Forskningsprojektbeskrivning FORMAS-ansökan April 5, 2005
Erik Levlin, Mark och Vattenteknik, Kgl. Tekniska Högskolan




An important goal in sustainable handling of municipal sewage sludge is to recycle nutrients such as phosphorus to agriculture without supply of harmful substances to humans or the environment (Hultman and Levlin, 1997). In year 2005 deposition of organic material including sewage sludge on landfill has been prohibited (SFS 2001:512). Incineration (ATV, 1997) and SCWO, Super Critical Water Oxidation (Gidner et al., 2000), are methods that eliminates all organic content and creates an inorganic sludge residue. The potential energy of the organic material can be utilised with SCWO which occurs in water of a supercritical phase at a temperature above 374 °C and a pressure higher than 22 Mpa.

Review of research area

This project is a continuation of research work made at the department. In earlier studies sludge incineration ash and SCWO-residues have been leached with acid and base (Hultman et al, 2002, Hultman and Löwén, 2001, Levlin et al., 2002, 2004a, 2004b, Stark, 2002, Stark and Hultman, 2003). Acid leaching gives a high degree of recovery but releases besides phosphate also the main part of other metals (including heavy metals). Leaching with base (NaOH) gives a less dissolution of metals however with lower degree of recovery (up to about 50 %). When leaching with base, the calcium content in the sludge probably binds phosphorus as calcium phosphate. Different methods to improve the leaching process have been suggested such as two stage leaching, leaching with hydrogen sulphide and combined leaching and separation with ion exchange.

Two stage leaching

A two-step leaching process has been tested experimentally (Levlin et al, 2005). The work including analysis of phosphate and dissolved metals wase made as a master thesis work at the Land and Water Chemical Laboratory, KTH. Alkaline leaching gives a phosphorus product with a lower metal contamination than leaching with acid. To increase the amount of leached phosphate at alkaline leaching calcium can before the alkaline leaching be dissolved with a weak acid at about pH-level 4. With a weak acid the dissolution reaction can occur at the desired pH-level without limitation caused by insufficient amount of acid. In the second leaching step the solid products was leached with 1 M sodium hydroxide. Two-step leaching gave for SCWO-residue 23% higher leaching of phosphate and for sludge incineration ash 11 % higher leaching. However, the degree of recovery was not high enough and aluminium was dissolved in the second leaching step, which has to be separated from the phosphate in a third process step.

Leaching with hydrogen sulphide

Iron phosphate is more soluble at anaerobic conditions there it is converted to iron(II) phosphate than at aerobic there it is in the form of iron(III) phosphate. A technology for recovery of phosphate from iron phosphate may therefore be based on natural reactions occurring in see sediment (Klee, 1985). At oxygen rich conditions iron is precipitated as ferric (III)phosphate. At oxygen free conditions iron is reduced to ferrous ions and some ferrous (II) phosphate is dissolved. If both ferrous iron and phosphate is released from the sediment, iron is oxidized to ferric iron and the phosphate is precipitated as ferric phosphate. However if hydrogen sulfide is present in the sediment the ferrous iron is precipitated as ferrous sulfide and the phosphate can be released from the sediment.

Rydin (1996) found at leaching sludge with rainwater at pH-level=5, that about 95 % of the phosphorus could be released at anaerobic conditions, while leaching at aerobic conditions only dissolved about 20 % - 30 %. If sulphide in the sediments reacts with divalent iron to ferrous sulphide and phosphate can be released from the sediment. Without sulphide iron is oxidised then the ions reach oxygen rich water and ferric phosphate is precipitated. The use of sulphide for phosphorus recovery has been studied by for instance Suschka et al. (2001). At addition of sulphate by anaerobic conditions sulphate was reduced by sulphate reducing bacteria to hydrogen sulphide. Ferrous phosphate was thereby dissolved and the iron was precipitated as ferrous sulphide.

Fe3(PO4)2 (s) + 3 H2SO4 —> 3 FeS (s) + 2 H3PO4 (aq) + 2 O2

In the Seaborne process ( hydrogen sulphide is used to remove heavy metals from leachate obtained by acid leaching of digested organic material. The hydrogen sulphide originates from biogas produced by anaerobic digestion of the leached material and a hydrogen sulphide free biogas is obtained. The process can be used for all kinds or organic waste inclusive sewage sludge. However chemical precipitated municipal sewage sludge which contains a lot of iron or/and aluminium require a larger amount of hydrogen sulphide than other types of organic waste.

Combined leaching and separation with ion exchange

Ion exchange can be used to separate chloride and ferric ions from solution after leaching with acid. The formed phosphoric acid will pass through the ion exchanger. The ferric and other metal ions can be separated with a cation exchanger and the chloride ions can be separated with an anion exchanger. After separation of metal and chloride phosphate will remain in the solution as phosphoric acid. The minimum amount of chemicals needed for processing one mole of ferric phosphate, is three mole hydrochloric acid for leaching, three mole hydrochloric acid for regeneration of the cation exchanger and three mole sodium hydroxide for regeneration of the anion exchanger. 1 mole FePO4, 6 mole HCl and 3 mole NaOH gives 1 mole H3PO4, 1 mole FeCl3 and 3 mole NaCl. If sulphuric acid is used the minimum amount of acid will be three mole per mole ferric phosphate. In the leaching process an excess of acid is needed for decreasing the pH-level to the acidity needed for dissolving the metal phosphate.

Use of ion exchange material for dissolution of phosphate from sludge or ash and separation of metals in the same process step, reduces the chemical need significantly compared to the amount needed for acid leaching followed by separation of metals in a following step. In mixing sludge with the cation exchange resin, hydrogen ions from the cation exchange material will dissolve the metal phosphate. The ion exchange material will take up the dissolved metal ions and phosphoric acid will remain in the solution.

CEX=H3 + FePO4 (s) —> CEX=Fe + H3PO4 (aq)

In this case there will be no need for an anion exchanger for removal of the anions from the leachate. Use of ion exchange material for dissolution of phosphate from sludge or ash and separation of metals in the same process step, reduces the chemical need significantly compared to the amount needed for acid leaching followed by separation of metals in a following step (Levlin, 2001). The minimum amount of chemicals needed for processing one mole of ferrous phosphate, is three mole HCl for regeneration of the cation exchange resin. 1 mole FePO4 and 3 mole HCl give 1 mole H3PO4 and 1 mole FeCl3.

Metals can be dissolved at higher pH-level since the ion exchange resin takes up dissolved metal ion and thus keeps the concentration of dissolved metal ions at a low level. This makes the need of acid for regeneration of the magnetic ion exchange resin smaller than the amount of acid needed for leaching. Since the ion exchange material in the process with combined leaching and separation is circulated an access amount of regeneration chemicals will not be necessary and the amount of chemicals needed for a process with separation after leaching with acid will be more than three times larger compared to the process with combined leaching and separation.

Project goals

The project goals are to further develop these proposed methods to improve the process for phosphorus recovery. The work will be made at the department Land and Water Resources Engineering. Some preliminary work has been made with the two stage leaching process. The other processes are less investigated and will be studied in a continuation of the project.

Two stage leaching

To get a higher degree of phosphorus recovery the second leaching step with base can be made at higher temperatures. Stendahl and Jäfverström (2003) dissolved up to 90% of the phosphorus at temperatures between 80-90 °C. However, aluminium was also dissolved at leaching with base, and has to be separated from the phosphate in a third process step. If iron is used as precipitation chemicals there will be less aluminium that can be dissolved in the second process step. Alternatively if aluminium is chosen as precipitation chemical, aluminium separated from phosphorus in a third process step can be used as precipitation chemical and the treatment plant can be self supplied with precipitation chemicals. To do these experiments magnetic stirrers with heating facilities has to be purchased.

Leaching with hydrogen sulphide

Use of sulphur reactions for leaching phosphorus from ash/SCWO-residues with hydrogen sulphide will be studied in one experimental arrangement. Hydrogen sulphide can be made by adding hydrochloric acid to iron sulphide. The hydrogen sulphide reacts with ash/SCWO-residues producing metal sulphide and phosphate is released. The liquid is removed for analyse of pH and phosphate concentration. A research question is if leaching with acid as in the Seaborne process before using hydrogen sulphide is necessary. Using hydrogen sulphide directly on the ash will create a simpler process. It will also be interesting to study how and to which degree the sulphur content in the sludge can be used for phosphorus recovery. The sulphate in the ash/SCWO-residue can be converted to hydrogen sulphide with use of sulphate reducing bacteria.

Combined leaching and separation with ion exchange

Phosphorus recovery with ion exchange material will be studied experimentally. On mixing ash or SCWO-residues with a cation exchanger, hydrogen ions from the cation exchange material will dissolve the metal phosphate and metal ions will be taken up by the ion exchanger. Thus a leachate containing phosphoric acid free from metal ions can be obtained. Methods to separate the ion exchange material will be studied. An ion exchange column can not be used if the fluid contains suspended solids which will be clogged by the solid particles. However, by using balls covered with ion exchange material, the ion exchange material can be separated by a sieve, which let the fine grinded ash to pass through. In an acid bath the ion exchange material is recharged with acid and the metal ions are released. The phosphoric acid is separated from ash or SCWO-residues and can be concentrated by evaporation. Use of ion exchange processes, make it possible to recover the phosphate as phosphoric acid, which is produced from apatite ore, thus preserving the limited apatite ore resources and also other resources, mainly sulphur, needed for producing phosphoric acid from apatite.

International cooperation

The work with phosphorus recovery from sludge have been presented at seven seminars within the Swedish-Polish Research Co-operation project, which the research group organized by professor Bengt Hultman annually have organized since 1997 together with scientists from different Polish Universities (


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