Struvite is a very pure mineral product and a safe phosphorus fertiliser

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Struvit is a recycled fertiliser that is recovered from wastewater. Struvite is primarily a phosphorus fertiliser and contains on average about 12.6% P, but also 5.7% nitrogen in the form of ammonium and 9.9% magnesium. read more about struvite and how you can use it as a fertiliser in the article:

Struvite - from wastewater to resource

Because struvite is derived from wastewater, many farmers and consumers have expressed concerns about contamination, but struvite is a very pure mineral product. Although it can contain small amounts of contaminants, the high phosphorus content also means that the application rate per hectare is low.

The amount of pollutants per kg of nutrient is low compared to other organic fertilisers. Therefore, struvite poses a lower risk compared to fertiliser alternatives such as cattle manure, pig manure and rock phosphate. Overall, struvite is considered a safe phosphorus fertiliser.

Heavy metal content is low in struvite

Sewage sludge has long been associated with heavy metal contamination. However, due to advances in wastewater technology, heavy metal contamination in sewage sludge itself has drastically decreased1^,2. this can be seen in the example of cadmium (Cd) in the UK, where Cd contamination changed from around 160 mg/kg in the 1960s to below 2 mg/kg from the 2000s ^onwards3.

Heavy metal contamination, especially zinc, copper and cadmium contamination from struvite in Europe (Table 1) is lower or comparable to animal manure. However, the heavy metal content of struvite is significantly lower compared to sewage sludge ash, showing how selectively struvite is formed.

Table 1: Heavy metal contamination (zinc (Zn), copper (Cu) and cadmium (Cd)) in different fertilisers in the EU (Source: Bünemann et al., 2024).

However, the extent of heavy metal contamination can vary between different struvite production systems and depending on the sewage sludge. A recent study tested 3 struvite samples from Denmark (Aaby, Herning, Marselisborg in Aarhus) using the Phosphogreen system. They all had levels below the detection limit of all regulated heavy metals except zinc. However, the detected zinc contamination was very low in the range 1.6 - 4.2 mg/kg dry matter, far below the legal limit of 1500 mg/kg dry matter ^4.5.

The Innovation Centre for Organic Farming recently had two different samples analysed for their heavy metal content. In both samples, the content of heavy metals was far below the limit values. However, it is important to note that due to variations in the analysis methods, the detection limits were lower, resulting in the detection of more values.

Table 2: Heavy metal contamination of five Danish struvite samples compared to the permitted EU limits (<DL=below detection limit; n.d.= no data) (Source Muys et al., 2021 and Innovation Centre for Organic Farming)

Organic contaminants are largely absent

Struvite is virtually free of organic pollutants. This is shown in several different ^studies5-7. Struvite has a naturally low content of organic compounds because it is a mineral precipitation product.

Organic carbon can be used as a measure of how much organic material, which can contain contaminants, is present in struvite. The legal limit in the EU is 1% of dry matter. The previously mentioned three Danish samples showed a range of 0.3-0.7% and were thus below the limit value4^,5. In addition, the slow build-up of organic contaminants in the soil along with minimal crop transfer poses minimal risk to consumer ^health2.

Pharmaceutical and antibiotic residues are rarely found in significant amounts in struvite, and are many times lower than in sewage sludge itself7^,8.

Per- and polyfluoroalkyl substances (PFAS) are less ^studied2. PFAS, a group of organic pollutants, have received significant media and research attention in recent years. A recent German study showed that PFAS levels are lower in struvite than in sewage ^sludge9. The EOF value (a measure of PFAS) in sludge samples from Germany and Switzerland ranged between 154 and 7209 µg per kg, while struvite had EOF values between 96 and 112 µg per kg. However, further research is needed in this area.

The microbiological health risk is low

The recycling of products derived from waste, especially human excreta, raises concerns about microbiological health risks such as contamination with pathogens, human viruses and bacteria. Low levels of these can be found in raw struvite, but drying struvite at temperatures above 35°C significantly reduces their viability5^,10.

In 2021, a Danish struvite sample was tested for 5 different microbial contamination indicators and they were all below legal limits (Table 3). Furthermore, their lifetime in soil is limited and only short-term transfer to plants could be measured, even with artificially added samples of untreated sewage ^sludge11.

Table 3: Microbial contamination indicators measured for a 9-month stored struvite sample from Denmark (Source Muys et al., 2021).

The article is a literature review and the small raised numbers in the text refer to the reference list (pdf file, 1 page)