Struvite - from wastewater to resource

Struvite precipitates from wastewater and has a phosphorus content that makes it suitable as a starter fertiliser for organic high-value crops.

By Karen Munk Hansen, Linda Rosager Duve

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Struvite is a granulated waste product from wastewater treatment plants and contains 12% phosphorus. In its raw form, struvite is relatively slow-acting because the phosphorus in struvite is not water-soluble. Plant availability increases the finer the particle size of the assigned struvite, as smaller particles have a larger surface area and solubility increases.

In 2024, we tested 3 different struvite products (pellets, powder and liquid) as starter fertiliser for potatoes. Struvite in pellet form is a commercial product, while struvite powder and liquid struvite were developed by the University of Copenhagen to increase P availability. Struvite powder is finely ground struvite and liquid struvite is pulverised struvite mixed in organic acid.

The trials were conducted in potatoes that in conventional cultivation have given significant additional yield for placed phosphorus fertiliser.

The struvite products were compared with the effect of triple superphosphate (TSP). All struvite products increased yield, but not significantly. A mere 7.5 kg P in a liquid product gave the highest tuber yield of 583 hkg/ha.

Treatment and phosphorus (P) kg/ha	Yield and additional yield, hkg/ha
Untreated	531
TSP 30 kg	13
TSP 15 kg	42
Struvite pellets, 15 kg	33
Struvite powder, 7.5 kg	27
Struvite liquid, 7.5 kg	52

Struvite as an organic fertiliser

The application and handling of struvite is like other granular phosphorus fertilisers. You get the best effect by applying struvite in connection with the establishment of the crop. You should pay attention to soil reaction rates and find the best possible balance between using as fine struvite granules as possible without compromising precise placement close to the seed.

The nutrient content of struvite:

Phosphorus 12%
Ammonium nitrogen 5%
Magnesium 10 per cent

Phosphorus content varies in Denmark, and in some places it is low and/or decreasing - especially on clay soils in Eastern Denmark. By fertilising with struvite, it is possible to maintain or build up the soil's phosphorus pool, thereby reducing the risk of yield loss.

Fertilisation with struvite is particularly interesting in Eastern Denmark, where there are very low phosphorus levels (see map of Denmark). An additional advantage of struvite is that it is a granulate that is easy and cheap to transport.

Struvite is a phosphorus-containing granulate that can be precipitated from wastewater.

Rules for the use of struvite

Struvite can be used in organic production when it fulfils the conditions and documentation described in Appendix 2.1 of the Guide to Organic Agricultural Production (Danish Agricultural Agency, 2025). Adequate documentation for the need on the individual field is soil analyses and phosphorus levels (Pt) below 3. The amount of phosphorus that can be applied is equivalent to fertilising up to phosphorus number 3.

The rules for marketing struvite are described in the article:
Struvite - rules for fertilisation in organic farming

Pure, high-quality fertiliser

The quality and purity of struvite is generally very high, especially in terms of heavy metal content compared to other organic fertiliser products. The content of heavy metals copper (Cu), zinc (Zn) and cadmium (Cd) is very low in struvite compared to rock phosphate, animal manure and sewage sludge (see table). Other problematic substances such as PFAS and microplastics found in sewage sludge may be present in struvite, but in such small amounts that they do not pose a problem.

Table 2. zinc (Zn), copper (Cu) and cadmium (Cd) content in different organic and inorganic fertilisers (mg per kg dry matter). Source: Bünemann et al, 2024.

Fertiliser type	Zn	Cu	Cd
Wastewater, EU	823	272	0,87
Dewatered sludge, EU	937	327	0,96
Struvite, EU	123	41,6	0,45
Struvite, ^DK1	198	37	0,12
Triple Superphospate	401	28,8	22,1
Rock Phosphate	163	16,3	21,3
Pig manure, DK	185	104	n.d^.2
Cattle manure, DK	93	10,2	n.d^.2

¹⁾ Average of 2 Danish struvite plants
²⁾ n.d. = not determined (not measured)

Production of struvite in Denmark is limited

In 2025, struvite is potentially produced at 4 wastewater treatment plants in Denmark (Aaby, Herning, Helsingør and Marselisborg wastewater treatment plants). However, struvite precipitation does not occur continuously at the four plants, and the amount of struvite currently produced corresponds to approximately 57 tonnes of phosphorus per year. If struvite is used as a starter fertiliser and applied in the same quantities as in the aforementioned trials, the current production of struvite is sufficient to fertilise between 6,000-12,000 hectares.

The production of struvite is therefore limited at the moment, but an increasing demand may mean that more wastewater treatment plants start producing struvite. 60% of Danish wastewater is treated at plants with digesters that could potentially set up plants for controlled precipitation of struvite. If all these plants produced struvite, the production of struvite could cover starter fertiliser for 140,000-280,000 hectares of organic high-value crops.

Importing struvite could also be an option in the long term. In Germany, Benelux and other EU countries, the production of struvite at wastewater treatment plants is more common than in Denmark. Struvite can be imported relatively easily and quickly if demand increases. An inventory of struvite for the entire EU shows that about 13% of the need for mineral phosphorus fertiliser can potentially be covered by struvite (Muys et al 2021).

Recycling phosphorus using struvite

Phosphorus is a scarce resource, but a necessary nutrient for crops. At the same time, phosphorus can pose an environmental problem if it ends up in rivers and lakes. Therefore, the circle must be closed to recirculate as much phosphorus as possible and minimise loss to the environment. Struvite can be part of the solution.

When phosphorus in wastewater can be returned to agriculture in the form of struvite, it benefits the circular economy and all links in the chain:

Better utilisation of a scarce resource - rock phosphate is mined in countries such as China, Morocco and Russia
Fewer deposits in the pipes of the wastewater treatment plant
More precise phosphorus fertilisation for the benefit of the aquatic environment
Geographical redistribution of phosphorus from surplus to deficit soils
Better establishment of phosphorus-demanding crops and potentially higher yields