Ansökan 2001 till FORMAS Forskningsrådet för Miljö, areella näringar och samhällsbyggnad

Erik Levlin, Mark och Vattenteknik, Kgl. Tekniska Högskolan

 

RECOVERY OF PHOSPHORUS FROM SEWAGE TREATMENT SLUDGE AND ASHES DEPENDING ON PRECIPITATION CHEMICALS

 

RESEARCH PROGRAMME

 

RELEVANCE FOR SOCIETY

 

Sustainable handling of sludge from municipal wastewater treatment has as an important goal to recycle resources without supply of harmful substances to humans or the environment (Hultman et al., 1997). Sludge use on agricultural land is at present the preferred and main alternative for the use of sludge as a resource. However, increasingly more stringent requirements of maximum concentrations of pollutants in sludge, resistance from food industry, farmers or the public may worsen the possibilities to use sludge for agriculture.

 

Recovery of nutrients such as phosphorus from incinerated sludge ash is an alternative that allows the nutrient to be used as fertilizer in agriculture without using sludge on agricultural land. Instead of removing the impurities as metals from the sludge the nutrient in the sludge can be recovered and used as fertiliser in the agriculture. The nutrients in the sludge are potassium, calcium, phosphate and nitrogen. Of these nutrients phosphate is most important to recover. Miljökommitén (2000) has in a proposal to the Swedish government set up a national goal that 75% of phosphorus in wastes should be recovered. The Swedish Environment Protection Agency (SEPA) has been given a commission from the government to evaluate the implementation of this goal and propose modifications.

 

Table 1. Goal set up by Miljökommitén (2000) for phosphorus recovery from sewage sludge.

 

1999

2000

2005

2010

of total

Phosphorus in sludge

2 440

100

650

1 600

 

Phosphorus from sludge hydrolysis

100

100

800

2 000

6 700

 

Phosphate fertiliser is produced by mining of phosphate ores. More than 300 different phosphate minerals are available, but only apatite (calciumphosphate, Ca3(PO4)2) is used for production of fertiliser (Corbridge, 1995). 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 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.

 

Another important goal is to avoid or reduce the amount of waste and sludge that has to be deposited on landfill. In Sweden a tax of 250 SEK/ton on all deposited solid waste is introduced from year 2000 (SFS 1999:673) and deposition of organic material will be prohibited from year 2005 (SFS 1998:902). Anaerobic digestion eliminates half of the organic content and half of the energy can be utilised as methane gas. To avoid release of phosphate due to the reduction of ferric phosphate to ferrous phosphate at anaerobic digestion, a large excess of precipitation chemical is needed. At Henriksdal treatment plant the iron phosphate ratio is about 1.9. Incineration is a method that reduces the volume by eliminating the organic content and the potential energy can be utilised (ATV, 1997 and Weibusch et al., 1997). The resources in sludge and solid waste remaining after incineration are nutrients and the inorganic material. Development of method to recover these resources from incineration ashes will make handling of municipal waste and sewage sludge more sustainable.

 

PREVIOUS WORK

 

Investigations have been made on phosphorus recovery from incineration ashes (Levlin et al., 1998 and 2000, Schmidt, 1998). Three types of ashes were used; ash from digested sludge incinera­ted in a laboratory oven, bottom ash from Högdalen incineration plant there municipal solid wastes were mixed with 10 % heat dried sludge and fly ash from Igelsta incineration plant where biofuel were mixed with 20 % dewatered sludge. The sludge was from Högdalen treatment plant which uses precipitation with iron salt.

 

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 about 20 % may be very difficult to remove from the ashes. The produced ashes during co-incineration showed often a much higher quotient of metal to phosphorus than the supplied wastewater sludge. The quotient in ashes and leachate is normally higher than for the sludge and also higher than in approved sludge. 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 other methods to recover phosphorus from sludge.

 

 

Table 2. Examples of coagulant and phosphorus recovery products.

Precipitation agent

Precipitate

Example of recovery methods

Recovered coagulant

Phosphorus product (examples)

CaO

Ca5(OH)(PO4)3
CaCO3

Combustion

CaO

Ca5(OH)(PO4)3

Al3+

AlPO4 Al(OH)3

Use of sodi­um hydroxide

Al(OH)4- (aluminate)

Ca5(OH)(PO4)3

Fe2+

Fe3(PO4)2 Fe(OH)2

Use of acids

Fe2+

Ferric phosphate
Phosphoric acid

Fe3+

FePO4 Fe(OH)3

Use of acids

Fe3+

Ferric phosphate

Phosphoric acid

 

 

Different precipitation chemicals can be used such as calcium oxide, iron and aluminium salts. Iron salt is the most used precipitation chemical. Table 2 shows examples of coagulant and phosphorus recovery products. The use of calcium oxide will give calcium phosphate as recovered product but it can be assumed that the solubility is to high to get efficient phosphate removal in the treatment process. When precipitation with iron salt is used, the phosphate is recovered as ferric phosphate, which is not regarded as a suitable raw material for the phosphate industry. The higher solubility of apatite makes the phosphate easily extractable with sulphuric acid or able to be converted to phosphorus gas by an electric furnace. Iron phosphate, that can be used if iron phosphate hade been a suitable phosphate raw material, is more frequent occurring mineral than apatite. The phosphate in the incoming wastewater originates from products produced from apatite ore. In the treatment process phosphate from a non-renewable resource, apatite, has been transformed to iron phosphate instead of recycled as apatite.

 

Table 3 shows solubility constant and equilibrium concentrations at dissolution/precipitation of different phosphate compounds. The very low equilibrium concentrations for trivalent iron and aluminium ions compared to divalent give an efficient phosphate removal in the wastewater treatment process. The equilibrium concentrations depend on pH-levels and decreases with increasing pH-level. However at high pH-level the solubility of aluminium and iron phosphate increase due to precipitation of hydroxides (see figure 1). However, released phosphorus from the iron phosphate may bee precipitated as calcium phosphate, which have a low solubility at high pH-levels, thus creating a mixture of iron hydroxide and calcium phosphate. Aluminium compounds can at high pH-levels (above pH 9) also be dissolved as aluminate ions, which can be separated from precipitated calcium hydroxide.

 

 

Table 3. Solubility constant, Ks (Corbridge, 1995) and equilibrium phosphate concentrations (Levlin, 1999) at dissolution/precipitation of phosphate compounds.

Phosphate compound

Solubility constant, Ks

Equilibrium concentration (mol/litre)

Mg3(PO4) 2

6.3*10-26

7.14*10-6(K´)-0,4

Ca3(PO4)2

1.4*10-29

1.33*10-6(K´)-0,4

Zn3(PO4)2

9.0*10-33

3.06*10-7(K´)-0,4

Pb3(PO4)2

8.0*10-43

2.99*10-9(K´)-0, 4

AlPO4

5.8*10-19

7.62*10-10(K´)-0,5

FePO4

1.3*10-22

1.14*10-11(K´)-0,5

K´ = (1+10(-pH+12.023) + 10(-2*pH+12.023+7.198) + 10(-3*pH+12.023+7.198+2.148))-1

 

 


AlPO4

FePO4

 


CaHPO4

Ca4H(PO4)3




Ca10(PO4)6(OH)2
Ca10(PO4)6(F)2

 

Figure 1. The solubility of metal phosphates (Stumm and Morgan, 1981).

 

 

 

 

 

 

 

 


Figure 2. A sketch of how precipitated calcium phosphate may be separated from precipitated hydroxide with a help of ion selective membrane.

 

 

Figure 2 shows a suggestion of how precipitated calcium phosphate can be separated from precipitated iron or aluminium hydroxide with ion selective membrane. Metal contaminants will probably remain in the hydroxide and pure calcium phosphate will be recovered. If recovered iron and aluminium is not polluted by metal it can be used as precipitation chemicals. An alternative use for aluminium hydroxide with metal contaminants is for production of aluminium cement. The high aluminium content of a sludge produced at water treatment with aluminium salts makes it useful for production of aluminous cement (Lubarski et al., 1996).

 

At leaching with acid aluminium can be extracted from zeolit A at pH-levels around 3 (Lass, 1997) and from digested sludge Jardin och Pöpel (2001). There is also special ion exchange technology to extract aluminium as a salt (Petruzzelli m fl, 2000).

 

 

PROPOSED WORK

 

Project leader is Erik Levlin who will perform the literature review and report writing and Monica Löwén will assist in the experimental work. This study will be focused on alternative precipitation chemicals as the use of aluminium salt or calcium oxide for phosphate precipitation. The work will be performed as follow:

 

(1) Complementing the literature review on recovery of phosphate then using of aluminium and calcium oxide. Separation of dissolved phosphate and aluminium by ion exchange will be evaluated.

 

(2)        Laboratory experiments performed in similar way as Culp och Culp (1971) with addition of aluminium salt to sewage water to get a phosphorus and aluminium rich sludge. The sludge and incinerated sludge is used to study:

§         Degree of dissolving aluminium and phosphate at different pH-levels

§         Precipitation of dissolved phosphate as calcium phosphate to evaluate degree of recovery

§         Precipitation properties of recovered aluminium salt

§         Metal contamination of recovered phosphate and aluminium salt

§         Evaluation of the process and the utilisation of phosphorus products

 

(3)               Study of methods for separation of aluminium and phosphate (see figure 2)

 

(4)               Study of usage of aluminous hydroxide for cement production. Studies on burning aluminium cement of aluminious sludge have been made in Moscow by Lubarski (dead) and Koroleva (Lubarski et al., 1996). The work has continued and can be a presumptive partner in a future project.

 

 

REFERENCES (More references)

 

ATV (1997). Klärschlammverbrennung Beseitigung oder Verwertung. Korrespondenz Abwasser, Vol. 44, No. 10, pp. 1880-1884.

Corbridge, D.E.C. (1995) Studies in Inorganic Chemistry 20, Phosphorus, An Outline of its Chemistry, Biochemistry and Uses, 5th ed., Elsevier Science, ISBN 0-444-89307-5.

Culp, R.L. och Culp, G.L. (1971). Advanced wastewater treatment. Van Nostrand Reinhold Co.

Hultman, B., Levlin, E., Löwén, M. and Mossakowska, A. (1997) Uthållig Slamhantering, Förstudie. Stockholm Water Co., R. nr 23 Sept-97.

Jardin, N. and Pöpel, H.J. (2001) Refixation of phosphates released during bio-P sludge handling as struvite or aluminium phosphate, 2nd International Conference on Recovery of Phosphates from Sewage and Animal Wastes, Holland, NL, 12-13 March 2001.

Levlin, E. (1999) Resources recovery from incineration ashes, Sustainable municipal sludge and solid waste handling, Report No. 5, Proceedings of a Polish-Swedish seminar, Stockholm August 24, TRITA-AMI REPORT 3063, ISBN: 91-7170-439-6. pp. 43-53

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.

Levlin, E., Löwén, M., Schmidt, E., Hultman, B. and Mossakowska, A. (2000) Phosphorus recovery from sewage sludge incineration ash. 1st World Water Congress of IWA, Paris, 2000.07.03-07.06, CD-ROM, ISBN: 2-9515416-0-0 EAN: 9782951541603.

Lubarski, V., Levlin, E. and Koroleva, E. (1996) Endurance test of aluminous cement produced from water treatment sludge. Vatten, 52(1), 39-42.

Miljökommittén (2000). Framtidens miljö – allas vårt ansvar (The future environment – our common responsibility), Statens Offentliga Utredningar SOU 2000:52

Petruzzelli, D., Volpe, A., Limoni, N. and Passino, R. (2000). Coagulants removal and recovery from water clarifier sludge. Water Research, Vol. 34, No. 7, pp. 2177-2182.

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

Stumm, W and Morgan, J.J. (1981) Aquatic chemistry, An introduction emphasizing chemical equilibria in natural waters, 2nd ed., John Wiley & Sons Inc., ISBN 0-471-04831-3.