generated in the European Union. But studies have shown that wastewater contains nearly five times the amount of energy that is needed for the wastewater treatment process. This mean that wastewater treatment facilities are able not only to produce the energy they need, but also to help heat and power the cities that produce the wastewater, thus decarbonising the economy.

    Advanced facilities typically recover only chemical energy in the form of biogas, which is produced from the anaerobic digestion of wastewater sludge. Biogas is one of the most important renewable energy sources. It does not rely on critical raw materials and does not disrupt wildlife. In addition, can be stored and distributed using the existing gas infrastructure network.

    Research into recoverable energy embedded in municipal wastewater, however, suggests that the potential for thermal energy (80% of energy recovered) is much higher than that for chemical energy (20%). Only a very small amount (<1%) of the embedded energy is in the form of hydraulic energy. This indicates that a significant portion of energy that could be recovered from wastewater currently remains unexploited.

    Hot and cold

    Thermal energy from wastewater, recovered through technologies such as heat exchangers and heat pumps, can be used for district heating/cooling, agricultural greenhouses, and even for drying sludge. This is because wastewater exhibits a relatively high temperature, as it originates from warm sources such as showers, dishwashers and washing machines.

    There are no technological limitations to the recovery and use of thermal energy from wastewater. Difficulties recovering it are attributed to the distance of the supply from wastewater plants. To utilise this thermal energy fully, authorities need to include it in municipal planning.