In der Debatte um Strategien des Phosphorrecyclings zeichnet sich immer mehr ab, dass ein Ende der bodenbezogenen Verwertung von Klärschlamm nicht zielführend ist, wenn Schlämme für dieses Recyclingverfahren qualitativ geeignet sind.

Whether or not there will be a phosphorus (P) peak within decades, centuries or millennia, (Cordell and White, 2011; Scholz and Wellmer, 2013) one thing is for sure – phosphorus is a limited and, in its function as a nutrient, an essential and irreplaceable resource (Asimov, 1959; Smil, 2000; Filippelli, 2008). The debate on P limitation is often mentioned as motivation to foster activities regarding P recovery and recycling. The ambition of the European Commission (EC) to establish a circular economy in Europe goes far beyond that and is not primarily motivated by limitations of certain raw materials. From the European perspective and in the light of having just one small mine in Finland, the geopolitics and economic vulnerability are issues to be taken seriously. Europe is highly dependent on phosphorus imports (De Ridder et al., 2012) as reflected by the quantities given in figure 1. In contrast to the above mentioned issues, the waste and dissipation of phosphorus that exists in developed countries may lead to a different conclusion. The global resource efficiency for P along the supply chain from mine to fork is only 20% (Schröder et al., 2010). Given the figures of 225 million tons P rock globally mined in 2013 (USGS, 2015) and assuming that 90% of the mined P is used for food production, only 45 million tons of the mined quantity finally ends up in form of food on our tables. So, what can we do to increase the resource efficiency of P? Recently, the implementation of a coherent package of nutrient management strategies and measures to close the European P cycle has been proposed – the 5R strategy (Withers et al., 2015). The five R’s are Realign P inputs, Reduce P losses to waters, Recycle P in bio-resources, Recover P from waste and finally Redefine our food system. So, recovery and recycling can play an important role in improving resource efficiency and sustainable nutrient management. Although, there are various relevant waste streams carrying huge quantities of phosphorus dissolved in liquids or fixed in solids like in manure or organic waste, the focus of P-REX was laid upon P recovery and recycling from wastewater and sewage sludge.

In recent years several ways of recovering phosphorous from municipal wastewater have been developed. Depending on the applied technology the recovered products vary significantly concerning the concentrations of heavy metals and organic residues. Within the boundaries of data quality and present uncertainties a comparative risk assessment of seven secondary phosphorus fertilizers, sewage sludge, raw ash and triple super phosphate has been conducted for PCDD/Fs, PCBs, PAHs, As, Cd, Cr, Cu, Hg, Ni, Pb and Zn. Local exposure assessment was done using the kinetic model of the European Union’s Technical Guidance Document for all substances accounting for both fertilization and average atmospheric deposition. For substances of concern (Cd and Zn) the exposure was additionally refined using a solute transport model (HYDRUS-1D) and a precipitation model (Visual MINTEQ-software). An annual fertilizer amount equivalent to 60 kg P2O5/ ha × year by these products is assumed. In order to account for potential accumulation a time span of 100 years is modelled. Results indicate that out of the selected 11 (groups of) chemicals only cadmium and zinc are of concern. Regarding soil organisms, zinc is of concern for sludge, raw ash and one of the seven secondary phosphate fertilizers in case of soil-pH above pH 6.0. Regarding groundwater, cadmium and zinc are of concern below pH 6.0 since mobilization at this pH level increase significantly. No risk is expected regarding the endpoint humans. Among the investigated products struvites have shown the lowest phosphorus-specific heavy metal contents. For ash related products more data from full scale operations are needed to reduce still existing uncertainties like the influence of raw wastewater quality and WWTP operation on the final product.

The recovery of phosphorus (P) from sewage sludge, sludge liquor, or ash from monoincineration can be realized with different processes which have been developed, tested or already realized in full-scale in recent years. However, these pathways and processes differ in their amount of P that can be recovered in relation to the total P content in sludge, in the quality of the recovered P product, and in their efforts in energy, chemicals, fuels, and infrastructure required for P recovery. This study analyses selected processes for P recovery from sludge, liquor, or ash in their potential environmental impacts, following the method of Life Cycle Assessment (LCA, ISO 14040/44). Based on available process data from technology providers and end users, these processes are implemented in a hypothetical reference system for sludge digestion, dewatering and disposal in mono-incineration, including potential side-effects on mainstream wastewater treatment with the return load from sludge dewatering. Recovered products (e.g. P or N fertilizer, electricity, district heating) are accounted as credits for substituting equivalent industrial products. Depending on the maturity of the investigated process, collected process data of process efficiency, product quality, and energy and material demand originates from full-scale plants, pilot trials, or prospective modeling (status in 2014). This data is validated with the technology providers, transferred to the reference system and evaluated with a set of environmental indicators for energy demand, global warming, acidification, abiotic resource depletion, eutrophication, and human and ecotoxicity. Results show that pathways and processes for P recovery differ heavily in their amount of recovered P, but also in energy and related environmental impacts (e.g. greenhouse gas emissions). As direct struvite precipitation in sludge or liquor relies on the dissolved amount of P in digested sludge, these processes are only applicable in wastewater treatment plants with biological P removal. Here, they can recover 4-18% of total P in sludge with a relatively low effort in energy and chemicals, reducing return load to the mainstream process and eventually improving sludge dewaterability in case of direct precipitation in sludge. Acidic leaching of P from digested sludge can yield up to 48% of P for recovery, but requires a significant amount of chemicals for control of pH (leaching and precipitation) and for minimizing heavy metal transfer into the product. The quality of products from sludge and liquor is good with low content on heavy metals, leading to a low potential toxicity for humans and ecosystems. Leaching of monoincineration ash with sulphuric acid yields 70% P with moderate chemical demand, but the leached ash and co-precipitated materials have to be disposed, and the product contains some heavy metals. Complete digestion of ash in phosphoric acid and multi-stage cleaning with ion exchangers yields high recovery of 97% P in a high-quality product (H3PO4) and several coproducts, having an overall low environmental impact. Thermo-chemical treatment of ash can recover up to 98% P with moderate energy input in case of integration into an existing monoincineration facility, but the product still contains high amounts of selected heavy metals (Cu, Zn). Metallurgic treatment of dried sludge or ash can also recover up to 81% of P, but the process has still to be tested in continuous pilot trials to validate product quality, energy demand, and energy recovery options. Sensitivity analysis shows that other pathways of sludge disposal (e.g. co-incineration combined with upstream P extraction, direct application in agriculture) may also be reasonable from an environmental point of view depending on local boundary conditions and political targets. In general, the use of life-cycle based tools is strongly recommended to evaluate and select suitable strategies for regional or national concepts of P recovery from sewage sludge.

Dieser Beitrag vermittelt einen Überblick über die im Rahmen des EU Projektes P-REX erzielten Ergebnisse und Schlussfolgerungen. Neben der Bewertung von praxisrelevanten Verfahren zur Phosphorrückgewinnung aus dem Abwasserpfad und den jeweiligen Recyclaten geht es vor allem auch um Aspekte zur flächendeckenden Implementierung und Marktentwicklung. Vor allem integrative Ansätze, die auf eine bessere Ausnutzung der bereits vorhandenen Infrastruktur zur Optimierung des Phosphorrecyclings abzielen, bieten vielversprechende und vor allem kurzfristig umsetzbare Lösungen. Um jedoch Anreize für deren Umsetzung zu schaffen, bedarf es Entscheidungen und verlässlicher politischer Weichenstellungen. Für den Fall des Phosphorrecycling haben Goethes Worte „Wissen ist nicht genug, wir müssen auch anwenden! Wollen ist nicht genug, wir müssen auch tun!“ höchste Aktualität.