Poster presentation [L088] at 1st World Water Congress at the International Water Association 3-7 July Paris 2000




Erik Levlin, Monica Löwén,
Eva Schmidt and Bengt Hultman
Div. of Water Resources Engineering,
Royal Institute of Technology,
S-100 44 Stockholm, Sweden

and Agnes Mossakowska
Stockholm Vatten AB,
S-106 36 Stockholm, Sweden



The objective was to study the possibility for phos­phorus recovery from ash obtained from co-incineration of sludge with solid waste in a grate oven and with biofuel in a fluidised bed reactor. Measurements of the phosphorus content before and after incineration of sludge samples between 550 and 1000 ºC showed that no phosphorus is lost by evaporation. Incinerated sludge and ash from the co-incinerations were leached with HCl at different concentrations and contact times. Leaching with 1 M HCl or higher concentrations and during 8 hours gave more than 75 % dissolution of phosphorus. For certain ashes the dissolution was nearly complete while other ashes seem to contain a phosphorus fraction that is very resistant towards leaching. During leaching of ashes for phosphate dissolution different metals are also released which complicates recovery of phosphates as a clean product. Metal contents of ash from the co-incinerations and in the leachate were measured and the ratio between metal and phosphorus was calculated. The ratio was higher for ash and leachate than for the sludge. For almost all metals the ratio for ash was higher than the limit for sludge approved for agricultural use. Bottom ash from the grate oven had lower ratios than ash from the fluidised bed reactor. Especially metals with low boiling temperatures were removed from the bottom ash of a grate oven



Sustainable sludge handling has as an important goal to recycle resources without supply of harmful substances to humans or the environment (Hultman and Levlin, 1997, Hultman et al., 1997) and much efforts are today performed at Stock­holm Water to evaluate different sludge strategies to use sludge as a resource (Mossakowska et al., 1998). Sludge use on agricultural land is at present the preferred and main alternative for the use of sludge as a resource. However, different factors may worsen the possibilities to use sludge for agriculture as increasingly more stringent require­ments of maximum concentrations of pollutants in sludge, resistance from food industry, farmers or the public or lack of suitable land areas near the plant. An alternative to sludge use on agriculture is to fractionate sludge into different components for production of nutrients and to separate metals into a small stream (Mossakowska et al., 1998).

Matsuo (1996) has in a laboratory scale studied leaching of phosphorus from ash with hot water. The ash was obtained by incineration of sludge from a biological phosphorus removal plant. A high fraction of phosphorus could be removed. The removal rate decreased with increasing temperatures between 670 and 1000 ºC. Addition of iron(III) salts before the incineration decreased the dissolution of phos­phorus. In leaching of incinerated sludge with acids between pH-level 4 and 6 Ozaki et al. (1997) obtained less than 10 % dissolution of phosphorus. Mederer (1998) reportted that the Phosphorus in ashes may be bound as mixed crystals of Ca - Al - Fe - phosphates and Ca - Mg - phosphates and also as an amorph phase composed of Si - Al - Ca - K - P - O.


Acid leaching; co-incineration; heavy metals; phosphorus recovery; sludge incineration



Mono-incineration of municipal wastewater treatment sludge is not used at present in Sweden. However, some tests have been performed with co-incineration of sludge at existing plants using other organic materials such as municipal solid wastes and biofuels.


The objective of this investigation was to study possibilities to recover phosphorus from ash after incineration of pre-precipitated sludge together with either municipal solid wastes or biofuels. Focus on the studies were put on:


§         Possible evaporation of phosphorus during the


§         Effects of acidity on leaching of phosphorus

§         Simultaneous release of metals during the

leaching process


Two incineration plants, Högdalen and Igelsta, were selected for full-scale co-incineration studies with sludge. In Högdalen incineration plant, municipal solids wastes from Stockholm are incinerated in a grate oven and in Igelsta incineration plant, biofuels are used in a fluidised bed reactor.


In the incineration experiments at the Högdalen plant (Älvesand, 1998) municipal solid wastes were mixed with 10 % heat dried sludge from Himmerfjärden treatment plant southeast from Stockholm. Most of the ashes are obtained as bottom ash and this ash was used for the leaching experiments.

Incineration experiments at the Igelsta plant (VAI VA-Projekt AB, 1998) were performed with a mixture of 80 % biofuel (recirculated wood) and 20 % of dewatered sludge from Henriksdal treatment plant. Most of the ashes were obtained as fly ash, which was used for the leaching experiments.

In the wastewater treatment plants pre-precipita­tion is used with ferrous sulphate as the coagulant. The sludge is treated by anaerobic digestion before dewatering with polyelectrolytes as conditioning agents.

The phosphorus content in ash from incineration of sludge from Henriksdal is about 8.3 % (VAI VA-Projekt AB, 1997). Phosphorus in ash from incineration of municipal solid wastes varies from 0.45 % (Bäverman, 1997) to 1.3 % (Kida et al., 1969) and the phosphorus content in ash from incineration of biofuel varies from 0.50 % (Bäverman, 1997) to 1.4 % (Sander and Andrén, 1997). Metal contents in sludge, solid wastes, biofuel and produced ash are shown in Table 1.







Phosphorus evaporation

To evaluate phosphorus loss through evaporation sludge was incinerated and the phosphorus content was measured before and after incineration. Sludge samples from Himmerfjärden treatment plant was incinerated in an oven at 550 ºC, 700 ºC, 850 ºC and 1000 ºC. Table 2 shows total phosphorus content in the ashes and Table 3 in the sludge before incineration. As the phosphorus content in the ash is a little larger than the phosphorus content in the sludge divided by the non-volatile solids contents, phosphorus was not evaporated in the incineration.

Phosphorus leaching with acid

Phosphorus was leached from ash with 0.5 M HCl. The acid was exchanged every second hour up to a total contact time of 8 hours. Figure 1 shows the percentage of phosphorus leached with 0.5 M HCl from sludge incinerated in a laboratory oven at 550 ºC, 700 ºC, 850 ºC and 1000 ºC, Högdalen bottom ash and Igelsta fly ash.

Ash from the test incineration at Högdalen and Igelsta were used in studies of dissolution of phos­phorus at different values of the acidity (Schmidt, 1998). Figure 2 shows dissolved phosphorus in percentage and mg/l depending on the acidity of the leaching liquid. For Högdalen bottom ash liquids with 0, 0.05, 0.1, 0.25, 0.5, 1, 2 and 4 M HCl were used and for Igelsta fly ash 0, 0.5, 1, 2 and 4 M HCl were used. The contact time was 4 hours.

Metal contamination of leachate

The content of Cd, Cr, Cu, Pb, Ni and Zn was measured in ashes used for leaching and in some of the leachates, to evaluate the metal contamina­tion of recovered phosphorus. Table 4 shows the percentage of dissolved metal depending on the acidity ranging from 0 to 4 M HCl. Leaching Igelsta ash with 0.5 M HCl during 8 hours dissolved totally 89.9 % Cd, 54.5 % Cr, all Cu and Pb, 39.1 % Ni and 59.2 % Zn






The leaching was done by mixing 1 g ash with 25 ml liquid. After a specific contact time the liquid was separated from the ash. The ash was centrifuged and the liquid was filtrated through a 0.45 mm membrane filter. Phosphorus content in solid phases was measured with plasma emission spectrometry ICP-AES. Dissolved phosphorus in leachate liquids was measured as orthophosphate with a colourimetric method by Aquatec (FIA-system, flow injection analyse method ASN 146-01/90). Orthophosphate is first converted with ammonium molybdate to heteropoly molybdophosphoric acid, which in a second step is reduced to phosphormolybdenum blue by stannous chloride in a sulphuric acid media. The compound has an intensive colour and the concentration is measured as absorbance of light at 690 nm. The light absorbance of the sample is calibrated against a standard solution and the instrument calculates the concentration of the sample. Filtration through 0.45 mm membrane filter separates suspended phosphorus from dissolved. Since colourimetric methods is sensitive to suspended particles the filtration also ensures correct measurements. However, the method was originally designed for water and seawater and has to be adapted for the too high pH-levels of the leachates. Neutralising with NaOH gives precipitation of particles which would disturb the measurements. The leachates were therefore diluted with distilled water to pH-level 3 to 4



Figure 1. Percentage of phosphorus leached during 8 hours with 0.5 M HCl from incinerated sludge,

Högdalen bottom ash and Igelsta fly ash.


Figure 2. Dissolved phosphorus as (a) percentage and (b) mg PO4-P/l versus acidity in M HCl of the leaching liquid.



Table 5. The ratio metal/phosphorus as mg/g P in sludge (3.0 % P), ashes from test incineration in Högdalen (2.42 % P) and Igelsta (2.79 % P), leachate (with 2 M HCl) from Högdalen and Igelsta ashes and for maximum metal content in sludge approved for agricultural use (3% P). Ratios higher than in approved sludge are marked bold




Phosphorus recovery


In order to recover phosphorus from the leachate methods must be developed to separate metals from phosphate, for instance precipitation of metals by use of sulphides. The high amount of acids needed for an efficient removal of phosphorus from ashes in combination of separation problems in further handling of the leachate may make it more advantageous to develop methods to recover phosphorus from the sludge before incineration.


The obtained result that phosphorus is not evaporated during incineration in the studied interval 550 - 1000 ºC is in accordance with results by Belevi et al. (1998) and Takaoka et al. (1997). Some experiments on leaching of ashes showed on a nearly complete removal of phosphorus and especially for sludge incinerated in a laboratory oven. Leaching of phosphorus from ashes from Igelsta indicates that a certain fraction (about 20 %) may be very difficult to remove from the ashes.


Metal – phosphorus ratio


The produced ashes during co-incineration showed often a much higher quotient of metal to phosphorus than the supplied wastewater sludge. This was also the case for the produced leachate from the ashes. Table 5 shows the ratio metal/ phosphorus as mg/g P in sludge, ashes and leachate from leaching with 2 M HCl for the test incinerations in Högdalen and Igelsta. The ratio for maximum metal content in sludge approved for agricultural use in Sweden for a phosphorus content of 3 % is also shown in the table. Most ratios in ashes and leachate is higher than for the sludge and also higher than in approved sludge. The ratio for most metals are higher in ash from Igelsta than in ash from Högdalen.


Figure 3. The ratio of the metal content before and after incineration versus boiling temperature for Pb 1743 ºC, Cd 767 ºC, Cr 2663 ºC, Cu 2573 ºC, Hg 357 ºC, Ni 2887 ºC and Zn 911 ºC, for Högdalen bottom ash and Igelsta fly ash.


Figure 3 shows the ratio of the metal content before and after incineration for Högdalen bottom ash and Igelsta fly ash versus metal boiling tem­perature. The diagram for bottom ash shows that metals with low boiling temperature are evapora­ted and transferred to the fly ash. Incineration in Högdalen grate oven has removed almost all mercury from the bottom ash. Metals with high boiling temperature are equally divided between bottom ash and fly ash. In ash from Igelsta fluidised bed reactor the ratio is independent of the metal boiling temperature.




Belevi, H., Langmeier, M., Mönch, H., Turban, Y., Müller, T. and Baccini, P. (1998) Stoff­buchhaltung für eine Müllverbrennungsanlage (Dust accountant for a waste incineration plant). Müll und Abfall, 2, 82-94.

Bäverman, C. (1997) Long-term leaching mechanisms of ashes and slags; Combining laboratory experiments with computer simulations. Doctor thesis in Chemical Engineering and Technology, Royal Institute of Technology, Stockholm, TRITA-KET R 65, ISSN 1104-3466, ISRN KTH/KET/R-65-SE.

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, Royal Institute of Technology, Stockholm, ISRN KTH/AMI/REPORT 3045-SE, ISBN 91-7170-283-0, Paper 5.

Hultman, B., Levlin, E., Löwén, M. and Mossa­kowska, A. (1997) Uthållig Slamhantering, Förstudie (Sustainable sludge handling, pre-study). Stockholm Water Co., R. Nr 23.

Kida, A., Noma, Y. and Imada, T. (1969) Chemical speciation and leaching properties of elements in municipal incinerator ashes, Waste Management, 16, 5/6, 527-536.


The results are reported in detail by:

Levlin, E., Löwén, M., Schmidt, E., Hultman, B. and Mossakowska, A. (1998) Fosforutvinning ur aska (Phosphorus recovery from ash). Stockholm Water Co., R. Nr 54.

Schmidt, E. (1998) Possibilities to recover phos­phorus from sewage sludge before and after incineration. Diploma work, Div. of Water Resources Engineering, Royal Institute of Technology, Stockholm, AVAT-EX-1998-04.


Matsuo, Y. (1996) Release of phosphorus from ash produced by incineration waste from activated sludge from enhanced biological phosphorus removal. Wat. Sci. Tech., 34, 1-2, 407-415.

Mederer, J. (1998). Bewertung der Spurenelement - bzw. Schwermetallgehalte von mineralischen Reststoffen im Vergleich zu naturlichen Gesteinen (Evaluation of trace elements – respectively heavy metal contents in mineral rest dust in comparision to natural rock). Veröffentlichung­en des Institutes für Siedlungswasserwirtschaft und Abfalltechnik der Universität Hannover, Heft 107, Tagungsbeiträge 7.

Mossakowska, A., Hellström, B.G. and Hultman, B. (1998) Strategies for sludge handling in the Stockholm region. Wat. Sci. Tech., 38, 2, 111-118.

Ozaki, M., Watanabe, H. and Weibusch, B. (1997) Characteristics of heavy metal release from incinerated ash, melted slag and their re-pro­ducts, Water Science Tech., 36, 11,.267-274.

Sander, M.-L. and Andrén, O. (1997) Ash from cereal and rape straw used for heat production: Liming effect and contents of plant nutrients and heavy metals, Water, Air and Soil Poll., 93, 93-108.

Takaoka, M., Takeda, N. and Miura, S. (1997) The behaviour of heavy metals and phosphorus in an ash melting process. Water Science Tech., 36, 11, 275-282.


The co- incineration tests are reported by:

VAI VA-Projekt AB (1998) Provförbränning av slam vid Igelstaverket. (Test incineration of sludge at the Igelsta plant) Stockholm Water Co., R Nr 44.

Älvesand, L. (1998). Provförbränning av rötslam i Högdalenverket. (Test incineration of digested sludge at the Högdalen plant) Stockholm Water Co., R Nr 12.