Types of recycled fertilisers and how to use them

Recycled fertilisers are recovered nutrients from manure, food scraps or waste. These alternatives are increasingly available and, thanks to improved technologies and quality certification, are becoming more reliable and safer to use.

These are the different types of recycled fertilisers and their uses.

is produced by anaerobic digestion of different substrates (animal manure, energy crops, sewage sludge, food industry waste,…). The composition depends on the input material but includes all the main crop nutrients (N, P, K, S) as well as stabilised organic matter.

is produced from animal manure that undergoes aerobic composting at temperatures of up to 70 °C. This reduces the volume of the manure, stabilises the carbon and reduces the short-term N availability. The high temperatures during composting kill germs and weed seeds. Due to the heating, sanitary risks are reduced.

has a high dry matter (around 60%) and organic matter content, whereas nutrient content is rather low. Green waste and household waste compost contain relatively little phosphorus (P). Compost has a high soil organic matter reproduction value due to the high stability of organic matter contained in compost.

is the liquid fraction of pig manure after primary separation in the barn (sloped floor underneath the slatted floor). Pig urine therefore mainly contains N and K and low concentrations of P and organic matter. N is readily available and application is recommended in spring.

are produced by dissolving N from manure or digestate with an acidic solution or washing it from barn air. Depending on whether nitric acid (HNO₃) or sulphuric acid (H₂SO₄) is used, ammonium nitrate or ammonium sulphate is produced. The recovery process technology determines whether the produced ammonium salts are available as liquid or solid/granular fertilisers. Ammonium nitrate and sulphate mainly contain N and usually only traces of P and K. Ammonium nitrate provides half of the N occurs in the nitrate form and half in the ammonium form. Moreover, ammonium sulphate contains substantial amounts of SO₃.

is produced by filtration and concentration of the liquid fraction of manure or digestate. N is mainly present in the form of ammonium. N content is relatively low (less than 1% N nowadays). However, technologies to upgrade these products are currently under development and products with a higher N content will be launched to market in the future. N content of currently available products is increased by adding urea or ammonium sulphate and they are used as a top-up fertiliser alongside animal manure

is widely used in Belgium and is obtained through biological treatment of animal manure. Processing converts N from the liquid fraction (LF) of manure, digestate or sewage sludge into N gas (N₂), which is absorbed by the atmosphere. Though this technique is commonplace in Belgium and produces an RDF, the actual process is not the most sustainable and circular, as N is emitted into the atmosphere while paradoxically having to fixed again from the atmosphere for the production of mineral N fertiliser. The effluent contains less than 10% of the original amount of P from the raw manure, as well as less than 5% of the original amount of N. Other soluble nutrients such as potassium, sodium and chlorides will largely remain in the effluent.

is produced by incineration or gasification of animal manure, sewage sludge, or wood. Ash is often used as a component in blended fertilisers. The main nutrients of ash differ depending on the source material:

P availability from ash varies greatly and depends on the soil conditions, climate, raw materials (with or without F) and the technology.

are produced by precipitating P from sewage sludge, urine, digestate, animal manure or effluent from agri-food industry activities. The resulting crystalline product is made available as granules, pure or blended as a component into a fertiliser. The main nutrients in struvite are P, Mg and a small amount of N (ammonium-N). Some types of struvite contain K instead of ammonium. P availability varies greatly but struvite was found to have the same efficiency as TSP, especially on sandy, slightly acidic soils, making it a promising alternative for P fertilisers using phosphate rock.

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