RECOVERY OF PHOSPHATE FROM SEWAGE SLUDGE

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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 anion

exchanger, which removes sulphate ion, and is regenerated with potassium chloride producing

potassium bisulphate, KHSO4. The third column is an anion exchanger, which removes

phosphate ions, and is regenerated with hydrochloric acid producing phosphoric acid, H3PO4.

The last column is a cation exchanger, which removes other metals, and is regenerated

hydrochloric acid producing metal chloride.

Figure 2. Regeneration of ion exchange columns for recovery of phosphoric acid.

Cation

exchanger

HCl for

regeneration

Anion

exchanger

H3PO4

(and HCl)

HCl for

regeneration

Metal chloride

(and HCl)

Cation

exchanger

H2SO4 for

regeneration

Anion

exchanger

Fe2(SO4)3

(and H2SO4)

KCl for

regeneration

KHSO4

(and KCl)

Leachate with

H+, H3PO4, Fe3+

and HSO4

-

H+

H3PO4

HSO4

-

H+

H2PO4

-

Cl-

(SO4

2-)

H+

Cl-

SO4

2-

H+ Cl-

SO4

2-

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At pH-levels below 2 is phosphate present as zerovalent H3PO4, which not can be removed by

ion exchange. The pH-level in the third column must therefore be above 2 there phosphate is

converted to anions, H2PO4

-, which can be removed by an anion exchange column. At pHlevels

above 2 is the potassium sulphate produced as K2SO4. The pH-level before the second

column is below 2 and by removing the hydrogen sulphate ion the pH-level increases until the

solution reach a pH-level there the phosphate is converted to monovalent ions which can be

removed in the third column.

However, regeneration in one step often requires an excess amount of regenerate (Helfferich,

1995). The effluent regenerate from the column may contain a mixture of the product realised

from the column and regenerate. The realised product must be separated from the regenerate

by a further precipitation step and the remaining regenerate can be reused for regeneration.

Alternatively can the need of excess amount of regenerate be reduced by reducing the degree

of recovery. In the case of BioCon the degree of phosphorus recovery is about 60 % (Balmér

et al, 2002). Using an anion exchange resin with a selectivity for chloride in the third

phosphoric acid recovery column, almost all chloride ions in the regenerate will be exchanged

for phosphoric acid at regeneration and an almost chloride free phosphoric acid will be

produced. However, since the resin is selective for chloride only a part of the phosphoric acid

in the leachate is taken up by the column,

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