ANAEROBIC DIGESTION OF PHOSPHORUS RICH SLUDGES
C.M. CARLIELL-MARQUET2 , A.D.WHEATLEY1* and J. CHURCHLEY3
1
Water Research Group, Civil Engineering Department,
Loughborough University. LE11 3TU
2
School of Civil Engineering, University of Birmingham, B15 2TT
Email: A.D.Wheatley@lboro.ac.uk
3
Severn Trent, Avon House, Coventry. CV3 6PR
ABSTRACT
The paper compares the anaerobic digestion of biological and chemical phosphorus
enriched sludges. The research has used sequential extraction and internal standards to
determine the speciation of P and metals in both laboratory and full-scale digestion.
Bioavailable iron and P were found to be very low in CPR digested sludge, compared
to the controls. Some of the biologically removed P was remobilised during digestion
but most was precipitated as calcium and calcium magnesium complexes, including
struvite. There was no evidence of deficiencies of important metal cofactors or
increased solubility of toxic metals.
Keywords:
Anaerobic digestion, speciating trace metals, struvite, biological
and chemical phosphorus removal.
INTRODUCTION
Phosphorus removal during wastewater treatment will change the nature of the sludges
fed to anaerobic digesters. Previous research on chemical phosphorus removal (CPR) has
shown adverse effects from the inorganic enrichment [1]. The phosphate from CPR is
retained within the digester and reduces the biodegradability of volatile solids [2]. The
digestibility of biological phosphorus removal (BPR) sludges are unaffected but between
20 and 50% of the phosphorus is resolubilised during anaerobic digestion [3] and this
may generate inorganic deposits within the digester or scale in downstream equipment.
The phosphate enriched precipates may also be suitable for recovery. There may be other
problems caused by the mobilisation of potentially toxic metals or the precipitation of
important cofactor trace metals. This research reports on changes in phosphorus and
metal speciation that occur in digesters treating BPR and CPR sludges and its effects on
digestion.
METHODS
Sequential chemical extraction methods based on those of Stover et al [4] and Uhlmann
et al [5] were developed to compare metal and phosphorus fractionation of anaerobically
digested BPR and CPR sludges. The extraction steps are shown in Tables 1 and 2.
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The metal and phosphate fractions were analysed by plasma emission spectrophotometry
(Thermo Jarrell Atomscan 16). Colorimetric analysis (ascorbic acid method) was
necessary to analyse soluble reactive phosphate [6]. Attempts to use ion chromatography
to measure soluble reactive phosphate were unsuccessful due to the strong extractant
solutions. A number of standard analar compounds were used as internal standards to
validate and adjust the methods. (Table 3). Laboratory scale research was used to
determine the effects of phosphorus sludges on digestion under well controlled conditions
and samples from full-scale digesters used to investigate the influence of complex
conditions on metal and phosphorus speciation. Sewage Treatment Works (STW) A was
a control digester with no phosphorus removal, STW B used BPR and STW C CPR (preprecipitation with ferric sulphate) (table 4). A more extensive methodology and results
are in the thesis of the work (7).
RESULTS AND DISCUSSION
Relative total average P concentrations were 31g kg-1 (of dried sludge) in the CPR
digester, 26g kg-1 in the BPR digester and 16g kg -1 in the control digester. The average
iron content was 72g kg-1 in the CPR digester, 13g kg-1 in the BPR digester and 12g kg-1
in the control digester (fig 1). The soluble P in the CPR digester was less than 0.1mg L-1
compared to 13mg L-1 prior to iron dosing. Soluble P in the control digester was 28mg
L-1 and 107mg L-1 in the BPR digester (fig 2). Analysis of the speciation of P in the
CPR digester showed that it remained bound to iron (represented by the NaOH fraction in
fig 3 and 4) but in the reduced ferrous form (represented by the EDTA fraction in fig 5
and 6. This confirmed laboratory-scale results, in which ferric phosphate was found to be
briefly solubilised under anaerobic conditions and re-precipitated as ferrous phosphate.
Iron is normally dosed at a 2:1 molar ratio with phosphorus [8] therefore sufficient iron is
present for re-precipitation of all solubilised phosphorus as Fe3(PO4)2, but if iron is dosed
at lower concentrations than 1.5:1, soluble phosphate could be released during digestion
[7]. Ferrous phosphate is not bioavailable [9], therefore P limitation in a CPR digester
could reduce digester performance [7, 10].
Loss of digester performance may also be caused by excesses or deficiencies of cofactor
metals. Copper and chromium for example are reported as causing toxicity problems [11,
12, 13]. Metal bioavailability and toxicity will decrease as the metal becomes more
difficult to extract. Metal fractionation results showed that copper in both the BPR and
CPR digesters shifted from the residual to the HNO3 fraction (fig 7 and 8). The internal
standards suggest that the residual fraction is CuS and CuFeS2. This may, therefore,
represent a decrease in the formation of chalcopyrite (CuFeS2) because the P enrichment
leaves less soluble iron. Copper speciation was most markedly changed in the CPR
sludge digester, the residual fraction decreasing from 26 % pre-CPR to 6 % post-CPR and
the Na4P2O7 fraction correspondingly increasing from 12 % pre-CPR to 25 % post-CPR,
indicating increased copper bioavailability in this digester. The trend towards increased
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solubility of metals in the phosphorus-rich digesters was more pronounced with
chromium (figs 9 and 10). In the control digester, 58% was in the residual and HNO3
fractions whereas in the P enriched digesters only 35 % (BPR) and 26 % (CPR) of the
chromium was represented by these fractions.
Iron, nickel and cobalt are methanogenic cofactors [14]. The concentrations of iron in the
control and BPR digester were similar (fig 5 and 6) but the control digester was the only
digester with easily available iron (iron in the solubles KNO3 and KF extracts or adsorbed
iron), the CPR digester having the least available iron. The results also show other
changes. In the control digester iron was also present in the HNO3 and residual fractions,
whereas the EDTA fraction (representing iron phosphate) was increasingly important in
the BPR and CPR digesters.
The BPR and control digesters had very similar nickel profiles (figs 11 and 12). The
CPR sludge contained a higher proportion of EDTA-extractable (52 %) and adsorbed (11
%) nickel than either the control and BPR digesters, however, this was not due to CPR as
a similar fractionation profile for Digester C was recorded prior to CPR. The total
concentration of nickel in the CPR digester, like the chrome, was much higher than in
either the control or BPR digesters. The CPR plant does include metal processing
industrial effluent.
The cobalt fractionation results were different for each digester (figs 13 and 14). All the
digesters contained similar concentrations of soluble cobalt but there was less cobalt in
the other labile fractions of the BPR and CPR digesters. The control digester contained
the highest proportion of cobalt in the soluble, KNO3 and KF extracts, i.e. 25 % in the
control 18 % in the BPR digester and 10 % in the CPR digester.
Anaerobic treatment of BPR sludge is known to release soluble phosphate, magnesium
and potassium according to the molar ratios of their uptake during BPR [15]. Phosphate
and magnesium are rapidly re-precipitated but the potassium remains in solution. This is
illustrated in our research (figure 15) the BPR digester contains soluble magnesium at 12
mg L-1 and potassium at 218mg L-1 compared to 35 mg L-1 soluble Mg and 57 mg L-1
soluble K in the control and 45mg -1 L soluble Mg and 60mg L-1 K in the CPR sludge
digester. Using the soluble potassium concentrations in a BPR digester, it is possible to
predict the total concentrations of magnesium and phosphate released from the BPR
sludge [15]. This is shown in fig 15, which indicates that the theoretical concentrations
of phosphate and magnesium released into the BPR digester were 590mg L-1 and 120mg
L-1, respectively. Thus, only 10 % of the released soluble magnesium and 20 % of the
soluble phosphorus remained in solution indicating precipitation of both magnesium and
phosphorus in this digester. In spite of this, the residual soluble P concentration in the
BPR digester was still ten times greater than that of the control and 100 times greater than
the CPR digester.
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Previous work has indicated that struvite [16] could be a problematic sink for some of
this released phosphorus. Analysis of the speciation results described here indicates that
the largest P fraction was calcium phosphate in both the BPR and control plant (extracted
in the HCl fraction) (figures 3 and 4). In the CPR digester, on the other hand, most of the
P is in the NaOH extract as soluble reactive phosphate (ferrous phosphate). This was
confirmed by the internal standards, the laboratory work and an analysis of the iron
fractionation. Model compound testing showed that magnesium from struvite was
recovered in the pyrophosphate (Na4P207 fraction), and the BPR digester does show the
highest percentage of magnesium in this fraction (20 %) (fig 16 and 17). These results are
supported by Jardin and Pőpel [15] who calculated that 20% of the P released from BPR
sludge formed struvite and Wild et al [17] who also noted that 15% of the re-solubilised
P formed struvite and 33% calcium phosphate. The relatively high proportions of
magnesium extracted in the EDTA fractions of both the BPR and CPR digesters (29 and
40 % in comparison to 15 % in the control digester) was attributed to precipitation of
magnesium-calcium phosphate in the BPR digester.
Laboratory experiments indicated that soluble P concentrations needed to be above
500mg L-1 or the pH to be more alkaline than the neutral for struvite to become more
important. Borgerding [18] found similar results but also noted struvite precipitated on
cooling down from 35ºC. The struvite formed in the BPR digester is twice that in the
control digester but it is suggested below these critical thresholds (fig 16 and 17).
There was little struvite formation in the CPR reactor (fig 16 and 17) which helps confirm
Mamais et al [19] suggestions that iron dosing can prevent struvite formation since
ferrous phosphate is formed preferentially to struvite. Water hardness may also be
important since calcium phosphate complexes are also precipitated more readily.
CONCLUSION AND RECOMMENDATIONS
1. Phosphorus in CPR digesters is precipitated as ferrous phosphate whereas in BPR
digesters calcium is the major sink.
2. CPR causes inorganic enrichment of anaerobic biomass and may lead to decreased
digester performance. There is less available iron and phosphate and this needs
further research to confirm its importance compared to encapsulation of the volatile
solids.
3. There were only minor changes in solubility of the potentially toxic metals chrome
and copper or the important nutrient metals nickel and cobalt.
4. Struvite was shown to be one of the by-products of BPR but iron and calcium are
antagonistic. Concentrations of soluble P above 500 mg L-1 and a pH greater than
7.8 were necessary to increase the proportion of struvite.
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5. Iron has a high affinity for phosphate in the anaerobic digester and small doses in
combination with BPR could be used to avoid any problems with struvite or recycling
of soluble P. This needs more research.
6. An Internationally agreed standard methodology for P and metal speciation is
suggested to allow comparison of results.
ACKNOWLEDGEMENTS
The work was supported by EPSRC grant GR/K96946. The authors would also like to
thank Severn Trent Water for additional financial support, help, information and samples
from working digesters.
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REFERENCES
1.
Yeoman S., Lester J.N. and Perry R., The effects of chemical phosphorus precipitation on
anaerobic digestion. Environmental Technology., 11, 709-720 (1990).
2.
Kindzierski W.B. and Hrudey S.E., Effects of phosphorus removal chemicals upon methane
production during anaerobic sludge digestion. Canadian Journal of Civil Engineering.,
13, 33-38 (1986).
3.
Pitman A.R., Management of biological nutrient removal plant sludges – change the paradigms?
Water Research., 33, 1141-1146 (1999).
4.
Stover R.C., Sommers L..E. and Silveira D.J., Evaluation of metals in wastewater sludge.
Journal of the Water Pollution Control Federation., 48, 2165-2175 (1976).
5.
Uhlmann D., Roske I., Hupfer M. and Ohms G., A simple method to distinguish between
polyphosphate and other phosphate fractions of activated sludge. Water Research., 24,
1355-1360 (1990).
6.
Eaton A.D., Clesceri L.S. and Greenberg A.E., (Ed) Standard methods for the examination of
water and wastewater (19th edition). APHA-AWWA-WEF (1995).
7.
Carliell-Marquet C.M., The effect of phosphorus enrichment on fractionation of metals and
phosphorus in anaerobically digested sludge. PhD thesis Loughborough (2001).
8.
Upton J., Nutrient removal in the UK – Now and in the future. Paper presented at the
International Conference on Phosphorus Recovery (CEEP), Warwick University, UK.
May 6-7 (1998).
9.
Frossard E. and Morel J.L., Phosphorus species and availability in FeSO4 treated sludges. In:
Proceedings of the 2nd ESA Congress, Warwick University, UK, August, 408-409
(1992).
10.
Alphenaar P.A, Sleyster R., de Reuver P., Ligthart G.J. and Lettinga G., Phosphorus requirement
in high-rate anaerobic wastewater treatment. Water Research., 27, 749-756 (1993).
11.
Mosey F.E., Swanwick J.D. and Hughes D.A., Factors affecting the availability of heavy metals
1
to inhibit anaerobic digestion. Water Pollution Control., 70, 668-679 (1971).
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Mehrota A., Mehrota I. and Tandon S.N., Role of Cu (II) and Zn (II) species on the anaerobic
digestion of synthetic activated sludge. Journal of Chemical Technology and
Biotechnology., 56, 185-190 (1993).
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13.
Barber W.P. and Stuckey D.C., Metal bioavailability and trivalent chromium removal in ABR.
Journal of Environmental Engineering., 126, 649-656 (2000).
14.
Speece R.E., A survey of municipal anaerobic sludge digesters and diagnostic activity assays.
Water Research., 22, 365-372 (1988).
15.
Jardin N. and Pöpel H.J., Phosphate release of sludges from enhanced biological P-removal
during digestion. Water Science and Technology., 30, 281-292 (1994).
16.
Sen D. and Randall C.W., Factors controlling the recycle of phosphorus from anaerobic digesters
sequencing biological phosphorus removal systems. Hazardous and Industrial Wastes.,
20, 286-298 (1988).
17.
Wild D., Kisliakova A. and Siegrist H., Prediction of recycle phosphorus loads from anaerobic
digestion. Water Research., 31, 2300-2308 (1997).
18.
Borgerding J., Phosphate deposits in digestion systems. Journal of the Water Pollution Control
Federation., 44, 813-819 (1972).
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Mamais D., Pitt P.A., Cheng Y.W., Loiacono J. and Jenkins D., Determination of ferric
chloride dose to control struvite precipitation in anaerobic sludge digesters. Water
Environment Research., 66, 912-918 (1994).
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Table 1: Successive stages of the metals extraction method (Stover et al., 1976).
Metal fraction
Reagent
Concentration
Extraction time
Liquid: solid ratio
(ml:g DS)
Exchangeable
metals
KNO3
1 M, (pH 6.5)
15 h
50 :1
Adsorbed metals
KF
0.5 M, (pH 6.5)
15 h
80:1
Organic-bound
metals
Na4P2O7
0.1 M, (pH 9.95)
15 h
80:1
Carbonate
precipitates
EDTA
0.1 M, (pH 4.63)
15 h
80:1
Sulphide
precipitates
HNO3
1.0 M, (pH 0.25)
15 h
50 :1
Residual metals
Aqua Regia
4%
Approx. 1 h
Sludge pellet + 1.3
ml conc. HNO3 + 2.7
ml conc. HCl
Table 2: Successive stages of the modified phosphorus extraction method.
Phosphorus
Fraction
Labile P
Struvite/ CaCO3 P
CaCO3 P
Fe P; Al P, Org P
Calcium P
Residual P
Reagent
Concentration
Time
Deionised water
(deoxygenated)
Acetate buffer
Deionised water
Acetate buffer
Deionised water
NaOH
Deionised water
HCl
Deionised water
Acid digestion
(boiling)
(pH 6.2)
20 min
0.1 M (pH 5.2)
45 min
5 min
30 min
5 min
18 h
5 min
18 h
5 min
Approx. 1 h
0.1 M (pH 5.2)
1 M (pH 13.76)
0.5 M (pH 0.6)
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Liquid: solid ratio
(ml:g DS)
0.5 : 30
0.5 : 30
0.5 : 30
0.5 : 30
0.5 : 30
0.5 : 30
0.5 : 30
0.5 : 30
0.5 : 30
Sludge pellet + 1.3
ml conc. HNO3 + 2.7
ml conc. HCl
Table 3: Model compounds used to evaluate the metal and phosphorus fractionation methods.
Phosphates
Ca3(PO4)2
CaHPO4
MgNH4PO4.6H2O
Mg3(PO4)2.8H2O
FePO4
Fe3(PO4)2.8H2O (crystalline)
Fe3(PO4)2.8H2O (amorphous)
AlPO4
MnHPO4.xH2O
Cr(III)PO4
Cu3(PO4)2.2H2O
Zn3(PO4)2.2H2O
Carbonates
CaCO3
3MgCO3.Mg(OH)2.3H2O
FeCO3
MnCO3
CuCO3.Cu(OH)2
ZnCO3.2Zn(OH)2.H2O
NiCO3.xH2O
Sulphides
FeS
MnS
CuS
ZnS
NiS
Table 4: Details of the full-scale anaerobic digesters.
Parameter
CHARACTERISTICS
Digester volume (m3)
Draw-off point
OPERATING CONDITIONS
Feed characteristics
pH
T (°C)
Retention time (d)
PERFORMANCE
Gas production (m3 m-3 day-1)
Methane %
Volatile solids removal (%)
Control
Domestic
390
Top
Site
CPR
Domestic and Industrial
3745 (1872.5 x 2)
Top
BPR
Domestic and Industrial
1000
Top
Mostly co-settled primary
and humus sludge; small
amount of waste activated
sludge
7.46  0.04
36-37
16
Primary and humus sludge.
Primary sludge (62 % by
weight); WAS (38 % by
weight).
7.66  0.09
35-36
22
7.28  0.03
33
15
0.53
Not measured
-
0.48
75 – 80 %
-
Not measured
72 %
42 %
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Concentration, mg/kg of dried sludge
Dig C
Dig B
Dig A
60000
40000
20000
0
Phosphorus
Calcium
Magnesium
Iron
Aluminium
Fig 1 Average total concentration of P, Ca, Mg, Fe and Al in the digested sludge from the
anaerobic digesters. A is control. B = BPR. C = CPR. Data is presented as mg/kg.
Concentration, mg/l in digested sludge
Dig A
Dig B
Dig C
300
200
100
0
Soluble P
Soluble Ca
Soluble Mg
Fig 2: Average concentrations of soluble P, Ca and Mg in the digested sludge of anaerobic
digesters A is control. B = BPR. C=CPR. Data is presented as mg/kg.
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Phosphorus, mg/kg of dried sludge
32000
810
617
Residual
4812
HCl
24000
4474
NaOH Org. P
8978
NaOH SRP
538
16000
Acetate 2
3903
6327
3410
8000
19473
Acetate 1
3027
2627
Water
1420
3862
Soluble
Dig A
Dig B
2589
0
Dig C
Fig 3: Phosphorus fractionation profiles of full-scale digesters . A is control. B = BPR.
C=CPR. Data is presented as mg/kg.
100%
Residual
16
80%
39
HCl
35
15
Phosphorus
NaOH Org. P
60%
NaOH SRP
15
19
Acetate 2
40%
13
64
16
20%
6
Acetate 1
5
10
Water
9
15
Soluble
7
0%
Dig A
Dig B
Dig C
Fig 4: Phosphorus fractionation profiles of full-scale digester. A is control. B=BPR. C=CPR.
Data is presented as % of the total phosphorus concentration in the digested sludge.
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70000
Residual
Iron, mg/kg of dried sludge
60000
HNO3
50000
39783
EDTA
40000
Na4P2O7
30000
KF
20000
25994
10000
2529
4222
KNO3
4229
Soluble
5293
0
Dig A
Dig B
Dig C
Fig 5: Stover Iron fractionation profiles of full-scale digesters. A is control. B=BPR. C=CPR.
Data is presented as mg/kg.
100%
13
Residual
11
HNO3
20
80%
13
57
60%
EDTA
33
Iron
22
Na4P2O7
40%
KF
37
41
20%
37
KNO3
Soluble
7
0%
Dig A
Dig B
Dig C
Fig 6: Stover Iron fractionation profiles of full-scale digesters. A is control. B=BPR. C=CPR
Data is presented as % of the total iron in the digested sludge.
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300
Copper, mg/kg of dried sludge
Residual
250
30
17
15
200
92
101
HNO3
EDTA
147
150
Na4P2O7
49
71
100
49
63
41
50
32
27
7
0
Dig A
Dig B
18
KF
KNO3
Soluble
Dig C
Fig 7: Stover copper fractionation profiles of full-scale digesters. A is control. B=BPR. C=CPR.
Data is presented as mg/kg.
100%
12
7
6
Residual
80%
HNO3
41
34
EDTA
Copper
58
60%
Na4P2O7
40%
20
27
KF
19
20%
15
25
KNO3
7
Soluble
13
10
0%
Dig A
Dig B
Dig C
Fig 8: Stover copper fractionation profiles of full-scale digesters. A is control. B=BPR.
C=CPR. Data is presented as % of the total copper in the digested sludge.
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Chromium, mg/kg of dried sludge
Residual
200
HNO3
17
150
27
EDTA
Na4P2O7
100
56
KF
50
9
7
18
11
68
KNO3
Soluble
10
22
Dig A
Dig B
0
Dig C
Fig 9: Stover chromium fractionation profiles of full-scale digesters. A is control. B=BPR.
C=CPR. Data is presented as mg/kg.
100%
10
Residual
16
HNO3
19
80%
37
16
Chromium
EDTA
60%
15
32
21
Na4P2O7
13
KF
40%
48
20%
40
21
Soluble
8
1
0%
Dig A
KNO3
Dig B
Dig C
Fig 10: Stover chromium fractionation profiles of full-scale digesters. A is control. B=BPR.
C=CPR. Data is presented as % of the total chromium in the digested sludge.
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300
Residual
Nickel, mg/kg of dried sludge
250
15
200
HNO3
EDTA
126
150
Na4P2O7
KF
100
51
KNO3
50
17
27
2
0
Dig A
Dig B
Soluble
Dig C
Fig 11: Stover nickel fractionation profiles of full-scale digesters. A is control. B=BPR.
C=CPR. Data is presented as mg/kg.
100%
6
25
Residual
21
HNO3
80%
21
24
52
EDTA
Nickel
60%
Na4P2O7
40%
30
27
KF
21
20%
10
12
8
9
0%
Dig A
KNO3
7
5
5
11
Dig B
Dig C
Soluble
Fig 12: Stover nickel fractionation profiles of full-scale digesters. A is control. B=BPR.
C=CPR. Data is presented as % of the total nickel in the digested sludge.
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6
Cobalt, mg/kg of dried sludge
Residual
5
1.12
HNO3
1.13
4
EDTA
1.59
1.25
Na4P2O7
3
0.68
1.02
1.05
2
KF
1.22
1
1.09
0.89
0.35
KNO3
0.00
Soluble
0
Dig A
Dig B
Dig C
Fig 13: Stover cobalt fractionation profiles of full-scale digesters. A is control. B=BPR.
C=CPR. Data is presented as mg/kg.
100%
Residual
21
23
22
80%
HNO3
25
29
60%
EDTA
Cobalt
40
Na4P2O7
40%
21
19
KF
7
6
12
20%
0%
22
16
KNO3
7
6
4
7
5
6
Dig A
Dig B
Soluble
Dig C
Fig 14: Stover cobalt fractionation profiles of full-scale digesters. A is control. B=BPR.
C=CPR. Data is presented as % of the total cobalt in the digested sludge.
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600
Concentrations in digested sludge, mg/l
590
500
400
300
200
120
116
Mg (sol) predicted
P (sol) measured
100
12
0
K (sol) measured
K (sol) predicted
Mg (sol) measured
P (sol) predicted
Figure 15 Prediction of soluble magnesium and phosphorus concentrations released into the
Digester B as a result of BPR sludge digestion.
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Magnesium, mg/kg of dried sludge
561
6000
Residual
314
1013
HNO3
5000
1535
383
EDTA
954
4000
2682
Na4P2O7
1768
731
3000
521
KF
410
2000
871
198
1218
KNO3
1000
1847
842
1536
Soluble
424
0
Dig A
Dig B
Dig C
Fig 16: Stover magnesium fractionation profiles of full-scale digesters. A is control. B=BPR.
C=CPR. Data is presented as mg/kg.
100%
80%
8
19
5
25
8
Residual
15
HNO3
Magnesium
15
EDTA
60%
10
40%
29
40
Na4P2O7
17
20
KF
6
3
KNO3
20%
31
14
27
Soluble
7
0%
Dig A
Dig B
Dig C
Fig 17: Stover magnesium fractionation profiles of full-scale digesters. A is control. B=BPR.
C=CPR. Data is presented as % of the total in the digested sludge.
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