RECOVERY OF PHOSPHATE FROM SEWAGE SLUDGE
Enviado por quiqueGOL • 13 de Febrero de 2014 • Tesis • 4.265 Palabras (18 Páginas) • 300 Visitas
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RECOVERY OF PHOSPHATE FROM SEWAGE SLUDGE
AND SEPARATION OF METALS BY ION EXCHANGE
Erik Levlin
Land and Water Resources Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden
Abstract
Ion exchange can be used to separate phosphorus from iron when phosphorus is recovered
from sewage sludge. A review of the use of ion exchange for phosphorus recovery is
presented followed by a discussion on how to use ion exchange for separation of metal and
phosphate. Example of processes for recovering of phosphate with ion exchange are
BioCon and RemNut. In the BioCon-process is ash from sludge incineration leached with
acid and the metal ions are separated from the phosphate with a ion exchange process,
producing iron chloride and phosphoric acid. In the RemNut-process is phosphorus
recovered from the effluent of the sewage treatment plant as magnesium ammonium
phosphate. Phosphate, which without a ion exchange process is recovered as iron
phosphate, has no commercial value as raw material for the phosphate industry. By mixing a
cation exchange resin with the sludge, hydrogen ions from the ion exchanger can dissolve
metal ions which are taken up by the ion exchanger. If a magnetic resin is used it can be
separated from the sludge with a magnetic drum. Using the acid released from the ion
exchange resin to leach the sludge decreases the consumption of chemicals needed for the
process. Ion exchange textiles and ion selective membrane can also be used for phosphorus
recovery processes.
Keywords
Ion exchange, phosphorus recovery, sewage sludge
BACKGROUND
A national goal has recently been proposed in a report to the Swedish government that at least
75% of phosphorus from wastewater should be recovered at latest 2010 without risks for
health and environment (Wallgren, 2001). In the sewage treatment plant phosphorus is
removed from the wastewater by precipitation with iron salt. However, since phosphorus is
needed as a fertiliser in the agriculture, a requirement for getting sustainable wastewater
treatment is to create method to use the phosphorus from the wastewater as a fertiliser in the
agriculture. Most of the phosphorus used in the agriculture originates from mining of
phosphate ores. The global deposits of economically mine able phosphate are estimated to be
109 ton phosphorus and the total amount phosphorus in the sediments is estimated to be 1015
ton (Butcher et al., 1994). Many different phosphate minerals are available, but only apatite
(calcium phosphate, Ca3(PO4)2) is used for phosphate production (Corbridge, 1995).
Phosphate can be economically produced by leaching apatite mineral with sulphuric acid
(McKetta and Cunningham 1990):
Ca3(PO4)2 (s) + 3 H2SO4 + 3x H2O 2 H3PO4 + 3 CaSO4·xH2O (s)
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In 1995 the world phosphate rock production was 160 000 ton per year (as P2O5), having
tripled over the last 40 years. About 90% of this amount is used as fertiliser. At this rate of
consumption the known apatite reserves have been estimated to last for a period up to 1000
years. However, if the present increase in world population and the increasing need for
fertiliser for food production is taken into account, the supply of phosphate may well be
crucial within a century. Apatite ore is thereby a limited resource that must be preserved by
phosphate recovery.
Phosphorus may be recovered from sewage sludge by leaching with acid (Levlin et al., 1998,
2000, Levlin, 1999). The solubility of phosphate compounds decreases with increasing pHlevel.
Phosphate compounds can depending on the pH-level be dissolved as:
MePO4 (s) + 3 H+ H3PO4
o + Me3+ (pH < 2.15)
MePO4 (s) + 2 H+ H2PO4
- + Me3+ (pH > 2.15 and < 7.20)
MePO4 (s) + H+ HPO4
2-+ Me3+ (pH > 7.20 and < 12.02)
MePO4 (s) PO4
3-+ Me3+ (pH > 12.02)
Two systems for phosphorus recovery from sludge in wastewater treatment plants with
chemical precipitation with iron salts is at present considered, KREPRO (Hansen et al., 2000,
Hagström et al., 1997) and Bio-Con which uses ion exchange processes. In the two systems
the iron content in the sludge is dissolved by acid together with the phosphate. After
dissolution, the leachate contains a mixture of different ions including iron, together with
phosphoric acid, which must be separated in a further step.
In the KREPRO-process is the heavy metals precipitated with sulphide and the phosphate as
ferric phosphate. Without removing the iron, phosphate will preferentially be precipitated as
iron phosphate, which have a lower solubility than calcium phosphate. Iron phosphate has no
commercial value as raw material for the phosphate industry, and the low solubility makes it
less favourable to use as fertilizer. Since the phosphate in the sludge originate from
phosphorus products produced from apatite ore, recovering the phosphate as iron phosphate
will not preserve the limited apatite resources. Iron phosphate is a much more common
mineral in the ground than apatite. Use of ion exchange processes as in the BioCon-process,
make it possible to recover the phosphate as phosphoric acid, which is produced from apatite
ore, thus preserving the limited apatite resources and the resources, mainly sulphur, needed for
producing phosphoric acid from apatite.
ION EXCHANGE PROCESSES USED FOR PHOSPHORUS RECOVERY
The BioCon-process
The BioCon-process shown by figure 1 (Svensson, 2000), in which ion exchange is used for
phosphorus recovery after leaching with acid, has been developed by a Danish company. In
Sweden, the municipality of Falun intend to apply for a permit for sludge incineration with Precovery
from the incineration ash, and a plant using the BioCon-process is expected be built.
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Ash silo
Ash mill
Ion exchangers
Sulphuric acid
Water
Dissolving
container
Sand
Sludge residue
FeCl3 KHSO4 H3PO4
Figure 1. Resources recovery from ash with BioCon system (Svensson, 2000).
The ion exchange process shown by figure 2, is performed in four columns. In the first
column, which is a cation exchanger is the ferric ions taken up. This column is regenerated
with sulphuric acid producing ferric sulphate Fe2(SO4)3. The second column is an
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