Poster
presentation [L088] at 1st World Water Congress at the International Water
Association 3-7 July Paris 2000
PHOSPHORUS RECOVERY FROM SEWAGE SLUDGE
INCINERATION ASH |
Erik Levlin, Monica Löwén,
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 |
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INTRODUCTION
Keywords |
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OBJECTIVES
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 incineration §
Effects
of acidity on leaching of phosphorus §
Simultaneous
release of metals during the leaching process
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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.
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EXPERIMENTAL
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EXPERIMENTAL PERFORMANCE
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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 |
DISCUSSION 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. |
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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. |
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REFERENCES 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. |
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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. |