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A New Thermo-Chemical Approach for the Recovery of Phosphorus from Sewage Sludge

In order to utilise sewage sludge as phosphorus resource, several wet-chemical and thermochemical techniques have been developed so far, yet only few seem promising for large-scale implementation. In the EU-funded research project RecoPhos starting in March 2012, the Montanuniversitaet Leoben, the University of Stuttgart and eight partners from the industry will develop a new P-recovery process that produces phosphoric acid, a calcium-silicate slag and an iron alloy, all resources for the industry. The chemical principle, the Woehler reaction, will be realised on the chemically reacting packed bed of the so-called InduCarb retort.The ashes can be pre-purified in a reductive melt reactor connected upstream, using alternative heat sources like dried sludge.

In the discussion about the sustainable use of natural resources the element phosphorus is gaining importance quickly. One of the most promising secondary phosphorus resources is sewage sludge, which has led many countries in and outside the EU to commit their near-future sludge strategy to phosphorus recycling instead of mere disposal.

The goal of most previous research works was to recover phosphorus directly either from a wastewater side stream or from liquid and dewatered sludge by wet-chemical processes. Many of them are combined with the removal of nitrogen from the wastewater, producing the ready-to-use N-P fertiliser MAP. Others precipitate the phosphorus as metal phosphates for the industry.

However, there is a general trend in Europe towards thermal treatment of sewage sludge, being the only way to destroy organic pollutants adsorbed at the sludge. Additionally, in the sludge disposal chain the highest concentration of phosphorus is found in the ashes from mono-incineration. With roughly 15 mass-% of P₂O₅ they can be considered as low concentrated rock phosphates. Hence both common ways of rock phosphate processing, the chemical digestion with sulphuric acid, as well as the thermo-chemical reduction, have in the past been considered appropriate to separate phosphorus from its mineral Matrix.

Several other thermo-chemical approaches using dried sludge or ash attempt to leave the phosphorus in a purified mineral matrix, which shall serve as fertiliser directly or after further treatment.

Since up to now the development of efficient and competitive P-recovery techniques has had limited success, the EU as well as national and regional authorities in Europe are encouraging and funding further research in this field.

Copyright:	© Thomé-Kozmiensky Verlag GmbH
Quelle:	Waste Management, Volume 3 (Oktober 2012)
Seiten:	7
Preis:	€ 0,00
Autor:	Matthias Rapf Univ. Prof. Dipl.-Ing. Dr.techn. Harald Raupenstrauch Dr.-Ing. Carla Cimatoribus o. Prof. Dr.-Ing. Martin Kranert
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