LCAs of nutrient recycling scenarios including harvesting mussels to reduce eutrophication and recycle nutrients.

A thesis compares LCAs of various nutrient recycling scenarios including harvesting of mussels to reduce eutrophication and then recycling as fertiliser, wastewater urine and blackwater separation, food waste and slaughterhouse waste.

Mussels (Mytilus edulis) are grown for food in Sweden in the Skagerrak sea. Mussels in the Baltic proper however are smaller, because of lower salinity, and cannot be used as food. Cultivation of mussels in the Baltic to remove nutrients and recycle as fertilisers was assessed. Field tests show that around 150 tonnes of mussels per hectare can be harvested after 28 – 30 months, cultivated on nets.

The mussels need to be processed, because fertiliser demand does not necessarily correspond to mussel production dates. Composting of mussels mixed with straw has been shown to be effective. Another process modelled but not tested would be crushing and storage as a pulp in water in anaerobic conditions to slow down decomposition: this would avoid the considerable nitrogen losses of composting, providing a fertiliser product adapted to the N:P requirements of crops.

Life Cycle Analysis
The LCA concludes that mussel harvesting for fertiliser use can be effective in removing nutrients from the Baltic and supplying renewable fertiliser nutrients. Cadmium levels in mussels can be significant, but are lower than in the mineral fertilisers and liming agents which were replaced by the composted mussels.

In the other scenarios considered in the thesis, choice of wastewater treatment system had a considerable impact on LCA conclusions. Urine separation reduced the environmental impact for the largest number of categories. Use of meat meal (slaughterhouse waste), blackwater, urine and mussels as fertilisers reduced greenhouse gas emissions and/or energy use compared to mineral fertilisers.

Slaughterhouse waste
LCAs of two different routes for valorisation of slaughterhouse waste (Animal By Products category 2) were compared:

  • Production of a meat meal fertiliser (Biofer, produced at Ortved, Denmark) for use as a fertiliser and use of fats for energy. This system was a net consumer of energy, because of energy used in processing the meat meal.
  • Combustion for energy of all the slaughterhouse waste together. This system was a net energy producer.

Biofer contains N and P (10:3 ratio) and comparable meat meal fertilisers contain c. 30% organic carbon. This can lead to excess phosphorus application to land because the N:P ratio is lower than crop needs. It contains c. 1.3 mgCd/kgP2O5 (compared to proposed EU Fertiliser Regulation limits of 20 – 60 mg).

Taking into account mineral fertiliser production, the LCA concludes that the production and use of meat meal as fertiliser can reduce greenhouse gas emissions, but that the environmental benefit depends on the available infrastructure.

Food waste
In a third study, different routes for treatment of food wastes were considered: anaerobic digestion and use of digestate as fertiliser, incineration and use of mineral fertilisers. The use of digestate scenario showed higher energy consumption, greenhouse emissions and other impacts, but the authors consider that this could be modified by improvements to the anaerobic digestion process.

“Bringing nutrients from sea to land e mussels as fertiliser from a life cycle perspective”, Journal of Cleaner Production 51 (2013) 234e244 J. Spångberg, H. Jönsson, P. Tidåker

“Environmental impact of meat meal fertilizer vs. chemical fertilizer”, Resources, Conservation and Recycling 55 (2011) 1078-1086 J. Spångberg, P.-A. Hansson, P. Tidåker, H. Jönsson

“Environmental impact of recycling digested food waste as a fertilizer in agriculture—A case study”, Resources, Conservation and Recycling 95 (2015) 1–14, Y. Chiew, J. Spångberg, A. Baky, P-A. Hansson, H. Jönsson.

Authors’ address: Swedish University of Agricultural Sciences, Box 7032, 750 07 Uppsala, Sweden.

“Recycling Plant Nutrients from Waste and By-Products. A Life Cycle Perspective”, thesis, Uppsala 2014, J. Spångberg

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