Urine – the best way to recycle plant nutrients

­ – Slaughterhouse waste, source separated toilet water and source separated urine reduced climate impact compared with using mineral fertilizers. But the source-separated urine is the fertilizer that will do best if you look at all categories of environmental impact, says Johanna Spångberg, at the Department of Energy and Technology, SLU.

Environmental impacts have been estimated from the recycling of nutrients to agriculture from the different categories of waste and by-products from the society. The study includes slaughterhouse waste, toilet waste fractions, digested food waste and mussels, cultivated in order to remove nutrients from the Baltic Sea.

The study included aspects such as energy, climate change, eutrophication, acidification, cadmium flow into soil and consumption of non-renewable phosphorus. Environmental impacts were estimated using life cycle assessment (LCA), where it is important to take into account all the important aspects and features of a product's lifecycle.

– If one takes into account that the mussels have a lime-acting effect on the fields, they contribute with less cadmium to arable land than mineral fertilizers and liming materials together. However, one should also try to reduce emissions from storage and distribution if they should be comparable to mineral fertilizer use, says Johanna Spångberg.

In some cases, waste fractions are useful in more ways than serving as fertilizer. Digested food waste produced also biogas, and source separated urine decreased the nitrogen and phosphorus loads in the wastewater. These effects were included in the calculations.

The use of each fertilizer was compared with a reference scenario where mineral fertilizer was used. The two main conclusions of these studies were that the influence of the choices in the surrounding structure, such as the choice of alternative energy sources, affected the results a lot. Emissions from storage and distribution of organic fertilizers also affected the results, foremost for eutrophication and acidification. Better technology for emission reductions needs to be developed and used.

The EU waste hierarchy sets material recycling before other forms of reuse and recycling, and organic farms without access to manure have great needs for organic fertilizers. Those are some of the motives to recycle the plant nutrients from agriculture that are found in society’s waste fractions.