PHOSPHORUS RELEASE AND RECOVERY FROM ...10421/...Phosphorus recovery from sewage sludge after incineration or Supercritical Water Oxidation. 1 st Conf. Volatization of Phosphates and

TRITA-LWR PhD Thesis 1024 ISSN 1650-8602 ISRN KTH/LWR/PHD 1024-SE ISBN 91-7178-123-4

PHOSPHORUS RELEASE AND RECOVERY

FROM TREATED SEWAGE SLUDGE

Kristina Stark

August 2005

Kristina Stark TRITA LWR PhD Thesis 1024

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Phosphorus release and recovery from treated sewage sludge

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Thou shalt not waste Arrhenius (1919)


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ABSTRACT

In working towards a sustainable society, recycling and recovery of products together with handling of scarce resources must be considered. The growing quantities of sludge from wastewater treatment plants and the increasingly stringent restrictions on landfilling and on agricultural use of sludge are promoting other disposal alternatives. Sludge fractionation, providing sludge volume reduction, product recovery and separation of toxic substances into a small stream, has gained particular interest. In this thesis, the potential for phosphate release and recovery from treated sewage sludge is investigated as an alternative for agricultural use in urban areas. Leaching and recovery experiments were performed on sludge residue from supercritical water oxidation, ash from incineration and dried sludge at different temperatures.

Results showed that acid or alkaline leaching is a promising method to release phosphate from sewage sludge treated with supercritical water oxidation, incineration, or drying at 300°C. The leaching is affected by a number of factors, including how the sludge residue has been produced, the origin of the sludge residue, the quantity of chemicals added and the presence of ions in the leachate.

The implementation of any particular sludge treatment technology would depend on cost, environmental regulations, and social aspects. The results of this thesis may be beneficial for minimizing the use and cost of chemicals, and give increased knowledge for further development of technology for phosphate recovery. Keywords: Ash; Phosphorus release; Phosphorus recovery; Sludge fractionation; Supercritical water oxidation; Sustainable sludge handling.


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SAMMANFATTNING Vägen till det uthålliga samhället går via skapande av kretslopp och utvinning av produkter

tillsammans med resurshushållning. De växande slammängder från avloppsreningsverk, fler restriktioner för deponi och jordbruksanvändning öppnar för andra alternativ av slamomhändertaganden. Speciellt slamfraktionering har visats stort intresse eftersom den möjliggör minskad slammängd, produktutvinning och separering av toxiska substanser i en liten delfraktion. Denna avhandling fokuseras kring potentialen av fosforfrigöring och utvinning från behandlat avloppsslam. Förutsättningen har varit att jordbruksanvändning inte är möjlig i den urbana staden. Laknings- och utvinningsförsök har genomförts med slamrest från superkritisk vattenoxidation (SCWO), aska från förbränning och torkat slam vid olika temperaturer.

Resultaten visar att lakning med syra eller bas är en lovande metod att frigöra fosfat från avloppsslam behandlat med SCWO, förbränning eller torkning vid 300ºC. Den största andelen frigjort fosfor var mellan 80-100% vid syralakning vid 0.1M från SCWO rest. Syralakning från aska vid 1M gav 75-90% frigjort fosfor. Alkalisk lakning av aska och SCWO rest vid 1M gav 40-70% respektive 50-70% frigjort fosfor. Resultaten visar att fosfat frigörs lättare från SCWO rest än från aska vid låga syrakoncentrationer. Det visades att förbehandling av aska kan vara viktig för bättre fosfatfrigöring och att SCWO resten kan ändras under en längre tids förvaring.

Lakningsresultaten påverkas av flera faktorer, såsom dess slamsammansättning, slambehandlingensmetod, tillsatt kemikaliemängd, närvaro av joner i lakningsvätskan. Lakning med syra ger ett högre metallinnehåll i lakningsvätskan jämfört med alkalisk lakning, förutom aluminium som löses vid både syra- och baslakning. Järn hade dessutom en låg frigöring vid både alkalisk och syralakning från aska och från SCWO rest. Den låga frigöringen av järn ger lägre förbrukning av kemikalier för lakningsprocessen och gör det möjligt att utvinna fosfor som produkt med högre marknadsvärde än järnfosfat. Resultaten indikerade att fosfat lakas mer effektivt vid experiment under pH kontroll. Kemikaliebehovet är mycket beroende av metod av fosforreduktion (biologisk eller kemisk) och ökar linjärt med doserad mängd av fällningskemikalier. Användning av biologisk fosforreduktion och slamfraktionering i två steg är fördelaktigt med hänsyn till lågt kemikalie- och energibehov samt hög utvinningsgrad.

Resultaten visar också att fosfatutvinning genom kalciumfosfat är möjlig från både aska och SCWO rest. Ungefär 50% av den totala fosformängden var utvunnen vid 1M NaOH.

Sammanfattningsvis har resultaten i denna avhandling visat på potentialen av fosforutvinning från SCWO eller förbränning samt hur olika parametrar kan påverka lakningsprocessen. Införande av en viss teknik kommer att bero på kostnader, miljölagar och sociala aspekter. Resultaten kan vara användbara för att minska användning och kostnad för kemikalier och ge ökad kunskap om fosforutvinning.


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TABLE OF CONTENTS ABSTRACT ............................................................................................................................. v SAMMANFATTNING ......................................................................................................... vii LIST OF APPENDED PAPERS .......................................................................................... xi ACKNOWLEDGEMENTS.................................................................................................. xv 1. INTRODUCTION.............................................................................................................. 1

1.1 Sustainable urban water systems................................................................................... 1

1.2 Sewage sludge as a resource.......................................................................................... 2

1.3 Possibilities for phosphorus recovery ........................................................................... 3

2. OBJECTIVES AND SCOPE.............................................................................................. 7 3. MATERIALS AND METHODS ....................................................................................... 9

3.1 Materials studied............................................................................................................ 9

3.2 Experimental procedure .............................................................................................. 11

3.3 Parameters and techniques investigated .................................................................... 12

4. RESULTS AND DISCUSSION ....................................................................................... 13 4.1 Chemical reactions and requirements ........................................................................ 13

4.1.1 Reactions in acid and base leaching and calcium phosphate recovery ......................................... 13 4.1.2 Chemical requirements in leaching.................................................................................... 14

4.2 Factors affecting phosphorus leaching....................................................................... 15 4.2.1 Different added amounts of acid or base ............................................................................ 15 4.2.2 Sludge residue composition.............................................................................................. 16 4.2.3 Temperature ............................................................................................................... 18

4.3 Optimising leaching and recovery .............................................................................. 19

4.4 The value and use of phosphorus products ............................................................... 20 4.4.1 Phosphorus products ..................................................................................................... 20 4.4.2 Quality of leachate........................................................................................................ 21 4.4.3 Treatment of remaining residue........................................................................................ 21 4.4.4 Cost .......................................................................................................................... 22

4.5 Future urban wastewater systems with recovery ....................................................... 22 4.5.1 Local, central and regional solutions ................................................................................. 22 4.5.2 Evaluation of systems.................................................................................................... 22

5. CONCLUSIONS ............................................................................................................... 25 6. FUTURE RESEARCH..................................................................................................... 27 REFERENCES ..................................................................................................................... 29


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LIST OF APPENDED PAPERS

This thesis is based upon following papers, referred to in the text by their Roman numerals. Published papers are appended and reproduced with kind permission of the publishers.

I. Stark, K., Levlin, E., Plaza, E. and Hultman, B. 2005. Development of phosphorus recovery technologies in European countries. Submitted to Journal of Resources, Conservation and Recycling.

II. Levlin, E., Löwén, M., Stark, K. and Hultman, B. 2002. Effects of phosphorus recovery requirements on Swedish sludge management. Water Science Technology, 46(4-5): 435–440.

III. Stark, K. 2002. Phosphorus recovery from sewage sludge by thermal treatment

and use of acids and bases. In: Hahn, H.H., Hoffmann, E., Ödegaard, H. (Editors). Chemical Water and Wastewater Treatment VII, IWA-publishing, London, pp. 331-338.

IV. Stark, K., Plaza, E. and Hultman, B. 2005. Chemical requirements for phosphorus

release from sewage sludge treated with supercritical water oxidation. Submitted to Water Research.

V. Stark, K., Plaza, E. and Hultman, B. 2005. Phosphorus release from ash, dried

sludge and sludge residue from supercritical water oxidation by acid or base. Chemosphere (in press).

VI. Stark, K., Hultman, B. and Levlin, E. 2002. New system technology for combined

phosphorus removal and recovery. Proc. 3rd IWA World Water Congress, Melbourne, Australia 7-12 April 2002.

VII. Stark, K. and Hultman, B. 2003. Phosphorus recovery by one- or two-step technology with use of acids and bases. In: Ödegaard, H. (Editor). Wastewater Sludge as a Resource. Proc. IWA Specialist Conf. Biosolids 2003, 23-25 June 2003 Trondheim, Norway, pp. 281-288.

VIII. Stark, K., Plaza, E. and Hultman, B. 2005. Phosphorus recovery from sewage

sludge treated with supercritical water oxidation or incineration. Submitted to Chemosphere.

Other publications related to this research are not appended. An additional list follows for those interested: International journal Hultman, B., Levlin, E. and Stark, K. 2001. Phosphorus recovery from sewage sludge: Research

and experience in Nordic Countries. SCOPE Newsletter, No 41 special edition, pp. 29-32. International conference proceedings Stark, K., Hultman, B., Levlin, E., Löwén, M. and Mossakowska, A. 2002. Calculation of

chemical needs in combined phosphorus removal and recovery at Henriksdal WWTP, Sweden. Proc. 3rd IWA World Water Congress, Melbourne, Australia 7-12 April 2002.


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Hultman, B., Levlin, E., Mossakowska, A. and Stark, K. 2001. Effects of wastewater treatment technology on phosphorus recovery from sludges and ashes. Proc. 2nd Int. Conf. Recovery of Phosphates from Sewage and Animal Wastes, Noordwijkerhout, the Netherlands, 12-13 March 2001.

International conference abstracts Levlin, E. and Stark, K. 2004. Phosphorus recovery from sewage sludge after incineration or

Supercritical Water Oxidation. 1st Conf. Volatization of Phosphates and Phosphorous Compounds Covaphos1, Marrakech, Morocco, 11-13 October 2004, pp. 262-264.

Stark, K. Levlin, E. and Hultman, B. 2004. Phosphorus recovery from sludge - national policies and assessments in Sweden, The Netherlands and Germany. 14th Stockholm Water Symposium, 16-20 August 2004, pp. 169-171.

Polish-Swedish seminar proceedings Stark, K. 2005. Sustainability in solid waste and sludge handling - Source separation or end-of-

pipe solutions. Proc. Polish-Swedish seminar Krakow, 17-18 March 2005. Integration and Optimisation of Urban Sanitation Systems. Final Report No 12. (in press).

Stark, K. 2004. Phosphorus recovery from sewage sludge – experiences from Europe. Proc. Polish-Swedish seminar Stockholm, 6-8 June 2004. Integration and Optimisation of Urban Sanitation Systems. Final Report No 12. (in press).

Levlin, E., Löwén, M. and Stark, K. 2004. Phosphorus recovery from sludge incineration ash and supercritical water oxidation residues with use of acids and bases. Proc. Polish-Swedish seminar, Wisla, 25-28 October 2003. Integration and Optimisation of Urban Sanitation Systems. Report No 11, TRITA-LWR REPORT 3007, pp. 19-28.

Mrowiec, B., Stark, K. and Poplawski, S. 2003. Release and recovery of phosphates from sewage sludge. Proc. Polish-Swedish seminar, Gdansk, 23-25 March 2003. Integration and Optimisation of Urban Sanitation Systems. Report No 10, TRITA-LWR REPORT 3004, pp. 35-46.

Hultman, B., Levlin, E., Plaza, E. and Stark, K. 2003. Phosphorus recovery from sludge in Sweden - possibilities to meet proposed goals in an efficient, sustainable and economical way. Proc. Polish-Swedish seminar, Gdansk, 23-25 March 2003. Integration and Optimisation of Urban Sanitation Systems. Report No 10, TRITA-LWR REPORT 3004, pp. 19-28.

Stark, K. 2001. Phosphorus release from sewage sludge by use of acids and bases. Proc. Polish-Swedish seminar, Nowy Targ – Zakopane, 24-26 October 2001. Wastewater Sludge and Solid Waste Management. Report No 9, TRITA-AMI REPORT 3088, pp. 19-30.

Hultman, B., Levlin, E. and Stark, K. 2000. Swedish debate on sludge handling. Proc. Polish-Swedish seminar, Krakow, 29 May 2000. Sustainable Municipal and Solid Waste Handling. Report No 7, TRITA-AMI REPORT 3073, pp. 1-16.

Licentiate thesis Stark, K. 2002. Phosphorus release from sewage sludge by use of acids and bases. Licentiate

thesis, Water Resources Engineering, KTH, Stockholm, Sweden, TRITA-AMI LIC 2005. Swedish journals Stark, K. 2002. Utvinning av fosfor vid superkritisk vattenoxidation av avloppsslam. (Recovery of

phosphorus in supercritical water oxidation of sewage sludge). Vatten 58 (2): 97-103. (In Swedish) (also in English in Licentiate thesis).

Stark, K., Hultman, B., Mossakowska, A. and Levlin, E. 2001. Kemikaliebehov vid fosforutvinning ur avloppsslam. (Chemical needs in phosphorus recovery from sewage sludge). Vatten 57 (3): 207-215. (In Swedish) (also in English in Licentiate thesis).


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Swedish publications (reports) 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 Report No 2004-03. (In Swedish).

Hultman, B., Levlin, E., Löwén, M., Mossakowska, A. and Stark, K. 2002. Utvinning av fosfor och andra produkter ur slam och aska. (Recovery of phosphorus and other products from sludge and ash). Stockholm Vatten AB, Report No 02. (In Swedish).

Hultman, B., Levlin, E., Löwén, M., Mossakowska, A. and Stark K. 2001. Utvinning av fosfor och andra produkter ur slam och aska. (Recovery of phosphorus and other products from sludge and ash). Stockholm Vatten AB, Report No 6. (In Swedish).

Levlin, E., Tideström, H., Kapilashrami, S., Stark, K. and Hultman, B. 2001. Slamkvalitet och trender för slamhantering. (Sludge quality and trends in sludge handling). VA-Forsk Report No 05. (In Swedish).


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ACKNOWLEDGEMENTS

I gratefully acknowledge the financial support from MISTRA and KTH. Many thanks to my supervisor Elzbieta Plaza for all her support, guidance and valuable help throughout this work. Many thanks also to my co-supervisor Bengt Hultman for accepting me as a PhD student and supporting me in all decisions. Thanks to both of you for all time you spent to give advices and learning me how the research world works. I am also very happy to have had Monica Löwén as advisor in the experimental set up. Many thanks for all help in laboratory, your involvement in my project and your support. Thanks to Erik Levlin for valuable and interesting discussions about phosphorus and sludge. I am very grateful to Aira Saarelainen for the wonderful assistance in administrative matters. Thanks to all colleagues at the Department of Land and Water Resources Engineering, especially those at the former Division of Water Resources Engineering for creating a nice and friendly atmosphere in the everyday life. Thanks to my Master thesis worker during my PhD-time; Claudio, Abdullai, Chen, Simonetta, Isabel and Yang. You gave me inspiration and help in the experimental work. All researchers and PhD-students involved in Urban Water. Mary for language review of this PhD summary and manuscript paper IV. Thanks to friends and family. Stefan, thanks for your comments of my articles and your support. Mom and Dad, who always support me and are the best parents. Tack! Many thanks to my best Sis Karolina for all pep talking, advices in article writing and your support. Dan, thanks for all valuable help in completing this thesis and thousands thanks for your support and patience in the difficult time of having two jobs. From here to the moon and… pok! Kristina Stark Stockholm, August 2005


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1. INTRODUCTION “Development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs”.

WCED (1987) 1.1 Sustainable urban water systems

The concept of sustainability became rapidly accepted internationally with the publication of the Brundtland Commission´s ‘Our Common Future’ in 1987. In sustainable development, the understanding of the relationship between environment and development is a key factor that cannot be achieved without political commitment. One approach is to regard the urban environment and urban areas as an ecological system (Sukopp and Wittig, 1998), where recycling and recovery of products together with handling of scarce resources must be considered. Sustainable development is also related to the appearance of the city. A future direction in planning research of the urban environment is a dense city, which can gather the resource flows and recycling or recovery processes in a central effective facility. In such development, it will be reasonable to also concentrate the suburbs. This creates strong motives to build further on the central systems already existing. However, the American trend in city development seems to be that the city is physically more spread out across sparsely residential districts, similar to the Swedish million programme residential areas (Berg and Tälleklint, 2005).

Sustainable city development demands an integrated handling of the physical, economic, biological, organisational, social and culturally-determined resources of the city. A concept of a multi-criteria basis intended to support decision-making for urban water and wastewater systems has been developed within the Swedish research programme Urban Water (Malmqvist and Palmquist, 2005). The approach uses a common conceptual framework with criteria, indicators and assessment tools (Figure 1).

The urban water system consists of the

technical structure (pipes, pumps, treatment plants, etc.), the users and the organisations (e.g. water companies, municipal boards, authorities) that interact. The system is analysed with five criteria groups established for sustainability assessment of urban water systems:

• Health and hygiene • Environment • Economy • Socio-culture • Technology

Figure 1. A framework for the integrated sustainability assessment of urban water and wastewater systems as suggested by the Swedish research programme Urban Water (Malmqvist and Palmquist, 2005).


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1.2 Sewage sludge as a resource

One crucial factor for traditional wastewater handling in central systems today is the difficulty in recycling resources in a reliable way. Agricultural use is often regarded as the best alternative if the pollutants in the sludge are below limiting and guidance values. However, a lack of acceptance from the food industry and the public is preventing widespread adoption of this option (Hultman et al., 2001a; Bengtsson and Tillman, 2004). Landfilling, land application and incineration methods are currently the dominant modes of disposing of the sludge from urban wastewater treatment plants (WWTP) (Svanström et al., 2004). Table 1 shows the sludge disposal options and costs in Swedish WWTPs. The most common option is land application (remediation of soils, parks, golf courses). The agricultural use (Table 1) forms part of a project called ReVAQ, which involves seven municipalities looking at how to produce sludge for agricultural use (Envisys, 2002). However, the Swedish farmers organisation, LRF, recommend Salix

(wood production) for sludge use, due to uncertainties about the sludge content. Sludge handling in Sweden has so far been regarded as a disposal problem (Bengtsson and Tillman, 2004), but the sewage sludge may be regarded not only as a threat to the environment but also as a resource. In sustainable sludge handling, resources are efficiently recycled without flows of harmful substances to humans or the environment (Harremoës, 1996). Sewage sludge has the potential to be used as a resource in a variety of options. The organic material may be used for soil conditioning, energy production, adsorption material or production of organic compounds as organic acids. The inorganic material may be used for reuse as precipitation chemicals, use in building materials and possible recovery of valuable metals. Nutrients, such as phosphorus, nitrogen and potassium, are also resources found in the sludge. Special attention may be given to the non-renewable phosphorus. Examples of sludge products for internal and external purposes in the WWTP are summarised in Table 2.

Table 1. Sludge disposal methods and cost in Swedish municipalities 2004 (modified from Stark, 2005) Sludge disposal % of the municipalities Cost* (Euro**/tonne) Land application 65 26.1 Agriculture 13 24.7 Landfill 11.5 71.9 Salix 8.5 24.2 Incineration 2 61.2 * Transport not included ** Exchange rate used: 1 Euro= 9.14 SEK


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1.3 Possibilities for phosphorus recovery Phosphate may be recovered from different sources (urine, wastewater, supernatant, sewage sludge) by direct use or by different technologies (Figure 2). There has been increasing interest in sludge fractionation, which includes sludge volume reduction, product recovery and separation of toxic substances into a small stream. Different options for sludge fractionation are

summarized in Table 3. A survey of the technical, economic and legal aspects concerning phosphorus recovery technologies from WWTPs in Europe is described further in Paper I, together with reasons to recover phosphorus. Detailed descriptions of the processes KREPRO, BioCon, Aqua Critox and Aqua Reci can be found in Papers II, III, VI and VII. The effects of Swedish proposals and goals concerning the percentage of phosphorus recovery from wastewater and sewage sludge

Table 2. Examples of products in sludge handling (Hultman and Levlin, 1997; Brett et al., 1997; Morse et al., 1998; Hultman et al., 2001b; Stark, 2002a) Type of product Internal use External use Gases Methane gas Energy production (heat, electricity) Energy production (heat,

electricity), use as fuel in vehicles Carbon dioxide Neutralisation of wastewater by use

of carbon dioxide produced for instance from anaerobic digestion or incineration

Carbon dioxide as a product

Gases from pyrolysis Energy production (heat, electricity) Energy production (heat, electricity)

Liquids Organic acids Production of methane gas, organic

carbon source in denitrification and biological phosphorus removal

Metal ions such as aluminium, ferrous or ferric ions

Reuse as precipitation chemicals

Solids Sludge Use in agriculture (depends on

the sludge content), reclamation of land, land building, etc

Compost Decrease of odour problems (compost filters)

Use as soil conditioner, fertiliser etc

Nutrients Use as fertiliser, e.g. as ammonium sulphate, magnesium ammonium phosphate and calcium phosphate

Activated carbon (from pyrolysis)

Adsorption material Adsorption material

Nitrification bacteria Seeding of nitrification bacteria produced in nitrification of supernatant

Sludge with a low volatile fraction

Weighting materials to improve sedimentation in e.g. the activated sludge process

Inorganic part of the sludge Use as building materials (cement, brick, glass ceramics etc)


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Figure 2. Possible pathways for phosphorus recovery based on Brett et al., 1997; Morse et al., 1998; Hultman et al., 2001a and b; Vinnerås, 2002; Jaffer et al., 2002; Doyle and Parson, 2002; Balmér et al., 2002; Roelevald et al., 2003; Kato et al., 2003; Klapwijk and Temmink, 2004; de-Bashan and Bashan, 2004; Papers I, VI.

Example of wastewater

treatment scheme

Example of

concentrated phosphate

stream

Example of phosphorus

product/commercial process

Water use (Households, etc.)

Urine

Different products/ Direct

use or Source separation technologies

Wastewater treatment Phosphate released

in anaerobic treatment of Bio-P

sludge

Calcium

phosphate/PhoStrip

Digestion Anaerobic release of

phosphate to supernatant from digested sludge

Calcium phosphate,

magnesium ammonium phosphate (MAP)

Release from residue by acid

Ferric phosphate

/KREPRO

Thermal hydrolysis

or

Supercritical water oxidation

or

Incineration

Release from residue

by acid Release from residue

by acid or base

Ferric phosphate

/Aqua Reci

Calcium phosphate, phosphoric acid

/BioCon


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are described in Paper II and an updated version of the proposal is described in Paper I. The latest update on this issue is published at www.regeringen.se. The recent environmental proposition from the Swedish government is in accordance with the

Swedish Protection Agency report (SEPA, 2002) and the organisation will not act on this issue before being given a new assignment by the Swedish government.

Table 3. Different options for sludge fractionation (Hultman and Levlin, 1997; Gidner et al., 2000; Hultman et al., 2001b; Stark, 2002a; Manhem and Palmgren, 2004; Saktaywin et al., 2005) Methods General purpose Physical Heat/pressure Solution of sludge components, sludge conditioning Mechanical Mechanical devices, ultrasonic Disruption of cells for improved sludge degradation Biological Enzymes Solution of sludge components, increased

biodegradability of the sludge Anaerobic treatment Production of organic acids, release of phosphates Sulphate reducing bacteria Production of sulphides for release of phosphates and

precipitation of metals

Sulphur, sulphide and ferrous oxidation bacteria

Production of hydrogen ions for release of metals from sludge

Chemical Acids, bases, oxidising agents (ozone, hydrogen peroxide, etc.)

Hydrolysis of sludge, release of different sludge components, conditioning of sludge, increased biodegradability of sludge

Complexing agents Release of metals, etc. from sludge


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2. OBJECTIVES AND SCOPE The overall aims of this thesis were to investigate the potential for phosphate release and

recovery from treated sewage sludge (Figure 3). The intention was to study solutions for a large WWTP within the long-term goal of phosphorus recovery when agricultural use is not possible in an urban area. Experiments were performed with sludge residue from supercritical water oxidation (SCWO), dried sludge and ash from incineration, followed by evaluations of leaching, recovery and possible systems. The main questions investigated were:

• What are the respective advantages and disadvantages of acid and alkaline leaching?

• How do differences in sludge residues affect phosphate release and recovery?

• How do different parameters affect the result (temperature, pH, etc.)?

• What degree of recovery can be expected from certain types of sludge?

• How should phosphorus recovery be introduced into an existing system?

System VI, VII

PHOSPHATE RECOVERY I

WWTP

Chemical and/or biological P-removal II, VI

Sludge

Sludge treatment Different technologies: -Incineration -SCWO -Drying

Sludge residue/Ash

Leaching III, IV, V VII, VIII Chemical requirement II, IV

Recovery VII, VIII

Effluent to recipient

Figure 3. A schematic diagram of conventional WWTP followed by recovery system. Points of investigation within the scope and aims of this thesis have highlighted references to Papers I-VIII.


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3. MATERIALS AND METHODS The experiments presented in this thesis were all conducted on laboratory scale at the Water

Chemistry laboratory at the Department of Land and Water Resources Engineering, KTH, Stockholm. The studies performed, materials used and experimental procedure are described in this chapter. Further details are described in Papers III-V, VII-VIII. Parameters and techniques studied are included to provide an overview of the content of the thesis.

3.1 Materials studied

The materials investigated were sludge

residues from the SCWO process, ash from incineration and dried sludge. The studies

conducted were divided mainly into acid, alkaline leaching and recovery experiments. The studies performed on SCWO residue,

Table 4. Summary of studies performed on SCWO residue, ash and dried sludge

Studies conducted Sources of sludge Material studied Papers WWTP

Acid leaching (at different pH) pH 2-5 Karlskoga 1 SCWO IV Acid leaching (at different molar concentrations of HCl) 0-2 M Bromma SCWO III, IV, V 0-2 M Borlänge SCWO IV 0-2 M Karlskoga 2 SCWO IV 0-1 M** Karlskoga 1 SCWO VIII 0-1 M Mora Ash V 0-1 M** Mora Ash VIII 0-1 M, 300ºC Himmerfjärden Dried sludge V 0-1 M, 550ºC Himmerfjärden Dried sludge V 0-1 M, 850ºC Himmerfjärden Dried sludge V Alkaline leaching (at different molar concentrations of NaOH) 0-2 M (0-5 M) Karlskoga 1* SCWO IV, V, VII 0-5 M Bromma SCWO III, IV, VII 0-1 M Karlskoga 1 SCWO VIII 0-1 M Mora Ash V 0-1 M Mora Ash VIII 0-1 M, 300ºC Himmerfjärden Dried sludge V 0-1 M, 550ºC Himmerfjärden Dried sludge V 0-1 M, 850ºC Himmerfjärden Dried sludge V Recovery studies (addition of calcium compounds) 0-5 M Karlskoga 1* SCWO VII 0-1 M Karlskoga 1* SCWO VIII 0-1 M Mora Ash VIII * Different run with Karlskoga 1 ** Also leaching with sulphuric acid (H2SO4)


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ash and dried sludge are presented in Table 4, while the composition of these materials is presented in Table 5 with further description of sludge origin in Papers IV, V and VIII.

Table 5. Composition of the SCWO residue, ash and dried sludge studied

Sources of sludge WWTP

Composition of SCWO residues, ash and dried sludge

Al2O3 %

CaO%

Fe2O3%

K2O %

MgO%

P2O5%

TS %

SCWO residue Bromma 11.7 6.2 23.3 0.48 1.0 10.9 5.4 Borlänge 3.6 6.0 12.9 0.94 1.1 8.2 0.7 Karlskoga 1 16.3 4.0 25.0 0.45 0.8 18.4 3.7 Karlskoga 2 4.5 4.3 19.3 0.61 0.92 6.3 3.4 Ash Mora 41.1 6.91 11.9 - 0.99 18.5 99.8 Dried sludge Himmerfjärden, 300ºC 7.06 6 16.8 - 1.17 13.4 94.4 Himmerfjärden, 550ºC 9.23 7.79 18.3 - 1.51 16.5 99.1 Himmerfjärden, 850ºC 11.5 9.61 30.1 - 1.85 21.2 99.7


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3.2 Experimental procedure

The experimental set-up and procedure are

presented in Figures 4 and 5. The development of the experimental procedure for estimation of phosphorus release and

recovery is described in detail in Papers III-V and VII-VIII, respectively, together with the methods of analysis used.

Chemical addition 25 ml

5 ml SCWO

residue or

0.5 g Ash

Solid phase

Centrifugation 4000 rpm

20 min

Shaking

2 h

Centrifugation

4000 rpm 20 min

Leachate

Filtration

Analysis

Figure 4. The procedure used for leaching from SCWO residue or ash at different molar concentrations.

Chemical addition 25 ml

5 ml SCWO

residue or

0.5 g Ash

Solid phase

Addition

of calcium

ions

Analysis

Centrifugation 4000 rpm

20 min

Shaking

2 h

Centrifugation

4000 rpm 20 min

Leachate

Shaking

2h

Filtration

Figure 5. The procedure used for recovery from SCWO residue or ash.


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3.3 Parameters and techniques investigated

The parameters and techniques investigated in this thesis are summarised in Table 6. Three main areas were studied; leaching, recovery and a systems approach. In the leaching procedure, different ions (mainly phosphate) and the chemical requirements of acid or base were examined. In recovery experiments, the possibilities of obtaining a precipitate were investigated. The systems

approach was based on literature studies of

the potential of systems and how improvements are possible. The different experiments were compared to identify differences and similarities between the materials studied, in order to better understand the release process from differently treated sludges.

Table 6. Summary of parameters and techniques investigated in Papers I-VIII Parameters

Papers: I II III IV V VI VII VIII

Leaching Phosphate release X X X X X Iron release X X Ca and Al release X X Heavy metals release X X X Chemical requirement X X Recovery Phosphate recovery X X Aspects of phosphorus recovery technologies

X X X

Systems Aspects of systems X X X Two step technology X X Costs X X


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4. RESULTS AND DISCUSSION This chapter includes complementary materials to the papers appended, interesting and

important findings arising during the research, comparisons between the results obtained and the literature. Moreover, how the process of phosphate release and recovery can be further developed in a WWTP and, finally, a discussion about recovery systems.

4.1 Chemical reactions and requirements

In chemical hydrolysis techniques, acids or bases are used to dissolve the material. Acids such as hydrochloric acid react with sludge components and with alkalinity, consuming soluble substances such as hydrogen carbonate and organic acids, and the remaining hydrogen ions decrease the pH. Bases such as sodium hydroxide react with different sludge components and soluble substances such as hydrogen carbonate and ammonium and the remaining hydroxide ions increase the pH value. The leached phosphate may be precipitated with chemical

additions, such as calcium oxide.

4.1.1 Reactions in acid and base leaching and calcium phosphate recovery

The main reactions for release and recovery of phosphorus from SCWO residue or ash are summarised in Table 7, together with other important inorganic sludge components that might be dissolved. As can be seen, the alkaline leaching of aluminium needs a lower chemical requirement (in moles) compared with acid leaching if aluminium is bound in the sludge as aluminium hydroxide while the opposite is

Table 7. Dissolution of inorganic sludge components (modified from Stark, 2001; Levlin and Hultman, 2005; Papers IV, VIII)

Component Leaching reaction

moles HCl or NaOH

per mole metal

g HCl or NaOH per g component

Leaching with acid Fe(OH)3 Fe(OH)3 + 3H+ → Fe3+ + 3H2O 3 1.024 Fe(OH)2 Fe(OH)2 + 2H+ → Fe2+ + 2H2O 2 0.812 Fe3(PO4)2 Fe3(PO4)2 + 6H+ →3Fe2+ + 2H3PO4 6 0.204 Fe3(PO4)2 Fe3(PO4)2 + 4H+ →3Fe2+ + 2H2PO4 4/3 0.136 FePO4 FePO4 + 3H+ → Fe3+ +H3PO4 3 1.379

FePO4 FePO4 + 2H+ → Fe3+ +H2PO4 2 0.484

AlPO4 AlPO4 + 3H+ → Al3+ + H3PO4 3 0.897 AlPO4 AlPO4 + 2H+ → Al3+ + H2PO4- 2 0.598 Al(OH)3 Al(OH)3 + 3H+ → Al3+ + 3H2O 3 1.403 CaCO3 CaCO3 + 2H+ →Ca2+ + 2HCO3– 2 0.729 MgCO3 MgCO3 + 2H+ →Mg2+ +2HCO3- 2 0.865 Leaching with base FePO4 FePO4 + 3OH- → PO43– + Fe(OH)3 3 0.796 AlPO4 AlPO4 + 4OH- → Al(OH)4– + PO43– 4 1.312 Al(OH)3 Al(OH)3 + OH- → Al(OH)4– 1 0.513 CaSO4 CaSO4 (s) + 2OH- → Ca2++ 2OH- 2 0.588 MgSO4 MgSO4 + 2 OH- → Mg(OH)2 2 0.665

Recovery of phosphate 3PO43- + OH- + 5Ca+ → Ca5(OH)( PO4)3

5CaO + 3PO43- + 5H2O → Ca5(OH)(PO4)3 (s) + 9OH-


14

the case if aluminium is bound as aluminium phosphate. Fewer chemicals are needed to dissolve ferrous than ferric ions with acid. The reactions taking place in the SCWO process are described in Paper IV. 4.1.2 Chemical requirements in leaching

In order to save leaching chemicals it is preferred to selectively leach phosphates and as little as possible of metals bound to other substances such as oxides, carbonates, silicates, etc. In Figure 6, chemical requirements and ratio of iron to phosphate for the different studies performed are summarized (Paper II; Stendahl, 2001; Stark et al., 2002). The results show that acid leaching of digested sludge from chemical-thermal hydrolysis (KREPRO) and from sludge incineration (BioCon) require approximately the same chemical usage for a certain Fe/P ratio (Paper II), while fewer chemicals were needed for the SCWO residue.

The essential factor is the incoming iron content of the sludge used when comparing KREPRO and BioCon. The lower use of chemicals in KREPRO is due to the iron being in the ferrous (II) form at dissolution, while the iron after incineration is in the ferric (III) form. The possibility of decreasing the chemical usage in acid leaching is more dependent on the technique used to decrease the iron content of sludge, i.e. increased use of biological phosphorus removal is favourable. Another possibility is selective release of phosphate without release of iron. The different behaviour of the SCWO process could be due to that the SCWO residue may be more reactive due to small particle size and the relatively low treatment temperature in the SCWO process compared to incineration. The temperature is around 450ºC for SCWO, while it is above 800ºC for the incineration process.

0100200300400500600700800900

1000

0 1 2 3 4kg Fe/kg P

kg a

cid/

tonn

e D

S

BioCon (Bröderslev)Paper II

BioCon system(Henriksdal) Paper II

KREPRO(Helsingborg) Paper II

KREPRO system(Henriksdal) Paper II

SCWO (Bromma)Stendahl, 2001

SCWO (Bromma)Stark et al., 2002

Figure 6. Effect of weight relationship Fe/P of total chemicalrequirements of acid for the systems KREPRO, BioCon and SCWO(modified from Stark et al., 2002).


15

4.2 Factors affecting phosphorus leaching

In this chapter, some of the behaviours observed in the experiments performed (Papers III, IV, V, VII and VIII) and possible explanations for these are discussed and compared with results in the literature. 4.2.1 Different added amounts of acid or base

The results of the experiments performed showed that phosphate was leached more easily with the acid (hydrochloric acid) at room temperature than with the base (sodium hydroxide), in accordance with the literature, e.g. Koutsoukos and Valsami-Jones (2004). The largest degree of released phosphate, 80-100% at acid concentrations of 0.1M HCl, was obtained by leaching SCWO residue. Alkaline leaching of SCWO-residue at 1M NaOH gave 50–70% released phosphate (Figure 7). Acid leaching of ash at

0102030405060708090

100

0 M 0.05 M 0.1 M 0.25 M 0.5 M 1 M

HCl or NaOH

% P

O4-

P re

leas

ed SCWO HCl (Paper IV)SCWO HCl (Paper IV)SCWO HCl (Paper IV)SCWO NaOH (Paper VII)SCWO NaOH (Paper III)

Figure 7. Phosphate released from SCWO residue by acid oralkaline leaching.

0102030405060708090

100

0 M 0.01M

0.05M

0.1 M 0.25M

0.5 M 1 M

HCl or NaOH

% P

O4-

P re

leas

ed

Ash HCl (Paper VIII)Ash HCl (Paper V)Ash NaOH (Paper V)Ash NaOH (Paper VIII)

Figure 8. Phosphate released from ash by acid or alkalineleaching.


16

1M HCl gave 75–90% released phosphate, while alkaline leaching of ash at 1M NaOH gave 40–70% released phosphate (Figure 8). The results showed that both added acid and base were sparingly consumed for leaching different components, thus, the data indicated that acid or base were added in surplus. A higher efficiency of consumed chemicals and released phosphate can be obtained by performing experiment under pH control (Paper IV). This corresponds to other studies showing a high release at low pH value (Hansen et al., 2000; Schaum et al., 2004). 4.2.2 Sludge residue composition

The composition of the sludge residue affects the leaching results and, thus, the recovery. The optimal situation for phosphate recovery is to obtain a high release of phosphate and a low release of heavy metals and metals that later on can give rise to separation problems.

Table 10 shows the behaviour of PO4, Al, Fe and Ca in different leaching studies on ash and SCWO residue (Schaum et al., 2004; Stendahl, 2005; Papers III, V, VIII). The different chemicals added, the precipitation agent in WWTP and the sludge used in the process are given. The leaching took place at room temperature. Phosphate released in these experiments (see appended Papers III, V, VIII) is in accordance with other studies

Table 10. Components leached in different studies on ash and SCWO residue PO4 (%) Al (%) Fe (%) Ca (%) References ASH 1M HCl Al, digested

87

23

6

83

Paper V

Al, digested 77 - - - Paper VIII 1M H2SO4 Al, raw sludge 88 65 32 5.6 Schaum et al., 2004 Fe, raw sludge 96 71 16 6 Schaum et al., 2004 Fe, digested 94 70 21 4.3 Schaum et al., 2004 EBPR* 100 84 19 8 Schaum et al., 2004 Al, digested 0.05 - - - Paper VIII 1M NaOH Al, raw sludge 31 31 - - Schaum et al., 2004 Fe, raw sludge 28 31 - - Schaum et al., 2004 Fe, digested 3 44 - - Schaum et al., 2004 EBPR 54 40 - - Schaum et al., 2004 Al, digested 70 9.5 0.04 0.19 Paper V Al, digested 42 - - - Paper VIII SCWO residue 1M HCl Fe, digested

100

-

2.80

-

Paper V

Fe, digested 82 - 5.3 - Paper IV Fe, digested 95 - 18 - Paper IV Fe, digested 48 - - - Paper VIII HCl pH below 2 Fe, digested

76

92

10

86

Stendahl, 2005

H2SO4 pH below 2 Fe, digested

84

100

12

17

Stendahl, 2005

1M H2SO4 Fe, digested

0.05

-

-

-

Paper VIII

1M NaOH Fe, digested

53

43

0.07

-

Paper V

Fe, digested 74 - 0.045 - Paper III Fe, digested 13 - - - Paper VIII


17

published. Unfortunately, all leached ions were not analysed and in most cases only iron and phosphate were analysed. As can be seen from Table 10, in most cases Al was released together with the phosphate in acid leaching, while iron had a lower release. Alkaline leaching also showed a release of Al and PO4, while Ca and Fe showed a low release. After the thermal treatment, iron is probably present as iron hydroxide, which is difficult to break. The leaching of iron was higher from ash than from SCWO residue. This may be explained by the higher temperature in the incineration than in the SCWO process. There was no significant difference in phosphate release between ash and SCWO residue at this leaching concentration (1M) However, a lower acid concentration has shown a better release from SCWO than from ash (Paper V, VIII). The experiments in Paper VIII showed a low release of phosphate from SCWO residue compared with the other studies (Papers III, VII). This is probably depending on a long storage time of the residue (about 3 years) before the leaching experiments (Figure 9). This may explain the differences in results from earlier studies comparing SCWO and incineration.

The metal/phosphorus ratio was higher with acid than with base (Papers III, V). This means alkaline leaching will preferentially dissolve phosphorus with a lower metal contamination compared to acid leaching and is in accordance with results presented by Cheeseman et al. (2003) and Stendahl and Jäfverström (2004).

The difference shown in leaching with acid or base may be due to the fact that the calcium present in the sludge was binding phosphorus as calcium phosphate in the case of alkaline leaching (Stendahl and Jäfverström, 2004). When leaching SCWO residues with sodium hydroxide, Stendahl and Jäfverström (2004) found that residues from Karlskoga sludge with 3% calcium released 90% of the phosphorus content, while residues from Stockholm sludge with 8% calcium only released 65% phosphorus. Thus, leaching with base (with higher calcium content) would give a lower release of phosphate than leaching with acid. However, this was not observed in the current studies (Papers III, VII) with alkaline leaching (Table 11). Another reason could be that the precipitation of calcium hydroxide can be favourable at high pH in systems containing calcium (Morel and Hering, 1993).

0102030405060

0M 0.5M 1M

NaOH

% P

O4-

P re

leas

ed

Year 2002(Paper VII)Year 2005(Paper VIII)

Figure 9. Phosphate release from freshand stored SCWO residue.

Table 11. Relationship between calcium content in SCWO residue and phosphate released with alkaline leaching

Ca (%) in SCWO residue

PO4 released

(%)

References

3 54 Paper VII 4.4 72 Paper III


18

4.2.3 Temperature

In many cases, higher temperatures increase the reaction process. However, this seems not to be favourable in phosphate leaching, as shown by the results from Stark (2002b), where the experiments performed at 20°C (Paper III) were compared with leaching studies performed at 90°C (Stendahl, 2001). The results show that iron was released differently with acid leaching at 20°C

and 90°C (Figure 10 and 11). At room temperature, 100% of the phosphate was released at 0.1M HCl and only 1% iron, while at 90°C, 64% phosphate was released and 71% iron. A possible reason is that the metal bonds break at different rates and dissolution is therefore likely to be a multi-tier process, the overall rate of which will be defined by the slowest breaking bonds (Koutsoukos and Valsami-Jones, 2004). The higher temperature together with acid dissolved the strong iron precipitates and caused precipitation of phosphate with dissolved ions. In the case of alkaline leaching, the release was similar at both temperatures, with a low iron release. This may be due to the high concentration of hydroxide ions and to the fact that FeOH3 is difficult to dissolve.

The role of temperature of the treated sludge is also an important factor. In Table 12, the relative leaching from ash at different temperatures is shown (Ek and Junestedt, 2005; Paper V). As can be seen from Table 12, the present studies presented in Paper V showed similar results to those of Ek and Junestedt (2005), namely that phosphate had a lower release at higher temperatures. This may be related to the phenomena of glassification and crystallization that may occur with increasing temperature.

0

20

40

60

80

100

0 0.1 0.2 0.3

HCl (M)

% P

O4-

P re

leas

ed

Fe 90°CFe 20°CPO4 90°CPO4 20°C

Figure 10. Comparison of releasedphosphate and iron at 20°C and 90°C with acid leaching (modified from Stark,2002b).

0

20

40

60

80

100

0 0.5 1 1.5

NaOH (M)

% P

O4-

P re

leas

ed

Fe 90°CFe 20°CPO4 90°CPO4 20°C

Figure 11. Comparison of releasedphosphate and iron at 20°C and 90°C with alkaline leaching (modified from Stark,2002b).

Table 12. Relative leaching from dried sludge at different temperatures Ash from °C

NaOH leaching (%)

HCl- leaching (%)

PO4 Al Fe PO4 Al Fe Ek and Junestedt, 2005:

325 30 58 0.02 - - - 550 24 52 0.01 84 82 72 800 16 6 0.004 86 30 12 Paper V:

300 53 - - 83 - - 550 38 - - 73 - - 850 24 - - 55 - -


19

4.3 Optimising leaching and recovery

One possibility to prevent the mixture of leached components is to have a pre-treatment step. This would include pre-washing of the sludge residue, followed by acid leaching and alkaline leaching. With pre-washing, soluble matter that would otherwise consume chemicals can be removed. The first step could also be an acid leaching step. The majority of calcium and magnesium compounds may be removed at slightly acid conditions. Some phosphate is also released during this acidic leaching and conditions should be chosen where the release of iron and heavy metals is low. This stream may be combined with supernatant from dewatering of digested sludge or side-streams from processes like Pho-Strip to recover part of the phosphorus supplied to the wastewater treatment plant (Stark, 2002b). Two-step leaching experiments performed on ash and SCWO residue (Levlin et al., 2005) showed that it was possible to first leach main part of calcium and a small part of phosphate. The first step included acid

leaching (pH 3-6) to leach calcium and phosphate, while the second step (alkaline) leached aluminium and additional phosphate. The results showed an around 10% increase in phosphate release from ash and SCWO residue compared to the use of only one step. However, the leached amount of phosphate was not higher than around 60%. A reason for that may be that the pH was not below value of 3 (Paper IV). Furthermore, aluminium was released between pH 3-4, while calcium was released at higher pH-values (higher values from the SCWO residue) during acid leaching.

Another option is to take advantage of the processes in the WWTP. The demand for chemicals is largely dependent on the type of method used for phosphorus removal (biological or chemical) and increases linearly with the dosage of precipitation agents (Paper II). A solution may be to use two-step technology (Papers VI, VII), i.e. firstly a recovery of phosphorus by biological technology and secondly a recovery of the rest of the phosphorus by an advanced technology system (Figure 12). Use of enhanced biological phosphorus removal and

Figure 12. Phosphate release from sludges with biologically and chemically bound phosphorus followed by phosphorus recovery (Paper VI).

Phosphorus recovery

Excess sludge

Sludge thickener

Phosphorus precipitation

Drying

Addition of chemicals or

heat for phosphorus

release

Incineration

Recycling of precipitation

agents or organic acids

Possible addition of post-

precipitated sludge

Phosphorus recovery

Phosphorus recovery

Anaerobic zone Anoxic zone Aerobic zone Settler


20

fractionation of the sludge in two stages is advantageous both with respect to lowering the demand for chemicals and energy and improving the recovery efficiency (Papers VI, VII).

4.4 The value and use of phosphorus products

The supply of phosphorus from human excreta and consumption of detergents is about 2.5 g P per person and day (Vinnerås, 2002). This value corresponds to about 8100 tonnes P per year for the Swedish population of 8.9 million inhabitants. With a connection degree of 87% and a phosphorus removal degree of 95% the annually available phosphorus from central municipal WWTPs is about 6700 tonnes P for potential recovery from sludge. In Henriksdal WWTP, Stockholm around 700 tonnes P is available in the sewage sludge (Stockholm Vatten, 2004). The environmental goal of 60% recovery (SEPA, 2002) implies that about 4000 tonnes of P will have to be recycled annually by the year 2015. In this thesis, acid leaching showed on average 80% phosphorus release and alkaline leaching 50% (Papers III-V, VII-VIII). The amount of fertiliser used in agriculture for arable land is 8 kg P per hectare and year, which is an annual total of 24000 tonnes for the Swedish arable area of 3 million hectares. Other figures show that around 17600 tonnes of P are sold annually in Sweden as fertiliser (Jönsson, 2003), but the phosphorus is added in surplus, causing eutrophication.

4.4.1 Phosphorus products

It is possible to produce different phosphorus products from the recovery technologies. These can be used as raw materials to the phosphate industry or for direct use in agriculture (Papers I, VI). This thesis studied calcium phosphate, which is seen as a potentially valuable phosphorus product according to the literature (Table 13).

The phosphate industries make phosphorus acid with calcium phosphate as the raw material. Since the phosphate in the sludge originates from phosphorus products produced from calcium phosphate ore, recovery of the phosphate as iron phosphate will not preserve the limited calcium phosphate resources. However, iron phosphate occurs in larger amounts in nature than calcium phosphate, but today exist no commercial method to produce phosphate from iron phosphate. Iron phosphate and magnesium ammonium phosphate only have a value in direct use as fertilisers. However, the plant availability of iron phosphate has been found to be limited (Krogstad et al., 2003), although other studies of ferric phosphate show that this product has a considerable fertilising effect (Hansen et al., 2000). Use of aluminium phosphate as a fertiliser is doubtful due to potential dissolution of aluminium ions at low pH-values, causing toxicity in the soil.

Table 13. Ranking of phosphorus products according to their market value Ranking Phosphorus product

(Hultman et al., 2001b) (Enskog and Johansson, 1999) 1 Phosphoric acid Calcium phosphate 2 Calcium phosphate Magnesium ammonium phosphate 3 Magnesium ammonium phosphate Iron (III) phosphate 4 Iron (III) phosphate Aluminium phosphate


21

4.4.2 Quality of leachate

In Table 14, the leachate composition from the studies in Paper IV is compared with sludge approved for agricultural use according to Swedish limits (SNFS, 1998). The ratio of metal to phosphorus in the leachate at low acid concentrations in Study B (Table 14) is below the permitted ratio for agriculture. The cadmium value is exceeded in several of the other studies, but it also has the lowest permitted value. Cadmium and other heavy metals may be removed by a step including precipitation with sulphide.

The phosphate industry has high demands on the raw material for phosphorus production. Around 20% of the raw material

currently originates from WWTP (Roeleveld et al., 2004). The phosphorus used comes from the PhoStrip process, where the phosphate released is precipitated as calcium phosphate (Schipper et al., 2001). The most important requirements for phosphorus as a raw material to the phosphate industry (Thermphos) are shown in Table 15. As can be seen, the leachate obtained from the experiments performed in the present study may be used as raw material. An intermediate step may be used to improve the quality.

4.4.3 Treatment of remaining residue

A separate stream of toxic material remains after recovery of phosphate and other valuable products, such as Al or Fe salts that can be processed further to precipitation agents. It is an advantage to have the toxic material concentrated, instead of treating the whole sludge amount. The remaining residue can be deposited in landfill or used as construction material, e.g. in bricks or as a component in the cement industry (Kikuchi, 2001; Weng et al., 2003; Boura et al., 2003). However, the material has to be properly enclosed since ashes contain mobile compounds that may give rise to significant pollution on disposal or during use (Reijnders, 2005).

Table 14. The metal to phosphorus ratio in the leachate at 0.5M acid or base, except 1M base for Study F, (Paper IV) and in sludge approved for agricultural use Study B

(acid) mg Me/g P

Study C (acid)

mg Me/g P

Study D (acid)

mg Me/g P

Study E (base)

mg Me/g P

Study F (base)

mg Me/g P

Approved sludge

mg Me/g P Cd 0.0271 <0.0986 0.0613 <0.0242 <0.2707 0.05 Cr <0.33 0.43 0.10 <9.68 <1.08 2.86 Cu 1.779 31.57 2.387 3.753 <1.081 17 Pb 0.65 2.56 1.70 0.56 0.31 2.86 Hg* nd nd nd nd nd 0.07 Ni <0.33 1.69 0.17 <9.68 <1.08 1.43 Zn 1.7 27.6 4.2 11.0 <1.0 22.9 *) nd =no data

Table 15. Requirements for products of phosphorus recovery at Thermphos International B.V. (modified from Roeleveld et al., 2004) Parameter Unit Thermphos

Elementary P-industry

Dry mass % > 75 P2O5 % of DS > 18 Fe mg/kg < 10 000

Zn mg/kg < 100

Cu mg/kg < 10


22

4.4.4 Cost

Phosphorus recovery can be seen as technically possible, but the economic feasibility of phosphorus recovery from sewage can still be regarded as dubious (Roeleveld et al., 2004). In Balmér et al. (2002) (Table 3 in Paper I), the cost analysis of six potential phosphorus recovery systems was compared. The most favourable system, apart from direct agricultural application, was the advanced technologies (PhoStrip, KREPRO and BioCon) that showed less cost to the reference alternative (incineration). The increased cost was about 1.5-5 Euro/person or 3.5-9 Euro/kg P. This would mean an increase of 1-3% in Swedish water and wastewater taxes. The leaching and recovery step are not involving the greatest cost. Instead, the largest investment required is in the technology for sludge treatment. In Paper I, the economic aspect is discussed, since phosphate recovery cannot be profitable without also taking into account the savings in the WWTP. In Papers I and VIII, total cost for technologies and estimations of cost for leaching phosphate from ash or SCWO residue are described and show about the same cost for the both processes. This is also presented by Levlin et al. (2005), namely the leaching cost was estimated to be 7 Euro/kg P for the ash and 7.2 Euro/kg P for the SCWO when all matter was leached. Leaching only phosphate, calcium and aluminium gave a cost of 0.32-1.43 Euro/kg P from incinerated ash or SCWO residue. 4.5 Future urban wastewater systems with recovery

“Will the sustainable water and wastewater systems of the future be improved versions of what exists today, or will there be some radical changes?”

Urban Water (2003)

4.5.1 Local, central and regional solutions

Improved sustainability of the urban infrastructure may be achieved with the help

of technical arrangements and suitable management systems. The direction of development is likely to include both improving the existing WWTP in a more sustainable way and introducing source separation technologies to a larger extent. It is probable that future urban wastewater systems will include local, central and regional solutions that have in common resource conservation and production of usable products.

In sustainable sludge handling, the sludge will have to be utilised as a raw material to produce different useful products (Tables 2 and 3). One scenario is that the WWTP will consist of a sludge treatment process followed by a recovery unit (Figure 13) and, depending on scale and size, treatment will be central or regional. In Sweden, the larger cities may have a particular treatment technology installed, such as SCWO or incineration, and may be able to receive sludge from surrounding towns.

4.5.2 Evaluation of systems

A comparison was made between a source separation system and a conventional system followed by recovery (Stark, 2005). The systems selected were a blackwater system with urine separation and a WWTP plant followed by the Aqua Reci process (using SCWO) as the energy and recovery unit for fertiliser. Selected indicators for each criterion were chosen according to Malmqvist and Palmquist (2005) (Figure 1). Data for the comparison were taken from Vinnerås (2002); Balmér et al. (2002); Stendahl and Jäfverström (2004); Ahlman et al. (2004); Hellström (2005); Berg and

WWTP

Sludge

treatment

Recovery

unit

Figure 13. Addition of a recovery unit to the WWTP.


23

Tälleklint (2005); Papers I, VII. Table 16 shows the comparison between the source separating system (S) and the recovery system (R) with selected indicators for each criterion. The system chosen was the one that gave lowest or highest effect depending on the indicator.

This simple comparison between the systems showed that the recovery system (R) obtained more positive indicators than the source separating system (S) when each indicator was seen as equally important. Weighted indicators and criteria may change the results, as has been investigated in different studies (Hellström et al., 2000; Söderberg et al., 2002). Source separation is motivated by easy recovery of nutrients for farmland, but need social acceptance and the economic factor is essential for the individual. Source separation systems are

preferred in small-scale units, and in residential areas outside the dense city (Berg and Tälleklint, 2005). The legislative and political scope for action is regarded as sufficient to complete either of the alternatives. However, future regulations are likely to be stricter for discharge and sludge use, but to promote alternative urban wastewater systems.

The following arguments for a central or regional urban wastewater system (R) have been identified (Stark, 2005). (These points may also be used in reverse order to improve small scale systems): • The future urban environment will be a

dense city, which creates strong motives to build further on the central systems already existing.

• Fewer organisational resources and dialogue between actors are demanded

Table 16. Comparison between a source separating system (S) and a recovery system (R) with selected indicators for each criterion (modified from Stark, 2005) CRITERIA HEALTH AND HYGIENE

ENVIRONMENT AND USE OF NATURAL RESOURCES

ECONOMY

SOCIO-CULTURE

TECHNICAL FUNCTION

INDICATORS WITH CHOSEN SYSTEM Lowest microbial risks: Exposure to pathogens

Lowest flows of heavy metals to water

Lowest annual cost

Best institutional capacity, incl. split of responsibilities and risks between actors

Highest technical robustness

R S R R R Lowest chemical risks: Exposure to pharmaceutical residues

Lowest flows of heavy metals to farmland

Lowest transition cost

Best possibilities for learning and participation

R R R S Highest reuse of

nutrients to farmland

Lowest financial cost

Best social robustness

S/ R R R Lowest use of

energy Best comfort

S R Lowest discharge of

nutrients to water

S


24

with conventional systems compared with what is demanded when introducing the source separating systems.

• It is cheaper to have the city fully connected to a large-scale system than to have local solutions.

• Less maintenance and less technical risks in households are considered with conventional system solutions.

This comparison is an example to show the complexity involved in evaluating different systems. Other factors as the recent focus on risks due to sabotage, terrorism and war

actions and possibilities to integrate urban water and wastewater handling with other infrastructure will also influence future systems for urban wastewater. As many ways are available for development of technical urban infrastructure one important aspect is to plan for flexibility related to investments in a local, central and regional perspective.


25

5. CONCLUSIONS Phosphorus recovery is not justified only based on market economy. However, the concern of

phosphorus as a non-renewable resource and diffuse disposal of phosphorus-rich sludge, residue or ash may justify phosphorus recovery based on environmental concerns. Other important aspects are the secondary effects of phosphorus recovery including less sludge amounts to handle and recovery of other components such as precipitation agents. What are the respective advantages and disadvantages of acid and alkaline leaching?

The choice of using acid or alkaline leaching is depending on the desired achievement. Acid leaching is advantageous in respect of high phosphate release and high sludge reduction, while alkaline leaching preferentially gives lower metal contamination of the leachate. The result in this thesis showed phosphate was leached more easily with the acid than with the base. The largest degree of leached phosphate was obtained by leaching SCWO residue with acid (80-100% at acid concentrations 0.1M HCl). Acid leaching of sludge incineration ash gave 75–90% released phosphate at the concentration 1M HCl. Alkaline leaching of SCWO residue gave 50–70% leached phosphate at the concentration 1M NaOH, while 40-70% was obtained from the ash. Both acid and alkaline leaching showed that iron was not released simultaneously with phosphate, which facilitates the further recovery step. How do differences in sludge residues affect phosphate release and recovery?

Prediction of phosphate release is a complex process, as the components leached from the sludge residue depend on its original composition and the presence of ions affects the phosphate release. Leached iron and aluminium ions together with phosphate give rise to separation problems in obtaining a phosphorus product. The type of bonds of phosphorus and metals during incineration and especially SCWO still need to be further studied.

The results showed that it was easier to release phosphate from the SCWO residue than from the ash at low acid concentrations.

(0.05M-0.5M HCl). The different behaviour for the SCWO process could be due to that the SCWO residue may be more reactive due to smaller particle size than the ash. It was found that pre-treatment of the ash may be important for better release of phosphate and storage of SCWO residue influenced the release of phosphate. The difference in composition of ash and sludge residue samples had no significant influence on release of phosphate at higher concentrations of acid (1M HCl). Better efficiency of release for any of the ash or SCWO residue samples at lower base concentration was not observed. Results for alkaline leaching showed higher release from ash at 1M NaOH than SCWO residue and dried sludge at 300°C. How do different parameters affect the result of phosphate release(temperature, pH, etc.)?

The result in this study has shown that the temperature used in acid leaching of SCWO residue influences the relative release of ferric and phosphate ions. The leaching was favoured at room temperature compared to 90°C. A reason for this might be due to the metal bonds breaking at different rates and iron hydroxide may have been dissolved at higher temperature. The temperature of thermal treated digested sludge, incineration ash and SCWO residue influences the release of phosphate, aluminium and ferric ions. The dried sludge treated at higher temperature showed less release of phosphate. This may be related to the phenomena of glassification and crystallization that may occur with increasing temperature.

The results showed that both added acid and base were sparingly consumed for leaching different components, thus, the data indicated that acid or base were added in


26

surplus. A higher leaching efficiency for phosphate was shown at experiments under pH control (values of pH below 3). What degree of recovery can be expected from certain types of sludge?

The results showed that phosphate recovery as calcium phosphate was possible from both ash and SCWO residues. Any differences in implementation of a certain technology would depend on cost, environmental regulations and social aspects. Approximately 50% of total phosphorus was recovered at 1M NaOH.

How should phosphorus recovery be introduced into an existing system

Phosphate recovery could be introduced into an existing system by addition of a recovery unit to the WWTP. Two-step technology is a promising concept consisting of use of enhanced biological phosphorus removal and sludge fractionation. This is advantageous both with respect to lowering the chemicals requirements and energy, as well as increasing the recovery efficiency.


27

6. FUTURE RESEARCH This thesis focuses on the potential for phosphorus recovery by leaching of treated digested

sludge (drying, incineration and SCWO) using acid or base, followed by recovery of phosphorus as a calcium phosphate product. More research work is still needed to optimise this process technology and gather more knowledge about: • the influence of the treated sludge

properties (e.g. temperature used in incineration, chemicals added in phosphorus removal (Al, Fe, Ca) and degree of biological phosphorus removal, storage time of the treated sludge before leaching, etc.),

• the pH value used in leaching, • conditions for calcium phosphate

precipitation and further handling of the precipitate obtained, and

• use of two-step technology for phosphate recovery.

An interesting possibility to recover phosphorus before digestion is the Pho-Strip process. However, this process scheme can be modified in many ways and these possibilities could be better evaluated, such as: • Other methods (than anaerobic

conditions) to release phosphates from the sludge, such as use of heat and addition of chemicals

• Thermal treatment of the precipitate to remove organic materials

• Enrichment of calcium phosphates by use of calcinations followed by separation of insoluble calcium phosphates and soluble lime (CaO)

• Combined treatment of phosphate enriched stream with ammonium rich supernatant to precipitate magnesium ammonium phosphate

• Precipitation of the phosphate enriched stream with aluminium salts followed by recovery of phosphorus and aluminium salts

• Precipitation with ferrous salt followed by recovery of phosphorus and ferrous salts

• Use of partial phosphorus recovery and its advantages/disadvantages for further sludge use as soil conditioner, for golf courses, and coverage of landfills.

In phosphorus recovery, it is important to also evaluate the possibilities for recovering other components, such as precipitation agents, nitrogen, sulphur and potassium. Moreover, side effects such as changes in sludge, ash or residue mass and volume and eventual production of polluted side-streams have to be studied further. In addition, the benefits of phosphorus recovery must be compared with the resources used for the recovery (costs, energy, chemicals, etc.).

As phosphorus recovery is a very complex area with many possibilities, it is important to develop different prediction tools to be able to make forecasts on such issues as leaching efficiency, degree of phosphorus recovery, environmental effects, costs, etc. Although much information already exists on different aspects of phosphorus recovery, a great deal of research is still needed to implement phosphorus recovery as a standard procedure in a sustainable society and in wastewater handling.


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