Poulsen, H.D., Møller, H.B., Klinglmair, M. & Thomsen, M. 2019. Husdyrs fosforudnyttelse og fosfors værdikæde fra husdyrgødning, bioaffald og spildevand. Faglig baggrundsrapport for fosforvidensyntese. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 84 s. - Videnskabelig rapport nr. 325
This report presents the status of the global phosphorus resources and investigates different options in relation to improving the utilisation of national phosphorus resources and thus reducing the consumption and emissions of phosphorus to the environment. This can be achieved by, among other initiatives, ensuring a better distribution of phosphorus at national level and optimising the utilisation of phosphorus resources in wastewater sludge and food waste in order to limit the purchase of commercial fertiliser. A reduction of phosphorus excretion from livestock to diminish the phosphorus content in farmyard manure is another way forward.
Raw phosphate has been mined for the production of mineral phosphorous fertiliser and feed phosphate for livestock for many years. EU's import of raw phosphate comes mainly from North Africa, Russia and the Middle East, which makes EU dependent on import from a relatively few producing regions, which in the long term could pose a strategic, i.e. a (commercial) threat. Today, there is no shortage of phosphorus at global level, but the concentration of phosphate production in a relatively few countries can potentially lead to shortage due to economic or political/strategic crises.
The known global phosphorus reserves comprise approximately 71 Gt with a world production of raw phosphate of about 0.2 Gt a year. In addition, there are large phosphorus resources, which today are not financially viable (i.e. too expensive considering the current trading prices), in among other countries. Saudi Arabia, the United States, Estonia, Kazakhstan and Peru. It is therefore likely that potential scarcity of raw phosphate on the global market will occur because of economic or political reasons rather than a real shortage of raw phosphate.
There is generally sufficient phosphorus in Denmark to cover the agricultural sector’s current need for phosphorus for crop cultivation without import of phosphorus as commercial fertiliser.
The problem is that the available phosphorus is not evenly distributed across the country; generally, there is a surplus in Jutland where there are many large livestock farms, while there is a deficit east of the Great Belt where the farms are dominated by crop production. Therefore, concentration of phosphorus in livestock manure is important in order to reduce the costs of transporting phosphorus from Jutland to Zealand and Funen.
Transport of phosphorus in slurry over long distances is cheaper if the solidpart of the slurry, where a very large proportion of phosphorus is found, is separated from the liquid part. This improves the possibilities for transport of the solid, and most phosphorus-rich, part over long distances. The liquid part, which is rich in nitrogen, can be used locally.
There is a wide range of technical solutions to separating the solid part from the slurry – from simple presses and centrifuges to more advanced methods such as membrane technique, evaporation, ultrafiltration and reverse osmosis. At present, only the simple methods provide an economic incentive that makes separation attractive, and it requires treatment of large volumes of manure. In this context, biogas plants play an important role in relation to the concentration of phosphorus from the livestock manure.
Livestock manure contains 80% of the potential Danish phosphorus resource, i.e. without considering commercial fertiliser. The remaining 20% comes from a number of minor sources where wastewater sludge and SSOW (source separated organic waste) constitute almost 18 out of the 20%. The other minor sources will not be discussed further here, but they may be of great local significance. As for livestock manure, there are different methods for utilising phosphorus in wastewater, sewage sludge and ash from sludge incineration. Common to these methods is that the costs per kg of phosphorus for sewage sludge cannot compete with phosphorus from commercial fertiliser, just as there is no willingness to pay for the nutrient value of the products at the present time.
If the use of commercial fertiliser should be replaced with phosphorus from, for instance, livestock manure or sewage sludge, the costs associated with the use of the various fertiliser types are essential.
Direct application of untreated liquid manure, degassed biomass or poultry manure provides a significant financial gain in relation to the use of phosphorus in commercial fertiliser due to the simultaneous supply of other nutrients. Regarding liquid fertiliser, local use (< 20 km) is required, while poultry manure can be transported over relatively long distances and still remain an inexpensive alternative to the use of commercial fertiliser due to the high dry matter content of poultry manure.
If the slurry is to be transported over long distances (e.g., 100 km), it is necessary to separate the slurry into solid and liquid fractions in order to reduce the costs of transport. However, the value of the nutrients does not offset the costs of separation and transport, and commercial fertiliser will therefore be a cheaper alternative.
As is the case with livestock manure, direct application of wastewater sludge is a possible way of recycling phosphorus to agricultural land. Today, approx. two thirds are applied directly even though the wastewater sludge has a negative market value when doing so. In addition, the sludge can be digested (in biogas plants), either separately or together with other organic matter (such as organic household waste), or mineralised (in planted tanks). In addition, struvite (magnesium ammonium phosphate) can be precipitated as a fertiliser product from sewage water or the sludge may be incinerated, after which the ash is processed to a phosphorus fertiliser. For struvite, and a few of the methods for processing of ash, some are commercialised or approaching market maturity. These technologies are able to supply end products that meet the quality requirements (especially regarding the heavy metal content) in the Waste to Soil Order (Statutory Order on Sludge) and the Bioash Order.
Livestock manure contains the vast majority of the Danish phosphorus amount, corresponding to about 80% of the total available phosphorus used for crop production. The phosphorus input to farmland can therefore be diminished by reducing the amount of phosphorus that livestock excrete in the manure. Over the years, livestock excretion of phosphorus has been reduced markedly through reduced feed consumption (improved feed efficiency) and through changes of the feed by use of, for instance, phytases to replace the feed phosphates. The possibilities of decreasing the excretion of phosphorus by livestock are not fully exhausted, although the potential differs greatly for the various livestock species. Overall, the possibilities of reduction are mainly associated with two factors:
(i) increased digestibility of phosphorus in the livestock feed.
(ii) improved adaptation of the feed’s content of digestible phosphorus to the actual need and genetic potential of the animals.
Some livestock holdings are challenged as to the relationship between nitrogen and phosphorus (N:P relation) in the livestock manure; especially in pig and poultry production this ratio is very low, while others are challenged by a high content of phosphorus in feed and crops. Most farms are also challenged by the demand for increased use of alternatives to soybean meal as a protein source since these often have a relatively higher phosphorus content than soy. This requires solid knowledge of the various initiatives before turning the potentials to practice.
Several of the strategies and technological solutions for recycling and use of phosphorus that are mentioned in this report may have a number of negative side effects in relation to, for example, climate and environment. This applies, for instance, to the production of biogas and composting of waste that can result in emission of ammonia and nitrous oxide. Also, there may be challenges in relation to the concentration of heavy metals by recycling of certain types of waste. A systematic analysis and quantification of these side effects are not part of this report. It is therefore important that an overall impact assessment including these elements is made prior to implementing the described strategies and technological solutions in practice.