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Contaminants were analysed and ecotoxicity tested for 3 struvites, 5 thermal recovered phosphates, and leachates, suggesting low risk to the environment from use in agriculture.

Three different struvites recovered from sewage sludge and five phosphate products recovered from sludge incineration ash were analysed for contaminants (organic and heavy metals) and tested for ecotoxicity (to aquatic plants, aquatic invertebrates, earthworms), with comparison to Triple Super Phosphate fertiliser. Leachates were also tested for ecotoxicity. The results were compared with a quantitative and relative risk assessment (Kraus & Seis 2015) conducted with single pollutants contained in the products.

The three struvites were precipitated from municipal sewage sludge (a) after digestion before dewatering (b) after digestion after dewatering (c) without digestion after sulphuric acid dissolution. The five thermal recovered phosphate products were raw sludge mono-incineration ash, MgCl2 and Na2CO3 calcination recovered P, metallic P slag and CaP/struvite precipitated after acid leaching of ash. Triple Super Phosphate (TSP) was purchased from Van Loon Hoeven B.V. (NL).

These nine products were analysed for heavy metals and the three struvites were analysed for six organic contaminants:

  • Benzotriazole – anticorrosive
  • Carbamazepine – pharmaceutical (anticonvulsive)
  • Diclofenac – pharmaceutical (analgesic)
  • Estrone – hormone
  • Mecoprop –pesticide (herbicide)
  • Sulfamethoxazole – pharmaceutical (antibiotic)

Benzotriazole, Carbamazepine and Diclofenac were detected in all three struvite samples at 1 – 113 ppb dry matter. Estrone, Mecoprop and Sulfamethoxazole were below detection levels in all three struvites. Metals analysed were lower in the struvites than in TSP, except for copper, iron, lead and mercury. Metals in the thermal recovered phosphate varied considerably.

Ecotoxicity tests

Ecotoxicity of the nine products, and also of copper chloride as reference, was tested using:

  • Chronic growth inhibition to duckweed (Lemna minor), test method ISO 200279, at five concentrations from 0.01 to 10 g DM/l
  • Acute toxicity and behavioural measurements with the aquatic invertebrate amphipod Gammarus fossarum, test method US EPA Gammarid Acute Toxicity Test with adjustments, at five concentrations 0.005 – 5 g DM/l)
  • Avoidance test with earthworms Eisenia fetida, test method ISO 17512-1 with adjustments, at five concentrations 0.05 – 5 % by dry weight in soil/compost.

Additionally, simulated leachate obtained by DIN 12457-1 (shaking 250 g DM of product in 0.5 l water for 24 hours at 20°C) was tested for toxicity on the gammarid as above, at dilutions of 0.001 – 1%. This was done for TSP and for the most recycled phosphate product with the most effect in the direct assessment. NOEC/LOEC (No/Lowest Observed Effect) concentrations were derived.

The authors note that TSP showed higher ecotoxicity at the higher concentrations tested, probably due to higher water solubility. The struvites however resulted in higher earthworm avoidance behaviour at lower concentrations: the authors consider that this may be due to the organic contaminants.

They conclude that agronomical relevant application levels of all of the recovered phosphate products and of triple super phosphate (TSP), in the worst case scenario, are unlikely to have acute toxic effects on earthworms, that the recovered phosphates might have a small impact on aquatic plants and TSP some negative impact on aquatic amphipods.

Overall, the authors conclude of the ecotoxicity with regard to the relative risk assessment (Kraus & Seis) that the recovered phosphate products, in particular struvite, showed to be of high quality and low hazard, and should be considered for agricultural phosphorus fertilisation.

“Ecotoxicological Assessment of Phosphate Recyclates from Sewage Sludges”, Water Air Soil Pollut (2017) 228:171

N. Rastetter, A. Gerhardt, LimCo International GmbH, Wollmatinger Str. 22, 78467 Constance, Germany N. Rastetter, K. O. Rothhaupt, Limnological Institute, University of Konstanz, 78464 Constance, Germany


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