Forskningsprojektbeskrivning FORMAS-ansökan Mars 5, 2006
Erik Levlin, Mark och Vattenteknik, Kgl. Tekniska Högskolan


RESEARCH PROGRAMME

PHOSPHORUS RECOVERY FROM ACID LEACHING OF INORGANIC SEWAGE SLUDGE RESIDUES

Introduction

Phosphorus removal from municipal wastewater has a long tradition in Sweden to protect receiving waters from eutrophication. Phosphorus removal by chemical precipitation with iron salts as dominating precipitation agent is needed to reach the high effluent standards. Phosphorus is removed with the sludge there it is strongly bound to metal ions. The phosphorus can be recovered as fertilizer by using sewage sludge in the agriculture. However, concern about the metal contamination of the sludge limits the use of sludge as fertilizer. Phosphorus recovery by spreading sludge on agricultural land has been stopped to a high extent due to resistance from the farmers´ organisation (LRF) and food industries. Development of methods to recover phosphorus from sludge will make handling of sewage sludge more sustainable.

Review of research area

Different methods for phosphorus recovery, such as KREPRO, BioCon or Aqua Reci have been suggested, which are based on dissolving phosphorus with acid or base. Most phosphates are soluble in acid, but some calcium phosphate compounds are insoluble in bases (Stumm and Morgan, 1981), which will reduce the degree of phosphorus recovery then leaching with base. In the KREPRO-process, sludge is leached with acid and phosphorus is recovered by precipitation as iron phosphate. On leaching with acid a leachate with phosphorus and metals is obtained. Most metals are more insoluble in bases than in acids, and leaching with acid should therefore give a phosphorus product with a higher contamination of metal. Since iron phosphate has the lowest solubility it is difficult to recover the phosphorus as other products than iron phosphate. Iron phosphate has no commercial value as raw material for the phosphate industry, and the low solubility makes it less favorable to use as fertilizer. For the production methods used in the phosphate industry iron phosphate is a pollutant, which binds phosphorus and reduces the amount of produced phosphorus. The global deposits of economically mineable phosphate minerals are estimated to be 109 ton phosphorus and the total amount in the sediments is estimated to be 1015 ton phosphorus (Butcher et al., 1994). Many different phosphate minerals are available, but only apatite (calciumphosphate, Ca3(PO4)2) is used for phosphate production (Corbridge, 1995). Since the phosphate in the sludge originate from phosphorus products produced from calcium phosphate ore, recovering the phosphate as iron phosphate will not preserve the limited calcium phosphate resources.

In the proposed BioCon-process sludge incineration ash is leached with acid and phosphorus is to be recovered as phosphoric acid by ion exchange. The ion exchange process has however been difficult to accomplish and recovery by precipitation as iron phosphate has been considered. The use of SWCO combined with phosphorus recovery is commercially developed as the Aqua Reci process (Stendahl and Jäfverström, 2003). In the Aqua Reci process the leaching is made with base, which selectively dissolves phosphorus, and phosphorus can be precipitated as calcium phosphate by addition of calcium.

In earlier research work phosphorus recovery technologies to dissolve phosphorus from inorganic sewage sludge residues has been studied. Super Critical Water Oxidation, SCWO and incineration are alternatives for complete removal of organic material from sewage sludge, which produces an inorganic rest product that can be deposited. Incineration requires drying the sludge while the SCWO reaction takes place in a liquid solution at a pressure above 221 bars and a temperature between 400 and 600 °C. SWCO may be environmentally favourable compared to incineration. Expected advantages are small amounts of pollutants in the effluent gas and liquid phase, efficient possibilities for energy recovery and the possibility to recover carbon dioxide. By use of SCWO, the energy of the organic material can be recovered as steam or hot water.

Leaching with acid and base is a promising method for phosphorus recovery from sewage sludge. Ash from incineration of different types of sludge such as aluminium precipitated raw sludge, iron precipitated raw sludge, iron precipitated digested sludge and excess sludge from a biological phosphorus removal with sulphuric acid and sodium hydroxide have been leached by Schaum et al. (2004). At leaching with acid the degree of dissolved phosphate was much higher than the degree of dissolved metals. Phosphorus recovery from SCWO-residues by leaching with acid and base in order to recover phosphorus from sludge has been studied in experimental works at KTH (Hultman and Löwén, 2001, Levlin et al., 2002, Stark, 2002, 2005, Stark and Hultman, 2003). The release of phosphate is 82 - 100 %, which is as high as from ash. The release of iron is 3- 18 %, which is much lower than from ash. However, the release of aluminium was not measured. Leaching SCWO-residues stored for three years gave a very low release.

Phosphorus recovery from sludge incineration ash by leaching with acid and base in order to recover phosphorus from sludge has been studied in experimental works in the Water Chemistry Laboratory at KTH (Levlin et al., 2004). Samples of ash were taken from the sludge incineration plant in the city of Mora, Sweden. The procedure of the leaching was performed in the following way (similar as for leaching of SCWO procedure described in Stark, 2002). Since the calcium content in the sludge was assumed to bind phosphorus as calcium phosphate when leaching with base, the possibility to recover phosphate by a two-step leaching process was also tested (Levlin et al., 2005). In the first step leaching was made with acid at pH-level 4, with the main purpose to dissolve calcium and thereby increase the amount of leached phosphate at leaching with base.

The costs of chemicals for leaching has been calculated for both acid leaching and two step leaching (Levlin, 2006, Levlin et a., 2005) Hydrochloric acid costs about 90 Euro/tonne and sodium hydroxide about 265 Euro/tonne. For the two step process it means that if all calcium is dissolved in the first step the acid consumption will be two equivalent of acid per mole calcium. For dissolving phosphate and aluminium in the second step the base consumption will be three equivalent of base per phosphate and one mole of base per mole aluminium. This gives a cost of 1.3 Euro/kg P for the used ash and 1.4 Euro/kg P for the used SCWO-residue. The acid consumption for phosphate recovery by leaching with acid will be one equivalent acid per equivalent of dissolved metal. To dissolve one mole phosphate three equivalent of acid is required independent of to that metal the phosphate is connected. If all metals are dissolved the cost will be same as for the two step process. However, since the degree of dissolved metals was lower then the degree of dissolved phosphate the cost is reduced. The hydrochloric acid consumption for leaching metal phosphate with acid is three mole acid per mol phosphate. This will with three mole acid per mole phosphate give a cost of 0.32 Euro/kg P. If also other metal compounds is dissolved the acid consumption and the cost will be higher. Leaching of the ash dissolved about 3.49 equivalent metal ions per mole phosphate, which gives a cost of 0.37 Euro/kg P for leaching with acid.

Project goals

Since leaching with acid was shown to be most cost effective the project will focus on to further develop the acid leaching methode to improve the process for phosphorus recovery. However, the acid leaching produces a liquid containing metals and phosphate from which the phosphate has to be separated as a convenient phosphate product. Phosphate products that are suitable for the industry are the product phosphoric acid, or calcium phosphate, which is used as raw material. This can be made for instance with ion exchange or by precipitation of metals with sulphide. The work will be made in a first step to evaluate different separation methods and in a second step there separation methods will be tested at the laboratory of department Land and Water Resources Engineering.

Separation with ion exchange

One possible method to separate dissolved metal ions and phosphate is to use ion exchange. This has been suggested for the BioCon process. The problem with ion exchange is to get a high degree of recovery without having to use a large excess of regeneration chemicals. An ion exchange material with large selectivity for metal ions can take up almost all metal ions but requires a large excess of regeneration chemicals to release the metals. An ion exchange material with low selectivity for metal ions can take up a small amount of the metal ions but requires a small amount of regeneration chemicals to release the metals.

Combined leaching and separation with ion exchange

Phosphorus recovery with ion exchange material is one possible separation methode. 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. An ion exchange column cannot 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. However, one question is if the metal uptake by the ion exchange material gives a higher dissolution of metal, causing a higher acid consumption.

Leaching with hydrogen sulphide

Use of sulphur reactions for precipitating metal in leachate from leaching ash/SCWO-residues with acid is another possible separation method. Hydrogen sulphide reacts with the metal ions in the leachate producing metal sulphide precipitation and a metal free phosphoric acid. Using hydrogen sulphide directly on the ash/SCWO-residues can create process there metal phosphate is converted to metal sulphide and phosphate is released. It can 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.

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 (http://www.lwr.kth.se/forskningsprojekt/Polishproject/index.htm). This work will in the future be made together with also scientist from Lviv, Ukraine.

References

Butcher S.S., Charlson R.J., Orians G.H. and Wolfe G.V. (1994) Global Biogeochemical cycles, 2nd ed., Academic Press Ltd, ISBN 0-12-147685-5.
Corbridge D.E.C. (1995). Studies in Inorganic Chemistry 20, Phosphorus. An Outline of its Chemistry, Biochemistry and Uses, 5th ed., Elsevier Science, ISBN 0-444-89307-5.
Levlin E. (2006) Phosphorus recovery with acid and base from inorganic sewage sludge residues. IWA pecialized Conference - Sustainable sludge management. State of the art, challanges and perspectives, 29-31 May 2006, Moscow Russia.
Levlin E., Hultman B. and Löwén M. (2005). Tvåstegslakning med syra och bas för fosforutvinning ur slam efter superkritisk vattenoxidation eller förbränning. (Two-step leaching with acid and base for phosphorus recovery of sludge after supercritical water oxidation or incineration) VA-forsk 2005-12
Levlin E., Löwén M. and Stark K. (2004). Lakning av slamrest från förbränning och superkritisk vattenoxidation, (Leaching of sludge residue from incineration and supercritical water oxidation) VA-Forsk, 2004-03
Levlin E., Löwén M., Stark K. and Hultman B. (2002). Effects of phosphorus recovery requirements on Swedish sludge management. 2nd World Water Congress of IWA, October 15-18, Berlin, Germany, 2001 and Wat. Sci. Tech. 46(4-5) 435-440
Schaum C., Cornel P. and Jardin N. (2004). Phosphorus recovery from sewage sludge ash. Chemical Water and Wastewater Treatment VIII, IWA Publishing ISBN 1-84339-068-X, pp. 355-363.
Stark, K. (2005). Phosphorus release and recovery from treated sludge. TRITA-LWR PHD 1024. Doctoral Thesis in Water Resources Engineering, KTH Architecture and the Built Environment, Stockholm.
Stark K. (2002). Phosphorus release from sewage sludge by use of acids and bases. Licentiate thesis, Water Resources Engineering, KTH, TRITA-AMI LIC 2005, ISBN 91-7283-307-6.
Stark K. and Hultman B. (2003). Phosphorus recovery by one- or two-step technology with use of acids and bases. Proceedings of IWA specialist conference Biosolids 2003 Wastewater sludge as a resource, June 23-25, 2003, Trondheim, Norway, pp. 281-288.
Stark K., Hultman B. and Levlin, E. (2002) New system technology for combined phosphorus removal and recovery. 3:d World Water Congress of IWA, Melbourne, Australia 7-12 April 2002. CD-rom.
Stendahl K. and Jäfverström S. (2003). Recycling of sludge with the Aqua Reci Process. Proceedings of IWA specialist conference Biosolids 2003 Wastewater sludge as a resource, June 23-25, 2003, Trondheim, Norway, pp. 351-358.
Stumm W. and Morgan J.J. (1981). Aquatic Chemistry, 2nd Ed, John Wiley & Sons Inc. ISBN 0 471 09173-1.