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Nutrient recycling

I was at a meeting here in Finland recently and someone from the audience asked the speaker (a fertilizer manufacturer) what will happen when phosphorus ceases to exist. I was amazed, because I didn’t know that the alchemists were back!

Phosphorus is not about to disappear, but it does spread to places where it should not go: Groundwater, the Baltic Sea, and so on. Unfortunately, it is much cheaper to excavate phosphate rock than to separate phosphorus from groundwater. There seems to be just one source for some 77% of the world’s phosphate rock reserves – in Morocco. The next largest producer is China with around 6%. To me this means one supplier only.

Phosphorus seems to be the thing that will limit our ability to feed mankind in the future, as nitrogen is all around us. Something must be done to enable phosphorus recycling.

A recent study performed here in Finland shows that all phosphorus fertilizers could be replaced by animal manure.

The only thing that is needed is an economical way of concentrating the phosphorus in manure and transporting it to where it is needed. It’s as simple as that. Of course, Finland may be an exception in that it has an abnormal ratio of animals to cultivated land (15 million animals to 2.2 million hectares of cultivated land). The total amount of animal manure contains some 17 million kilos of phosphorus. A large part of it is, of course, spread over grasslands by the producers, cows. No treatment or transportation needed here!

The problem with chickens is that research suggests that the more chickens there are per hectare, the higher the concentration of phosphate in the soil. This means that chicken litter cannot be applied on nearby fields – instead it must be transported to other locations. Although the litter is relatively dry, particularly when compared to cow slurry, the cost of transportation is excessively high.

Ductor Oy has a solution. We take chicken litter and convert it into renewable energy as well as a nitrogen fertilizer, ammonium sulfate, and a solid phosphorus fertilizer.

Why are we at Ductor unique? We have developed a fermentation process in which microbes convert organic nitrogen into ammonium that can be easily removed from manure using conventional stripping equipment. This way the unsuitability of chicken litter as anaerobic digestion substrate is eliminated.

This means that chicken litter with suitable nitrogen levels can now enter the biogas digestion process and around 20% of the organic matter will turn into biogas. This in fact concentrates phosphorus in the digested material. There is then a solid-liquid separation process and the digestate is dried in a super-heated steam dryer to form up to 80% dry matter. This can be pelletized for more convenient spreading. This means that we have a phosphorus fertilizer that can be transported and stored economically. The phosphorus content is increased from 1.7% (17 kg per ton of fresh chicken litter) to 6.6% in the dried digestate. In addition, the nitrogen removed from the litter is turned into ammonium sulfate, a nitrogen fertilizer.

This is nutrient recycling, not alchemy.

Written by Ilkka Virkajärvi, CTO Ductor


World Fertilizer Trends and Outlook to 2018, FAO, Rome, 2015

Luonnonvara- ja biotaloudentutkimus 45/2017, National Resources Institute Finland, Helsinki 2017