Abstract
Between 2004 and 2011 the German Government funded 17 different projects to develop techniques of phosphorus recycling from wastewater, sewage sludges, and sewage sludge ashes. Several procedures had been tested, such as precipitation, adsorption, crystallization, nano-filtration, electro-dialysis, wet oxidation, pyrolysis, ion exchange, or bioleaching. From these techniques, 32 recycling products were tested by five different institutes for their agronomic efficiency, that is, their plant availability, mainly in pot experiments. This manuscript summarizes and compares these results to evaluate the suitability of different technical approaches to recycle P from wastes into applicable fertilizers. In total, 17 products of recycled sewage sludge ashes (SSA), one meat and bone meal ash, one sinter product of meat and bone meal, one cupola furnace slag, nine Ca phosphates from crystallization or from precipitation, Seaborne-Ca-phosphates, Seaborne-Mg-phosphate, and 3 different struvites were tested in comparison to controls with water soluble P, that is, either single super phosphate (SSP) or triple super phosphate (TSP). Sandy and loamy soils (pH: 4.7–6.8; CAL-P: 33–49 ppm) were used. The dominant test plant was maize. Phosphorus uptake from fertilizer was calculated by the P content of fertilized plants minus P content of unfertilized plants. Calculated uptake from all products was set in relation to uptake from water soluble P fertilizers (SSP or TSP) as a reference value (=100%). The following results were found: (1) plants took up less than 25% P in 65% of all SSA (15 products); (2) 6 products (26%) resulted in P uptake of 25 and 50% relatively to water soluble P. Only one Mg-P product resulted in an uptake of 67%. With cupola furnace slag, 24% P uptake was reached on sandy soil and nearly the same value as TSP on loamy soil. The uptake results of Ca phosphates were between 0 and 50%. Mg-P products from precipitation processes consistently showed a better P supply in relation to comparable Ca-P compounds. With struvite the same P uptake as for water soluble P was reached. The fertilizer effect of the tested P recycling products can clearly be differentiated: TSP = struvite > Mg-P = sinter-P > Ca-P, cupola-slag > thermally treated sewage sludge ashes > meat-and-bone meal ash = Fe-P.
Highlights
Introduction and OutlinePrice fluctuations of phosphorus fertilizers since about 2005 indicate that the market for P has become volatile
These results indicate that ashes reheated to about 1000 ◦C together with CaCl2 to remove heavy metals are much less available to plants than super phosphate (SSP) or triple super phosphate (TSP)
A phosphate availability comparable to TSP or SSP was only given for struvite, that is, magnesiumammonium-phosphate, if it was poor in Fe impurities
Summary
Price fluctuations of phosphorus fertilizers since about 2005 indicate that the market for P has become volatile. Some reasons were the closing of two phosphate rock mines in the USA and the export stop for P in the USA and China. P markets in India and Southeast Asia increased. Soils in sub-saharan Africa are still poor in plant available P; new markets will develop in future. Prices had dropped again after this tremendous increase, they never reached their old level and a steady increase can be expected as already outlined in the CRU-report ‘Phosphate rock: ten year outlook (2011)’ (refer to scope newsletter No 81, November 2011, https://phosphorusplatform.eu/images/download/ScopeNewsletter81.pdf)
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