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Technology

2.1.05 Phosphorous recycling – wastewater and sewage sludge sources of a valuable resource

Phosphorous is essential to all life. It requires a great deal of energy to take mined phosphate ore and produce mineral phosphorus fertiliser. What’s more, these ores are finite: current knowledge indicates that the known reserves economically viable for mining will be exhausted in around 100 years. Scientists have therefore been working for some years now on procedures enabling the efficient recovery of phosphorus from wastewater and sewage sludge.

One of these research projects was funded by the Federal Ministry of Education and Research (BMBF). Entitled Recycling of Phosphorus – Ecological and Economic Evaluation of Different Processes and Development of a Strategical Recycling Concept for Germany” (PhoBe), it involved five institutes from various faculties and was led by the Institute for Environmental Engineering (ISA) at RWTH Aachen University. PhoBe (completion date: end of 2011) was an all-encompassing project summarising the results of the projects funded under the BMBF “Recycling management of plant nutrients, especially phosphorus” initiative and providing global analyses. This funding initiative was launched in 2004 in conjunction with the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU).

As the price of mineral raw phosphate is a deciding factor in evaluating the economic feasibility of recycling procedures, a medium to long-term assessment (2030) of the global price development was conducted within one of the eight work packages. The relevant phosphate-rich material flows in Germany were also identified and qualitatively measured. The phosphate products obtained from the recovery processes developed as part of the funding initiative were analysed for impurities and evaluated for their effectiveness as a fertiliser compared with conventional phosphate fertilisers (such as triple superphosphate).

In an additional step, the specific production costs of the developed processes were determined using a cost assessment and the processes were balanced in terms of ecological aspects. The step to follow this was to use the results gained to develop a recovery concept for Germany that demonstrates which material flow is appropriate for recycling. Another key focus was to look ahead from a technology perspective, by means of a survey of experts and the identification of future prospects for phosphorus recycling in Germany.

Price development of phosphoric acid

Price development of phosphoric acid
Price development of phosphoric acid
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Forecast for price development

A methodical approach was used for the medium and long-term price development of phosphate, beginning by analysing the fundamental data separately – the development of supply and demand – and then merging them for a subsequent price development. Two scenarios were examined, assuming a rise in phosphate consumption (one or two percent a year). The slow increase mirrors the development of recent years (Business as Usual, BAU); the faster increase would be due to increasing cultivation of plants for biofuels.

As phosphoric acid is the base substance in producing phosphate-based fertiliser, the price development of both raw phosphate and phosphoric acid was assessed: the first scenario saw an increase in the price of phosphoric acid to USD 660 a tonne by the year 2030 and the second saw an increase to USD 760 (see graph on the price development of phosphoric acid).

Magnesium ammonium phosphate (MAP) and sewage sludge ash (small image)

Magnesium ammonium phosphate (MAP) and sewage sludge ash (small image)
Magnesium ammonium phosphate (MAP) and sewage sludge ash (small image)
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The analysis of the secondary phosphate obtained in the funding initiative showed that all the magnesium ammonium phosphate (MAP) produced was beneath the limits (and almost all products beneath the level for labelling obligation) of the 2008 German Fertiliser Ordinance. The effectiveness of the fertiliser was tested on the first and second crop of maize planted in sandy and clay soil in comparison with triple superphosphate, raw phosphate and a zero control. Results so far indicate that the recovered secondary phosphate is in no way significantly different from triple superphosphate and is therefore comparable with conventional fertilisers in this regard.

Secondary phosphate is not yet competitive

The cost assessment of the processes developed during the funding initiative showed that the specific production costs for a kilogram of secondary phosphate are – depending on the technical effort required and the recovery potential of the process – between EUR 2 and 13 per kilogram and are therefore not yet in a position to compete with conventional phosphate fertiliser (approx. EUR 1.50/kg). And yet phosphate recycling is worth it even today: in cases where there is an additional benefit – such as avoiding pipe blockages due to deposited MAP or improving the drainage of sewage sludge.

The material flow balance produced for Germany states that the amount of phosphorous in wastewater and sewage sludge theoretically available for phosphorous recycling is around 70,000 tonnes a year. The contribution from sewage sludge is particularly significant, around a quarter of which is currently disposed of in mono sewage sludge incinerators. The combustion process destroys most of the germs, odorous substances and organic matter, but the phosphorous content is still fully present as a residue in the ashes. Tests have shown that the proportion of phosphorous in sewage sludge ashes is around 6% and thus the highest concentration of phosphorus compared with other sources (sewage plant outflow, sludge liquor, sewage sludge).

Potential of up to 45,000 tonnes

The mono-incineration capacities in Germany are around 520,000 tonnes of sewage sludge a year, with current operation at more than 90%. If all sewage sludge ashes produced from mono incineration were fed into phosphorous recovery, around 13,000 tonnes of phosphorous could be recovered every year. If sewage sludge that does not undergo mono incineration or agricultural recycling were to be included, the major sewage plants (for >100,000 residents) could recover a further 5,000 tonnes. Assuming that agricultural recycling of sewage sludge will be further restricted in future and thus that heat recycling will increase, a scenario has been calculated in which all sewage sludge is burned in mono-incineration facilities and then used for recovery: this would recover around 45,000 tonnes of phosphorous a year, equating to around a 60% substitution of phosphorus fertiliser.

Experts believe that phosphate recycling could be implemented in industrial countries between now and 2030 and be economically viable – this is the result of a survey called “Dringlichkeit der Phosphorrückgewinnung, Erfolgspotenzial der Phosphorrückgewinnung aus Abwasserbehandlung und Klärschlamm, Potenziale der Rückgewinnung aus Klärschlammasche und Phosphatrückgewinnung im Kontext eines Systemwandels in der Wasser- und Abfallwirtschaft” (phosphate recovery as a priority, success potential of phosphorus recovery from wastewater treatment and sewage sludge, potential of recovery from sewage sludge ash and phosphate recovery within the context of change of system to a water and waste economy).

Project website www.phosphorrecycling.eu/index.php?lang=en

RWTH Aachen
Institute for Environmental Engineering

Univ.-Prof. Dr.-Ing. Johannes Pinnekamp
Mies-van-der-Rohe Straße 1
52074 Aachen, Germany
Tel.: +49(0)2 41/80-2 52 07
Fax: +49(0)2 41/80-2 22 85
E-mail: isa@isa.rwth-aachen.de
Internet: www.isa.rwth-aachen.de
Funding reference: 02WA0805 – 02WA0808>
Ressource Wasser
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www.ph...g.eu/?lang=en