Leon Korving
Scientific Project Manager
Wetsus, European centre of excellence for sustainable water technology
The Netherlands
Leon.Korving@wetsus.nl
The European countries have been able to significantly reduce nutrient emissions to surface waters over the last decades. This has certainly also been true for the countries around the Baltic see. Nevertheless, we still have remaining challenges to further reduce these loads even though significant steps have been taken. On the other hand, our society is also increasingly realizing that access to nutrients is also of enormous strategic importance because without these nutrients are crucial to produce our food and feed the European population. Therefore, we need to realize better that controlling nutrients in our environment is not only about the quality of our surface water but also how we manage our food production in a more sustainable and circular way. Although current approach to control of nutrient discharges to surface water and the sea has been mostly effective from a water quality point of view it is not sustainable nor circular.
Issues in the current control of nutrients
Nitrogen and phosphorus are the most important nutrients that need to be controlled and both nutrients require different approaches. Our sewage treatment plants destroy more than 80% of the bound nitrogen in the sewage by a biological process called nitrification and denitrification using a lot of energy and part of the dissolved carbon in the sewage which is then not available for other uses. Also, in the last years we have learned that this process can produce significant amount of the potent greenhouse gas nitrous oxide (N2O) if not controlled properly. On the other hand roughly 1% of the world’s energy use is related to the conversion of nitrogen gas from the atmosphere into bound nitrogen for use in our fertilizers and a similar amount of energy is used for the removal and destruction of bound nitrogen in sewage treatment plants.
Phosphorus is now mainly removed by concentrating the phosphorus in the sewage sludge produced in sewage treatment plants, either by stimulating biological uptake (enhanced biological phosphorus removal or EBPR) or by dosing coagulants like iron salts which bind with the phosphorus. Thus, phosphorus removal increased the production of waste and requires a sensible destination of the sewage sludge loaded with the phosphorus. In countries where the sludge is incinerated or landfilled the phosphorus is lost for future generations. In other countries the sludge is used in agriculture but not always is the application rate truly in balance with the phosphorus need of the crops. Too high applications of sludge would then again lead to eutrophication issues. On the other hand phosphorus is considered a critical raw material by the European Union since it is strategic for our food production because EU countries do not have significant sources of phosphate rock in Europe. In the current situation the cost for removal of phosphate from sewage sludge is nearly as high as the production of phosphorus fertilizer from phosphate rock.
Upgrade or redesign?
As of January 1 of 2025 the revision of the Urban Waste Water Treatment Directive has entered into force and this revision will require EU member states not only to comply to stricter effluent limits but it also intends to stimulate circular use of nutrients. Therefore now is the time to plan for a new generation of sewage treatment plants and water utilities will have to decide if it is sufficient to upgrade existing plants or should we consider radical new concepts.
At Wetsus we focus on the development of new water technology in close collaboration with more than 50 academic chairs and 100 European industrial stakeholders. In this role we are at the forefront on developing new concepts to address the described challenges. What we see is that there are indeed options to become more circular. Some of these options can be used to retrofit existing sewage treatment plants while others focus on more radical changes. In the end we will need a healthy mixture of both retrofit and far fetching solutions to grow to more sustainable use of nutrients.
Examples of innovative concepts
Iron salts are commonly added to sewage treatment plants to bind phosphorus which then ends up in the sewage sludge as an iron-phosphorus mineral. It is expected that increasingly higher dosages of iron salts will be needed in the near future to comply to future effluent limits. Research at Wetsus has shown that a blueish mineral called vivianite is formed in the sludge and that it can bind up to 80% of all phosphorus in the sludge. This mineral is paramagnetic and that makes it possible to extract it from the sludges using magnetic equipment that is also used in conventional mining. In this way up to 60% of all phosphorus in sewage can be recovered while also recovering most of the iron. The recovered vivianite can be splitted to produce a phosphorus fertilizer while also recovering the iron for reuse. Also, there are interesting perspectives to use the material for production of lithiumironphosphate (LFP) batteries, a part of our current research. Our Finish partner Kemira is now commercializing this approach under the name of ViviMag®.
Another approach would be to adsorb the phosphorus on ironoxide adsorbents that can be regenerated after they are fully loaded with phosphorus. In this way the adsorbent can be reused continuously preventing the continuous dosage of iron salts. Our research has shown that via this approach one can achieve ultra low phosphorus concentrations (less than 50 ppb P) while also recovering the phosphorus. Our Dutch partners Aquacare and Royal HaskoningDHV have demonstrated this concept at pilot scale and are now working towards a first full scale application. This concept may also be used in a different way to remediate eutrophicated lakes by binding and recovery of the phosphate in a lake, either via a pump and treat approach or a more passive “tea bag” approach. New research is now underway to combine this with nitrogen adsorbents and thus providing for a more sustainable way to remove nitrogen from sewage water.
Finally an even more radical approach would be to rethink our sanitation system. Reuse of sewage sludge in agriculture is hindered by concerns of a whole range of pollutants in the sludge. By staying closer to the source and separation of urine, black water and grey water one can prevent that human manure gets polluted by other pollutants and this could produce a new and more sustainable way to bring nutrients back to agriculture. Important real life examples of this concept have been introduced in Helsingborg (Sweden), Hamburg (Germany) and Sneek (The Netherlands).
Create room for experimentation
These are just examples from our own research but more solutions are possible of course. Our society faces challenges not only to maintain the the quality of our rivers, lakes and seas but will also need to addressing security of our food production and other resources. The examples show that we can do both at the same time. However, it does require room for experimentation to mature these new concepts, learn lessons and improve them. We see many of these concepts now reaching puberty and now water authorities, regulators and financers should create space for these adolescent technologies to explore the real world and become adult technology.