Iemke Bisschops and Marissa Boleiji: Closing the loop - recovering nutrients from our wastewater

Wastewater. We all produce it, and we’d rather not think about what happens after we flush the toilet, empty our bath or rinse toothpaste down the drain. But it certainly deserves our attention. Throughout the ages, faeces and urine have been seen as valued resources for agriculture (fertiliser) and industry (ammonia). Later, when health risks became apparent and other sources more available, usage gradually declined. Since the late 19h century, access to clean water and the implementation of sewerage are probably the largest contributors to improved human health and our cities are unthinkable without. Sewage treatment started to be systematically implemented in the 1950s and since then advances in technology resulted in progressively cleaner water, protecting human and environmental health.

The clean water is mostly discharged to the surface water, where it becomes part of the natural water cycle. This sounds like a circular solution, but with the wastewater a wealth of resources was flushed down the drain: organic matter, nutrients like nitrogen and phosphorus and essential elements such as zinc. We excrete the majority of nutrients consumed with our food, disposing them into our wastewater. Recovering these nutrients as fertiliser for food production would be truly closing cycles.

Currently the world economy relies on finite resources, concentrated in certain parts of the world. Phosphate rock is probably best known in this regard, but the same is true for potash ore and elements like magnesium or cobalt. With the air consisting for almost 80% of nitrogen, this may seem an infinite supply. However, the process used to harvest it relies on finite natural gas. Making use of the resources contained in our wastewaters is therefore not only a nice circular idea, it is a vital part in ensuring their reliable supply.

Most sewage treatment plants (STPs) are based on energy intensive aeration of wastewater, with biodegradation of organic matter yielding sludge and carbon dioxide. Nitrogenous compounds are converted into nitrogen gas emitted to the air, and phosphorus and metals are removed with the sludge. The fate of nutrients in sludge depends on the local sludge management practices. The most direct way to use at least part of the organic matter and nutrients from our wastewater is through applying STP sludge as a fertiliser. In some countries this is normal practice, whereas in others little or no sludge is recycled due to strict legal limits to reduce risks for human and environmental health.

In society, there is a growing awareness about pollutants and associated health risks. Sewage contains varying levels of ‘classic’ pollutants like pathogens and heavy metals, as well as contaminants of emerging concern such as pharmaceuticals, antibiotic resistance and pfas. In the transition towards integrated nutrient recovery from domestic wastewaters, appropriate management of pollutants should be taken into account, making sure that any associated health risks are acceptable.

The wastewater sector has been making significant progress in the development of nutrient recovery technologies, but implementation is hindered by economic motives. Installing and operating additional technologies requires extra funds, that are not compensated by selling the recovered nutrients. At the same time, market acceptance of recovered resources and the regulatory framework are only just developing. Scarcity and geopolitical dependence of finite resources are not yet reflected in fertiliser prices. When this happens, recovery becomes economically feasible.

Recovery can have additional benefits: at some STPs, the phosphate mineral struvite is recovered, preventing its uncontrolled formation in piping and equipment. The reduction in maintenance costs makes the practice cost-effective. Struvite recovery is a mature technology, and the EU fertilising regulation was adapted to include struvite as a possible fertiliser product component. The same is true for phosphates recovered from sludge ashes. The advantage of incineration is that most pollutants are destroyed, an attractive aspect for many stakeholders. However, this process only targets phosphorus, whereas the other nutrients are not recovered at all.

The difficulty with sewage is that this is a mixed flow of all kinds of domestic, commercial and industrial wastewaters, combined with storm water. Useful resources are diluted and non-domestic pollutants are added. For the most efficient nutrient recovery we should be aiming for resource recovery close to the source, in this case: the toilet. Examples of source-separated wastewater systems with resource recovery are found in for example Helsingborg in Sweden (‘Oceanhamnen’ and ‘ReCoLab’), Ghent in Belgium (‘De Nieuwe Dokken’) and Sneek in the Netherlands (‘WaterSchoon’). In the applied concepts, highly concentrated toilet wastewater is collected with low-flush vacuum toilets and treated for biogas and struvite recovery. Depending on the local legislation, the produced sludge can be further processed for use as a fertiliser.

Although our wastewater is not an everyday conversation starter for most people, it is clear we can’t continue to ‘flush and forget’. The nutrients in our wastewater are vital for the future of our agrifood system, so we might want to ask ourselves a few questions. How can we achieve integration of nutrient recovery in our society? What risks do we find acceptable, when using recovered nutrients? And maybe more importantly: what about the risks we face if we don’t?