Phosphorus extraction from dairy cattle slurry with low acid dose and precipitation of calcium phosphate not struvite.

Phosphorus recovery from waste streams by acid/alkaline extraction and sub-sequent chemical precipitation is a widely researched and practiced method. This Portuguese study does not attempt to differ from the path, but investigates P extraction and precipitation from organic fraction of municipal solid wastes (MSW) after anaerobic digestion and from dairy cattle slurry.

This paper presents (duplicated) laboratory extraction (phosphorus dissolution) experiments performed on 40 gDM biowaste samples using nitric acid (HNO3) or sodium hydroxide (NaOH) at a liquid to solid ratio of 25 for 48 h. 100 ml, 1 hour, stirred beaker precipitation tests were then carried out, adjusted to conditions expected to result in struvite precipitation: pH 8.0 (using NaOH) and molar ratio Mg:N:P of 1:1:1 (using NH4Cl as nitrogen source and MgCl2 as magnesium source). The precipitate was analysed with X-ray Diffraction (XRD) and Scanning Electron Microscopy coupled with Energy Dispersion Spectroscopy (SEM-EDS).

MSW (municipal solid waste) digestates were collected from an anaerobic digester fed with mechanically separated organic fraction of MSW (Portugal). The sample was collected after the centrifugation process. The dairy cattle slurry sample was collected from a local farm (Coimbra, Portugal). The sample, comprising scraped cattle excreta and wash down, was collected from the ditch connecting the animal housing to the slurry storage pit. The samples were refrigerated and subsequently analysed for water content, ash content, organic matter, pH, electric conductivity, total P, Ca, Mg, K and heavy metals (Cu, Zn, Cd and Pb).

P-recovery potential
The concentration of P in MSW digestate is 0.8% (dry weight) and in the dairy cattle slurry 0.4% (dry weight). Although the P levels are lower than, for example, in sewage sludge or the ashes thereof, the P recovered from MSW represents a significant yearly contribution of 36 tonnes P from a city with 100,000 inhabitants, or over 1000 tonnes P from a city with 3.4 million inhabitants (e.g. Madrid, Berlin). Ca is the most abundant macro-element in both MSW digestate and dairy cattle slurry. MSW digestate contains higher amounts of heavy metals compared to dairy cattle slurry, in particular Pb, but both waste streams are well within the heavy metal limiting values for sludge application in agricultural soil in Portugal.

For both wastes P extractions were higher using HNO3 (acid) than using NaOH (alkali). The P extraction from dairy cattle slurry reached 90-100% (depending on pH), and sufficient extraction was reached already at a relatively high pH of 4.5, reducing the need for acid reagent. Reagent consumptions were not reported in the article. The P extraction from MSW digestate reached about 90%, and contrary to dairy cattle slurry, pH below 2 was required to reach sufficient extraction. The difference is due to the different form P is present in these wastes. In MSW digestate P is incorporated into the cellular structure of the microorganisms, making extraction more difficult, whereas in dairy cattle slurry 60-90% of P is in inorganic form and is more readily available. The P extraction with NaOH did not exceed 22% for dairy cattle slurry and 9% for MSW digestate. Similar to P extraction, the extraction of heavy metals was also higher in dairy cattle slurry compared to MSW digestate, Cd extraction being the highest compared to other heavy metals. In the precipitation experiments the reduction of P in solution was around 94-98% (depending on extraction solution) for dairy cattle slurry and 96-99% for MSW digestate.

Calcium phosphate not struvite
According to the XRD and SEM-EDS results, the extracted P reacted with Ca forming non-crystalline calcium phosphates instead of struvite. Similar to several other studies on dairy manure, the high Ca:Mg ratio of 12:1 in this study could have inhibited the formation of struvite. The authors suggest that the calcium phosphate can be used as a fertilizer or as a raw material to fertilizer industry, provided that the presence of heavy metals does not exceed acceptable levels. SCOPE editor’s note: whereas the fertiliser effectiveness of struvite is well documented (see SCOPE Newsletter n° 121) this is not the case for recovered calcium phosphates. The study does not give an economic outlook for the proposed process.

References
"Valorisation of Phosphorus Extracted from Dairy Cattle Slurry and Municipal Solid Wastes Digestates as a Fertilizer" Waste and Biomass Valorization: 1-9.2015 http://dx.doi.org/10.1007/s12649-015-9466-0

V. Oliveira (1), L. Ottosen (2), J. Labrincha (3), C. Dias-Ferreira (3). 1 = Research Centre for Natural Resources, Environment and Society (CERNAS), College of Agriculture, Polytechnic Institute of Coimbra, Bencanta, 3045-601 Coimbra, Portugal. 2 = Dept. Civil Engineering, Building 118, Technical University of Denmark, 2800 Lyngby, Denmark. 3 = Materials and Ceramic Engineering Department, CICECO, University of Aveiro, Campus Universitario de Santiago, 3810-193 Aveiro, Portugal.

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