Method development for sustainable and integrated sewage and organic waste handling - Phosphorus recovery by ion exchange from incineration ash and Supercritical Water Oxidation residues.

PROJECT OBJECTIVES

Background

Organic waste exist both as sewage sludge from wastewater treatment and as municipal organic waste from for instance households. Sustainable handling of municipal organic waste and sewage sludge has as an important goal to recycle resources without supply of harmful substances to humans or the environment (Levlin 1999, Hultman and Levlin, 1997). Another important goal is to avoid to deposit waste and sludge on landfill. In Sweden a tax of 250 SEK/ton on all deposited solid waste was introduced year 2000 (SFS 1999:673). Deposition of incinerable waste has been prohibited and in year 2005 there will be a ban on deposition of all organic material on landfill (SFS 2001:512). Incineration (ATV, 1997) and SCWO, Super Critical Water Oxidation (Gidner et al., 2000), are methods that eliminates all organic content and the potential energy of the organic material can be utilised. SCWO occurs in water of a supercritical phase at a temperature above 374 °C and a pressure higher than 22 Mpa. Sludge incineration requires that the sludge has to be dried to 40 % dry substance. The energy produced from the incineration is consumed by the drying and there is therefore no net energy recovery from sludge incineration. With use of SCWO, energy can be recovered also from sewage sludge and organic waste with too low dry substance for incineration. Anaerobic digestion eliminates half of the organic content and half of the energy can be utilised as methane gas. Increasing the energy recovery from sewage sludge and organic waste decreases the need of fossil fuel and makes the society more sustainable. Handling of sludge and organic waste can be integrated by use of food waste disposers and transporting the organic waste to the wastewater treatment plant with the sewer net (Karlberg and Norin, 1999). The resources in sludge and solid organic waste remaining after SCWO or incineration are nutrients such as phosphorus. Development of method to recover these resources from incineration ashes will make handling of municipal organic waste and sewage sludge more sustainable.

Since phosphorus is needed as a fertilizer in the agriculture, a requirement for getting sustainable wastewater treatment is to create method to recover the phosphorus from the wastewater. Most of the phosphorus used in the agriculture originates from mining of phosphate ores which thus is a limited resource. The global deposits of economically mineable phosphate are estimated to be 10⁹ ton phosphorus and the total amount phosphorus in the sediments is estimated to be 10¹⁵ ton (Butcher et al., 1994). Many different phosphate minerals are available, but only apatite (calcium phosphate, Ca₃(PO₄)₂) is used for phosphate production (Corbridge, 1995). Phosphate can be economically produced by leaching apatite mineral with sulphuric acid (McKetta and Cunningham 1990):

Ca₃(PO₄)₂ (s) + 3 H₂SO₄ + 3x H₂O ¾® 2 H₃PO₄ + 3 CaSO₄·xH₂O (s)

In 1995 the world phosphate rock production was 160 000 ton per year (as P₂O₅), having tripled over the last 40 years. About 90% of this amount is used as fertiliser. At this rate of consumption the known apatite reserves have been estimated to last for a period up to 1000 years. However, if the present increase in world population and the increasing need for fertiliser for food production is taken into account, the supply of phosphate may well be crucial within a century. Apatite ore is thereby the limited resource that must be preserved by phosphate recovery.

Phosphorus recovery through leaching with acids and bases

Phosphorus recovery from ash and SCWO-residues by leaching with acids and bases in order to recover phosphorus from sludge or ash has been studied in previous projects (Hultman et al,. 2002, Levlin et al., 2002, Stark 2002). Use of acid leaching dissolves phosphorus but also metal ions thus creating a mixture phosphoric acid, metal ions and anions from the acid. If hydrogen chloride is used to dissolve ferric phosphate the solution will contain zero-valent phosphate together with ferric and chloride ions.

FePO₄ (s) + 3 H⁺ + 3 Cl^-¾® H₃PO₄^o + Fe³⁺ + 3 Cl^-

The chemical consumption for recovery through leaching with acid was found to be a function of the molar ratio of iron and phosphorus in the sludge (Hultman et al,. 2002, Levlin et al., 2002):

Chemical consumption in equivalents	= 5000 + 6000 *	mole Fe in the sludge/tonne DS
tonne DS (dry solids)	= 5000 + 6000 *	mole P in the sludge/tonne DS

Half of the amount of chemical is needed for leaching and half of the amount is needed for separating phosphorus from the leachate. In Sweden, chemical precipitation by use of iron salts is due to stringent restriction on phosphorus content in effluent the most used method for phosphorus removal.

To produce phosphoric acid the other ions has to be separated in a further step. In the BioCon process for phosphorus recovery from incineration ash, an ion exchange process has been proposed (Svensson, 2000). In the KREPRO process proposed for Malmö, phosphorus is after leaching with sulphuric acid recovered by precipitation as ferrous phosphate. In the BioCon process intended for Falun, the ion exchange process has been abounded and phosphorus is intended to be recovered as in the KREPRO process as iron phosphate. Without removing the iron, phosphate will preferentially be precipitated as iron phosphate, which has a lower solubility than for instance calcium phosphate. However, iron phosphate has no commercial value as raw material for the phosphate industry, and the low solubility makes it less favourable to use as fertilizer. Since the phosphate in the sludge originate from phosphorus products produced from apatite ore, recovering the phosphate as iron phosphate will not preserve the limited apatite resources. Iron phosphate is a much more common mineral in the ground than apatite.

Leaching with bases to recover phosphorus has the advantage that a phosphorus product without metal contamination is obtained. In the AquaReci process is SCWO-reside leached with base and recovered as calcium phosphate through precipitation with lime. However, many phosphate compounds are insoluble in base, which reduces the degree of recovery then leacing with base.

Phosphorus recovery with ion exchange material

In this project use of ion exchange material for recovery of phosphorus and separation of metals from ash and SCWO-residues will be studied. Phosphorus recovery with ion exchange material produces a phosphoric acid free from metal ions. In mixing ash or SCWO-residues with a cation exchange material, hydrogen ions from the cation exchange material will dissolve the metal phosphate. The ion exchange material will take up the dissolved metal ions and a metal ion free phosphoric acid will remain in the solution.

CEX=H₃ + FePO₄ (s) ¾® CEX=Fe + H₃PO₄^o

After mixing the ion exchanger with ash or SCWO-residues, metal ions will be transferred to the ion exchanger and a phosphoric acid containing leachate will be obtained. Since dissolution of phosphate and separation of metals is made in the same process step the needed amount of chemical will be half of the amount needed for acid leaching followed by separation of metals in a following step. The next step in the process (see figure 1) is to separate the ion exchange material. By using balls covered with ion exchange material, the ion exchanger can be separated by a sieve which let the fine grinded ash or SCWO-residues to pass through. An other alternative is to use magnetic ion exchange material which can be separated magnetically. A process there magnetic ion exchange resin is mixed with sludge and separated with a magnetic drum has been studied by Swinton et al. (1989). In an acid bath the ion exchange material is recharged with acid and the metal ions are released. By centrifuge the phosphoric acid containing leachate is then separated from ash or SCWO-residues and the phosphoric acid 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.

Figure 1. Process scheme for leaching of ash or SCWO-residues and separation of phosphoric acid from metals.

PROJECT METHODOLOGY

Literature studies on phosphorus recovery with ion exchange (see enclosed paper: Ionexchn.pdf) have been made in previous work about sustainable sludge handling and presented at seminars (Levlin, 2001). The ion exchange process has also been presented as parts of other reports (Balmér et al., 2002 and Hultman et al., 2002). The work in this project is mainly laboratory experiments based on the literature study.

In the first phase the project be performed through experimental work in the Water laboratory at the Department of Land and Water Resources Engineering, KTH, there phosphorus recovery with use of different cation exchange materials shall be studied. Different methods for mixing and separation of ion exchange material with ash and SCWO-residues will be tested. The achieved degree of phosphorus recovery will be measured as well as the metal content of produced phosphoric acid and remaining ash and SCWO-residue. Some of the analysis can be made at the Water laboratory at KTH, and other has to be done on external analyse laboratories. In this project phase is salary for laboratory personal, analysis, ion exchange material and other commodities needed for the experimental work included in the budget.

During the last phase of the work the results will be evaluated and presented in reports, conference presentations and articles in international scientific papers. The potential of the process for recovery of phosphorus from sewage and organic waste will thereby be evaluated.

REFERENCES

ATV (1997). Klärschlammverbrennung Beseitigung oder Verwertung. Korrespondenz Abwasser, Vol. 44, No. 10, pp. 1880-1884.

Balmér P., Book K., Hultman B., Jönsson H., Kärrman E., Levlin, E., Palm O., Schönning C., Seger A., Stark K., Söderberg H., Tiderström H. and Åberg H. (2002) System för återanvändning av fosfor ur avlopp. Naturvårdsverket Rapport 5221.

Butcher S.S., Charlson R.J., Orians G.H. and Wolfe G.V. (1994). Global Biogeochemical cycles, 2^nd 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, 5^th ed., Elsevier Science, ISBN 0-444-89307-5.

Gidner, A., Almemark, M., Stenmark, L. and Östengren, Ö. (2000). Treatment of sewage sludge by supercritical water oxidation. IBC´s 6th Annual Conference on Sludge. Feb. 16th-17th 2000, London, England.

Hultman, B. and Levlin, E. (1997). Sustainable sludge handling, Advanced Wastewater Treatment Report No. 2, Proceedings of a Polish-Swedish seminar, KTH, Stockholm, May 30, 1997, Joint Polish - Swedish Reports, Div. of Water Resources Engineering, KTH, TRITA-AMI REPORT 3044, ISBN 91-7170-283-0, KTH 1997, Paper 5.

Hultman, B., Levlin, E., Löwén, M., Mossakowska, A. and Stark, K. (2002). Utvinning av fosfor och andra produkter ur slam och aska, Slutrapport. (Extraction of phosphorus and other products from sludge and ashes, Final report) Stockholm Vatten AB, R nr 02, feb 2002.

Karlberg T., and Norin E. (1999). Köksavfallskvarnar – effekter på avloppsreningsverk. (Food waste disposers – effects on wastewater treatment plants) VA-Forsk Rapport 1999-9.

Levlin E. (1999). Resources recovery from incineration ashes, Proceedings of a Polish-Swedish seminar, Join Polish Swedish Reports Report No. 5, Div. of Water Resources Engineering, KTH, TRITA-AMI REPORT 3063, ISBN: 91-7170-439-6. pp. 43-53.

Levlin, E. (2001). Recovery of phosphate and separation of metals by ion exchange. Proceedings of a Polish-Swedish seminar Nowy Targ Poland, 2001.10.24-10.26, Report No 9. Joint Polish - Swedish Reports, Div. of Water Resources Engineering, KTH, TRITA-AMI REPORT 3088, ISBN: 91-7283-190-1, pp. 81-90.

Levlin E., Löwén M., Stark K. and Hultman B. (2002). Effects of phosphorus recovery requirements on Swedish sludge management. Wat. Sci. Tech. Vol. 46, No. 4-5, pp. 435–440,

McKetta J.J. and Cunningham W.A. (1990). Alloy selection, Phosphates, Encyclopedia of chemical processing and design, 35 Petroleum fractions properties to phosphoric acid plants, Marcel Dekker Inc., ISBN 0-8247-2485-2, 429-495.

Stark K. (2002). Phosphorus release from sewage sludge by use of acids and bases. Dep. Land and Water Resources Engineering, KTH, licentiate thesis, ISBN 91-7283-307-6.

Svensson A. (2000). Fosfor ur avloppsslam – en studie av KREPRO-processen och BioCons process ur ett livscykelperspektiv, Master Thesis Kemisk miljövetenskap, Chalmers University of Technology, Gothenburg Sweden.

Swinton E.A., Eldridge R.J. and Becker N.S.C. (1989). Extraction of heavy metals from sludges and muds by magnetic ion-exchange. Sewage sludge treatment and use: new developments, technological aspects and environmental effects. Elsevier science publ. ISBN 1-85166-418-1, pp. 394-404.