Wastewater is not a problem. It is a highly valuable source of water, nutrients, and energy. Yet, more than 80% of all wastewater is currently treated as waste. We need to change the way we process it, making wastewater resource recovery a priority.
2.3 billion people live in water-stressed countries, meaning that water availability is less than 1700 m3 per person per year. Every year, 380 billion m3 of municipal wastewater is produced globally. It is projected that by 2030, the amount of wastewater will increase by 24% and by 2050 – by 51%. Wastewater is not a problem, but an opportunity.
Wastewater is a valuable and sustainable source of water, energy, and nutrients. Some countries have already realised this. In recent years, the wastewater sector in developed countries has moved away from the idea of simply treating wastewater as a pollutant, starting instead to see the potential for wastewater treatment plants to become water resource recovery facilities. These facilities can produce clean water, recover nutrients, and reduce carbon dioxide emissions by producing and using renewable energy.
This represents a significant change from the current situation. At present, more than 80% of the world's wastewater is discharged into the environment without adequate treatment. According to UN Water data, high-income countries treat on average about 70% of their wastewater. In upper-middle-income countries, this figure drops to 38%, and to 28% in lower-middle-income countries. In low-income countries, only 8% of generated wastewater is treated. We cannot afford to waste the resources contained in wastewater. Here is how to change this situation!
Water reuse
The first and 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 fresh water, of which only 1% is easily accessible. Despite the fact that fresh water is scarce, only 3% of it is used for drinking. The rest is mostly used in agriculture, which accounts for nearly 2/3 of fresh water consumption.
If fresh water resources are limited, reclaimed water can be a sustainable and effective solution. Reclaimed water is wastewater that has been treated to meet specific water quality standards suitable for its intended use. This water can be reused for various purposes; we distinguish two main types of water reuse – non-potable reuse and potable reuse.
Reuse
Water reuse is an answer to water scarcity and the impact of extreme climate events, such as prolonged drought. Non-potable reuse refers to the use of reclaimed water for purposes other than human consumption, most commonly for agricultural purposes. Reclaimed water is also used in industrial sectors and for irrigating urban green areas, as well as for street cleaning.
In hot weather and periods of drought, reclaimed water is used to restore or maintain river flow, raise lake levels, or restore wetlands, preserving biodiversity. A particular benefit of reclaimed water is the fact that it is a reliable source of water supply that does not depend on seasonal droughts and weather fluctuations.
A way of environmental enhancement that is becoming increasingly popular is managed aquifer recharge. This involves the deliberate injection of treated wastewater into groundwater layers for subsequent use.
New water
There are two types of potable water reuse. Indirect potable reuse is the augmentation of natural drinking water resources with recycled water. It can be both planned and unplanned. Planned reuse is becoming more common in Australia and the USA, whereas in the European Union, there is only one full-time potable water reuse system, namely at the Torreele plant in West Flanders, in northern Belgium, where reclaimed water is used to artificially recharge the nearby St-André dune aquifer, which in turn supplies water to 60,000 local community residents.
In fact, a large proportion of treated and untreated wastewater worldwide ends up in water supply systems as unplanned indirect potable use. On the other hand, direct potable reuse is the introduction of reclaimed water directly into the water supply distribution system. However, the use of reclaimed water for drinking is not widespread.
In Namibia, the city of Windhoek has been doing this for 50 years. Currently, reclaimed water accounts for approximately 30% of the city's current drinking water supply, serving 400,000 residents. In Singapore, reclaimed water covers 40% of its water needs. The country's NEWater process recycles treated water into ultra-clean, high-quality reclaimed water. It is expected that by 2060, NEWater will meet up to 55% of Singapore's water demand.
Despite the growing need, the use of reclaimed water remains controversial. The instinctive aversion associated with the idea of recycling wastewater, and the fear that reclaimed water is unsafe, is known as the “yuck factor”. This perception, while not always based on the real risk associated with recycled water, can create real problems for integrating water reuse into water supply.
Israel: From water scarcity to water sufficiency
Israel struggles with water scarcity. Its annual renewable water volume per capita (86 m3) is much lower than the 500 m3 threshold that defines absolute water scarcity* (If annual water stocks are less than 1000 m3 per person, the population faces water stress, but if they are less than 500 m3 – “absolute scarcity”. (UNWater))
However, the country has managed not only to secure the supply of drinking water for its population but to achieve a water surplus, which it exports to neighbouring countries, such as Jordan. Israel has utilised non-traditional water resources, reclaimed water, and desalination, which supplement natural water sources. Nearly 90% of the country's wastewater is currently treated for agricultural reuse, which accounts for about half of the total water used by farmers nationwide. Reusing reclaimed water in agriculture means that scarce fresh water resources are primarily used for drinking and domestic needs.
Furthermore, managed aquifer recharge with treated wastewater during months of low water demand means that aquifers are used as reservoirs to store water for periods of high demand and/or drought.
In Israel, large-scale seawater and brackish water desalination provides 85% of all drinking water. The European Investment Bank-funded Sorek II desalination plant will be the sixth desalination plant in the country and one of the largest in the world to use reverse osmosis.
Hamburg: Success in plant modernisation
In 2020, the European Investment Bank funded the modernisation of Hamburg's wastewater treatment plants, increasing their energy recovery capacity. Although the treatment plant already generates 107% of its required electricity and 113% of its required heat, these figures will improve even further, making the plant not only energy self-sufficient but also capable of providing electricity for approximately 5,700 households.
Nutrient recovery
Sewage sludge is a by-product of wastewater treatment. This sludge contains metals and microplastics, as well as pathogenic organisms such as viruses and bacteria. However, sludge is rich in nutrients, such as nitrogen and phosphorus, which originate from human waste, food, and certain soaps and detergents. These nutrients are valuable components of fertilisers in agriculture.
In urban areas without modern wastewater treatment services, the high concentration of these nutrients causes pollution. Excessive nitrogen and phosphorus are still the main cause of water quality deterioration in Europe. This further justifies the need to recover nutrients from wastewater.
Phosphorus crisis
It is not often talked about, but phosphorus is a vital element for sustaining life on the planet. Why? It is necessary for food production. Phosphorus in the form of phosphates is required for soil fertilisation. We are running short of it. It is projected that within the next 50-100 years, mineral phosphorus resources will become insufficient or even depleted. At the same time, we waste a lot of phosphorus, and it pollutes our waters. That is why phosphorus recovery is so important. By recycling only the world's municipal wastewater, we could meet 22% of global phosphorus demand. Due to the expected future shortage of phosphorus, interest in phosphorus recovery from wastewater has prompted extensive research in this area. These technologies are already being used in several places. The world's largest nutrient recovery facility operates near Chicago, USA. Its phosphorus recovery system is capable of recovering more than 85% of phosphorus and up to 15% of nitrogen from wastewater.
Nitrogen cycle
Unlike phosphorus, which is a limited and non-renewable resource, nitrogen is abundant in the atmosphere. Since the invention of the Haber-Bosch process in 1909, when atmospheric nitrogen was successfully converted into ammonia, nitrogen fertilisers have contributed to the historically largest increase in food production capacity.
The increase in food production volume achieved through these fertilisers has resulted in record amounts of nitrogen entering our wastewater. Therefore, energy-intensive methods must be used in wastewater treatment plants to remove nitrogen.
The methods used for nitrogen removal from wastewater also generate nitrous oxide, which is a greenhouse gas. However, full nitrogen recovery, rather than just removal, solves this problem, not only promoting a circular economy but also reducing greenhouse gas emissions. Unfortunately, current technologies only allow for the recovery of 5-15% of nitrogen. The fact that nitrogen is abundant in the atmosphere is also a commercial challenge.
Making recovery cost-effective
Technologies are evolving, but not enough to be cost-effective, and business opportunities remain limited. The low nutrient content in biological waste, particularly nitrogen, makes it impossible to sell them on the market at a profit. Only 5-15% of the nitrogen in wastewater can be recovered, but it is possible to extract 45-90% of phosphorus.
Scientists and engineers are also considering a range of other resources in wastewater that are not being fully utilised, including bioplastics, enzymes, metals, and minerals, but further work is needed to make their recovery economically viable.
Energy recovery
The wastewater treatment sector consumes a large amount of energy – about 0.8% of all electricity produced in the European Union. However, studies show that wastewater contains nearly five times more energy than is required in the wastewater treatment process. This means that wastewater treatment plants can not only produce the necessary energy but also help to heat and power the cities where wastewater is generated, thereby decarbonising the economy.
Advanced plants usually recover only chemical energy in the form of biogas, which is produced in the anaerobic digestion process of sewage sludge. Biogas is one of the most important sources of renewable energy. It does not depend on critical raw materials and does not interfere with nature. Moreover, it can be stored and distributed using the existing gas infrastructure network.
However, studies on recoverable energy in municipal wastewater show that the thermal energy potential (80% of recoverable energy) is much greater than the chemical energy potential (20%). Only a very small fraction (<1%) of the embedded energy is hydraulic energy. This indicates that a significant portion of energy that could be recovered from wastewater is currently still not used.
Hot and cold
Wastewater thermal energy recovered using technologies such as heat pumps can be used for district heating/cooling, agricultural greenhouses, and even sludge drying. This is because wastewater has a relatively high temperature, as it originates from warm sources such as showers, dishwashers, and washing machines.
There are no technological limitations to recovering and using wastewater thermal energy. The difficulties in recovering it are related to the supply distance from wastewater treatment plants. To fully utilise this thermal energy, authorities must include it in municipal planning.
Wastewater and climate
If we want to achieve the UN Sustainable Development Goals, we must change the way we treat wastewater. Wastewater management will play a crucial role not only in providing clean water to everyone, eradicating hunger and poverty, but also in reducing emissions.
Water and wastewater utilities generate between 3% and 7% of greenhouse gas emissions. Its potential contribution to mitigating climate change should not be neglected. Wastewater treatment plants consume 0.8% of all energy used in the European Union. Energy recovery systems could make all these plants self-sufficient. Water reuse could also reduce the amount of energy associated with extraction. The new European Union agricultural regulation, which will come into effect in 2023, could increase water reuse sixfold – from 1.7 billion m³ to 6.6 billion m3 per year – and reduce water scarcity by 5%.
Wastewater solutions
To promote water reuse without compromising public health, water quality standards suitable for the specific purpose must be established. Protecting public health must be an integral pillar of any reuse system.
We need new and better technologies that will enable more cost-effective recovery of larger quantities of nutrients. We must also take into account other resources in wastewater that have not yet been utilised.
To achieve this, investment in water infrastructure is needed. To reach the 2030 climate and energy goal, the European Union needs an additional €90 billion in investment just in the water and waste sector. But wastewater resources can help. It is estimated that 60-70% of the potential value of wastewater across the European Union is still not being used (i.e., heat, energy, nutrients, minerals, metals, chemicals, etc.).
To achieve the transition from pollution reduction to resource recovery, this concept should, if possible, be set as a goal at the early stages of planning new investments. There is no time (nor wastewater) to waste.
What is “fit for purpose” or “fit for use” water?
Fit for use water is recycled water that is specifically adapted to meet the requirements of its intended final consumption. Water quality depends on how it will be reused. For example, the quality of reclaimed water used in agriculture must be high enough to maintain soil health and food safety. Water reclaimed for food use will need to be treated even further.