Are we retrieving phosphorus for recycling, are we just purifying the water, or even worse: are we just hiding the problem away, pretending Retention is the same as purification and that eutrophication is not a real threat?
A reflection: Some people tends to think Climate Change is somebody else’s problem. ‘My possible contribution to reducing global CO2 footprint is so negligible, so I leave it to someone else – the politicians, my neighbors… – to handle the problem’. Do we see the same attitude to the phosphorus challenge?
Since the 1990ies peak-phosphorus is a highly discussed topic. However, there is as of today no consensus how long the reserves for commercial quality mineral phosphate will last, with predictions ranging from 50 to several hundreds of years. Predictions are dynamic and can change over time, depending on the what types of mineral deposits are regarded as a commercially viable reserves. Just to give you an idea refer to the table below which summarises historic extraction volumes for mineral phosphate, as well as the historically and currently known size of the world reserves*.
* Source: Bundesministerium für wirtschaflche Entwicklung und Zusammenarbeit und Entwicklung (BMZ), 2013 (German ministry for development and collaboration)
However, the majority of raw phosphate is located in politically unstable regions, and phosphorus mining is often associated with significant interventions in the landscape and environment. The European Commission has added phosphate rock to the list of “critical raw materials” in 2014 [EUWL]. Another important aspect is the fact that the remaining phosphate reserves have a higher amount of heavy metal contamination, which could affect their usefulness for the production of fertilizers.
The major challenges using phosphate rock are:
The most widely used sources for the recycling of phosphorous are wastewater and sludge, other sources are ashes or manure. The majority of approaches are based on precipitation of Struvite, i.e. MgNH4PO4 x 6H2O. Another common approach is the precipitation of phosphorous in the form of calcium minerals, i.e. hydroxylapatite Ca5(PO4)3(OH) or calcium phosphate Ca3(PO4)2.
Polonite filters are a realistic alternative to the above mentioned common approaches.
The main components of Polonite are silica and calcium oxide, of which the latter is the active component responsible for the sorption of phosphate from wastewater. The phosphate sorption capacity depends on the particle size and surface area. For particles of up to 0.5 mm the maximum phosphate sorption capacity measured was 119.6 mg/g, corresponding to 12 weight-% phosphate or 5 weight-% phosphorous.
The Polonite filter that is going to be used in municipal WWTPs has a particle size between 2-6 mm, and a phosphorous sorption capacity between 4% to 2%. Of those two values the latter was regarded as more realistic and is the one used for further discussions. Also, it should be noted that some of the calcium oxide is washed out during the filtration process, however, the exact amount is unknown. The table below shows the material contents of fresh and saturated Polonite.
* Source: G. Renman, Z. Brogowski, Characterisation of Opoka, Polish Journal of Environmental Studies, Vol. 13, No. 1, 2004