Struvite – wastewater to fertiliser

Marc Spiller senior researcher at the University of Antwerp in Belgium provides some background on a Phosphorus (P) containing mineral that can be recovered from wastewater called struvite.

Struvite is regarded as a good slow-release P fertiliser. Producing struvite can help recycle the critical raw material P back to agriculture and reduce the mining of non-renewable P resources.

Struvite – recovered phosphorus source

There is a lot of interest about struvite being used as an alternative source of phosphorus, especially since the spike in gas prices in 2022 dramatically affected fertiliser prices.

Currently, most of the struvite on the market originates from municipal wastewater that come from countries such as the Netherlands, Belgium and Germany.

But could struvite be used widely in agriculture as a source of P for fertilisers?

A scientific paper published by SUSFERT authors titled: A systematic comparison of commercially produced struvite: Quantities, qualities and soil-maize phosphorus availability  pointed out that even under the most optimal recovery assumptions, the amount of P derived from struvite could only replace 13% of the P used in European agriculture as fertiliser.

Ultimately, the potential for struvite could be influenced by European legislative measures that would make it mandatory to recover P in wastewater installations, which some countries in Europe have set a timeline for.

SUSFERT Communications spoke to Dr Marc Spiller from the Sustainable Air, Energy and Water Technology group of the University of Antwerp about research that assessed the quality of 25 industrial struvite samples from across Europe in relation to the EU Fertilizer Products Regulation (EU) 2019/1009 of the European Parliament and of the Council of June 5, 2019 that came into force on 16 July, 2022.

A summary of the regulation that aims to create a common EU market for recovered fertilisers can be found HERE.

Spiller will be speaking at the SUSFERT NextGen Bio-Based Fertilisers Webinar examining the fertiliser components developed and tested on the project. Spiller specifically will address struvite. Register HERE to join the conversation.

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Why is struvite important?

Struvite (MgNH4PO4.6H2O) is best known as the mineral that is contained in some types of kidney stones and is regarded as a source of P as it contains theoretically 12.6% of phosphorous when fully dry.

Phosphorus is an essential macronutrient for all living organisms. That means, without it we cannot survive. We need it to store energy. It is in our bones and in our DNA.

P is also classified as a critical raw material by the European Commission because about 90% of commercially available P is sourced from phosphate rock, a non-renewable and geographically restricted resource, with no meaningful reserves in the European Union (EU).

Because of this there is a big initiative in Europe to re-use P as much as possible. This means that we should recover it from sources where it is currently not re-used to produce food and feed.

Where can you recover it from?

In Europe, large P losses occur through domestic wastewater (15% of total EU import). That makes capturing P as struvite from wastewater a good idea. But the reason people started to make struvite had nothing to do with the recovery of P. Rather, it arose from a practical reason.

Starting early this century, wastewater industries mainly implemented struvite recovery reactors to avoid spontaneous precipitation of struvite that led to the blockage of pipes and pumps. In doing so, companies could reduce maintenance costs and therefore save a lot of money.

Under certain conditions, struvite precipitation can improve the so-called dewaterability of the solids that are remaining from treatment. As companies pay for the solids to be disposed of by weight, considerable savings were made this way.

Only somewhat later was there an increasing interest to use struvite as a fertiliser. Since then, research has shown that struvite is a good slow-release fertiliser and has a relatively low contamination with heavy metals such as cadmium, which is a concern particularly in mined P fertilisers.

Can you explain the struvite technology?

Struvite can, in principle, be sourced from all P-rich liquid streams. It is currently mostly applied in municipal wastewater treatment (after anaerobic digestion of the solids), and for potato wastewater as these streams are rich in P.

Nearly always, it is implemented after a process called anaerobic digestion, mainly known for the production of biogas. This process brings P into the solution by breaking down the organic matter.

The next key thing for struvite to be produced is then to add magnesium and to control the pH to between 7.5-9. In combination with some type of mixing of the solution struvite crystals will form and grow. Struvite can then be extracted from the water as it sinks down. In the figure above you can see two different types of struvite and how they look. They can be really round in shape, or more rod shaped.

What was your research about?

In SUSFERT we wanted to find out what the quality of struvite was and how it would comply with the new EU Products Fertilizer Regulation. As mentioned, the Regulation came into force in July, 2022 and set quality standards of recovered fertilisers and seeks to create a common market for these products in the EU.

What was your research finding?

We gathered samples from 25 industrial scale and pilot scale installations from wastewater and the potato industry.

We found that all but three of the samples we collected would meet all the legal requirements of the new EU fertilizer Regulation.

This is good news for the use of struvite as a fertiliser.

The SUSFERT project also investigated for the first time the granule properties of the struvite, showing that particle sizes ranged from <0.2 to 3.5 mm. As far as I know, nobody has described granules as large as 3.5mm on average.

Struvite granule size, shape important parameters

Granule size and shape are important quality parameters. They are important for farmers because they often use machines to apply fertilisers to land.

So, farmers look for specific sizes and shapes of granules that match current fertilisers, which are suitable for the agricultural equipment to disperse. Our research shows that farmers can pick and choose from different granule sizes.

Our work also confirmed that struvite is a good slow-release fertiliser. We could show in pot experiments that there was a continuous availability of P over the sample period between 3.5–6.5 mg P/litre of soil (mean: 4.8 mg P/l). This was very different from the single super phosphate we tested, where there was a release peak at the start, and a rapid decline in the first few days.

Struvite as good as single super phosphate fertiliser

Slow release can be a good thing as this can avoid the loss of P to the environment, since P is released more gradually as the plant grows and takes it up. But this is very case dependent.

In any case, we showed that struvite is at least as good as single super phosphate when it came to the measured biomass of maize seedlings in our experiments. So, from our findings we can say that the P recovered as struvite can replace mined P.

How is struvite processed for use as a fertiliser?

To be used as a fertiliser, struvite does not require further processing, as the struvite granules themselves already have good slow-release properties. If struvite were to be used as a raw material for the production of N-P-K fertilisers, the recommended method would be to blend it with other Nitrogen (N) and Potassium (K) containing granules.

Any other (chemical) processing is likely to alter the mineralogic composition of struvite resulting in changes to its unique slow-release properties, while potentially leading to additional emissions, such as ammonia to the air.

In SUSFERT, we will answer the questions of how the further processing of struvite as a raw material in a fertiliser plant could change its physio-chemical, mineralogical and fertilising properties.

So, will every farmer soon use struvite?

Well yes and no! Let’s start with the yes. Struvite is currently marketed as a fertiliser by several companies based in the Netherlands, Belgium and elsewhere. Prices for struvite are extremely variable, depending on the source and the distributor. From what I know, prices range between 50 – 1,000 Euros per tonne of struvite (2019).

However, over the past year the prices of struvite have risen because there is now a legal basis for its trade in Europe within the Fertilizer Regulation, and also because of increasing raw material prices. Recently, the price of struvite was substantially higher than the prices in 2019.

What are the challenges ahead?

The major bottleneck for struvite is its low availability and its relatively low potential. Currently, the struvite available would only cover less than 0.1% of the P demand of EU agriculture.

The first challenge would be to widely apply struvite precipitation at wastewater treatment plants, and other liquid waste-streams that are high in P, such as in manures, and other P rich effluents, such as potato wastewater.

Our estimates show that, for instance, recovery from municipal wastewater would only result in a 13% replacement of EU agricultural P fertiliser (2017 data).

One reason for this is that struvite precipitation cannot capture some of the P that is contained in the sludge and liquid that wastewater treatment produces. This P will then be lost.

Therefore, other technologies such as the burning of sludge in dedicated installations and the subsequent treatment of the remaining ash to recover the P are of great importance.

Implementation of these technologies is likely to be stimulated in the future by changes in legal requirements such as in Germany, where P recovery will be required by 2029 (Klärschlammverordnung (AbfKlär) for installations that treat the wastewater of at least 100,000 population equivalent.

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Table 2 below shows the estimated amount of struvite produced in 2020 in the European Union (including plants with anticipated commissioning in 2020; *excluding sample that did not meet legal requirements). Muys et al. (2021)

Item	Min	Max
Total struvite (t struvite /year]	9,784	12,057
Total P equivalent [t P /year]	1,095	1,353
P equivalent of plants not meeting legal limits  [t P /year]	996	1,254
Municipal struvite [t P /year]*	698	956
Municipal struvite [%]*	64%	71%
Potato struvite [t P /year]*	272	370
Potato struvite [%]*	25%	27%
NL [%]*	35%	43%
DE [%]	15%	15%
BE [%]	16%	20%
Sum of NL, DE, BE [%]*	69%	74%

Struvite comes from human wastewater, is that not a problem?

Well, you would be surprised at how many agricultural fields are fertilised with animal manure currently. But yes, something that was almost not done before was to investigate the contamination of the struvite with pathogens.

The sampling we did was a bit too small to be conclusive, but we think that pathogens mainly could be found where organic contamination was present, and we could show that the values dropped when struvite was stored for some months.

What will happen next in SUSFERT?

In upcoming work, SUSFERT will analyse the behaviour of struvite in the fertiliser formulation processes and its effect on emissions and the fertilising properties.

SUSFERT will demonstrate further the use of struvite in multi-functional fertilisers that include bio-based coatings, as well as P and iron releasing bio-stimulants. Finally, we will carry out a Life Cycle Assessment (LCA) of all the products that we have developed.

If you want to know more, read:

Muys, M., Phukan, R., Brader, G., Samad, A., Moretti, M., Haiden, B., Pluchon, S., Roest, K., Vlaeminck, S.E. and Spiller, M.  2021.  A systematic comparison of commercially produced struvite: Quantities, qualities and soil-maize phosphorus availability. Science of The Total Environment 756, 143726 https://doi.org/10.1016/j.scitotenv.2020.143726

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This project has received funding from the Bio Based Industries Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 792021.

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Marc Spiller is a post-doctoral researcher at the University of Antwerpen (Belgium), with a focus on resource recovery from wastewater, P and N re-circulation between cities, industry and agriculture, including Life Cycle Assessments (LCAs) in the bio-economy.